arXiv:astro-ph/0409490v1 21 Sep 2004 h oie ula niomn nNC95a seen as 985 NGC in environment nuclear ionized The .Krongold Y. noalrenme fclouds. of co a number be large fragmenta might a requires wind into scenario a pressure-confining such si The that medium AGN. suggests 3-phase in further a and to 3783, points NGC componen analysis in hotter our third Hence, a absor phases. that both two of requires f opacity but arises in observations, change absorption the the the explain that du can suggest component scenario that Such we absorbing is Thus, two analysis the pressure. our increa same observations, in to result BeppoSAX striking seems the most component and phot var The hotter of continuum the drops. picture the of flux simple to state a ionization response with the consistent in is detected component is colder absorbers the of ra ad(.-0 e)cniumi h epSXdt a b can data BeppoSAX (Γ the powerlaw in continuum a keV) (0.1-100 band broad e) nXrylmnst aito yafco . sobserv is 2.3 factor a and by variation luminosity X-ray An keV). C diin h o oiaincmoetpoue significan components produces absorption component the ionization low of the velocities addition, outflow the with and hs n 197 and phase u h aade o lo ocntanispoete.Thou (581 properties. components its the constrain of to velocities allow outflow not the does data the but h .- e nrni otnu a ewl ersne by represented well be can continuum intrinsic keV 0.6-8 the oie bobradasf xes ealdmdloe the over indica model model, detailed A powerlaw excess. single soft a a and absorber to ionized residuals strong exhibit o oiaincmoeti vdne ya eMselunres M-shell Fe states an charge by by densi column evidenced produced in is (UTA) 3 component and parameter ionization ionization low in 29 the of in difference observed a features absorption the compone fit absorption to Two keV). (kT=0.1 component blackbody a ytepeec fbodasrto etrsaiigfo se from arising features absorption (Fe broad transitions of presence the by Chandra iv eivsiaeteinzdevrneto h efr gala 1 Seyfert the of environment ionized the investigate We Chandra hc ugssta igeoto rdcsteU n X-ray and UV the produces outflow single a that suggests which HTSosrainada rhvlBpoA bevto.B observation. BeppoSAX archival an and observation -HETGS 1 .Nicastro F. , bevtos(eaae yams er) aiblt in Variability years). 3 almost by (separated observations ± ≈ xvii-xxii 8 ms km 184 1 yCadaadBeppoSAX and Chandra by . ) lcbd k=. e) n iheeg uo (E cutoff energy high a and keV), (kT=0.1 blackbody a 4), .Atidhgl oie opnn ih lob present, be also might component ionized highly third A ). − 1 o h o oiainoe r ossetwt ahother, each with consistent are one) ionization low the for 1 .Elvis M. , ABSTRACT vii Zezas to xiii 1 1 Chandra ..Brickhouse N.S. , h ihinzto hs srequired is phase ionization high The . 1 ± 0 ms km 206 pcrm h opnnshave components The spectrum. mut fO of amounts t − dbtenteBeppoSAX the between ed ino h ofie phases confined the of tion spesr-ofiigthe pressure-confining is t oelw(Γ powerlaw a ea lnso eL-shell Fe of blends veral 1 to opnnsduring components ption Chandra o ut-hs wind. multi-phase a rom igtepeec fan of presence the ting inzto equilibrium, oionization ewietecontinuum the while se ia otewn found wind the to milar yNC95wt new a with 985 NGC xy bevdi h V In UV. the in observed o h ihionization high the for y h rsneo the of presence The ty. igbt the both ring hpol constrained, poorly gh t r lal required clearly are nts le rniinarray transition olved ain u hl the while but iation, pert aethe have to appear s mncharacteristic mmon aaeeie by parameterized e 1 .Mathur S. , aasosthat shows data etrs The features. vi t spectra oth h opacity the ≈ N , 1 Chandra . )plus 6) c v and , ≈ 2 70 A. , 1. INTRODUCTION vational data are certainly pointing to a common nature for the UV and X-ray out- Highly ionized (or ‘warm’) absorbers flows (see §5.1). (Halpern 1982) have been observed in New studies based on high quality data about half of all UV and X-ray spectra of have shown that the absorbers can be de- AGNs (Reynolds 1997, George et al. 1998, scribed in a surprisingly simple way. For Crenshaw et al. 1999). The absorption example, Krongold et al. (2003) mod- lines of these systems are blueshifted with eled the 900 ksec Chandra HETG expo- respect to the optical emission lines, which sure on NGC 3783 (Kaspi et al. 2002) implies outflow of material. The mass loss and found that most of the absorption fea- rates inferred from these outflows can be tures present in the spectrum could be re- a substantial fraction of (Mathur, Elvis & produced by a model with only two ab- Wilkes 1995) or even larger than (Behar et sorption components, and that these com- al. 2003, Ogle et al. 2004) the accretion ponents appeared to be in pressure bal- rates needed to power the AGN contin- ance. This result was confirmed by Net- uum, indicating that these ‘quasar winds’ zer et al. (2003), who found a third highly play an important role in the mass and ionized component, also in pressure equi- energy budgets of AGNs. librium with the other two. Is this single The UV and X-ray absorbers must be object result only a coincidence? Or, on closely related, as exactly the same AGNs the contrary, does it suggest that promis- show both (Mathur, Elvis, & Wilkes 1995; ing advances in our understanding of the Mathur, Wilkes, & Elvis 1998; Crenshaw physical conditions of AGN winds can be et al. 1999; Monier et al. 2001). Further- achieved, carrying important repercussions more, their kinematic properties also ap- in our understanding of the structure of pear to be related, as both absorbers show quasars (e.g. Elvis 2000)? similar outflow velocities (∼1000 km s−1). Here, we present Chandra HETG and Given the intrinsic complexity of UV and BeppoSAX observations of the Seyfert 1 X-ray spectra, along with the large differ- galaxy NGC 985 and show that the X-ray ence in spectral resolution in the two bands spectra of this source can also be well de- (with an order of magnitude lower spec- scribed by a simple model with two absorp- tral resolution in the X-ray region), it has tion components. Furthermore, as for the been extremely difficult to connect these ionized absorber toward NGC 3783, these absorbers in detail, leading to highly com- components are consistent with pressure plicated models to describe them. Never- equilibrium, with one phase having suffi- theless, recent studies using more compre- ciently low ionization to produce substan- hensive models have shown that the obser- tial absorption in the UV region. We fur- 1Harvard-Smithsonian Center for Astro- ther discuss the implications of these re- physics. 60 Garden Street, Cambridge MA sults. 02138 2Department of Astronomy, Ohio State Univer- sity, 140 West 18th Avenue, Columbus, OH 43210

2 1.1. NGC 985 1996, NGC 985 was observed by ASCA with a total exposure of 40 ksec. The anal- NGC 985 (Mrk 1048) is a Seyfert 1 ysis of these data confirmed the presence of galaxy located at redshift 0.04274±0.00005 an ionized absorber in the spectra of this (12814±15 km s−1; Arribas et al. 1999, source (Nicastro et al. 1998, 1999). Nicas- based on stellar absorption features). NGC tro et al. found that photoelectric bound- 985 is a peculiar galaxy with a promi- free edge absorption could not account for nent ring shaped zone at several kilopar- all the features observed, and suggested secs from the nucleus (de Vaucouleurs & that resonant bound-bound line absorption de Vaucouleurs 1975), suggesting that the played an important role. galaxy is undergoing a merging process. Further evidence for a merger has been This possibility was supported by HST- found through optical and near-IR ob- STIS and FUSE ultraviolet observations of servations, which have revealed the pres- the nucleus in NGC 985, which have re- ence of a double nucleus (Perez Garcia & vealed the presence of six narrow absorp- iv Rodriguez Espinosa 1996, and references tion components detected in Ly α, C , v vi therein). The nuclei are separated by 3” N and O (Arav 2002). Five of these (2.5 kpc of projected linear distance; Ap- systems are observed in outflow, with ve- pleton et al. 2002), which indicates that locities (with respect to the centroid of the narrow emission lines) of 780, 670, 519, the intruder galaxy responsible for the for- −1 mation of the ring is sinking in the nuclear 404, and 243, km s (hereafter compo- nents 1 to 5). The sixth component is potential of the primary galaxy (Perez- −1 Garcia & Rodriguez-Espinosa 1996). As observed inflowing at −140 km s . As in other galaxies undergoing a merger, demonstrated by Arav, small to moderate the IR luminosity of NGC 985 is extreme, saturation is present in the Ly α trough 11 of these components. Once the effects of LIR =1.8×10 L⊙, which puts this object in the LIRG group. saturation are accounted for, at least com- ponent 4 is consistent with gas that also An ionized absorber has been observed produces X-ray absorption, i.e. a highly in the X-ray spectra of NGC 985. This ionized or ‘warm’ absorber, with a column source has been observed in the X-rays density of 1021.4 cm−2. with different satellites and is a strong X- ray source. The first detection was with In this paper we present 2 moderate ex- the Einstein Observatory, measuring an posure observations of NGC 985 (∼ 100 0.2-4.0 keV flux of 1.76×10−11 erg s−1 cm−2 ksec each) with Chandra and BeppoSAX. (a luminosity of 1.4×1044 erg s−1; Kruper, In §2 we describe the reduction and spec- tral analysis of the Chandra-HETGS data. Urry, & Canizares 1990). Subsequently, 1 spectral analysis of ROSAT-PSPC data The analysis was carried out in Sherpa revealed complex features (Brandt et al. (Freeman, Doe, & Siemiginowska 2001), 1994). A simple power law could not fit with the inclusion of the code PHASE the PSPC spectrum of NGC 985, and the (Krongold et al. 2003) to model in a presence of a warm absorber, as well as a self-consistent way the ionized absorber continuum soft excess, was suggested. In present in the spectra. In §3 we present the

