Reports from Observers

, REPORTS FROM OBSERVERS

Monitoring of Active Galactic Nuclei: the Why and the How

D. ALLOIN1, M. SANTOS-LLE01, G. STlRPE2, and B.M. PETERSON3

1Departement d'Astrophysique et de Cosmologie, URA 173 CNRS, Observatoire de Paris-Meudon, France; 20sservatorio Astronomico di Bologna, Italy; 3Department ofAstronomy, The Ohio State University, Columbus, Ohio, USA

1. Introduction the inner structure of AGNs is provided by collaboration (Ulrich et al., 1984; Clavel et measuring in detail how the emission-line al., 1990). One of the major surprises of Over the past twenty years, ground fluxes change in response to changes in the monitoring campaigns of the eighties based optical observations as weil as the continuum flux. The broad emission was that the BLR seemed to be an order ultraviolet, X-ray, and y-ray observations lines respond with small but measurable of magnitude smaller than the value Irom space-borne telescopes have re time delays (days to weeks) to variations generally predicted by photoionisation vealed the variable nature 01 the conti of the central continuum source, making equilibrium calculations. This conclusion nuum and emission lines in the spectra it possible to use the technique of demanded even denser sampling for 01 active galactic nuclei (hereafter re "reverberation mapping" to probe the AGN variability programmes. lerred to as AGN). Variability is now structure and kinematics of the BLR. In In 1987, two successive workshops in recognized as one of the distinctive general, the narrow lines do not vary in Segovia and Atlanta featured lively dis leatures of these important but poorly flux since the size of the NLR is usually cussion of results obtained from AGN understood objects. too large to provide a coherent response variability studies. It became apparent to to changes in the level of the continuum the community that the goals of spectro flux. scopic monitoring programmes could in 2. The Why of Monitoring The fundamentals of reverberation fact be achieved only if sufficient observ Campaigns mapping were described by Blandlord ing time could be devoted to such an and McKee (1982), but it has been only approach. 2. 1 Some basics over the last five years or so that the Cooperation of observers on a scale first tentative applications of this tech that was unprecedented in extragalactic In the study 01 AGN, variability affords nique to real AGN have been possible, astronomy, i.e. with very large collabora a potentially valuable probe of the as severe conditions on the amount and tions involving around 100 astronomers, properties of both the continuum source quality of the data have to be met (Pe became a necessity. itself and the broad-line emitting region terson, 1994). To deal with a collaboration of this surrounding it. Some experiments, undertaken by the size and a highly time-constrained pro Knowledge of the continuum variabili "International AGN Watch" collaboration, gramme, new ways of working and ty pattern in different wavebands from y have been conducted in part with tele cooperating had to be invented. The ray to radio wavelengths can provide a scopes at the European Southern Obser International AGN Watch was therefore way to probe the various physical pro vatory (ESO), and these form the subject established with the goal of focusing cesses at the origin of the continuum of this report. attention on a few AGN for intensive emission. The variation time scales in Similar programmes, albeit with a monitoring efforts and maintaining com particular give some indication of the size sometimes different overall emphasis, munication among the various individu of the emitting regions, an indirect clue to have been undertaken by other informal als and groups that carried out the actual the likely emission mechanisms. Of organisations du ring the same time observations. A key factor in the success potentially greater interest is the possibil Irame. For example, the European con of these efforts has been the ability to ity of eventually measuring time lags sortium LAG ("Lovers of Active Galax communicate and exchange information between continuum variations in different ies") wh ich was initiated by the late MV promptly via modern computer networks. wavebands because this can tell us about Penston, has carried out a spectroscopic The role of the AGN Watch has been the connection between various mecha and photometric monitoring of several multifold: (a) to define the scientific nisms producing continuum photons in AGN on the Canary Islands telescopes questions to be addressed, conceive the these systems. within the framework of the CCI 5% observational projects and coordinate In the framework of the so-calied international time programme (Robin the submission of the appropriate observ standard model, based on a massive son, 1994). ing proposals, (b) to ensure that data are black hole and accretion-disk system, we collected in a manner consistent with the assume that accreting material is distrib 2.2 A bit of recent history scientific goals, (c) to reduce the observa uted throughout the line-emitting regions: tional data and make these data sets the broad-line region (hereafter BLR) and In the early eighties, a number 01 available to the entire community, and (d) the narrow-line region (NLR), wh ich are groups involved in AGN studies under to perform the measurement and analy Somewhat arbitrarily distinguished by the took ultraviolet and optical monitoring sis of the data and publish the primary width of the lines they emit (ranging from programmes in an effort to probe the scientific results. as much as a few 10,000 km/s for the physics of AGN (for a review, see It was decided that detailed and broadest lines to only a few 100 km/s for Peterson, 1988). We note in particular the model-dependent interpretation would the narrow lines). An important probe 01 results of the so-calied NGC 4151 be left to individuals or sub-groups of the

