, 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).