3 BeppoSAX data, and in §4 we analyze the der spectra to improve the signal to noise variability effects of the source on the ab- (S/N) ratio of the data. Similarly, after sorber. In §§5 and 6 we discuss the differ- checking for wavelength scale consistency, ent scenarios consistent with the absorber the HEG and MEG spectra and their re- and their implications. sponses were coadded in the overlapping region, to further increase the S/N ratio. 2. THE Chandra-HETG SPECTRUM The S/N ratio per MEG resolution element (≈0.02 A)˚ varies from ≈ 7 to ≈ 2 in the 2-20 A˚ range, a factor ≈ 6 lower than the On 2002 June 24, the High Energy highest S/N ratio HETG spectrum of a Sy Transmission Grating Spectrometer (HETGS, 1 galaxy (NGC 3783; Kaspi et al. 2002). Canizares et al. 2000) on board the All the spectra presented throughout the Chandra X-ray Observatory (Weisskopf et paper (except where otherwise stated) cor- al. 2000) observed NGC 985 (see Table respond to a binning factor of 20 (i.e we 1). The observation was carried out with binned the spectrum in bins of size 0.1 the Advanced CCD Imaging Spectrometer A,˚ about 5 times the MEG resolution el- (ACIS; Garmire et al. 2003) in the focal ement), giving a S/N ratio ranging from plane, and had a duration of 80 ksec. We 14 to 3 per bin (in the 2-20 A˚ range). The processed the event files of this observa- spectrum is shown in Figure 1a. tion with the Chandra Interactive Analysis of Observations (CIAO2; Fruscione 2002) 2.1. Spectral Analysis software (Version 2.3), following the on- line data analysis “threads” provided by In the calculations reported here, we at- the Chandra X-ray Center (CXC3), and tenuated the continuum by an equivalent 20 −2 extracted source and background 1st or- hydrogen column density of 3.0×10 cm der spectra (positive and negative orders) to account for the Galactic absorption by and responses for the High-Energy Grat- cold gas in the direction to the source ing (HEG) and Medium-Energy Grating (Starck et al. 1992). We have explored (MEG) configurations. We accounted for only photoionization equilibrium models, the ACIS efficiency degradation (Marshall and we have assumed solar elemental abun- et al. 2003) using the script contamabs4 dances (Grevesse & Noels 1993). (version 1.1). The net exposure of the ob- servation is 77.7 ksec and the total num- 2.1.1. Fitting the Continuum ber of counts in the dispersed spectrum is To model the intrinsic continuum of the 13,979. source, we first used a simple power law. We then combined the -1st and +1st or- This model could not fit the data over the entire energy range (0.6-8 keV, 1.5-20 A),˚ 1 http://cxc.harvard.edu/sherpa as can be seen from the residuals (Fig. 2a). 2 http://asc.harvard.edu/ciao The model shows both positive and neg- 3 http://asc.harvard.edu/ciao/documents threads.html ative deviations (with significance > 2σ) 4 http://space.mit.edu/CXC/analysis/ACIS Con- typical of the ionized absorber and soft ex- tam/ACIS Contam.html cess commonly observed in Seyfert 1 galax-

4 ies (e.g. Reynolds 1997). by Galactic absorption, and (c) the ob- We repeated the power law fit using served continuum (i.e. the continuum fur- only data between 3 and 8 keV (we also ther attenuated by the bound-free transi- excluded the Fe Kα line region around tions from the ionized absorber, see details 6.4 keV). At these energies, absorption by in Fig. caption). moderately large columns of ionized gas (i.e. < 1023 cm−2) or low columns of neu- 2.1.2. Modeling the Ionized Absorber 21 −2 tral gas (i.e. < 10 cm , Galactic gas) We ran PHASE with three free parame- do not modify significantly the shape of ters for each component necessary to fit the the intrinsic continuum (e.g. Krongold et ionized absorber: (1) the ionization param- al. 2003). The results of this model are eter (defined over the entire Lyman contin- summarized in Table 2, and the residuals 2 uum, as U = Q(H)/4πr nH c with Q(H) (extrapolated to the whole spectral range) being the rate of H ionizing photons, r the are presented in Figure 2b. The presence distance to the source, nH the H density, of strong positive and negative deviations and c the speed of light.), (2) the equiva- below 3 keV in this model shows that both lent hydrogen column density NH , and (3) an ionized absorber and a soft excess are the outflow velocity. Other parameters of likely to be present. To account for the the code that we kept fixed are the internal presence of the excess, we further included micro-turbulent velocity of the absorbers a blackbody component in our continuum and the intrinsic Spectral Energy Distri- determinations, and the positive residuals bution (SED) of the source (Fig. 3a). The decreased significantly (see Fig. 2c). How- turbulent velocity of the gas is very diffi- ever, the model still shows strong nega- cult to constrain, since (1) single absorp- tive residuals at energies characteristic of tion lines in the spectrum are unresolved an ionized absorber. by the HETGS (which has a FWHM res- Finally, we fitted the continuum and olution of ≈ 400 km s−1at 17 A)˚ and (2) the absorption together (see §2.1.2 for the most of the observed features are blends details of the ionized absorber). In this of several transitions. We set this parame- model we left all the continuum compo- ter to 350 km s−1, the value most suitable nents (power law and blackbody) free to to the data after several tests comparing vary, and we assumed that the absorbing the measured equivalent widths (EWs) of gas also covers the soft excess. The re- a few unblended lines (Si xiv, Si xiii, Mg sults obtained for the power law with this xii, Mg xi, Fe xvii) with the EWs pre- approach are in excellent agreement with dicted by our model. the results obtained when considering only To approximate the SED, we proceeded the 3-8 keV range. Table 2 presents the as follows: in the X-ray region we used values for the best continuum fit. Figure the power law Γ, as well as the blackbody 1a shows the fluxed data over the entire contribution, as inferred from the fits to spectral range (1.5-20 A,˚ 0.6-8 keV), to- the continuum presented in §2.1.1. To ob- gether with (a) the intrinsic continuum of tain the exact parameters for the black- the source, (b) the continuum attenuated body component several iterations were

5 performed. The BeppoSAX data of NGC considered). 985 show the presence of a high energy cut- off in the intrinsic continuum (see §3.1.1), 2.1.3. The Ionized Absorber which we modeled with a break in the X- As is the case for other ionized absorbers rays at around 70 keV, and a powerlaw in AGNs (Kaspi et al. 2001; Blustin et. al with Γ = 5 above this energy. Based 2002; Krongold et al. 2003; Netzer et al. on HST-STIS observations of NGC 985, 2003 for NGC 3783; Kaastra et al. 2002 Bowen, Pettini & Blades (2002) found a for NGC 5548; Steenbrugge et al. 2003 for flux of 2.54×10−26 erg s−1 cm−2 hz−1 at NGC 4593; Blustin et al. 2003 for NGC 1230 A.˚ In order to determine the flux at 7469) the ionized absorber in the spectrum the Lyman limit, we extrapolated to 912 of NGC 985 could not be well fitted by A˚ assuming a power law with photon in- a single absorbing component (χ2/d.o.f.= dex Γ =2. The flux at 912 A˚ turned out 201.6/177). Therefore, we attempted to to be 1.83 ×10−26 erg s−1 cm−2 hz−1. Ex- fit the spectrum with the next simplest trapolation of the X-ray continuum down model, by including a second component. to the Lyman limit gives a flux two or- In this case, we obtained an acceptable fit ders of magnitude smaller than measured to the data (χ2/d.o.f.= 179.03/174; an F- by HST, which indicates the presence of a test indicates that the second component is soft X-ray-UV energy break, with a steep- required at a significance level larger than ening of the spectrum between the two 99.9%). Table 3 lists our results, and the bands (Fig. 3a). We assumed that this best fitting model is presented in Figure 4 break occurs at 0.1 keV. The actual en- (and also in Fig. 1b, where we show the ergy of this break is unknown (due to the model for the full spectral range plotted unobservable region in the UV-X-ray range over the fluxed spectrum). Most of the nar- associated with Galactic absorption). We row features observed in Figure 4 have a restricted the SED at low energies using significance < 2σ, but still the absorber is (not simultaneous) data from NED5 (see clearly detected and constrained through Fig.3a). For simplicity we extended the ex- the presence of relatively broad features trapolation between 1230 A˚ and 912 A˚ up due to both photoelectric bound-free ab- to 100 µm (i.e. we considered a power law sorption and blends of resonant bound- with Γ =2 from 912 A˚ to 100 µm). Below bound transitions (i.e. Fe L-shell and Fe 100 µm we introduced a low energy cut- M-shell absorption). To highlight the pres- off with Γ = −3.5. The final SED used ence of these features (which are significant to model the Chandra data is presented in at a level > 2.5σ), in Figure 5 we present Figure 3a. The differences in the infrared our model and the data in the 12-18 A˚ region between NED data and our assumed range, with a binning size of 0.15 A˚ (bin- SED do not have any effect on our analysis ning factor of 30). (the only effect such differences produce is an increase of the Compton temperature Two different ionization absorbing com- by a factor <1.1, when the NED data are 5 NASA/IPAC Extragalactic Database, http://nedwww.ipac.caltech.edu

