arXiv:astro-ph/9809086v1 8 Sep 1998 PASJ: ihntepaeo h torus. the possibly of or plane particles to the close scattering within very edge- lie that photons an broad-line providing through polarizing torus, seeing as obscuring this on NGC interpreted in They detected were 7582. lines (1997), broad-emission spec- al. scattered et optical Heisler no recent by out a carried In survey tropolarimetry luminosity. bright line X-ray it relatively emission variable have a “narrow spectrum, and to called as appear optical so which categorized of (NELG) its class galaxies” is small to a it form- according joins Though galaxies also galaxy spiral 2 Quartet. four Seyfert the Grus of the one ing sky, southern the .Introduction 1. 3 nXrybn,teeaeanme fsuiso NGC on studies of number a are there band, X–ray In G 52i ery( nearby a is 7582 NGC siuoprl enlgeeSui aizoiExtraterres Radiazioni Studio e Tecnologie le per Istituto 96rvae h aibiyat variablity the revealed 1996 10 evtos oee,teifre oundniyicesdby increased density column inferred the However, servations. EW h itr ftetasiso fncerXrycniumthroug continuum X–ray is keV) nuclear iro (2–10 of observed luminosity transmission s X–ray The the namely of clouds. torus”, picture “patchy individual the many of comprises terms line-of-sight in our interpreted be could variation hc stesm ee sta fSyet1glxe ftesm lumino same va the similar of showed source galaxies The 1 1994. Seyfert constant. in almost of stayed that flux as the level keV) same the is which infiatol ntehr –a ad(-0kV ihtenormalized the with keV) (2-10 band X–ray hard the in only significant fatp efr aaywihehbt oha bcrdadscat and obscured an both model. exhibits unification variability. which AGN X–ray galaxy the soft Seyfert from rapid centr 2 of scattered type lack the a the with of consistent as is mostly This interpreted region. be could contribution, 23 ul srn o.Japan Soc. Astron. Publ. 1 epeetterslso w SAosrain fteSyet2 Seyfert the of observations ASCA two of results the present We osgicn aitosi h vrg otnu n ieflxswere fluxes line and continuum average the in variations significant No h vrl rabn 051 e)setu scmlx tshows It complex. is spectrum keV) (0.5-10 broadband overall The e words: Key par least at though spectrum, X–ray soft keV 0.5-2 constant The ∼ cm h nttt fPyia n hmclRsac RKN,2- (RIKEN), Research Chemical and Physical of Institute The nOsue n ctee iwo h idnNucleus Hidden the of View Scattered and Obscured An Sui-Jian 7 V eo e,a“otecs”eiso oiae h spectr the dominates emission excess” “soft a keV, 2 Below eV. 170 SAOsrain fteSyet2Glx G 7582: NGC Galaxy 2 Seyfert the of Observations ASCA − 2 n a (Γ flat and ) Xue aais niiul(G 52 aais efr aais X–ra Galaxies: – 2 Seyfert Galaxies: – 7582) (NGC individual Galaxies: 1 , 2 z Chiko , ∼ 0 = 2 1 . ejn srpyisCne BC,Biig107,China 100871, Beijing (BAC), Center Astrophysics Beijing Rcie 98Jnay1;acpe 98Ags 24) August 1998 accepted 14; January 1998 (Received )pwrlwcniumpu lgtybodio K iron broad slightly a plus continuum power-law 5) . 03)atv aayin galaxy active ) 0053 50 > Otani ∼ 9 ofiec ihtesots iecl of timescale shortest the with confidence 99% ?–0(2018) ???–10 , 3 × 1 10 Tatehiro , 42 -al [email protected] E-mail: rss ergs Abstract − Mihara 1 iia otetpclvle o efr galaxies. 1 Seyfert for values typical the to similar , ceei hsseilojc.Ufruaey hs re- bright these several because Unfortunately, interpreted, unification be AGN to object. difficult the are special sults test this to along in as matter scheme well absorbing as the key line-of-sight, of important the nature the the be investigate could to some results between These or one observations. during in occurred as flux well continuum as of the density factors column absorption striking large the that some in variabilities especially However, revealed, galaxy. also were Seyfert results 2 type typical a otisa bcrdo ihyasre wt column (with absorbed probably highly These most or density 7582 obscured 1993). NGC an al. that contains et suggested (Warwick have Ginga studies recent 1989), more Pounds, & and (Turner EXOSAT 1985), 1981; al. Perola et & Reichert HEAO-1/A2 (Maccacaro e.g. Einstein missions, 1982), space (Mushotszky previous on based 7582 r IeR) N,ii oet 0,I419Blga Italy Bologna, I-40129 101, Gobetti iVia CNR, (ITeSRE), tri 1 Massimo , N ∼ H lcniumfo ptal extended spatially a from continuum al 4 > h ou-iegoer.Tenuclear The geometry. torus-like the h iblt naohrsotrobservation shorter another in riability ioaa ao atm 5-1 Japan 351-01, Saitama Wako, Hirosawa, 1 × ee msincmoeta expected as component emission tered ilyacutdfrb oestarburst some by for accounted tially aayNC78.Teosrainin observation The 7582. NGC galaxy getn htasrigmtra on material absorbing that uggesting iefauei locnitn with consistent also is feature line n 10 hsNC78 satpclexample typical a is 7582 NGC Thus Cappi 10 iy ntesf -a ad(0.5-2 band X-ray soft the In sity. aiblt amplitude variability 23 22 evl bobd(N absorbed heavily a cm cm um. eetdbtentetoob- two the between detected 1 , − 3 − ∼ n Masaru and , 2 2 -a msinnces like nucleus, emission X-ray ) 2 α rm19 o19.This 1996. to 1994 from × ieat line 10 4 .Vrain were Variations s. y ∼ Matsuoka . e with keV 6.4 σ RMS H ∼ ≈ 1 . 