25 16 ultraviolet spectra at a higher rate than in the original programme, once every two days with IUE between March 16 and ,...... May 27, 1993; during the second half of I 1I 0« 1 this campaign, HST spectra were ob N 14 I tained with an even higher frequency, 8 once per day. Detailed results of the ü h campaigns on NGC 5548 can be found I in Clavel et al. (1991), Peterson et al. Vl IIIHII It (1991), Korista et al. (1995) and refer QJ) 12 l-o r ences therein. The main conclusions llJ I I reached are as folIows: ~ I 1. The ultraviolet and optical continua ...... 0 vary with little, if any, phase difference '--' 1 between them. The continuum becomes u10 r.r... ~ 1 bluer as it becomes brighter and the shorter-wavelength continuum bands show sharper variations. 2. The variations of the highest ionisa tion lines (He 11, NV) lag behind the 1300 variations of the ultravioiet continuum by ...... N slightly less than 2 days, implying an I 8 inner radius of somewhat less than 2 Ü1200 light-days for the BLR. Its outer radius, ~ ~ 11 from the CIII] and Balmer lines, is I l Vl li 1 1 1 somewhat larger than 20 Iight-days. QJ) q\#lt I I 3. There are some indications that the l-o 11 llJ1100 ~ I jl 11 higher radial-velocity gas (line wings) ~ I I fIlII responds more rapidly than the lower I radial-velocity gas (Iine core), suggesting 0..... 11 "-" 11 1 a virialised BLR c10ud system. ..--..1000 co. I ::c: ~l 3.2 Where ESO comes on the "-" I r.r... tl I stage, the NGC 3783 campaign 900 In order to improve our understanding of the size and structure of the BLR and to test the generality of the NGC 5548 results, it was deemed to be desirable to Figure 1: The light-curves ot the optical continuum (top panel) and ot Hß (10 wer panel) trom the carry out similar programmes on other AGN in NGC 3783 during the ESO campaign. AGN in order to map the AGN luminosity vs. BLR size plane. Therefore, two other targets with different absolute luminosities were se AGN Watch collaboration, as weil as to ground-based and space-based obser lected, NGC 3783 (AGN Watch 11) and other interested parties. The AGN Watch vatories. Fairall 9 (AGN Watch IV), both observ data are at the disposal of the entire able from the southern hemisphere. community once the primary scientific 3. 1 First experience, the NGC 5548 These AGN Watch campaigns relied results have been published by the campaign heavily on ESO telescopes for the collaboration. ground-based component. The first AGN Watch project was an The AGN Watch campaign 11 was set eight-month monitoring campaign (AGN up to monitor NGC 3783 with IUE for 69 The How Watch campaign I) on the Seyfert 1 epochs from December 21, 1991 to July gaiaxy NGC 5548. IUE observations 29, 1992, once every 4 days for the first As many of the strongest and most were made once every four days be 172 days and once every 2 days for the important broad lines in AGN spectra are tween December 14, 1988 and August 7, final 50 days. Simultaneous optical and located in the ultraviolet domain, space 1989, for a total of 60 epochs. near-infrared observations were collect based observations are critical for under Ground-based observations were col ed from ESO and CTIO (Chile), CASLEO standing the BLR. Therefore, the initial lected with various telescopes in the (Argentina), Lowell Observatory (USA), focus of the AGN Watch efforts was UV northern hemisphere during this entire Vainu Bappu Observatory (India) and spectroscopy using the International UI period, and the ground-based compo SAAO (South Africa). The ground-based traviolet Explorer (IUE) and, later, the nent of this programme is in fact still campaign started on December 3, 1991 Hubble Space Telescope (HST). Howev continuing. The success of this original and was completed on August 9, 1992. er, concurrent observations of the optical programme (see Peterson, 1993, for a An application for the ESO observing lines and continuum are essential for a detailed summary) led to a similar programme was submitted through the complete description of AGN behaviour, programme of NGC 3783 (AGN Watch 11, normal Observing Programmes Commit and therefore each of the AGN Watch described below), and to a follow-up tee (OPC) channel, with special require campaigns was organised to ensure that programme on NGC 5548 (AGN Watch ments regarding the dates to be sched adequate data would be obtained at both 111) using both IUE and HST to obtain uled and the time-slot to be allocated to