6 ponents are clearly required by the data. fixing the velocity of both HIP and LIP The presence of significant features pro- at the velocity of the strongest UV com- duced by both Fe L-shell transitions (Fe ponent (4, outflowing at 404 km s−1; see xvii-xxii) and Fe inner M-shell transitions §1 and Fig. 8) yields an acceptable fit to (Fe vii-xiii, the unresolved transition ar- the data, with parameters fully consistent ray or UTA) is incompatible with a sin- with those obtained before. gle ionization degree for the absorbing gas. In the former analysis we have assumed This is shown in Figure 6, where the con- that the soft excess detected in the data tribution from the two components to the is covered by the absorber. Soft X-ray ex- absorption is presented separately. The cesses have been studied in low resolution high ionization phase (hereafter HIP), with spectra for more than a decade (e.g., Elvis logU=1.34, gives rise to the absorption by et al. 1991, and references therein) and Oviii, Ne ix-x, Mg xi-xii, Si xiii-xiv, and they have been found to vary on timescales Fe L-shell. The low ionization phase (here- of tens of days. This effect has been con- after LIP), with logU=-0.12, is responsible firmed in high resolution spectra (Netzer for the absorption by charge states O vii, et al. 2003), and suggests that the bulk Ne v-ix, Mg ix, and the Fe M-shell UTA. of the soft X-ray emission comes from a The components are well separated in ion- compact region (less than 1016 cm across; ization degree, the HIP has a value ≈ 29 Elvis et al. 1991). It has been suggested times larger in U than the LIP. Accord- that the emission might have a thermal na- ingly, the HIP has a higher temperature ture and might originate in the inner edges (logT=5.8 [K]) than the LIP (logT=4.5 of accretion disks (e.g. Czerny & Elvis [K]) by a factor of 21. These temperatures 1987). Therefore, it is reasonable to as- were calculated assuming photoionization sume that the absorbing material lies fur- equilibrium in each component. Figure 7 ther away from the central engine than the shows the theoretical spectral features pre- soft X-ray emission. Nevertheless, we note dicted for the absorption components. Ta- that the data does not allow to distinguish ble 5 presents the predicted column densi- whether the soft component is covered by ties for the dominant charge states in the the absorber or not. A fit assuming an un- LIP and the HIP. covered excess is statistically indistinguish- As has been observed in other ionized able from a fit which assumes full cover- absorbers (see above), the HIP has a larger ing. If the excess is not absorbed, then equivalent H column density than the LIP, the column density of the LIP is 1.6 times in the case of NGC 985 by a factor of larger and the normalization of the black- 3. The outflow velocity of the compo- body 2.3 times smaller. The rest of the nents is 581±206 km s−1 for the HIP and parameters in this model are fully consis- 197±184 km s−1 for the LIP. These veloc- tent with the values obtained before. We ities, though ill-determined, are consistent stress that whether the soft excess is cov- with each other (within ∼ 1σ) and with ered or not covered by the absorber, this those observed in the HST and FUSE sys- has a negligible effect on the conclusions tems (see Fig. 8 for more details). A fit presented in this paper.

7 We find no significant evidence of an (arising from Fe xix-xx), is missing in the ionized emitter in the spectrum. Resonant model. Thus, the HIP cannot contain less and forbidden Ovii lines have a statistical ionized gas. significance . 1 sigma. We estimate upper The ionization state of the LIP is also limits for the equivalent width of 218 mA˚ limited by the data, particularly by the for the resonant line and 185 mA˚ for the UTA. Figure 9b shows the comparison of forbidden line. Although an emitter might the data with a second model that includes be present, better data are required for its a factor 4 more ionized LIP (logU=0.5). detection. This model shows an excess of absorp- tion at 15- 16 A˚ produced by Fe xiv- 2.2. Constraining the Ionized Ab- xv, and a lack of absorption at longer sorber wavelengths (due to a diminished fraction vii-x Despite the limited S/N ratio of the of Fe in charge states ). A third Chandra spectra, the physical properties model consisting of a factor 5 less ionized of the ionized absorber in NGC 985 can LIP (logU=-0.8) is presented in Figure 9c. be constrained. As demonstrated by Be- This model is inconsistent with the data har, Sako, & Kahn (2001) and Krongold also. A low value of U shifts the UTA to et al. (2003), the UTA is a robust indica- higher wavelengths (peak of absorption at tor of the ionization level of the gas, as the 17 A)˚ because an important fraction of Fe iv-vi general shape and position of this feature is now present in charge states . At x-xii strongly depend on U for a given ioniza- the same time, the lack of Fe pro- tion component. The 12-15 A˚ Fe L-shell duces an underestimation of the absorption absorption complex also helps to limit the around 15.9-16.4 A.˚ From this analysis is physical state of the gas. evident that even lower values of U for the LIP (logU<-0.8) would produce even more To illustrate this point, we have con- shifted UTAs (produced by even lower Fe structed a series of models with ionization charge states), and therefore, higher dis- parameters different from those deduced crepancies between data and model. Thus, for our best fitting model (see §2.1.3 and the data impose important restrictions on Table 3). The comparison of these models the ionization state of the absorbing gas. with the data reveals clear discrepancies (see Fig. 9). The first model we considered A third absorber with an intermediate is presented in Figure 9a, and consists of value of the ionization parameter (between a factor 3 less ionized HIP (logU=0.8). In the HIP and the LIP) and high H col- this model, lower charge states of Fe would umn density can also be ruled out. Fig- be significantly present in the HIP (e.g. Fe ure 9d illustrates the inconsistencies that xv-xvi), and these would produce abun- arise when a third absorber with logU=0.6 −2 dant Fe M-shell UTA absorption at wave- and logNH =21.5 [cm ] is included in our lengths between 15 and 16 A˚ (see Fig. 9a). model. Such an absorber severely overpre- This is clearly not observed in the data. dicts the absorption by the UTA from 15 At the same time, the deep absorption fea- to 16 A˚ (because of a high contribution xiv- ture observed in the data around 12.85 A˚ to the absorption in the model by Fe

8 xvi). However, it is not possible to rule out physical properties. the presence of additional absorbing com- ponents with lower H column density. 2.2.1. The Effects Induced by the Lack of To quantify the limits imposed by the Accurate Dielectronic Recombina- data to additional absorbing material we tion Rates included a third absorption component in Our analysis of the LIP is based mostly our model and refit the data. In Figure 10a on the Fe M-shell UTA, and low tem- we show the logU-logNH confidence region perature Dielectronic Recombination Rate for this hypothesized absorber. The plot (DRR) coefficients have not been calcu- shows that gas with U between the HIP lated or measured yet for these ions. Kron- −2 and the LIP and logNH > 21.1 [cm ] (half gold et al. (2003) and Netzer et al. (2003) the column density of the LIP and 20% reported that the models were unable to that of the HIP) can be ruled out (at the simultaneously reproduce the absorption 3σ level). Figure 10a also shows that the by the UTA and two lines of Si (Six at presence of material less ionized than the λ6.850 and Sixi at λ6.775). Netzer et al. −2 LIP and logNH > 21 [cm ] can also be (2003) proposed that this was an effect in- ruled out (at 3σ significance). troduced by the lack of these DDR. This On the other hand, a third component means that, for a given value of the ion- with very high ionization parameter and ization parameter, there is a systematic considerable column density can be present shift in the ionization balance of Fe (to- in the spectra of NGC 985. Figure 10b ward higher charge states) with respect to presents the confidence region for a 3rd the rest of the species. Netzer (2004) and component consisting of high column den- Kraemer, Ferland, & Gabel (2004) have sity and high ionization gas. As observed estimated these rates. Their results show in the figure such a component is not well that there is indeed an increase in the ion- constrained by the data, but in order to ization parameter at which the Fe M-shell have larger column density than the HIP ions peak, offering a solution for the mis- −2 (logNH >21.8 [cm ]) requires logU>2 (i.e match between the UTA and Si lines in the 5 times larger U than the HIP). In fact the spectrum of NGC 3783. discrepancies (with significance < 2σ) be- In the spectrum of NGC 985, besides tween 1.6 and 1.7 A,˚ and near 10.6 A˚ (see the UTA, there are no other significant ab- Fig. 11) hint at the presence of such mate- sorption features that allow us to constrain rial, as they are consistent with absorption the ionization parameter of the LIP. Ac- xxiv-xxvi by Fe . cepted at face value, this means that we are It is clear, then, that the gas tends to underestimating the ionization parameter clump in two different degrees of ioniza- (and temperature) of the LIP, because of tion, with the majority of the low ionized the lack of Fe M-shell DRR. However, from material around the LIP. A third, very high our previous analysis on NGC 3783 (Kro- ionization component is hinted at by the ngold et al. 2003), we estimate that the data, but better S/N ratio data are needed error cannot exceed a factor of 2 in U (i.e. to establish its existence and constrain its ∆ log U < 0.3), a factor consistent with

9 the one obtained by Netzer et al. (2003) to fit the Fe L-shell lines, along with K- and Kramer, Ferland, & Gabel (2004). We shell lines arising from other elements, like have repeated our analysis shifting up by Si, Mg, and Ne (see Fig. 4). Although the a factor of 2 the ionization parameter of significance of these narrow lines is limited, the LIP in all elements except Fe. With it is clear that the ionization state of the the quality of the present data, this dis- gas is consistent with the observed charge tinction does not modify our results in any states. We note that the same was found significant way. In fact, estimates with this for the high S/N ratio spectrum of NGC new approach of the column density and 3783 (Krongold et al. 2003; Netzer et al. ionization parameter of possible additional 2003). Therefore, we do not expect a sys- absorbing components yield to limits fully tematic underestimation of U in the HIP. consistent with those presented in the for- mer section, showing again that the gas 3. BeppoSAX DATA clumps in two, or maybe three, absorbing components. To further study the nature of the ion- ized absorber, in particular to constrain While our main conclusions will not the absorber response to the changes in change due to this underestimation of U, the continuum of the central source, we this effect should be kept in mind. There- retrieved and reprocessed the previously fore, we have set the positive error bar in unpublished NGC 985 data obtained by the ionization parameter of the LIP to be BeppoSAX (Boella et al. 1997; Piro et consistent with a factor of 2 change in U, al. 1997) on 1999 August 29 (see Table in order to reflect the underestimation in- 1). The data have a total MECS expo- troduced by the lack of low temperature sure of 90 ks. We followed the standard DRR for Fe. reduction procedure (Fiore et al. 1999) for Recent calculations by Gu et al. (2003), the LECS, MECS, and PDS data. We ex- and measurements by Savin et al. (2002a, tracted LECS and MECS source spectra 2002b, 2003) have produced more accu- from circles of radius 8’ and 4’ respectively, rate values for Fe L-shell DRR. At tem- and the same extraction regions were used peratures close to the one derived for the 6 to extract background spectra from the HIP ∼ 10 K(∼ 86 eV) the differences be- LECS and MECS “blank fields.” The PDS tween the old estimates and the new ones instrument has no imaging capability and are relatively small (although these differ- is a collimated detector with a field of view ences become larger at lower temperatures, o 5 (FOV) of 1 .3 (FWHM). It makes use of i.e. 10 K). Thus, although the new calcu- rocking collimators for background moni- lations have not been incorporated into the toring. Using the ROSAT All Sky Survey, photoionization codes, we expect the un- we searched the FOV of the PDS in the certainties introduced into the HIP model direction to NGC 985, and did not find by the uncertainties of the Fe L-shell DRR any other source in this region capable of a currently used in the codes to be within significant contamination of the PDS spec- the error bars derived in our analysis. In trum (Mrk 1044 is the closest bright X-ray fact, our model for this component is able source to NGC 985 in the sky, at an angu-