0 0 . × 3, 1 , 2 S. J. Xue et al. [Vol. 50, X–ray sources were detected by Einstein and ROSAT ASCA data of NGC 7582 did not suffer contaminations HRI in the immediate proximity of NGC 7582, includ- from any confusing sources noted in section 1. Source ing cluster S´ersic 159-03 (Abell S1101), active galaxies counts of both observations were extracted from circu- NGC 7590, NGC 7552 (Charles & Philips, 1982), and a lar regions of radius ∼ 3 arcmin and ∼ 5 arcmin for the BL Lac object PKS 2316-423 (Crawford & Fabian 1994), SIS and GIS, respectively. All the backgrounds were ex- which probably contaminated the former non-imaging X- tracted from the source-free regions in the same detec- ray observations. Therefore, to clarify the possible vari- tors. ability properties of NGC 7582, multiple, spectral imag- ing X-ray observations (with spatial resolution sufficient 3. Timing analysis to separate out the confusing sources in the field) are ideally required. Light curves of each observation were extracted for In this paper, we present the results of two ASCA all the four spectral instruments in XSELECT (V1.3), observations of NGC 7582. The imaging capability of with short (128 s) and long (5760 s, ∼1 satellite orbit) ASCA clearly separated these 5 sources. The broad en- binnings. Then the same instrumental-type data, SIS0 ergy band (0.5–10 keV) X–ray spectra obtained with and SIS1, GIS2 and GIS3 were combined to improve the ASCA provide a good opportunity to improve our under- signal-to-noise ratio. The resulting light curves show that standing of this object significantly. We describe the ob- the SIS and GIS data are well consistent in each obser- servations and data reduction in section 2. The high qual- vation. ity data enables temporal and spectral analysis, which The variability properties were further investigated for are presented in section 3 and section 4, respectively. The SIS light curves in the soft (0.5–2 keV) and hard (2–10 results are discussed in section 5. The conclusions of the keV) energy bands (figure 1). The result indicates that paper are given in section 6. significant variation (∼ a factor of 2) was only detected in the hard X–ray band with typical timescale of ∼ 2 × 104 2. Observations and data reduction s, which could be clearly seen on both 128 s and 5760 s temporal analysis (figure 1). On the other hand, the NGC 7582 was observed twice with the ASCA satel- soft X–ray curve almost remained constant during each lite. One observation was carried out on November 21-22, observation. There was no sign of correlation between 1996 (AO4, hereafter), with Solid-state Imaging Spec- the hard and soft light curves. According to the spectral trometers, SIS-0 (S0) and SIS-1 (S1) operating in 1-CCD study in the next section, a break-point of the soft and Faint mode. The other one was made on November 14, hard components is ∼ 2 keV. Together with the tempotal 1994 (AO2 hereafter), with the SIS instruments oper- analysis, we can conclude the existence of the two inde- ating in a mixture of Bright and Faint 2-CCD modes. pendent spectral components in the spectrum of NGC For each observation, Gas Imaging Spectrometers, GIS-2 7582. (G2) and GIS-3 (G3) were operated in normal PH mode. In order to check whether there was a possible corre- All data were selected from the intervals of high and lated spectral variability, such as a change of N , within medium telemetry rates. The SIS data were screened H the hard band during each observation, we have calcu- using the following criteria: a) The data was taken out- lated the hardness ratio HR (defined as the ratio of the side of the South Atlantic Anomaly, b) the angle between count rate in 4–10 keV to the count rate in 2–4 keV) for the field of view and the edge of the bright and dark each bin of the light curve. The resulting HR is plotted earth exceeded 25◦ and 5◦, respectively, and c) the cutoff in figure 2 as a function of count rate for the SIS light rigidity was greater than 4 GeV c−1. After these selec- curve of AO4. It shows no significant spectral variabil- tions, we also deleted data if d) there were any spurious ity with the intensity. The same result was obtained for events or the dark frame error was abnormal. For GIS AO2 data. data, no bright earth angle and cutoff rigidity criteria were applied. More details concerning the performance and instrumentation of ASCA were reported in separate 4. Spectral analysis papers (ASCA satellite: Tanaka et al. 1994; SIS: Burke et al. 1991; GIS: Ohashi et al. 1996). Since no significant spectral changes were detected in The resulting effective exposures of the observation in the individual soft and hard spectral components in ei- 1996 are ∼ 41.4ks and ∼ 42.4ks for the SIS and GIS, re- ther observation, we added together the data from each spectively. For the SIS data in mixture mode obtained in observation and performed spectral fitting of the four 1994, the data was treated in BRIGHT (including con- time-averaged spectra simultaneously. These spectra are: verted FAINT data) mode, and the resulting effective ex- pairs of the SIS(0/1) in the energy of 0.5–10 keV, and posures of the data are ∼ 22.5ks for the SIS and ∼ 24.9ks the GIS(2/3) in 0.7–10 keV. The data was analyzed for for the GIS. each year; and independent normalizations were used for No.5] ASCAObservationsoftheSeyfert2GalaxyNGC7582 3
