Multiwavelength observations of GeV blazars

Ast roparticle Physics ELSEVIER

Astroparticle Physics 11 (1999) 169-176 www.elsevier.nl/locate/astropart

Multiwavelength observations of GeV blazars Ann E. Wehrle Infrared Processing

and Analysis

Center, Jet Propulsion Laboratory and California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA

Abstract Multiwavelength observations of GeV blazars have been used to constrain physical models of the relativistic jets. Using the EGRET and COMPTEL instruments on the Compton Observatory, in concert with other spacecraft and ground-based telescopes, we can form complete spectral energy distributions from 10’ to IO**Hz. Such “snapshots” of the spectral energy distribution, especially during flares, show changes attributable to varying conditions in the jet. @ 1999 Elsevier Science B.V. All rights reserved.

1. Introduction The goal of multiwavelength campaigns is to obtain uniform, well-calibrated data across the electromagnetic spectrum to constrain models of blazars, e.g. Tiirler et al. [ 401, Fig. 1. The ultimate aim is to understand the propagation of energy in relativistic jets from a few Schwartzschild radii of the central black hole through the broad line region. In simple terms, we wish to relate the jet’s input energy to the output energy (cooling), despite the complicating factors of possible thousandfold amplification from Doppler boosting and unknown orientation to the line of sight. The underlying physical properties include shocks, reacceleration of particles, opacity, interactions with the local ambient medium, radiation from the accretion disk, instabilities in magnetic fields, and variation in the black hole accretion rate. We also do not know what the jet is made of (electron/positron pairs, hadronic cascades, etc.) or the central black hole mass or angular momentum. A multiwavelength “snapshot” of the spectral energy distribution (SED) yields information on phys-

ical conditions in the jet at a given moment. A series of such “snapshots” tracks variability through the electromagnetic spectrum, with models constrained by both the shape of the SED and time delays. The predictions of such models, categorized roughly by their principal source of seed photons for inverse-Compton scattering as external Compton, “EC” or internal synchrotron self-Compton, “SSC”, are reviewed in this conference by Marscher and Dermer [ 18,7]. Ideally, we could use a flare at some waveband as an alarm for impending activity at other wavebands (preferably at least a year in advance, so we can write proposals for intensive observations!). We have not, however, figured out what the best “alarming” waveband is. In the meantime, we have three separate strategies for assembling multiwavelength SEDs-coordinated campaigns to get simultaneous (within 1 day) data on a few objects, targets of opportunity campaigns, and literature-based SEDs (i.e., based on archival data in NED and literature searches), e.g. [40]. All three methods have been useful: the coordinated campaigns yield data about individual “archetypes”, the target-ofopportunity campaigns capture unusually bright flares,

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170 energy

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Physics I I (1999) 169-I 76

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Fig. 2. Spectral energy distributions uary-February 1996 flare.

Fig. 1. Nearly 20 000 observations of 3C 273 are presented here in the form of 70 normalized light curves (forming vertical columns) from the radio to the y-ray domain. The chart shows the observing date versus the frequency with the relative intensity (log( v * Fy) ) encoded as grayscale. Variability across the spectrum is emphasized by normalizing each light curve as (F, - ( Fv))/a. For example, the flare beginning in 1982 in the ultraviolet can be traced (horizontally and vertically) by 1984 as an increase in mm and radio bands. Figure courtesy of M. Ttirler, private communication.

and archival database SEDs reveal information about classes of blazars. The logistics of coordinated multiwavelength campaigns are quite complex because both space-based and ground-based observations are required to cover the range from IO9 to 10” Hz. Observing dates are constrained primarily by spacecraft: the target may not be too close to the sun, nor at such an angle as to prevent illumination of the solar panels. Most objects are visible twice a year to spacecraft with 70-120” sun constraints. Coordinating with TeV observatories requires that the moon be absent from the night sky during the observations. The length of the observing campaigns for y-ray blazars has been determined by the least sensitive telescope convolved with the fewest emitted photons, namely, the y-ray telescopes. In round numbers, EGRET detected one photon every 15 minutes from a bright source. Ideally, the shortest time scale of variability would be Nyquist-sampled. Close coordination of the spacecraft scheduling has been made possible by cooperation

of 3C279

during

the Jan-

between mission schedulers. Individual multiwavelength campaigns include those on 3(3279,3C273 and 0716+714 [ 17,48,43,45]. The largest campaign to date has involved 7 spacecraft and 18 ground-based observatories [ 481, including contributions by undergraduate students and amateur astronomers. The resulting SEDs are shown in Fig. 2. In this review I pose nine outstanding observational questions about GeV blazars and summarize what is known from multiwavelength observations to date. Prospects for the future are briefly discussed.