10 lar distance of 1.2o, but it was outside the The BeppoSAX data indicate a flatter FOV of the PDS during the observation). power law photon index (∆Γ ≈ 0.2 ± 0.07) Modest variability was observed dur- and a larger flux in both the power law ing the BeppoSAX observation of NGC and the blackbody components (by a factor 985. In the 0.1-2.0 keV band (LECS data), ≈ 2.3) with respect to the Chandra data. source variations were detected up to a fac- These differences are reflected in the SED tor 1.45 on a time-scale of about 50 ksec. used to fit the ionized absorber in the Bep- Variability in the 2.0-10 keV range (MECS poSAX data (see Fig. 14). data) was detected up to a factor of 1.3 in the same time-scale. The quality of the 3.1.2. Modeling the Ionized Absorber Present data does not allow for a separation in low in the BeppoSAX Data and high state spectra, therefore we mod- Modeling of absorption features in low eled the full time integrated dataset. resolution data is extremely difficult be- cause the lack of detail does not allow 3.1. Spectral Analysis the determination of the exact ionization 3.1.1. Fitting the Continuum state of the gas and the number of ab- sorbing components. Because of this, we To model the broadband (0.1-100 keV) decided to use the solution found for the BeppoSAX continuum of NGC 985, we high resolution Chandra data, to constrain first followed the same approach used with the physical properties of the gas at low the Chandra data, constraining the intrin- resolution. We assumed that two compo- sic powerlaw in the 3-8 keV range. The nents contribute to the absorption, as in- residuals of the extrapolation of this model ferred from the Chandra data, and fixed to the whole BeppoSAX band are pre- the outflow velocity and the turbulent ve- sented in Figure 12. The presence of both locity of the absorbers to the values de- an ionized absorber and a soft excess is ev- duced in §2.1.3 (these parameters are es- ident, together with that of a high energy sentially unconstrained when modeling low cutoff at energies larger than 60 keV. We resolution data). then refitted the data including an expo- First, we left the ionization parameter nential cutoff to model the continuum at and the column density of both compo- high energies, as well as a blackbody and nents free to vary while fitting the data. an ionized absorber (see §3.1.2) to model An acceptable fit was obtained (reduced the soft excess and the absorption at low χ2 = 1.063 for 112 degrees of freedom), energies.6 Results of our best continuum with column density values in both absorp- fits are shown in Table 2. The intrinsic tion components closely consistent with continuum of the source further attenuated the values obtained modeling the Chan- by Galactic absorption is shown in Figure dra data: we find column density values of 13a (dashed line).

6 is not required by the BeppoSAX data of NGC The inclusion of a cold reflector, often observed 985. We find a 2σ upper limit for the amount of in the high energy spectra of Seyfert galaxies (e.g. reflection R< 0.7, see Table 2. NGC 3783, Kaspi et al. 2001, de Rosa et al. 2002),

11 −2 logNH =21.95±.24 [cm ] (∆ logNH =0.14 effects between the two instruments. We −2 [cm ]) for the HIP and logNH =21.38±.26 will follow the analysis assuming Γ=1.4 for −2 −2 [cm ] (∆ logNH =0.01 [cm ]) for the LIP. the BeppoSAX data, noticing that this dif- Hence, we next froze the column densities ference does not have a significant effect on to the values obtained with Chandra, and our conclusions. fit the data letting only the ionization pa- To investigate the significance of the rameters vary. The results are reported in variability of the soft emission component, Table 4, and are discussed in the next sec- we attempted to fit the BeppoSAX data tion. Figure 13 shows the data and the while freezing the blackbody component best fit model and Table 5 shows the pre- used to parameterize the soft excess to the dicted column densities for the dominant values obtained for our best fitting Chan- charge states. dra model. Every model tried imposing this constraint led to an underestimate of 4. VARIABILITY ANALYSIS the continuum level below 1 keV, pointing to a genuine variation of the flux of the soft 4.1. Intensity and Spectral Changes component between the two observations. in the Continuum We note from Table 2 that only a change in A comparison of the 2002 Chandra and the intensity of the emission is required by 1999 BeppoSAX observations indicates a the data, as the best fit solutions to both flux drop by a factor of 2.3 in the 0.1-10.0 datasets are consistent with a blackbody keV range. Variability is observed in both temperature of 0.1 keV. the soft X-ray (0.1-2.0 keV) and hard X- ray (2.0-10.0 keV) bands. Since the hard 4.2. Changes in the Opacity of the X-ray region is barely affected by the ion- Ionized Absorber ized absorber, the change of flux is indica- The ionization degree of the ionized ab- tive of intrinsic source variations. Our con- sorber along the line of sight to NGC tinua fits indicate a steepening in slope of 985 has also changed between the Chan- ∆Γ ≈ 0.2 ± 0.07. Any attempt to fit both dra and BeppoSAX observations. An F- datasets with a single photon index value test comparing the opacities deduced from yields unsatisfactory results. For instance, 2 the Chandra best fitting model with those we find a χ = 59.1 for 50 degrees of free- for the BeppoSAX best fitting model shows dom when fitting (between 3 and 8 keV) that the difference is statistically signifi- the Chandra data with a value of Γ frozen 2 cant at the 99% confidence level. To better to 1.4, this imply a ∆χ = 12.4 with re- quantify the changes in the ionized mate- spect to the best fitting model (an F-test rial, it is convenient to introduce a different gives a significance larger than 99.9% for ionization parameter, Ux, defined through this difference). Fitting the BeppoSAX the number of ionizing photons between data with a photon index equal to 1.6 pro- 0.1 and 10.0 keV (Netzer 1996). This def- duces even worse results. This difference inition makes the changes in the ioniza- could be indicating a real slope variation tion parameter (now relevant only to the or could be the result of cross-calibration X-ray species) directly proportional to the

12 change in the X-ray flux, as opposed to time scale is inversely proportional to the bolometric flux changes, which are not di- density in the gas during both increasing rectly observable. Thus, it is preferable in and decreasing flux intensity phases (e.g. studies of the effects of flux variability over Nicastro et al. 1999) . If the density is ionized gas in the X-ray regime. The two low enough, this timescale can be larger different definitions of the ionization pa- than the variability timescale of the central rameter scale as logUx=logU - 2.43 for the source, and the medium does not have time Chandra SED, and logUx=logU - 2.08 for to adapt to the changes in the continuum. the BeppoSAX SED. In such conditions, delays in gas responses The best fit to the Chandra data gives may lead to scenarios where the ionization a value of logUx=-2.55±0.09 for the LIP. degree of the gas decreases while the flux If the gas is in photoionization equilibrium increases. Such cases have been already and is not physically confined, a change of found in other ionized absorbers around flux by a factor 2.3 would induce a change AGN, for instance NGC 4051 (Nicastro et in the X-ray ionization parameter by the al. 1999). (2) The gas is not in photoion- same factor, yielding a value of logUx=- ization equilibrium because another mech- 2.20. The best fit value of logUx for the anism, collisional ionization (due, for in- BeppoSAX data is -2.30±0.10, consistent stance, to shock heated gas) dominates. with the predicted value. We conclude that In this case, obviously, changes in the gas the LIP is responding to the changes in opacity are essentially unrelated to changes the continuum and thus is probably both in the source flux. (3) The gas is indeed in photoionized and in equilibrium. photoionization equilibrium, and has un- dergone a significant change in its density. The value of logUx for the HIP in the Chandra data model is -1.09±0.10. If the This is possible if the gas is pressure con- absorber is in equilibrium, the flux change fined by an external medium. (4) Finally, observed between observations implies a it is possible that the absorbing gas is not the same during the two observations. X- value of logUx=-.73 for the HIP present in the BeppoSAX spectrum. The value ob- ray absorbers can change independently of tained in our best model for the BeppoSAX flux variations (see George et al. 1998) be- cause of transverse motion of the material data is logUx=-1.25±0.13. These two val- ues of the ionization parameter are clearly or because of condensation and dissipation inconsistent with each other. Thus, this of clouds out of a hot medium (Krolik & component has indeed changed ionization Kriss 2001). In the following sections we state between the two observations, but will explore further the validity of these does not seem to be following the changes different scenarios. on the continuum. On the contrary, the degree of ionization of this gas seems to 5. ABSORBER PHYSICS DISCUS- have increased while the flux decreased. SION Such behavior has several possible ex- The ionized absorber present in NGC planations. (1) The gas is out of equi- 985 is suggestive of a simple picture, being librium: the photoionization equilibrium constrained by two strong absorption com-