Fig. 1. The comparison of SIS hard and soft band light curves of NGC 7582. Light curves in both short (128 s) and long (5760 s) bins are shown for AO4 (upper panel) and AO2 (lower panel), respectively. The time axis of AO4 starts from 18:24:13, November 21 (UT), 1996; and AO2 starts from 05:47:32, November 14 (UT), 1994. For both observations, the rapid variability is seen in the hard (2–10 keV) band, and no significant correlated variability in the soft (0.5–2 keV) band.
Fig. 2. The hardness ratio (4–10 keV)/(2–4 keV) as the function of SIS count rate (2-10keV) in AO4. No spectral variability, such as a change of NH, was detected in the hard component.
the SIS and GIS data, because there are small uncer- We then attempt to fit the spectra by adding an extra tainties in the relative flux calibrations of different de- spectral component to the single power-law to represent tectors. The spectra were grouped so that each energy the upturn soft X–ray excess below ∼2 keV. The follow- channel contains at least 20 counts allowing minimization ing continuum models were firstly investigated: 2 χ techniques. Spectral analysis have been performed us- a) Scattering model. This model consists of the sum of ing XSPEC(V9.1) program (Arnaud et al. 1991). two power-laws having the same photon index but differ- Throughout this paper, the Galactic line-of-sight col- ent absorptions and normalizations. One power-law is a × 20 −2 umn density is fixed at NHgal =1.47 10 cm (Stark direct component absorbed by a highly intrinsic column et al. 1992). All errors reported below are quoted at the density, and the other is a scattered component which 90% confidence level for one interesting parameter (i.e. is free from intrinsic absorption. The “non-absorbed ” 2 ∆χ =2.7). scattered continuum mimics the soft excess. This inter- pretation is supported by unified models of AGNs (An- 4.1. Modeling of the soft excess component tonucci, 1993), which predicts that the direct continuum The broad-band spectra were initially modeled with a of Seyfert galaxies is strongly obscured by a thick torus single absorbed power-law model. Residuals of the fit- (Ghisellini, Haardt, & Matt 1994), whereas a part of the ting clearly indicate the presence of significant “excess” nuclear emission is scattered by free electrons surround- emission that dominates the spectrum below ∼2 keV. At ing the broad-line region (e.g. Antonucci & Miller 1985; higher energies, some small residuals around ∼6.4 keV Awaki et al. 1991). The fraction of the scattered con- indicate the presence of an iron Kα emission and/or an tinuum is evaluated by the ratio of the two power-law absorption edge. normalizations As/Ap. The spectral fit of this model is The poor description of the single power-law and the shown in figure 3, and the best-fit parameters are listed complexity of overall spectrum might be expected in the in table 1. physical picture of unified AGN schemes. In the scheme, It should be noted that the “partial covering model” is we might expect to see some fraction of the primary con- numerically equal to the scattering model. In this model, tinuum (power-law like) through the heavy obscuration of the “non-absorbed” power law component, which might the putative torus, with some components superimposed possibly explain the soft excess, physically represents a upon this, which could represent the indirect central con- fraction “leakage” of the continuum from a partially ob- tinuum scattered into our line-of-sight, and/or the leak- scured central X–ray source. In contrast to the scat- age of the central continuum through a non-uniform (e.g. tered scenario, the soft component in the partial covering partial covering) absorber. In addition, hot gas due to model is expected to show the same rapid variations as the starburst activity or in the host galaxy might also the direct continuum. From a correlation analysis of the make an extra contribution to the soft X–ray emission. soft (0.5–2 keV) with hard (2–10 keV) count-rate, the 1σ Thus, both the data and the possible physical picture upper limit of contribution from partial covering (i.e. the lead us to explore some complex models for NGC 7582. leakage of variable hard X–ray continuum) to the soft 4 S. J. Xue et al. [Vol. 50, emission was not more than 10%. Therefore, the par- % of the flux at 1 keV, which is consistent with the value tial covering model is immediately excluded by the data. expected from the FIR/X–ray luminosity correlation for The constancy of the soft component is consistent with normal and starburst galaxies (see section 5.3.1). the scattering model because any rapid variations in the Therefore, the plausible model which explains the data direct continuum would be smeared out in the emergent is that the soft emission consists of a mixture of the scat- spectrum. tering component and the thermal component; the hard In the scattering model, the soft-emission is physically component is a heavily absorbed power-law continuum related to the hard-emission component; however there plus a slightly broad cold iron Kα line. For