26 the programme, i.e., once every 4 nights Year, it was possible to arrange a nearly observation times, with special care being (concurrent with the IUE observations), 2 simultaneous multi-wavefength snapshot taken to maintain the regular sampling hours of time placed such that NGC 3783 of NGC 3783which included observations that is desirable in these monitoring would be at its meridian transit (in orderto from GRO, ROSAT, Voyager, IUE, HST, programmes. The AGN Watch observa minimise the air mass). optical and infrared ground-based tele tions were scheduled even when the The project was recommended by the scopes, and the VLA. These data were telescope was otherwise idle, and we OPC and carefully scheduled by ESO on also essential to a better understanding of greatly benefitted from this high level of the 1.5-m telescope. All Pis of other the continuum source in NGC 3783 and cooperation by ESO. regular successful 1.5-m telescope pro complemented the AGN Watch data sel. The on-site organisation of the cam posals were informed in advance by ESO The results of AGN Watch campaign 11 paign was roughly similar to that set up for about these two-hour blocks of time and are described in detail by Reichert et al. AGN Watch 11, with Dr. C. Mendes de were required to schedule their own (1994), Stirpe et al. (1994), and AIIoin et Oliveira and Dr. E.Chatzichristou being observations around this interruption. A al. (1995). We note here the salient successively responsible for the interac detailed handbook had been prepared for conclusions: tion with Pis of the regular proposals the AGN Watch observations, describing 1. As in the case of NGC 5548, scheduled on the 1.5-m telescope at briefly the purpose of the observations, significant variations were detected, both every AGN Watch epoch, and forthe data the experimental conditions to be strictly in the continuum and in the emission-line collection. Again, the campaign went on followed, and providing information for all f1uxes. We observe in NGC 3783, how smoothly as the ESO staff and most of the the necessary contact persons. A group ever, rapid f1uctuations of relatively high regular observers were extremely coop of ESO postdocs, students, and cooper er amplitude than in NGC 5548, while the erative and helpful. ants present at La Silla over this period of longer-term modulations appear to be Data from the ground-based cam time was organised under the responsi comparatively less weil defined. paign, from all participating observato bility of Dr. B. Altieri to actually take care 2. The continuum fluxes appearto vary ries, are now being reduced and will be of the AGN Watch observations. Dr. B. simultaneously in all four measured compared soon to the ultraviolet continu Altieri was also in charge of collecting and ultravioletloptical bands. The slope of the um and emission-line Iight-curves. Pre reducing all the AGN Watch data on-line ultraviolet continuum is found to vary in liminary results from the IUE campaign in order to ensure that the programme the sense that the fractional amplitude of show that Fairall 9 did vary significantly was being carried out as designed and the continuum variations decreases with and that high trequency variations are was producing useful data. increasing wavelength. superimposed on the longer-term trend. This organisation turned out to be 3. Cross-correlation analysis indicates extremely satisfactory and efficienl. The strikingly short time delays for most of the ESO staff was found to be very coopera strong emission lines. The peaks of the 4. Conclusions tive, wh ich was certainly one of the cross-correlation functions occur at lags primary reasons for the success of the of 0 ±3 days for He 11+0111], 4 ± 2 days for Although the observational effort de campaign. The observers of regular Lya and CIV, 8 ±3 days for Hß, and 8-30 manded in such campaigns is quite large, proposals had to deal with some interrup days tor Si 111] and CIII]. the AGN community is convinced that the tion of their own programmes and consult 4. The continuous emission of the scientific returns are sufficiently impor with the ESO AGN Watch team with genuineAGN in NGC 3783 appears to be tant that such campaigns are worth the regard to details of the programme in rather flat from soft y-ray to infrared trouble. Through these large-scale coor order to carry out both the PI and AGN wavelengths with index a '= 1. The dinated efforts, we have been able to Watch programmes as efficiently as ultraviolet and near-infrared excesses acquire data sets of high quality and Possible. We are pleased to report that we can be understood in terms of thermal reasonable homogeneity which are suit found a highly cooperative spirit among emission from an accretion-disk surface able for further statistical analysis. The the regular observers. Altogether, the and a hot dust component, respectively. AGN Watch data sets are available to the experience has been quite positive in our entire community, and there is no doubt relations with the ESO staff and the 3.3 A high luminosity object, that they will be used by many more European astronomical community. We the Fairall 9 campaign astronomers in the future. owe them many thanks and certainly The AGN Watch campaigns have some part of the credit for the success of AGN Watch campaign IV was devoted demonstrated that there is no delay, to the campaign. to an AGN of much larger absolute the accuracy measurable so far, between After reduction, the ESO data were luminosity, Fairall 9, which was already the ultraviolet and optical continua. This then merged with similar data sets known to exhibit long-term large-ampli result rather argues in favour of repro collected at other ground-based facilities tude variability (Clavel et al., 1989). In the cessing models and seems to rule out (Fig. 1). There was in particular a very ultraviolet, this object was observed with simple, geometrically thin, optically thick close collaboration with CTIO, as re IUE once every 4 days from April 30 to accretion-disk models for AGN. In at least searchers in charge of the AGN Watch at December 26, 1994. Because Fairall 9 is three AGN, it has been confirmed that the both sites were in continuing contacl. a southern source, again the 1.5-m ESO size of the BLR is an order of magnitude The NGC 3783 campaign was also telescope played a key role in the ground smaller than predicted on the basis of distinguished from the AGN Watch cam based effort. Again the standard proce the standard photoionisation equilibrium paign I by two important features: dure of time application through the OPC models of AGN. In NGC 3783, it is found 1. The availability of HST allowed us to channel was followed, without calling for that the BLR extends from about 1 to 2 obtain a high-resolution, high signal-to a key projecl. Time was granted to the light-days upwards to around 30 light noise ultraviolet spectrum that proved to campaign, 2.5 hours once every 4 days days, and is radially structured with the be crucial in disentangling features in the from May 2 to September 27,1994 but highly ionised material closest to the IUE data by using the HST spectrum as a with two consecutive epochs missing due centre. model. to a large block of time scheduled with Carrying on such large campaigns 2. Under the auspices of the "World an instrument wh ich was not suitable for requires good will, excellent organisation Astronomy Days", sponsored by ESA in our projecl. Again, the ESO schedule and communication among the astrono the context of the International Space matched as weil as possible the IUE mers involved in the pilot group, and a