13 ponents. The same situation was found for C iv, and H i column densities inferred the ionized absorbers in IRAS 13349+2438 from the LIP of our Chandra best fitting (Sako et al. 2001) and NGC 3783 (Kron- model. It is evident that this absorbing gold et al. 2003). In all three objects, an component would produce absorption in Fe vii-xiii UTA and several Ovii lines are the UV band. Arav (2002) measured col- −2 the signature of the cooler phase, and the umn densities of log NNv = 14.2 [cm ], Oviii xvii-xxii −2 and Fe are the main fea- and an log NHI = 14.7 [cm ]. These val- tures of the hotter phase, with high ioniza- ues are smaller than the ones derived from tion iron dominating the spectrum below our model by factors 29 for N v and 5 for H 15 A.˚ These ionized absorbers are inconsis- i (see Table 6). Finding consistent column tent with substantial amounts of material densities for the UV and X-ray absorbers with intermediate values of the ionization has been historically difficult as has been parameter. Furthermore, the ionization discussed by Krongold et al. (2003). In degree of the LIP is consistent with a cooler the case of NGC 985, a few reasons can be medium producing significant amounts of responsible for the discrepancy: (1) The O vi and other lower ionization species, predicted column densities inferred by any which should also imprint their signature model in the UV strongly depend on the in the UV band (see also §5.1). In the exact shape of the SED assumed in the case of NGC 3783, a third very high ionized analysis (Kaspi et al. 2001; Steenbrugge component has also been detected (Netzer et al. 2003). Thus, the overestimation in et al. 2003). In NGC 985 the presence of the model could be an effect induced by such component is hinted by the data, al- our chosen SED (Fig. 3a). In fact, the de- though it cannot be well constrained (see rived factors reduce to 15 for N v and 4 for Fig. 11). H i when considering the SED presented The similar characteristics in the ab- in Figure 3b (see Table 6). The reason for sorbers of these objects suggest that this the decrease in ionic column density in this an intrinsic property related to the struc- SED is that it has a larger number of pho- ture of the nuclear environment. In the tons in the extreme UV, the relevant region following sections, a possible scenario re- for the abundances of the ions absorbing garding the nature of these absorbers is in the UV (note the small decrease in H discussed. i, since the photon flux assumed in both SEDs at 912 A˚ is the same). However, the 5.1. The UV X-ray Connection of intrinsic SED in this region cannot be con- the Absorbers strained due to Galactic absorption. (2) In addition to SED effects, the fact that the NGC 985 has been observed with HST- observations are not simultaneous should STIS and FUSE. Arav (2002) modeled be kept in mind. It is possible that the these data, and showed that, once satura- absorber has changed in opacity because tion is accounted for, the component out- −1 the gas responds to flux variations (or be- flowing at 404 km s (i.e. component 4) cause the absorbing gas is not the same produces absorption in the X-ray band. at the two different epochs). For instance, In Table 6 we present the O vi, N v, the ionization parameter for the LIP in the

14 BeppoSAX model is larger than the one derived from our fit to the Chandra obser- in the Chandra model. Then, the differ- vation, the ionization parameter Ux, and ence in N v column density between Arav’s the recombination rate for Ovii (Shull & measurements and our BeppoSAX best fit van Steenberg 1982) since it is the dom- model is only 13, if the SED presented in inant charge state. We estimate a lower Fig. 14 is used (this SED is similar in the limit for the electron density of the LIP of −3 extreme UV to the one presented in Figure ne(LIP)>610 cm . This translates into 3a for the Chandra analysis, with a small an upper limit of R(LIP)< 21.6 pc for the flux in this region). Therefore, the differ- distance between the central source and ent columns can reflect only the ionization the ionized gas, and a maximum thickness state of the gas. (3) Finally, it is possible ∆R(LIP)< 1.3 pc for the gas. that other UV components (besides com- ponent 4) contribute to the absorption ob- 5.3. Thermal Instabilities and the served in the X-rays. Absorbing Components Quantifying these differences would re- Several models have been proposed to quire simultaneous UV and X-ray obser- explain the nature of the ionized absorbers vations with high resolution and high S/N around AGN. The different scenarios in- ratio. However, we note that NGC 985, clude a continuous range of ionization pa- along with NGC 5548 (Kaastra et al. 2002; rameters (Krolik & Kriss 2001), different Arav et al. 2003; Steenbrugge et al. 2003), clouds in different regions (including for in- NGC 3783 (Kaspi et al. 2001; Gabel et al. stance different UV and X-ray absorbing 2003; Krongold et al. 2003; Netzer et al. regions), or similar locations for the ab- 2003), and NGC 7469 (Blustin et al. 2003; sorbing winds (Krongold et al. 2003). Kriss et al. 2003) are good examples of the The data presented here suggest that a UV and X-ray absorbing connection. continuous range of U does not adequately describe the observed spectra (see discus- 5.2. Density and Location of the sion in §2.2). The presence of different Absorbing Gas clouds in different regions seems unlikely Our analysis of the LIP indicates that (at least for the objects discussed here), this gas may be in photoionization equi- not only because of the similar characteris- librium and responding consistently to tics of the absorbing components in differ- the changes in the continuum. Using the ent AGNs, but also because pressure bal- elapsed time between the high state and ance is observed (to within the errors) be- low state observations (1029 days) as an tween the components (see 5.4). upper limit to the recombination time, a An intriguing possibility is that the dif- lower limit on the density of the recom- ferent components form because of ther- bining gas and an upper limit on the dis- mal instabilities in the thermal photoion- tance from the central source can be set ization equilibrium S-curve (Krolik, Mc- for this gas. For this calculation, we used kee, & Tarter 1982). This curve marks the photoionization equilibrium temper- the points of thermal equilibrium in the ature, the flux in the 0.1-10 keV range T, U/T plane, where T is the equilibrium

15 temperature of the gas, and U/T is a quan- guishable within 25%, (the pressure of the tity inversely proportional to the pressure phases is consistent with a single value at of the gas (see Fig. 15). The wiggles a level well within 2σ). in the S-curve are due to changing heat- We stress again the fact that the absorb- ing and cooling rates, as different ions be- ing material does not span all the points come dominant. The shape of this curve in the stable branches of the S-curve, but is also affected by the ionizing SED and rather it is concentrated in separate phases the metallicity of the absorbing gas (e.g. (see §2.2). Therefore, although the effect Komossa & Fink 1997). The high tem- of thermal instabilities may be partially re- perature branch lies where only Compton sponsible for the formation of the phases heating is important. Gas in regions of the (§5.3), this cannot alone explain the ob- curve with negative derivative is unstable servations. One possibility is that these because any isobaric perturbation will be phases form as the gas tends to go to the amplified, leading to net cooling or heat- stable branches of the thermal equilibrium ing. Then, the different components ob- curve, but keeping pressure balance be- served form simply, when the gas is driven tween them (see below). to the stable (positive slope) branches of There are two special considerations re- the curve, while tending toward equilib- garding the validity of the pressure equi- rium. librium result that have to be discussed at this point: 5.4. Pressure Balance between the Absorbing Components (1) The shape of the SED: Kaspi et al. (2001) and Steenbrugge et al. (2003) have Krongold et al. (2003) reported that the shown that the shape of the SED in the two components in the ionized absorber far UV (the unobservable region of the of NGC 3783 are in pressure balance to spectrum) has a negligible effect on the within 7%, which suggested that the ab- charge states absorbing in the X-ray re- sorption could arise from two phases of the gion. However, the curve of thermal equi- same medium. As we shall point here, this librium does depend on the shape of the also seems to be the situation for the ion- SED at these wavelengths. Thus, it is pos- ized absorber of NGC 985. sible that SEDs with a UV-X ray break dif- As can be observed in Figure 15a, the ferent to the one assumed here would result HIP and the LIP in the best fit model in two phases lying out of pressure balance. of the NGC 985 Chandra data have very In other words, our result could be reflect- different equilibrium temperatures but lie ing specific characteristics of our chosen close to each other in the U/T axis. As SED. To further investigate this possibil- mentioned before, with the adopted defini- ity, we have repeated our entire analysis tion of the ionization parameter (U), this with the SED presented in Figure 3b (with ratio is inversely proportional to P, the the UV-X ray break at 1 keV instead of 0.1 gas pressure. Hence, assuming that the keV). This choice gives the maximum effect HIP and the LIP have the same location, introduced in our results by SED uncer- the gas pressure between them is indistin- tainties since the break cannot be at larger

16 energies because then, the observed spec- to be shorter than the timescale in which trum would be directly affected. We find the gas reacts to the changes of the con- that pressure equilibrium is found regard- tinuum. The sound velocity vs for media − less of the assumed shape of the far UV at temperature ∼< 106 K is ∼< 100 km s 1, SED (see Figure 15b). and the hydrodynamic timescale can be es- (2) The analysis of the LIP is based mostly timated as tH ≥ ∆R/vs, where ∆R is the on the UTA: As discussed in §2.2.1, the scale size of the medium. For a single gas lack of reliable DRR for low Fe charge cloud with conditions typical of ionized ab- 4 6 states may be producing an underestimate sorbers (10 K few 21 −2 of the ionization parameter of the LIP. 10 cm ), the hydrodynamic timescale Even if this effect is real, it does not mod- will always be larger than the photoioniza- ify the analysis presented here in any way. tion timescale, which prevents the gas pres- More accurate DR rates would shift the po- sure from responding to the change in the sition of the LIP to higher values of both incident continuum quickly enough to re- pressure and temperature in the S-curve main in pressure equilibrium. This is true (∆ log U < +0.3, see §2.2.1). However, for any density inferred using the ioniza- the shift is insufficient to move the LIP out tion parameters from our model, assuming of pressure equilibrium with the HIP. On that the gas is located anywhere within the contrary, such a change would make 10 parsecs of the central source. Thus, the pressure of both components even more an important conclusion is that the phases closely matched (Figure 15). can only be in true pressure balance if the confined medium is composed by a large 5.5. Pressure Confinement of the number of small clouds (∼ 104 clouds), so Absorbing Components that the hydrodynamic time scale can be shorter than the photoionization timescale. A striking result is that pressure equilib- The total column density of the absorber rium between the two phases is also found would result from the individual contribu- in the BeppoSAX observation of NGC 985. tions of the clouds. This can be observed in Figure 16, where (2) The photoionization equilibrium the S-curve for the SED used in the Bep- timescale of the media has to be shorter poSAX analysis is shown, and the positions than the variability timescale of the central of the HIP and the LIP in this model are source, in order for the absorbing material also marked. Although it is not clear what to be close to photoionization equilibrium. drives the change of ionization in the HIP, This condition is assumed in our analysis. both phases again align close to a single Clearly, if the gas is out of photoionization value of logU/T, as would be expected if equilibrium, then the ionization parame- one of the two components confines the ters and temperatures derived here are not other one (or as if both components are meaningful, and the absorbing gas is not pressure confined by a third phase). even constrained to lie on the S-curve. We There are two necessary conditions for note that photoionization equilibrium of this to happen: (1) The hydrodynamic the confining medium is not required for timescale (tH ) in the confined medium has