2. Nine outstanding

observational

questions

2.1. What is the shape of the inverse Compton peak? The change in spectral index (“break”) between X-ray and y-ray wavebands can be used to constrain theoretical models for Compton cooling of injected relativistic electrons. In simple models of incomplete cooling, the spectral index change should not exceed 0.5 [ 32,6], however, for the group of EGRET blazars with the highest radio luminosities, Fossati et al. [ 81 find the spectral change is about 0.7 (their Table 5). Hartman et al. [ lo] have recently reviewed y-ray observations of blazars. They find that the average photon spectral index of “normal” y-ray blazars is 2.1 (U = 0.3). Establishing the existence of turnover at the high energy end is difficult because of small-number statistics for the few detected photons, however, interactions of very high-energy TeV y-rays with am-

A. E. Wehrle /Astropnrticle

bient infrared photons will reduce the transmission of y-rays from distant blazars (e.g., [ 34,291). The high energy component of “normal” y-ray blazars peaks in the range 10” and 1024 Hz. Two unusual blazars, CR0 JO5 16-609 (probable counterpart PKS 05066 12) and PKS 0208-512, form the archetypes of the class of “MeV blazars” [ 5,4,3] whose high-energy SEDs peak at MeV energies. OSSE, COMPTEL and EGRET data have shown that the spectral index break for two highredshift blazars, PKS 0528+ 134 and CTA 102, exceeds the value predicted by simple models.

Dhysics II

(1999)

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10 2.2. What is the relationship between the frequencies at which the synchrotron emission and the inverse Compton emission peak? It has been known for some time that the synchrotron SED peak frequency and the inverse Compton SED peak frequency are directly correlated. For example, X-ray selected BL Lacs (“HBL’s”) have SEDs that peak at ultraviolet and near-TeV energies, while FSRQs have SEDs that peak at infrared and IOOMeV energies. Classes of blazars have been studied by Sambruna and Fossati in their PhD theses [ 3 1,8]. Fossati et al. formed average SEDs by binning 51 y-ray detected blazars (Fig. 3, from Ghisellini et al. [ 91). The spectral index in the X-ray waveband is key: a negative slope in the SED results from synchrotron domination or the high energy tail of the accretion disk radiation; a positive slope results from inverseCompton domination. A comprehensive analysis of ASCA X-ray data on 18 blazars is presented by Kubo et al. [ 141. An attractive model combining external Compton and synchtrotron self-Compton emission has recently emerged from statistical analysis of the average SEDs of Fossati et al. From the model SED fits, Ghisellini et al. have proposed a unified scheme wherein increasing the external radiation field, the total energy density, and the injected jet power yields the sequence of HBL, LBL, HPQ, and LPQ. Observationally, the ratio of inverse-Compton peak to synchrotron peak amplitudes increases in the same sequence, while the yp& decreases (see Fig. 7 of Ref. [ 93 ) .

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2.3. How variable is the synchtrotron

component?

In general, the optical and ultraviolet wavebands vary much faster than emission longward of the synchrotron peak because of synchrotron self-absorption at long wavelengths. Multiwavelength studies of gamma ray blazars have benefitted enormously from the long term monitoring programs at radio, millimeter, submillimeter and optical wavebands. Two recent conferences, “Blazar Continuum Variability” in Miami, Florida [46], and “The BL Lac Phenomenon” in Turku, Finland [ 381, showcased such work. Generally speaking, the radio emission of FSRQs is self absorbed at centimeter wavelengths, with variations up to factors of 3-5 [ 2,1] on time scales of months to several years; “intraday variability” has been observed in a few objects (e.g., [ 441) such as 0716+714. BL Lac objects may be optically thin at centimeter wavelengths with higher amplitude variability on shorter time scales than FSRQs. Flares initially visible at short submillimeter wavelengths have been tracked through the spectrum to centimeter wavelengths via multi-observatory cooperative work for several objects (e.g., [ 35,151). Optical emission has been monitored at a number of observatories