this model, are also a few possibilities that the soft emission is of ther- confidence contours of the photon index of the primary mal origin, and is completely independent on the hard- continuum vs. column density are shown in figure 5. emission component. For example, b) Bremsstrahlung Finally, we have searched for the presence of an iron and c) Raymond-Smith model could account for the soft edge in excess of that modeled by the best-fit absorption component. In these cases, it is assumed the soft X–ray in each dataset. Adding two extra parameters (threshold emission is produced by an optically thin hot plasma as- energy and the depth of the edge) to the best-fit model sociated with the host galaxy, e.g. starburst activities. produced a slight improvement to the fit (∆χ2 ≃ 4.6) ≃ +0.15 ∼ The best-fit of these two thermal models have minor dif- for AO4, and gave result of τ 0.15−0.10 at 7.1keV. ference, thus only the result of model c was quoted in However, the AO2 data does not suggest any additional table 1. iron edge probably due to its lower statistics. In this For each model we have also tested the significance for model, the excess absorption edge implies either an iron the presence of a narrow iron Kα line, which was firstly overabundance of factor ∼ 2 or an additional X-ray ab- fitted with free energy, and fixed width (σ). In all these sorption (partially covering) column in the line-of-sight models, thawing the line width could improve the line (see section 5.3.2). fitting at > 90% confidence level (∆χ2 = 2.7 for one interesting parameter) than that of narrow line fitting, 5. Discussions we then let this parameter free. It is clear from table 1 that both the scattering and 5.1. Short-term variabilities during the observations thermal models give acceptable fits to the data with 2 ∼ Rapid X-ray variability is a very common property χν = 1.0 1.1, and these models yield similar values of column density, hard photon index and the similar of Seyfert 1 galaxies, and the variability amplitude was iron Kα feature in each observation dataset. Therefore, found anti-correlated with the source luminosity (e.g. statistically these two type of models cannot be discrim- Nandra et al. 1997; Lawrence & Papadarkis 1993). inated. However, as we will discuss in section 5.3.1, the Therefore, to examine whether or not Seyfert 2 galaxies soft continuum in NGC 7582 cannot be dominated by (for which we can detect their direct nuclear emission) the thermal component. This suggests that the soft con- also show the rapid variability, is a crucial test for the tinuum of the source is most likely dominated by the AGN unification scheme. scattered component. Furthermore, it is widely examined that the complex X–ray spectra of type 2 Seyferts in the ASCA energy band have different physical origins. To discriminate 4.2. The Best-fit Model them, variability studies of various spectra components The complexity of soft component is also suggested by are important. NGC 7582 is a rare example of Seyfert 2 the data. Although the scattering model gives an ac- galaxies that clearly shows rapid X–ray variability. Most ceptable fit of the data, it is not perfect. There are still compellingly, the hard and soft X-ray emissions of this some significant residuals below 1 keV (see figure 3), es- source have different variability properties that indicate pecially a small bump around 0.85 keV that might be of two distinct emission regions: hidden nuclear hard X–ray thermal origin, i.e. probably iron L-shell complex lines. emission and extended soft X–ray emission. Therefore, we modeled this extra component using the To test the idea that the observed highly variable con- Raymond-Smith model, the best fit gives a the temper- tinuum comes from a hidden Seyfert 1 nucleus, we added ≃ − 2 ature of kT 0.8 keV, and abundance of 1.0 2.0 solar our data points to the σRMS (0.5-10 keV) vs. LX (intrinsic (table 1). Although the abundance is poorly constrained luminosity in 2–10 keV) plot of Seyfert 1 samples (Nan- owing to the lack of unambiguous line features in soft dra et al. 1997) (figure 6). The rapid-variability charac- X-ray spectrum, the extra two free parameters improved teristics of the hard X–ray component in NGC 7582 at the fit of the scattering model at > 99.9 % significance two different epochs are very similar to those of Seyfert level with ∆χ2 = 21. The best-fit results are listed in ta- 1 galaxies. This suggests that the nuclear emission of ble 1 and shown in figure 4. In this complex model, the NGC 7582 is similar to that of Seyfert 1 galaxies. Turner Raymond-Smith emission contributes at most ∼ 10 − 20 et al. (1997a) also reported three other Type-2 objects, No.5] ASCAObservationsoftheSeyfert2GalaxyNGC7582 5
Fig. 3. The folded spectrum of NGC 7582, fitted simultaneously for SIS(0/1) and GIS(2/3), with the scattering model. Related residuals for AO4 (upper panel) and AO2 (lower panel) are also shown. The cross marker represents the SIS data, and the void-square denotes the GIS data. The model gives acceptable description of the data (see table 1), but still some residuals remain around ∼1 keV, indicating that the soft spectrum is more complex than just scattered component.