17 pressure confinement (see §5.5.3). sity is larger (if the phases are at the same We will follow the discussion assuming distance, the difference in U between them that these two conditions are indeed sat- should be mostly an effect of the differ- isfied (but see 5.6). In such a scenario, ent densities). This would require the HIP the confining medium would be respond- to be more extended than the LIP, and at ing to the changes in the continuum, and the same time, the HIP would have to be moving freely on the S-curve. The con- made up of several small clouds immersed fined medium, on the other hand, would in the LIP. We find it extremely unlikely not be able to move freely in the S-curve, that these two conditions can be satisfied rather it would have to follow at all times simultaneously. the position of the confining medium in the U/T axis. This means that the confined 5.5.2. Pressure Confinement of the LIP medium would have to adapt to the ioniz- by the HIP ing continuum with a constrained value of A second scenario is that the HIP is con- the pressure at all times. Thus, to reach fining the LIP. However, the HIP would the photoionization equilibrium tempera- have to be out of photoionization equi- ture, the confined phase would have to ex- librium, in order to explain the changes pand or contract to keep pressure balance in ionization parameter. This picture is with the confining medium. Then, the ion- not possible, since as explained before, if ization parameter (and thus, the opacity) this phase is not in photoionization equilib- of the confined phase would be determined rium, then its position on the T-U/T plane not only by the change in flux of the cen- is unknown. Thus, the alignment on the tral source (as in an isolated system), but S-curve would have to be considered just a also by the change in density due to the bias induced by our analysis (see §5.6). change in volume. 5.5.3. Pressure Confinement of the HIP 5.5.1. Pressure Confinement of the HIP and the LIP by a Third Component by the LIP

This scenario can easily explain the ob- A third scenario is that a more ionized served behavior of the HIP between the component is confining both the HIP and BeppoSAX and Chandra observations: as the LIP. Being more ionized, this compo- the flux drops, the LIP gets colder and its nent would require a density lower than pressure drops. Hence, the HIP would have that of the HIP (which is already 30 times to expand to decrease its pressure. The smaller than the density of the LIP). In drop in density of the HIP would increase this case we have two possibilities. The its ionization parameter and temperature first is that despite the low density, this (even though the flux is dropping), until component still has time to adapt to the photoionization equilibrium is reached. changes in the continuum, and therefore is However, an inconsistency for this pic- in photoionization equilibrium. The sec- ture is that the column density of the LIP ond is that this is not the case. In either is smaller than that of the HIP and its den- scenario, the HIP and the LIP would have

18 to follow the position of this hypothesized following (or not even responding to) the phase in the U/T axis. In the photoioniza- continuum. tion equilibrium situation the third compo- nent has to be located on the curve of ther- 5.6. The Absorbing Components mal stability. In the non-photoionization Are Not in Pressure Balance equilibrium case, the component could be located off the curve, as long as the pres- From the last sections, it is clear that a sure imposed by this phase on the other scenario of pressure-confining is appealing two is compatible with the region of the S because, due to the regions of thermal sta- curve where multiple phases can survive, bility in the S-curve, it can explain the dif- otherwise the phases would dissolve. ferent absorption components observed in This scenario is appealing because the the spectra of Seyfert galaxies. However, presence of this phase could easily explain such scenario also imposes strong physical the observed change of ionization parame- conditions on the media that could be dif- ter of both the LIP and the HIP, between ficult to satisfy. There is not enough evi- the BeppoSAX and Chandra observations, dence currently to rule out or confirm such as we show below. In addition, the data a scenario and other possibilities have to hints the presence of more ionized gas (see be considered. Fig. 11). In NGC 3783, the presence If the pressure confinement is not real, of such absorber has already been estab- why do the phases align close to a single lished, and found to be in pressure balance value of U/T? The answer to this ques- with the less ionized components (Netzer tion may rely on the shape of the S-curve. et al. 2003), which further suggests that For several SEDs, the S-curve can be ex- this could be the case in NGC 985. Evi- tremely steep in a large range of temper- dently, much better data and time evolv- atures (between 104 to 106 K) where pho- ing photoionization models (e.g. Nicastro toionized gas absorbs in the X-ray range et al. 1999) are needed to explore further (Krolik & Kriss 2001). Large gradients of this scenario. temperature (factors 10-50) can then arise If a third confining component is the from small changes of the gas pressure. correct picture, then the HIP and the LIP This is clearly illustrated in Figures 15 have to be close to photoionization equilib- and 16. Thus, if several absorbing compo- rium, but may not respond “as expected” nents were in photoionization equilibrium to the flux variations because their pres- and located in the “vertical region” of the sure is fixed to the pressure of the hottest S-curve, but were completely disconnected phase. If this hot phase does not change from each other, they would appear as if its position on the (U/T,T) plane dramat- they were in pressure balance. While this ically in response to continuum changes is certainly plausible, we note that there (because it is out of photoionization equi- are also SEDs that do not show such a pro- librium or because the gas lies on a steep nounced vertical branch (Komossa & Fink part of the S-curve), then the HIP and/or 1997). In those cases, it is possible to dis- the LIP may appear as if they were not entangle true pressure balance from appar-

19 ent pressure balance (see for example the (1) The first dataset was produced using S-curve obtained for the SED derived by the HIP plus gas less ionized than the LIP Krongold et al. 2003 in their analysis of (i.e. gas located to the left of the LIP NGC 3783, their Fig. 16). Thus, discrimi- in the horizontal branch of the S-curve −2 nating true pressure balance from apparent with log U=-0.8 and logNH =21.5 [cm ]). pressure balance strongly depends on (1) We modeled these data with PHASE and the quality of the X-ray spectrum to con- found that the presence of colder gas was strain better the position of the phases in easily detected and constrained. the (T,U/T) plane, and (2) the true shape (2) The second dataset was simulated us- of the SED, which is, however, always un- ing 3 absorption components: the HIP and certain. the LIP (Table 3), plus a third cooler ab- Evidently, if the gas is out of photoion- sorber with relatively large column den- −2 ization equilibrium, the curves of thermal sity (log U=-0.8 and logNH =21.5 [cm ]). stability discussed here are not relevant to First, we modeled these data including describing the physical state of the gas. only two absorption components. This ap- Then, we would not know if the phases proach gives an acceptable fit, but the ion- are or are not in pressure balance. We ization parameter and column density of note that the excellent fits obtained for the the low ionization gas are only upper lim- high quality data of NGC 3783 (Krongold its, which allows the presence of the ad- et al. 2003, Netzer et al. 2003) suggest ditional cold ionized material. Then, we that the gas is at least close to photoioniza- fitted the simulated data with three ab- tion equilibrium. Time evolving photoion- sorbing components. With this model the ization models applied to high quality data values of the column density and ioniza- are the best way to determine if the data tion parameter of the LIP can be recov- are in photoionization equilibrium, and to ered within a significance of ≈ 1.5σ, as ob- further study possible scenarios of pressure served in Figure 17a, and the parameters confinement out of equilibrium. of the HIP can also be recovered within Additional complexity is present be- a level ≈ 1σ. The third component is cause the X-ray detectors are most sen- well constrained in NH (21.4

20 would be basically unconstrained (log U < 2003) forming part of the biconical flow -0.5). Thus, even with low S/N ratio data, observed for the narrow emission line re- we feel confident that there is no addi- gions (Kinkhabwala et al. 2002; Behar et tional absorbing material along the hori- al. 2003)? Or, on the contrary, are they zontal branch of the S-curve (i.e. gas out of closer (at subparsec distances, e.g. Nicas- pressure balance) in the spectrum of NGC tro et al. 1999; Netzer et al. 2002; Gabel 985, or it would be detected. Evidently, et al. 2003; Reeves et al. 2003) and part the high quality data of NGC 3783 is less of a disk wind (Elvis 2000)? susceptible to this kind of bias. Answers to these questions will help us All these tests indicate that, while the understand what is the genesis of these pressure-confining scenario might be ap- winds and what is their relation to the propriate, it requires fragmentation of black hole and the accretion disk. the confined absorber, and thus, the idea should be tested even further. Any model 6. ALTERNATIVE SCENARIOS of the ionized absorbers around AGN has FOR THE IONIZATION MECH- to explain several points: (1) Why is there ANISM not a significant amount of gas on the hor- izontal branch of the S-curve, at temper- The solution found for the ionized ab- atures lower than the one of the LIP? (2) sorber through photoionization models is Why does the gas tend to give preference to satisfactory and carries important implica- special values of the ionization parameter? tions for the nuclear environment of AGNs. (3) How do the observed components re- Nevertheless, an alternative to photoion- late to the stable branches of the S-curve, ization by the central source deserves to be assuming that the gas is indeed in pho- explored for the HIP, since this phase does toionization equilibrium? (4) What is the not respond simply to continuum changes. relation between the UV and the X-ray ab- The ring located in NGC 985 at a dis- sorbers? In several objects, the observed tance of kiloparsecs from the nucleus has X-ray absorbing components do not oc- a large inclination angle, which brings the cupy all the points in the stable branches possibility that our line of sight to the of the S-curve, and the absorption seems to central source crosses through collisionally be produced from similar charge states. A ionized material in this ring. Such colli- simple model with absorbing components sions would be produced by an expanding located at random cannot explain these wave triggered by a pole-on merger (see properties. Rather they seem to point to for instance Karovska et al. 2002 for Cen- real properties of the structure of active taurus A). Hence, there is a chance that nuclei. the material producing the absorption ob- A perhaps more basic question that still served in X-rays is collisionally ionized (the has to be answered is ‘where are the X- HIP is too hot to be photoionized by the ray absorbers located?’ Are they located starburst induced by the merger). If colli- at large scale distances (parsecs from the sions drive the ionization, the temperature nucleus; Netzer et al. 2003; Behar et al. of the HIP is at least an order of magni- tude hotter than our estimate, and thus