27 broad consensus in the community on the References Peterson, B.M., Balonek, T.J., Barker, E.S., et importance of the project. We have been al., 1991, ApJ368, 119. Reichert, GA, Rodriguez-Pascual, P.M., AI fortunate in benefitting from the interest, Alloin, D., Santos-L1eo, M., Peterson, B.M., et loin, D., et al., 1994, ApJ425, 582. encouragement, and support of the staff al., 1995, A&A 293,293. Blandford, R.D., McKee, C.F., 1982, ApJ255, Robinson, A., 1994, in Reverberation Mapping of various observatories at which these 419. of the Broad-Line Region in Active Gatactic observations have been made. In addi Clavel J., Boksenberg A., Bromage G., et al., Nuctei, ed. P.M. Gondhalekar, K. Horne, tion to the tangible scientific return from 1990, MNRAS 246, 668. and B.M. Peterson, ASP Conference Se these programmes, we believe that the Clavel, J., Reicher!, GA, AIIoin, D., et al., ries, 69, 147. Stirpe, G.M., Winge, C., Altieri, B., et al., 1994, large-scale international collaborations 1991, A pJ 366, 64. Clavel, J., Wamsteker, w., Glass, 1., 1989, ApJ ApJ 425, 609. in the AGN field have greatly enhanced 337,236. Ulrich M.H., Boksenberg A., Bromage G., et the mutual interactions of the astrono Korista, K.T.,Alloin, D., Barr, P., etal, 1995, ApJ al., 1984, MNRAS206, 221. mers involved in the project, have led to a Suppt. Ser., April 15. much more efficient use of telescope Peterson, B.M., 1988, PASP 100, 18. time, and have resulted in a better Peterson, B.M., 1993, PASP105, 247. Peterson, B.M., 1994, in Reverberation Map- coordination of programmes, thus lead ping of the Broad-Line Region in Active ing to faster and unquestionable pro Gatactic Nuctei, ed. P.M. Gondhalekar, K. gress in our understanding of the AGN Horne, and B.M. Peterson, ASP conference D. Alloin, e-mail: phenomenon. Series, 69, 1. ALLOIN%[email protected]

On the Variability of Narrow-Line Seyfert 1 Galaxies

M.E. GIANNUZZ01 and G.M. STIRPE?