Table 1. Spectral fit parameters
a b 2 Data kT Abundance NH Γ As/Ap ARS/As EKα σ EW χν /d.o.f − [keV] [solar] [1023 cm 2] % % [keV] [keV] [eV]
Scattering + Absorbed Power Law :
+0.08 +0.14 +1.7 +0.09 +0.35 +80 1994 - - 0.94−0.08 1.49−0.14 3.9−1.2 - 6.31−0.10 0.12−0.07 170−65 1.07/288
+0.06 +0.09 +0.9 +0.06 +0.11 +47 1996 - - 1.32−0.06 1.64−0.09 3.2−0.7 - 6.34−0.06 0.15−0.06 192−46 1.05/578
Thermal (RS) + Absorbed Power Law :
+1.54 +0.05 +0.16 +0.09 +0.12 +70 1994 1.62−0.51 < 0.09 0.75−0.04 1.33−0.16 - - 6.30−0.1 0.12−0.08 170−77 1.04/288
+1.40 +0.11 +0.13 +0.07 +0.11 +53 1996 2.48−0.60 < 0.08 1.20−0.08 1.52−0.13 - - 6.33−0.06 0.15−0.06 180−47 1.04/578
Scattering + RS + Absorbed Power Law :
+0.07 +1.6 +0.08 +0.12 +0.5 +7 +0.09 +0.16 +75 1994 0.78−0.12 2.6−1.6 0.86−0.08 1.38−0.11 3.8−1.5 16−7 6.32−0.10 0.12−0.10 163−66 0.99/286
+0.07 +0.5 +0.06 +0.09 +0.3 +5 +0.06 +0.10 +50 1996 0.79−0.09 1.4−0.4 1.24−0.08 1.52−0.07 3.3−0.4 14−6 6.33−0.05 0.15−0.07 182−40 1.02/574
a As/Ap = (normalization of the scattered component at 1 keV)/(normalization of the primary continuum at 1 keV) b ARS /As = (normalization of the RS component at 1 keV)/ (normalization of the scattered component at 1 keV)
NGC 526A, MCG-5-23-16 and NGC 7314 that showed ∼ 0.6 × 10−11 erg cm−2 s−1 (Warwick et al. 1993); and 23 −2 the same property as seen in figure 6. This result suggests that a substantial increase in NH from 1.6×10 cm to that rapid X-ray variability is also common in Seyfert 2 4.6×1023 cm−2 occurred in the ∼4 year interval between galaxies. the EXOSAT and Ginga observations. All these results indicated that a complex long term spectral fluctuation might exist accompanying with a larger factor change in 5.2. Long-term variabilities between the observations absorption column density. Almost the same average 2–10 keV flux (∼ 1.5 × 10−11 The two ASCA observations separated 2 years pro- erg cm−2 s−1) was obtained in two ASCA observa- vided a valuable chance to check for spectral variability tions. This flux is consistent with the HEAO 1 result over a longer time scales. Comparing the contour plots of 1.4 × 10−11 erg cm−2 s−1 (Mushotzky, 1982), if as- of photon index vs. column density between the two ob- sumed that the power-law component in their spectral servations in figure 5, the spectral index does not show model represents the source emission; and also consis- a significant difference. However, the absorption column tent with the EXOSAT result of 1.7×10−11 erg cm−2 s−1 density evidently increased by ∼ 44% from AO2 to AO4, (Turner & Pounds 1989). Among the previous studies, confirming previous findings (Warwick et al. 1993). The Ginga observation reported the lowest continuum level as present ASCA observations are amongst the best avail- 6 S. J. Xue et al. [Vol. 50,
Table 2. Flux and luminosity for the best-fit model
− − − − Dataset Observed Flux [10 13 erg cm 2s 1] Intrinsic Luminosityb [1040 ergs s 1]
0.5-2 keV 2-10 keV 2-10 keVa 0.5-2 keVc 2-10 keVa
+0.49 +36 +73 +0.64 +89 1994 3.63−0.67 151−42 227−54 4.54−0.71 273−65
+0.34 +24 +46 +0.66 +57 1996 4.18−0.40 155−22 272−40 5.06−0.26 327−46
a b −1 −1 c The absorption corrected. Assuming H0 = 50 km s Mpc and q0 = 0.5. The Galactic absorption corrected.