21 the two components cannot be in pressure ionization parameter, and by a factor of equilibrium. Although this scenario can- 3 in H column density. The hotter phase not be excluded, we notice that the change is clearly identified through broad absorp- in ionization structure of the HIP observed tion features produced by several blends of in the 3 year period elapsed between the Fe L-shell absorption, and the cooler com- observations, requires an increase in the ponent is detected due to the presence of electron temperature by at least a factor a deep Fe M-shell UTA. However, the sig- of 2 in the HIP (the temperature variation nal to noise ratio of the Chandra data does is significant at a level 2.6σ), which sug- not allow the identification of significant gests that this gas is probably not ionized (σ > 2) narrow (unblended) absorption by collisional mechanisms. Furthermore, if features. This results in a poor determi- the absorption of the HIP arises in the ring, nation of the outflow velocities of the com- then the pressure balance found between ponents (581 ± 206 km s−1 for the high the phases would have to be interpreted as ionization phase and 197 ± 184 km s−1 for a pure coincidence. We stress again that the low ionization one). pressure balance is also observed in NGC (3) Despite the limited signal to noise 3783, a relatively undisturbed galaxy. ratio of the Chandra data, the ioniza- tion state of both components is well con- 7. SUMMARY strained thanks to several features of Fe L-shell absorption, but mainly because of A new Chandra observation and an the Fe UTA. The lack of accurate dielec- archival BeppoSAX observation are used tronic recombination rates of low charge to study the nuclear environment of NGC states of Fe (Netzer et al. 2003) intro- 985. Absorption features consistent with duces an uncertainty by a factor less than the presence of ionized gas in the nuclear 2 in the ionization parameter of the phases; environment of this Seyfert 1 galaxy are however, our results are robust and highly detected in both datasets. We have used insensitive to this source of error. the code PHASE (Krongold et al. 2003) to model in a self-consistent way the ionized (4) The outflow velocity of both compo- absorber, and to further study possible nents is consistent with most of the absorp- changes in the opacity of the gas to con- tion components found in the UV spec- tinuum variability. Our main results are trum of this object. In particular, both as follows:. components are consistent with the UV component with outflow velocity of 404 km (1) The intrinsic continuum of the s−1, which is hot enough to absorb in the source is well reproduced by a power law X-ray regime (Arav 2002). In addition, (Γ ≈ 1.4 − 1.6) and a thermal component the low ionization component in the X-rays (kT=0.1 keV). This continuum was atten- produces significant amounts of O vi, N v, uated by an equivalent hydrogen column and C iv, which strongly suggests that the density of 3.0 × 1020 cm−2 to account for absorption features in both bands are the Galactic absorption. manifestation of a single wind. (2) The data requires two absorption (5) The data set tight limits on the pres- components different by a factor of 29 in

22 ence of additional cold absorption compo- plain the change in opacity of both compo- nents, showing that gas with temperature nents during the observations, but requires < 106 K clumps around the two absorbers that a third hotter component is pressure- detected. On the other hand, the presence confining the two phases. Thus, our anal- of a third component, hotter than the other ysis strongly points to a 3-phase medium, two (T> 106 K), and producing Fe K-shell just like the wind suggested in NGC 3783. absorption, is hinted by the data. How- (9) An important implication of our ever, data with better signal to noise ra- analysis is that, if the pressure-confining tio is needed to constrain the properties of scenario is real, then fragmentation of the such component. confined phases in a large number of clouds (6) The gas detected in the spectrum of is required. NGC 985 has strikingly similar character- The analysis presented in this paper istics to the gas found in NGC 3783 and provides further evidence that warm ab- IRAS 13349+2438: (1) absorption pro- sorbers in AGN are simple systems that duced by cold gas (Fe vii-xiii), (2) ab- can be relatively well understood. The sorption by hot gas, dominated by Fe xvii- high resolution spectroscopic capabilities xxii, and (3) no significant absorption con- of the new generation of X-ray observa- sistent with Fe xiv-xvi. Then a continu- tories have revealed warm absorbers as ous range of ionization parameters is dis- strong winds, now recognized as an im- favored (at least in these objects). These portant feature of AGN structure. Here, similar characteristics further suggest that we have used high and low resolution data this is an intrinsic property of the ab- to constrain and characterize the physical sorbers, probably related to the unstable conditions and structure of these winds. branches of the thermal equilibrium curve. Measuring the response in the opacity to (7) The X-ray luminosity drops by a fac- variability of the ionizing flux, we suggest tor 2.3 between the BeppoSAX and Chan- that the nature of the ionized absorber is dra observations (separated by ∼ 3 years). that of a multi-phase wind, with one phase We detect (model dependent) variability in confining the other two. The implications the opacity of the absorbers. While the of this result are extremely important to colder component is consistent with a sim- our understanding of quasar energy bud- ple picture of photoionization equilibrium, gets, as well as to our understanding of the ionization state of the hotter compo- the interaction between these winds and nent seems to increase while the flux drops. the interstellar medium of the host galaxy. (8) The most striking result in our anal- However, our analysis is based on a com- ysis is that the two absorbing phases are parison between high and low resolution consistent (to within the errors) with pres- data. Only a direct comparison between sure equilibrium. This result is obtained high resolution data will allow further test- during both the Chandra and the Bep- ing of the scenario presented here, leading poSAX observations. Based on this, we to a deeper understanding of the nature of speculate that the absorption arises from a AGN and quasars. multi-phase wind. Such a scenario can ex-

23 We thank the anonymous referee for valuable comments that helped improve our work. This research has been partly supported by NASA Contract NAS8-39073 (Chandra X-ray Center), NASA grant NAS G02-3122A, and Chandra General Observer Program TM3-4006A. This re- search has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Labora- tory, California Institute of Technology, under contract with the National Aero- nautics and Space Administration.

24 Table 1 Observation Log of NGC 985

Observatory Sequence Number UT Start Exp Time (ks)a

BeppoSAX 50862001 1999AUG2006:38 95.0 Chandra 700449 2002JUN2419:33 77.7

aSum of good time intervals.

25 Table 2 Continuum Parameters

Parameter Chandra Data BeppoSAX Data 0.7-8 keVa 3-8 keV 0.1-110 keVa

Γb 1.60±0.03 1.62±0.02 1.40±.04 Normc 1.22±0.04 1.22±0.04 2.47±0.04 BB kTd 0.10±0.01 ··· 0.10±0.02 Norme 3.7±0.8 ··· 8.8±1.2 f Ec (keV) ······ 67 ± 24 Flux(.1-2keV)g 0.63±0.12 ··· 1.34±0.08 Flux(2-10keV)g 0.54±0.09 ··· 1.49±0.12 Rh ······ 0.0+0.35 χ2/d.o.f. 179.0/174 46.7/49 116.4/110

aContinuum fits carried out with the self-consistent inclusion of an ionized absorber. See Tables 3 and 4. bPower Law Photon Index cPower law normalization in 10−3 photons keV−1 cm−2 s−1 at 1 keV dBlackbody Temperature in keV

e −5 2 Blackbody Normalization in 10 L39/D10, where L39 is the 39 −1 source luminosity in units of 10 erg s and D10 is the distance to the source in units of 10 kpc f High Energy Cutoff as inferred from the SAX data gIn units of 10−11 ergs cm−2 s−1 hReflection component factor (R=∆Ω/2π, with ∆Ω the solid angle subtended by the reflector).

26 Table 3 Two Phase Absorber Parameters for Chandra Data

Parameter High-Ionization Low-Ionization HIP LIP

a +0.30 Log U 1.34±0.10 -0.12−0.09 −2 a Log NH (cm ) 21.81±.22 21.39±0.19 −1 b VTurb (km s ) 350 350 −1 a VOut (km s ) 581±206 197±184 [Log T (K)]c 5.79±0.11 4.47±0.04 d +0.30 Log U/T (∝1/P ) -4.45 ±0.21 -4.59−0.13 e Log Ux -1.09±0.10 -2.55±0.09

aFree parameters of the model. bFixed parameter. cDerived from the column density and ionization pa- rameter, assuming photoionization equilibrium.

d The gas pressure P∝neT. Assuming that both phases lie at the same distance from the central source ne ∝1/U, and P∝T/U. eIonization parameter defined in the 0.1-10 keV range (Netzer 1996).

27 Table 4 Two Phase Absorber Parameters for BeppoSAX Data

Parameter High-Ionization Low-Ionization HIP LIP

a +0.30 Log U 0.83±0.13 -0.22−0.10 −2 b Log NH (cm ) 21.81 21.39 −1 b VTurb (km s ) 350 350 −1 b VOut (km s ) 581 197 [Log T (K)]c 5.48±0.12 4.56±0.05 d +0.30 Log U/T (∝1/P ) -4.65 ±0.25 -4.78−0.15 e Log Ux -1.25±0.13 -2.30±0.10

aFree parameter of the model.

b Fixed parameters, assuming NH , FWHM, and VOut values from the best fitting Chandra model. cDerived from the column density and ionization pa- rameter, assuming photoionization equilibrium.

d The gas pressure P∝neT. Assuming that both phases lie at the same distance from the central source ne ∝1/U, and P∝T/U. dIonization parameter defined in the 0.1-10 keV range (Netzer 1996).