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(Colgate University, Tuorla, Torino, Rosemary Hill, La Silla, IAC/Las Canarias). Optical flares usually occur on time scales of days to weeks, with historical variations of up to 5 magnitudes. The small optical and ultraviolet variations observed during the 1996 X-ray and y-ray flare of 3C279 may result from a shift in the frequency of the synchrotron peak, as expected from increased cooling on external photons. The curious phenomenon of “microvariability” (variations up to 50% within an hour) has been observed by Miller and collaborators [ 21,221. The origin of microvariability is not known: it may be due to short-lived synchtrotron flares in the jet, or to variable thermal emission from the central accretion disk, or to more exotic phenomena such as magnetic loops in the accretion disk [49]. Clearly, we would like to sort out how much optical variability at a given time is due to jet-related processes, so that we may correlate changes in the intrinsic synchtrotronemitting electron population with the production of inverse Compton gamma rays. Optical monitoring of changes in the polarization angle, the polarized flux, and the total flux (as the Allers do at centimeter wavelengths) would be of enormous benefit. The region of the synchrotron peak with the least amount of data is the infrared; the compilation of 1983 IRAS data on blazars [ 111, formed by both survey and pointed observations, has not been superseded by any new space mission. New studies of infrared variability were made with IS0 from 1996 to 1998; results should be available in the next year as calibration problems are addressed. The NASA Space Infrared Telescope Facility (SIRTF, launch 2001) will conduct pointed observations which could monitor blazars periodically over its expected lifespan of five years. A major step forward in obtaining very broad coverage across the width of the synchrotron peak will be made by the ESA Far Infrared and Submillimeter Telescope (FIRST) mission, anticipated for launch in 2006. 2.4. How variable component?

is the inverse-Compton

Two blazars have shown tremendous y-ray variability: by factors of N 80 (PKS 1622-297, and N 100 (3C279, [ 481) . A long term EGRET light curve of 3C279 is shown in Fig. 4 [ 391. Observations of other y-ray blazars showed lower amplitude variabil-

Physics I I (1999) 169-l 76

ity. The most comprehensive study of y-ray variability in EGRET blazars has been made by R. Mukherjee and collaborators [ 251. Long term “light curves”, consisting of snapshots of y-ray activity at widely spaced intervals (N 1 year), show that variability of factors of a few is common in FSRQs. Mukherjee et al. found that 76% of the FSRQs were variable and 16% nonvariable in their sample. In contrast, only 50% of the BL Lacs were variable while 21% were nonvariable. 2.5. What is the duty cycle offlares? There are two parts to this question; first, how often do flares occur, second, what is their duration? No blazar has been monitored continuously for longer than 7 weeks with EGRET, COMPTEL or OSSE. COMPTEL and EGRET pointings in the Virgo region have generally included both 3C279 and 3C273 simultaneously, while the fortuitous “yray constellation” of the Crab nebula, the Geminga pulsar, and PKS 0528+134 can also be covered in a single pointing. Despite our lack of long continuous data trains on large numbers of objects, we can roughly estimate the y-ray duty cycle to be about 20-30% by comparing relative frequency of high and low flux measurements in more-or-less random observations. For example, two out of eight independent EGRET observations of 3C279 showed a flare as we see from examining Fig. 4 [ 391, while five independent EGRET observations of 3C273 yielded only a single high state (Mukherjee et al. [25], their Fig. 1; the remaining three observations of 3C273 are not yet published). Of course, we reobserved 3C279 often because it was in a flare state just after the EGRET launch. If flares come in bunches, we’re biassing our statistics by following up flaring objects and not following up nonflaring objects. Interestingly, Mukherjee et al. show that BL Lacs may have a lower duty cycle than FSRQs because BL Lacs had flux variations greater than f3a less often than FSRQs. The duration of a y-ray flare may be 24 hours to several weeks, if we assume that a high level of photon flux integrated over a full EGRET pointing represents a single flare. Fig. 5 shows the January-February 1996 flare in 3C279 [48] where the doubling time was N 8-12 hours. The fastest rise time (doubling time 8 hours) observed by EGRET was during the flare of

A. E. Wehrle /A.stropariicle Physics 1 I (I 999) 169-l 76

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PKS 1622-297 [ 201. The variability time scale may be used with the gamma-gamma transparency argument to constrain relativistic beaming parameters [ 19,481.