Fig. 4. The unfolded spectrum shown for AO4, fitted with the model of “absorbed power-law + scattered power-law + Raymond-Smith + Fe K line”. The best-fit parameters of this model and flux measurements are listed in table 1 and table 2.
able in providing evidence for the variability of X–ray ab- where M8 is the mass of the central blackhole in the unit 8 sorbing column within a Seyfert 2 nucleus, because most of 10 M⊙. Rpc and Dpc are in the unit of pc. previous results (see Warwick et al. 1993) are based on On the other hand, if the iron line comes from the a comparison of two measurements made by different in- same absorbing matter, the ionization parameter ξ ≡ struments. L/nR2 of the matter should be ξ < 100 erg cm s−1, Variations in X–ray absorbing column are not common since the energy of the detected iron K line was 6.4 keV events in AGNs. To our knowledge, so far it was only (see section 5.4) suggesting that the matter was cold or reported for a Seyfert 1.9 galaxy ESO103-G55 by War- slightly ionized (Kallman & McCray 1982). Here n is wick et al. (1988) based on multi-observations made with the density of the plasma. Assuming the number of the EXOSAT. In that case, the significant variation in the clouds in the line-of-sight to be Nc, the observed column column occurred over a 90-day period, the author inter- density NH was the sum the densities of the clouds, NH = preted it in terms of an X–ray absorbing screen composed NcnD. Substituting the source X-ray luminosity of L = 43 −1 23 −2 of broad-line clouds moving out of the line-of-sight. 3 × 10 erg s and NH =1 × 10 cm , However, the significant amount of X–ray absorbing 2 −1 column in type 2 AGN is generally associated with the Dpc < 1.0RpcNc . (3) putative obscuring torus. The partial obscuring of the Eq. (1) and (3) give the upper limit of D as nucleus of NGC 7582 would occur in the outer region pc close to the edge of the torus (see section 5.4). If this − 2/5 − D < 5 × 10 3M N 1/5. region contains clumpy clouds (i.e. “patchy torus”), its pc 8 c orbital motion around the nucleus could cause the ob- 1/5 2/5 The maximum value is at Rpc =0.07M Nc . served variability of the line-of-sight absorption. 8 Eq. (2) and (3) give the lower limit of Rpc as If this is the case, the transverse time scale, D/v, of a cloud with the size, D, and the velocity, v, should be 1/5 2/5 Rpc > 0.011M8 Nc . smaller than ∆t ∼ 2 years. ∆t should also be larger than 1 week, since no variation of the column density was ob- Since the dependencies of M8 and Nc are weak, Rpc is served within one previous observation. As the velocity consistent with the putative obscuring torus. is estimated by (GM/R)1/2, where R is the distance from 1/5 −3/5 Eq. (1) and (3) also give D/R < 0.07M8 Nc , the nucleus and G is the gravitational constant, this gives which is consistent with the view that there are many (∼ 10) small (compared with the size of the torus) clouds D ∼< ∆t(GM/R)1/2 ∼ 1.3 × 10−3M 1/2R−1/2, (1) pc 8 pc near the edge of the torus. The transverse motion of the clouds could cause the observed change of the column > × −5 1/2 −1/2 Dpc ∼ 1.3 10 M8 Rpc , (2) density. The number of the clouds in our line-of-sight No.5] ASCAObservationsoftheSeyfert2GalaxyNGC7582 7
Fig. 5. Confidence contour levels (68%, 90% and 99%) for the spectral slope vs. the intrinsic absorption column density. The solid and dashed lines are for AO4 and AO2 data, respectively. The photon-indices of the hard continuum are in the range of 1.3-1.6 in the two datasets. The intrinsic absorption changed evidently 44% in the 2 year interval.
Fig. 6. Comparison of the rapid-variability characteristics of NGC 7582 seen in 2–10 keV with Seyfert 1 galaxies (Nandra et al. 1997), on the basis of 128 s temporal analysis. Also shown in the figure, three other type-2 objects: NGC 526A, MCG-5-23-16 and NGC 7314 (Turner et al. 1997a). For these Seyfert 2 galaxies, the rapid variability alone suggests that the hidden Seyfert 1 nuclei are directly seen in the hard X–rays.