28 Table 5 Ionic Column Densitiesa predicted by the Models

HIP LIP Ion Chandra BeppoSAX Ion Chandra BeppoSAX

C vi 16.53 16.88 C v 17.31 17.01 C vii 18.34 18.36 C vi 17.64 17.68 N vii 16.46 16.80 N vi 17.03 16.94 N viii 17.76 17.73 N vii 16.95 16.97 O vii 16.18 16.94 O vii 18.03 18.07 O viii 17.75 18.09 O viii 17.48 17.69 O ix 18.61 18.54 Ne vi 17.16 16.97 Ne ix 16.52 17.09 Ne vii 16.70 16.77 Ne x 17.38 17.54 Ne viii 16.44 16.75 Ne xi 17.66 17.45 Ne ix 16.20 16.78 Mg xi 16.55 16.99 Mg viii 16.51 16.21 Mg xii 17.10 17.02 Mg ix 16.54 16.57 Mg xiii 16.91 16.56 Mg x 16.10 16.45 Si xiii 16.96 17.09 Si viii 16.29 15.76 Si xiv 16.98 16.79 Si ix 16.52 16.34 Si xv 16.47 16.03 Si x 16.37 16.53 Fe xvii 16.16 16.60 Fe ix 16.01 15.28 Fe xviii 16.69 16.83 Fe x 16.24 15.77 Fe xix 16.81 16.70 Fe xi 16.34 16.11 Fe xx 16.72 16.36 Fe xii 16.19 16.33 Fe xxi 16.28 15.68 Fe xiii 15.92 16.28

alog of column density in cm−2

29 Table 6 Column densitiesa of low ionization LIP ions from the Chandra best fit model.

Ion SED1b SED2c

O vi 17.23 17.13 N v 15.67 15.39 C iv 15.82 15.23 H i 15.37 15.33

alog of column density in cm−2 bSED presented in Fig. 3a aSED presented in Fig. 3b

30 Fig. 1.— Fluxed, combined (MEG+HEG) first-order spectrum of NGC 985. (a) Continuum fitted to the spectrum. The three solid lines stand for (from upper to lower): intrinsic continuum of the source, contin- uum attenuated by Galactic absorption, and continuum further attenuated by the bound-free photoelectric absorption edges. The dotted line represents the predicted power law without the contribution from the blackbody component. (b) Two phase absorption model plotted for comparison. Typical errors (per bin) are marked with green solid ranges.[See the electronic edition of the Journal for a color version of this figure]

31 Fig. 2.— Fit residuals for continuum determinations over Chandra HEG+MEG data of −2 NGC 985. All models are attenuated by cold gas with logNH = 20.5 [cm ] due to Galactic absorption. (a) Model including a continuum power law fitted over the entire spectral range (0.6-8keV). (b) Power law fitted only in the 3-8 keV range. (c) Model in Panel (b) plus a soft excess parameterized with a blackbody component. The data are binned to > 100 photons in each channel to make evident the overall features. [See the electronic edition of the Journal for a color version of this figure]

32 Fig. 3.— Spectral Energy Distribution used to model the ionized absorber in the Chandra- HETG data. Filled circles (blue) mark the observed SED of NGC 985 (obtained from NED). The plot shows the slope adopted in each energy range. This slope relates to the photon index as Γ=s+2. (a) Model with low energy far UV cutoff at 0.1 keV and including the contribution of a blackbody component. (b) Model with high energy far UV cutoff at 1 keV. [See the electronic edition of the Journal for a color version of this figure]

33 100

50

0 2 3 4 5

100

50

0 5 6 7 8 9 10 100

50

0 10 11 12 13 14 15 40

20

0 15 16 17 18 19 20

Fig. 4.— Two phase absorber model plotted against the first-order combined MEG+HEG spectrum of NGC 985. Absorption lines predicted are marked along bars at top for each element (red). The continuum level (including edge continuum absorption) is overplotted for comparison (dotted green line). The spectrum is presented in the rest frame system of the absorbing gas. Below 10 A˚ the data are presented in bins of size 0.05 A,˚ above 10 A˚ in bins of size 0.1 A.˚ [See the electronic edition of the Journal for a color version of this figure]

34 60

40

20

0 12 13 14 15 16 17 18

3 2

1

0 -1

-2

-3 -4 12 13 14 15 16 17 18

Fig. 5.— Upper panel: MEG+HEG spectrum of NGC 985 grouped in bins of size 0.15 A˚ (binning factor of 30). The solid line (green) marks the continuum level, including photoelectric bound-free absorption from our model. The final model including resonant absorption is marked with the dotted line (red). The feature in the continuum (solid green line) between 13.3 and 13.8 A˚ is produced by a chip gap in the detector. Lower panel: Residuals (data minus continuum green solid line) in units of standard deviations showing the presence of deep Fe L-shell resonant absorption (significance ≈ 3σ per bin) and Fe M-shell inner shell resonant absorption (significance ≈ 2σ per bin).[See the electronic edition of the Journal for a color version of this figure]

35 60

40

20

0 12 13 14 15 16 17 18

60

40

20

0 12 13 14 15 16 17 18

Fig. 6.— Upper Panel: Resonant bound-bound absorption produced by the low ionization phase (LIP) of our model, plotted against the MEG+HEG spectrum of NGC 985 grouped in bins of size 0.15 A˚ (binning factor of 30). The dotted line (green) marks the continuum level, including bound-free photoelectric absorption from our model. The main feature of this component is the Fe UTA. Lower panel: As upper panel, but showing the contribution from the high ionization phase (HIP). Below 15 A˚ the spectrum is dominated by Fe L-shell absorption.[See the electronic edition of the Journal for a color version of this figure]

36 Fig. 7.— Theoretical transmission spectrum of the two absorption components at 0.001 A˚ resolution. The low ionization component produces most of the bound-free photoelectric absorption and the UTA near 16 A.˚ The high ionization component produces the Fe L-shell absorption below 15 A.˚ [See the electronic edition of the Journal for a color version of this figure]

37 UV6 UV5 UV4 UV3 UV2 UV1

Fig. 8.— Outflow velocities for the HIP and the LIP, and for the 6 absorption components observed in the UV. The HIP is consistent within 2σ with UV components 1 to 5, and the LIP with components 3 to 6. Both the HIP and the LIP are consistent with component 4, which was expected to absorb in the X-ray region (Arav 2002).

38 60 80 60 40 40 20 20 0 0 11 12 13 14 15 16 17 18 19

60 80 60 40 40 20 20 0 0 11 12 13 14 15 16 17 18 19

60 80 60 40 40 20 20 0 0 11 12 13 14 15 16 17 18 19

60 80 60 40 40 20 20 0 0 11 12 13 14 15 16 17 18 19

Fig. 9.— Different values for the ionization parameters of our two phase absorber showing inconsistencies with the observed UTA. The left panels present the data in the range 11-14.5 A˚ in bins of size 0.15 A˚ (binning factor of 30), the right panels the data in the range 14.5-19 A˚ in bins of size 0.25 A˚ (binning factor of 50). The green dotted line shows the best fit model for comparison. Panel (a): Less Ionized HIP, logU=0.8. Note that the absorption from Fe L-shell is underestimated. Panel (b): More ionized LIP, the ionization parameter of the low ionization component has been set to logU=0.4. Panel (c): Less Ionized LIP, logU=-0.8. Panel (d): A third absorber has been included in our model with logU=0.6 and logN h=21.5 [cm−2]. Including a new component overpredicts the absorption by the UTA, which further suggests that the absorbing gas tends to clump. [See the electronic edition of the Journal for a color version of this figure]

39 Fig. 10.— Confidence regions constraining the presence of a third absorber in the Chandra spectrum of NGC 985. The solid lines correspond to the 1σ, 2σ, and 3σ levels. Panel (a): A low ionization absorber with low column density (logU=-1.0 and logNh=20.8 [cm−2]). Panel (b): A high ionization component with high column (logU=2.1 and logNh=21.8 [cm−2]).

40 6 4 2 0 -2 1.2 1.4 1.6 1.8 2

2

0

-2

10.4 10.6 10.8 11

Fig. 11.— Residuals to our best fit model over Chandra data in the ranges 1.1-2 A˚ and 10.2- 11 A.˚ The presence of absorption by material with significant amounts of Fe xxiv-xxvi is suggested in the figure, but better S/N ratio is required to constrain the physical properties of such component. [See the electronic edition of the Journal for a color version of this figure]

41 4 Chandra Data

2

0

-2

-4

1 10 100

Fig. 12.— Fit residuals for a model including only a continuum power law constrained in the 3-8 keV range and further attenuated by Galactic absorption, over BeppoSAX data of NGC 985. [See the electronic edition of the Journal for a color version of this figure]

42 Chandra Data

Fig. 13.— BeppoSAX spectra of NGC 985, plotted against an ionized absorber model for comparison. (a) Fluxed spectrum. The blue dashed line shows the continuum further attenuated by Galactic absorption. (b) Empirical Spectrum. [See the electronic edition of the Journal for a color version of this figure]

43 Fig. 14.— Spectral Energy Distribution used to model the ionized absorber observed in the BeppoSAX spectrum. Filled circles (blue) mark the observed SED of NGC 985 (obtained from NED). The plot shows the slope adopted in each energy range. This slope relates to the photon index as Γ=s+2. The model assumes a low energy far UV cutoff at 0.1 keV and includes the contribution of a blackbody component.[See the electronic edition of the Journal for a color version of this figure]

44 Fig. 15.— Curve of thermal stability for the two different SEDs used in the analysis of Chandra data, as described in the text. (a) Far UV cutoff at 0.1 keV plus blackbody (Fig. 3a). (b) Far UV cutoff at 1 keV (Fig. 3b). The positive error bar of the LIP represents the maximum effect in U induced by the lack of low temperature DRR. The HIP and the LIP are consistent with pressure equilibrium. [See the electronic edition of the Journal for a color version of this figure]

45 Fig. 16.— Curve of thermal stability for the SED used in the BeppoSAX analysis, as described in the text. The positive error bar of the LIP represents the maximum effect in U induced by the lack of low temperature DRR. Pressure balance between the LIP and the HIP appears to be present in the BeppoSAX data, even though the ionization degree of the gas is different than the one obtained for Chandra data (see text for details). [See the electronic edition of the Journal for a color version of this figure]

46 Fig. 17.— U vs. NH confidence regions for a model fitted to simulated data including 3 absorption components: the HIP and LIP from the Chandra best fit model plus a third colder −2 absorption component with logU=-.8 and logNH =21.5 [cm ]. The solid lines correspond to the 1σ, 2σ, and 3σ levels. In both panels the crossed-circle point marks the value used to produce the simulated data and the cross point marks the value obtained in the model. Panel (a): Confidence region for the LIP. Panel (b): Confidence region for the third component. Clearly, a third component would be detectable in the data, however, the ionization state of such a component would be difficult to constrain.

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