2.6. How does the spectrum change before, during and afrer a jlare? Changes in the spectral index during a flare could yield information on energy-dependent acceleration and loss processes as well as the relative dominance of one physical mechanism over another at various locations in the jet. The lack of such changes may suggest that the jet emission consists of a rapid series of flares, rather than a dormant, quiescent state with occasional, widely spaced flaring events. Well-determined spectral indices require lots of detected photons; possibly the only blazar with sufficient data is 3C279, where was no difference in the spectrum between the flare rise (Jan 1996) and the flare peak (Feb 1996) despite the factor-of-three difference in brightness. In 1997, we spent several weeks monitoring 3C279 in a very weak state; it will be useful to compare the spectral index so derived to that of the flare state. Early work by Sreekumar et al. suggested evidence for spectral hardening during a flare for two objects, PKS 1222+216 and 1633+382. Evidence for such hardening on PKS 0528+ 134 [ 251 using more extensive data [ 25 1 has vanished.

Fig. 5. Multiwavelength light curves of 3C 279 during the EGRET campaign (1996 January 16-February 6): (a) EGRET fluxes at > 100 MeV binned within 1 day (open squares) and 8 hours (filled squares); (b) X-ray fluxes at 2 keV: besides the RXTE data (open squares), the isolated ASCA (filled square) and ROSAT-HRI (cross) points are reported with horizontal bars indicating the total duration of the observation; (c) IUE-LWP fluxes at 2600 A: (d) ground-based optical data from various ground-based telescopes in the R band; (e) JCMT photometry at 0.8 mm (open squares) and 0.45 mm (filled squares); (f) radio data from Metsiihovi at 37 GHz (open squares) and 22 GHz (filled squares ). and from UMRAO at 14.5 GHz (crosses). Errors. representing 1-o uncertainties, have been reported only when they are bigger than the symbol size.

2.7. Is there a real quiescent state, or do we observe an endless series of flares? Variability exists on every time scale on which we have observed blazars, in fact, it is the defining characteristic of the class. In optical wavebands in particular, we see a series of flares in the y-ray blazars. Longterm light curves from the Rosemary Hill Observatory show that the sources do not conveniently hover about a characteristic optical magnitude, instead, they have slowly varying ( N 5 years) “baseline levels” on which faster flares are superimposed [ 331. At y-ray

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bands, we know that flares may have rise times less than a day (PKS 1622-297,3C279 [ 20,481)) yet usually we needed to integrate for a week or more to get an EGRET detection. It is quite possible that such long integration times smooth over low-amplitude flares as described by Urry (this volume). As mentioned above, the detection of a characteristic difference in the y-ray spectral index during bright and dim states could provide evidence of the existence of a quiescent state. Flare power spectra could be obtained with the proposed Gamma-Ray Large Area Space Telescope (GLAST) in scanning and pointed modes. This could tell us much about the nature of (episodic) energy release in various classes of blazars, and by extension, their parent populations, assuming we also know the amount of relativistic beaming (e.g., from VLBI studies). Specifically, is most of the jet energy on subparsec scales carried by many small flares or by a few large ejection events?