is large enough that NGC 7582 almost always shows the soft X-ray component in the thermal (e.g. Raymond- similar absorption feature. Smith) model (see section 4.1). Furthermore, the ac- In the present case, the significant column variations ceptable fits obtained with a RS thermal model need an were observed on a timescale of years, therefore support- abnormally low metal abundances, less than 0.08 solar ing the association of the X–ray absorbing matter with values (see table 1), this is unlikely to be real, because the putative torus. However, more tight constraints on all known abundance gradients in spiral galaxies suggest the physical conditions of the absorbing material can be that the nuclei should be somehow enriched in heavy ele- expected from the future observations with time scales of ments with respect to solar (e.g. Vila-costas & Edmunds, week ∼ year. 1992). There is one possibility to explain this low metal abundance, i.e. the soft emission is a mixture of thermal component with a featureless continuum, e.g. the scat- 5.3. The spectral components tered nuclear emission. However, in this case, the ther- 5.3.1. The origin of soft emission mal component with normal metal abundance no more The lack of rapid variability in the soft X–ray band dominates the soft emission. All these results make the suggests that it has a different origin from the hard con- hypothesis of a major thermal origin for the soft excess tinuum. Some origins, either electron scattering, thermal to be unlikely in NGC 7582. emission, or a mixture of the two, are expected to explain As shown in section 4.2, the soft excess emission of it. NGC 7582 showed complex and composite spectrum, ei- There are good evidence for starburst activities un- ther the unique thermal or scattered origin can be re- dergoing in the center region of NGC 7582. For exam- jected by the data. We concluded that the soft spectrum ples, the optical spectrum of NGC 7582 shows high-order is dominated by the scattered component with 10–20% Balmer absorption features, which Ward et al. (1980) in- contribution from the starburst emission. The “scatter- terpreted as evidence for a significant population of hot ing efficiency”, as defined by the normalization ratio of stars; also its infrared spectrum is dominated by dust the scattered to the primary power-law continuum, was emission, and shows both dust emission and silicate ab- found to be of 3-4%. Similar properties of the spectrum sorption features (Roche et al. 1984). However, we found at low energies were also found for many others Seyfert 2 that the thermal emission related to this starburst ac- galaxies (Ueno 1997; Turner et al. 1997a), with the sta- tivities can not explain the total soft X–ray excess. Ac- tistical distribution of scattering efficiencies in the range cording to the far-infrared(FIR)/X–ray luminosity corre- of 1–10%. lation for normal and starburst galaxies (David, Jones & Forman, 1992), we estimated, from the FIR luminosity The scattered view of the major soft X–ray emission of NGC 7582, the maximum expected starburst contribu- of NGC 7582 was also expected by the presence of non- tion to the 0.5–4.5 keV luminosity is ∼ 1.7×1040 erg s−1, thermal UV emission (Kinney et al. 1991; Mulchaey et al. which is 3–5 factors below the derived luminosity of the 1992), which could not be seen through an absorption of 8 S. J. Xue et al. [Vol. 50, ∼ 1023 cm−2. Thus it should be a scattered light of nu- the continuum source is completely covered by a torus of clear emission. One can expect that several soft emission nonuniform density (see Weaver et al. 1994; Hayashi et lines will be produced in the warm gas which is respon- al. 1996). We found that a dual-absorber model with an ≃ +3.5 × 23 −2 sible for the scattering, as seen in the ASCA spectra of additional column of NH 6.3−2.8 10 cm could ex- Mrk 3 (Iwasawa et al. 1994; Turner et al. 1997b). It plain the data with a steep power-law component similar is, however, hard to discriminate such photoionized lines to the case of reflection model. The only constraint on in the present data, because of the poor statistics and such a complex model comes from the iron K absorption contamination by the thermal starburst emission. edge at ∼ 7.1 keV expected by neutral absorption. The The present data showed evidently a scattered and ob- model predicts an absorption edge with optical depth scured view of a hidden Seyfert 1 nucleus. This naturally τ ≃ 0.16 − 0.33 (90 % confidence), which is still consis- +0.15 suggested that a hidden broad line region (HBLR) should tent with the measured value of τ =0.15−0.10 (see section also be detected in the polarized spectrum in the con- 4.2). Therefore, it seems that composite dual-absorber text of the unified model for Seyfert galaxies. However, model is another plausible explanation for the flat con- Heisler et al. (1997) reported recently a non-detection of tinuum of NGC 7582. The same model has also been polarized broad emission lines (PBLs) for NGC 7582 in a applied for several other sources observed with ASCA, well defined infrared-selected sample of Seyfert 2 galax- like NGC 5252 (Cappi et al. 1996), NGC 2110 (Hayashi ies. On the one hand, there are difficulties in measuring et al. 1996) and IRAS 04575-7537 (Vignali et al. 1998). weak PBLs, and we have little knowledge on how well The remaining possible interpretation is that the spec- the visibility of HBLR in Seyfert 2 galaxies is related to trum is intrinsically flat. This will pose a serious problem their X–ray properties. for theoretical models which are successful in the expla- nation of AGN’s canonical X–ray slope at the value of Γ ≃ 1.9 − 2.0 (e.g. see Haardt & Maraschi 1991; 1993). 