10ipartimento di Astronomia, Universita degli Studi di Bologna, Italy 20sservatorio Astronomico di Bologna, Italy

1. Introduction The distinction between type 1 and larger than that of the forbidden lines1 1 type 2 Seyferts is by no means clearcut. (FWHM ~ 1000 kms- ). Studies of NLS1 s Narrow-Iine Seyfert 1 (NLS1) galaxies Spectropolarimetry (e.g. Antonucci & have shown that the broad components are characterised by the relatively low Miller, 1985) has revealed that several of the lines have ratios similar to those of projected velocities of their line-emitting (though not necessarily all) Seyfert 2's 'normal' Seyfert 1's and, on average, nuclear gas. We describe a spectroscop contain BLRs which are hidden to con lower equivalent widths (Osterbrock & ic programme based on a search for ventional spectroscopy by obscuring Pogge, 1985, Goodrich, 1989); this last variability, which attempts to constrain material (possibly a dust torus). This has property, however, is the extension to low the causes of their difference with sparked a debate on the possibility that all FWHM of a trend observed throughout respect to other Seyfert 1 galaxies. Seyferts may be described within a the Seyfert 1 population. Some NLS1 Active galaxies which are classified as unified model, in which the orientation of galaxies present in their spectra high Seyferts (characterised by a Juminous the nuclear axis determines the aspect of ionisation iron lines like [FeVIIl )",5721, nucleus of stellar appearance, with a a source's spectrum, and therefore its )",6087 and [Fe Xl )",6375 (Osterb rock, non-stellar blue continuum and strong classification. Within this framework, 1985, Osterbrock & Pogge, 1985), in emission lines) are divided into two Seyfert 2 nuclei are viewed at large some cases with high intensity: these are categories according to the widths of inclination angles, and Seyfert 1's at properties common in Seyfert 1 galaxies, their lines: in Seyfert 2 galaxies, forbid medium and small ones. but quite rare in Seyfert 2's. NLS1 s den and permitted lines all have the same comprise approximately 10% of optically 1 width (- 1000 kms- ), while in Seyfert 1 selected Seyfert 1's, but a significantly galaxies the permitted fines have an 1.1 What is a narrow-Iine higher percentage - 16-50%) of soft X additional component of much greater Seyfert 1 galaxy? ray selected Seyfert 1 sampies (Steph 4 width (- 10 km S-1). The difference is ens, 1989, Puchnarewicz et al., 1992). attributed to the presence of both a broad The broad components of Seyfert 1's Boiler et al. (1995) report the observation line region (BLR) and a narrow line region display a great variety of profiles and with ROSAT of a sampie of NLS1 s, (NLR) in the nuclei of Seyfert 1's, while widths (e.g. Osterbrock & Shuder, 1982, finding that the objects in this class have only the latter is present, or visible, in Stirpe, 1990), and it is tempting to explain generally steeper soft X-ray continuum Seyfert 2's. The BLR is characterised by it on the basis of projection effects. In slopes than normal Seyfert 1's, and rapid higher densities, higher velocities of the particular, the so-called 'narrow-line Sey soft X-ray variability. gas which forms it, and by a smaller size fert 1 galaxies' (Osterbrock & Pogge, than the NLR: in fact, BLRs are so 1985) are at the lower end of the broad compact (<< 1 pc) that even in the closest line width distribution in the Seyfert 1 'It is important to realise that we are reterring to active galactic nuclei (AGN) they cannot class. While they are clearly distinct from objects whose maximum observed velocities tram be resolved spatially. The large velocities Seyfert 2's because of the different widths the BLR are low, not to objects in wh ich the broad component 01 the emission lines is very weak present in the BLR are generally attribut of permitted and forbidden Iines, and compared to the narrow component, but also very ed to the gravitational effects of a because of the presence of Fe 11 lines broad: the FWHM 01 the permitted line (broad + massive (> 107 M,,) accreting black hole, (which are not emitted by the NLR and are narraw component) can be similar in objects 01 these two types, and sometimes a low signal-to-noise ratio which is the prime cause of the nuclear therefore absent in Seyfert 2 spectra), the in a spectrum can mask a weak but very braad activity. width of their broad components is barely component, and cause an object to be misclassilied.