2.8. Are alljlat y-ray bands?

spectrum

radio quasars active at

Comparison of redshift distributions of flat spectrum radio quasars and EGRET quasars (Hartman et al. [ lo], their Fig. 7) show that the two populations are similar, given the small number of EGRET blazars in the sample. Not all blazars in the 1-Jy sample [37] have been detected with EGRET. Perhaps EGRET detected only those blazars that were increasing their activity level as suggested by Valtaoja and Terasranta [ 4 1,421; certainly the lack of y-rays from well-known but currently weak blazars like 3C345 has been puzzling. A comparison of EGRET Phase 1 detections and non-detections of HPQs with radio light curves obtained with the Metasahovi telescope at 22 and 37 GHz showed that y-rays were detected only in the initial phases of a radio outburst [ 4 1,421. This implies that the y-rays are produced in the shocked regions of the jet. Data obtained with the 100 m Effelsberg telescope quasi-simultaneously with EGRET observations in Phases 1 and 2 confirm that FSRQs were preferentially detected when the radio flux was increasing [ 24,231.

Physics I I (I 999) 169-I 76

2.9. How is y-ray activity related to the parsec-scale jet? y-ray activity probably occurs well within the radio photosphere that radio astronomers call the core. The (re)acceleration of particles to relativistic speeds can occur in shocks within the parsec-scale jet. If y-ray activity is unique to the conditions in a few unusually bright blazars, we might expect that the parsec-scale radio jets bear a distinct signature, such as smoother, clumpier, or more polarized jets. This has not been borne out by observations. Kellermann et al. [ 121 found no difference in the parsec-scale morphology of EGRET blazars and bright FSRQs, as evidenced by 15 GHz VLBA imaging. It has been suggested that some y-ray flares were followed by the ejection of relativistic blobs. This seems plausible if y-ray flares are correlated with radio flares. We do know that centimeter-wavelength flares are associated with energetic events in parsecscale radio jets; such flares give rise to blobs, interpreted as shocked plasma moving at relativistic speeds. Isolated blobs may be tracked with VLBI for years. After a year or two of high frequency (22, 43, 86 GHz) VLBI monitoring, their movements can be extrapolated backwards to establish a date of spatial coincidence with the core, assuming uniform speed and straight trajectory. The blazars for which some evidence of a y-ray flare/VLBI blob connection exists are 3C279 [47,48] PKS 0528+134 [28,13], 3C454.3 [13], S5 0836+710 [13,26], and NRA0 190 [ 161. Lower resolution monitoring of three other quasars (PKS 0202+ 149, CTA26 (PKS 0336-019)) PKS 1606+106) has not shown any y-ray flare/VLBI blob connection [ 271. How often the “average” flat spectrum quasar ejects a blob is not known because VLBI observers have traditionally concentrated their monitoring efforts on a few bright superluminal objects. A good measurement of blob ejection rates in a statistically well-defined sample may be made when third-epoch VLBA data are analyzed (Kellermann et al., in progress). This kind of statistical information is essential to properly evaluate the probability of coincidence for the birth of VLBI blobs to be associated with y-ray flaring (“alarming”), assuming that blobs last long enough to be tracked between VLBA observing runs. In addition, we will

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need GLAST’s near-continuous the y-ray flare duty cycle.

coverage to evaluate

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3. Prospects The next few years will see the launch of new spacecraft, the activation of robotic, remotely operated telescopes like SARA, and operation of new and upgraded TeV facilities. New monitoring programs such as those with RXTE and BeppoSAX have joined longterm ground-based programs to provide more comprehensive databases to search for correlations, for example. collaborations led by McHardy on 3C273 and 3C279, and Urry and Maraschi on PKS 2155-304. New ground-based TeV telescopes can monitor many more BL Lac objects and serve as focal points for multiwavelength campaigns. In addition, proposed new imaging air Cerenkov telescopes like MAGIC could bridge the observational gap between TeV and GeV energies; this would open up the possibility of monitoring many blazars in the EGRET catalog from the ground. A particularly powerful combination will be TeV telescopes plus GLAST because the GLAST survey mode (which will scan most of the sky every 90 minutes) could trigger observations of flares, moreover, pointed mode observations can provide highly sensitive data on new TeV candidates.

Acknowledgements I am grateful to my collaborators Meg Urry, Laura Maraschi, and Elena Pian and to all my multiwavelength colleagues. I thank Dave Thompson and Bob Hartman for providing the long term EGRET light curve of 3C279. This work has been supported in part by the NASA Long Term Space Astrophysics Program. Portions of this research were carried out at the Jet Propulsion Laboratory which is managed by the California Institute of Technology, under a contract with NASA. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

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