5.3.2. The flat continuum There seems increasing number of Seyfert 2 galaxies with The best-fit model for both observations require a flat spectra revealed by Ginga and ASCA, where Ginga strongly absorbed hard power-law continuum. Figure 5 data suggests that many of them can not be explained shows that, within the 99% error range, the two observa- by the reflection model (Smith & Done 1996); however, ∼ − tions are consistent with a photon index of Γ 1.3 1.6. ASCA data suggests that many of them tend to be ex- This indicates that the primary X–ray spectrum of NGC plained by the dual-absorber model. Actually, so far no 7582 is rather hard compared with the averaged slope any case could clearly claim to exclude both possibilities ≈ (Γ 2.0) found for most of other Seyfert 2s (Turner et due to the limit of data quality. Therefore, whether or al. 1997a). It is well established that presence of reflect- not there are Seyfert galaxies with intrinsic flat spectra ing matter in the nuclear region could strongly flatten is still an open question. the observed AGN continuum (Lightman & White 1988; George & Fabian 1991; Matt Perola & Piro, 1991), and Ginga observations of some Seyfert galaxies supported 5.4. The iron Kα line feature this idea (Matsuoka et al. 1990; Pounds et al. 1990). An iron Kα line emission was significantly detected in We have tried to add a reflection component (“plrefl” both observations. The best-fit line energy for both ob- model in XSPEC) in the best-fit model to test the possi- servations are consistent with the iron fluorescence line bility that the apparent flat spectrum could be due to an of 6.4 keV at rest from nearly-cold matter (< Fe XVI) intrinsic steep continuum plus some cold reflection. The (e.g. Makishima 1986). The observed equivalent width of result (e.g. for AO4 data) showed that an intrinsic steep EWKα ≃ 100 − 200 eV is slightly larger than the predic- continuum with Γ ≈ 1.85 ± 0.28 plus a certain reflection tion (∼70-90 eV) of the model where gas with inferred (R ≈ 0.88 − 4.22) slightly improved the spectral fit at ∼> column density uniformly distributed around an isotropic 68% confidence (∆χ2 = 2 with one additional free pa- X–ray source (e.g. Inoue 1990). It, however, can be easily rameter). Though the reflection ratio R, which describes interpreted by the model that the surrounding matter is the relative strength of reflection component to the di- not uniformly distributed, but in “torus” like geometry rect component, was not tightly constrained, the present (Inoue et al. 1990; Awaki et al. 1991). In such a case, reflection model predicts an equivalent width of 250±150 since the observed absorption is Compton-thin, and it is eV for the iron line. This value is slightly larger than, but well believed that absorption tori in prototype Seyfert ≈ +30 > 24 −2 still consistent with the observed value of EW 135−90. 2 galaxies are Compton-thick (i.e. NH ∼10 cm ), we An alternative possibility to flatten a steep intrinsic probably observe the central source through the edge of continuum is through the presence of an additional more the torus. The observed complex configuration of X–ray strongly absorbed power-law. This scenario usually be absorbing material along our line-of-sight, as suggested referred as “Dual-Absorber” model, which can be re- by its column variation as well as the sign of “dual- lated to a physical scenario for type 1.5/2 AGNs in which absorber”, can be understood by such a scenario. No.5] ASCAObservationsoftheSeyfert2GalaxyNGC7582 9 We note that observed equivalent width of iron Kα is A line feature was fitted with the center energy of ∼ 6.4 very similar to the average result of
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Fig The comparison of SIS hard and soft band light curves of NGC Light curves
in b oth short s and long s bins are shown for AO upp er panel and AO lower
panel resp ectively The time axis of AO starts from November UT
and AO starts from November UT For b oth observations the rapid
variability is seen in the hard keV band and no signi cant correlated variability in
the soft keV band 4
3
2
1
0.1 0.2 0.3
Cts/s (2-10 keV)
Fig The hardness ratio keV keV as the function of SIS count rate
keV in AO No sp ectral variability such as a change of N was detected in the hard
H comp onent
Fig The folded sp ectrum of NGC tted simultaneously for SIS and GIS
with the scattering mo del Related residuals for AO upp er panel and AO lower panel
are also shown The cross marker represents the SIS data and the void square denotes the
GIS data The mo del gives acceptable description of the data see table but still some
residuals remain around keV indicating that the soft sp ectrum is more complex than
just scattered comp onent
Fig The unfolded sp ectrum shown for AO tted with the mo del of absorb ed p ower
law scattered p ower law Raymond Smith Fe K line The b est t parameters of this
mo del and ux measurements are listed in table and table
Fig Con dence contour levels and for the sp ectral slop e vs the
intrinsic absorption column density The solid and dashed lines are for AO and AO data
resp ectively The photon indices of the hard continuum are in the range of in the
two datasets The intrinsic absorption changed evidently in the year interval
Fig Comparison of the rapid variability characteristics of NGC seen in keV
with Seyfert galaxies Nandra et al on the basis of s temp oral analysis Also
shown in the gure three other type ob jects NGC A MCG and NGC
Turner et al a For these Seyfert galaxies the rapid variability alone suggests that
the hidden Seyfert nuclei are directly seen in the hard X rays