F. Makino

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

ASCA Observation of an X-Ray/TeV Flare from the BL Lacertae Object Markarian 421

We observed the BL Lac object Mrk 421 with the X-ray satellite ASCA in 1994 as part of a multifrequency observation. The 24 hr observation was conducted 1 day after the onset of a TeV flare detected by the Whipple Observatory and detected an X-ray flare, with no apparent variability in the optical, UV, and EGRET GeV flux. The ASCA 2-10 keV flux peaked at 3.7 ? 10-10 ergs cm-2 s-1 and then decreased to 1.8 ? 10-10 ergs cm-2 s-1 with a doubling timescale of ~12 hr. The shape of the X-ray spectrum varied during the observation, such that the hard X-rays always led the soft X-rays, both in brightening and dimming of the source, with a lag of the 0.5-1 keV photons versus those in the 2-7.5 keV band of ~1 hr. The rapid TeV variability indicates a compact TeV-producing region, suggesting relativistic beaming with a Doppler factor ? ? 5. The correlation of the flux in the X-ray and the TeV bands indicates that a high-energy tail of a single electron population is responsible for both X-rays and TeV ?-rays, with radio, IR, UV and X-rays produced via the synchrotron process and GeV and TeV ?-rays produced via Comptonization. Under the assumption that the soft lag observed in the X-ray band is due to the synchrotron-lifetime effects, with ? = 5, we calculate the magnetic field for the X-ray-producing region to be ~0.2 G. The Lorentz factors ?el of the electrons responsible for the emission in the keV and TeV bands are ~106, consistent with the values implied by the Klein-Nishina limit.

ASCA Observations of Blazars and Multiband Analysis

We present data for 18 blazars observed with the X-ray satellite ASCA, half of which were also observed contemporaneously with the EGRET instrument onboard Compton Gamma Ray Observatory as parts of multiwavelength campaigns. The observations show a clear difference in the spectra between three subclasses of blazars, namely, high-energy peaked BL Lacertae objects (HBLs), low-energy peaked BL Lac objects (LBLs), and quasar-hosted blazars (QHBs). The ASCA X-ray spectra of HBLs are the softest, with the power-law energy index α ~ 1-2, and they form the highest observable energy tail of the low-energy (LE, synchrotron) component. The X-ray spectra of the QHBs are the hardest (α ~ 0.6) and are consistent with the lowest observable energy end of the high-energy (HE, Compton) component. For LBLs, the X-ray spectra are intermediate. We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Doppler-boosted synchrotron self-Compton (SSC) emission, with the Doppler factor δ consistent with the VLBI and/or γ-ray variability data. For many QHBs, on the other hand, the γ-rays cannot be solely a result of the SSC mechanism unless δ is significantly in excess of that inferred from VLBI data. We consider an alternative scenario consistent with the measured values of δ, where the SSC component is still present in QHBs and dominates in the X-ray band but is below the observed γ-ray spectrum. With an assumption that the peak of the SSC emission is on the extrapolation of the X-ray spectrum, and adopting a value of 10 for δ, we infer the magnetic field B to be 0.1-1 G and Lorentz factors γb of electrons radiating at the peak of the νF(ν) spectrum of ~103 for QHBs; this is much lower than γb ~ 105 for HBLs, even though the values of B are comparable in the two subclasses. This difference of γb is most likely due to the large photon density expected in QHBs (e.g., from thermal components visible in these objects) as compared with that of HBLs; Compton upscattering of these photons may well provide the observed GeV flux.

Multiepoch multiwavelength spectra and models for blazar 3C 279

Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.

HIGH-ENERGY EMISSION FROM THE TeV BLAZAR MARKARIAN 501 DURING MULTIWAVELENGTH OBSERVATIONS IN 1996

We present the results of a multiwavelength campaign for Mrk 501 performed in 1996 March with ASCA, EGRET, Whipple, and optical telescopes. The X-ray —ux observed with ASCA was 5 times higher than the quiescent level and gradually decreased by a factor of 2 during the observation in 1996 March. In the X-ray band, a spectral break was observed around 2 keV. We report here for the —rst time the detection of high-energy c-ray —ux from Mrk 501 with EGRET with 3.5 p signi—cance (E ( 100 MeV). Higher —ux was also observed in 1996 AprilMay, with 4.0 p signi—cance for E ( 100 MeV and 5.2 p signi—cance for E ( 500 MeV. The c-ray spectrum was measured to be —atter than most of the c-ray blazars. We —nd that the multiband spectrum in 1996 is consistent with that calculated from a one-zone synchrotron self-Compton (SSC) model in which X-rays are produced via synchrotron emission and c-rays are produced via inverse Compton scattering of synchrotron photons in a homogeneous region. The —ux of TeV c-rays is consistent with the predictions of the model if the decrease of the Compton scattering cross section in the Klein-Nishina regime is considered. In the context of this model, we investigate the values of the magnetic —eld strength and the beaming factor allowed by the obser- vational results. We compare the 1996 March multiwavelength spectrum with that in the —are state in 1997 April. Between these two epochs, the TeV —ux increase is well correlated with that observed in keV range. The keV and TeV amplitudes during the 1997 April —are are accurately reproduced by a one-zone SSC model, assuming that the population of synchrotron photons in 1996 are scattered by newly injected relativistic electrons having maximum energies of However, the TeV spectrum c max D 6 ) 106. observed during the 1996 March campaign is —atter than predicted by our models. We —nd that this cannot be explained by either higher order Comptonization or the contribution of the ii seed ˇˇ IR photons from the host galaxy for the —rst-order external radiation Comptonization, but we cannot exclude possible eUects of the IR photons that may arise in the parsec-size tori postulated to exist in active galactic nuclei. Subject headings: BL Lacertae objects: individual (Markarian 501) ¨ gamma rays: observations ¨ radiation mechanisms: nonthermalX-rays: galaxies

Variability Pattern and the Spectral Evolution of the BL Lacertae Object PKS 2155–304

The TeV blazar PKS 2155(304 was monitored with the X-ray satellite ASCA in 1994 May as part of a multiwavelength campaign from the radio to X-ray bands. At the beginning of the two-day continuous observation, we detected a large —are, in which the 2¨10 keV —ux changed by a factor of 2 on a time- scale of 3 ) 104 s. During the —are, the increase in the hard X-ray —ux clearly preceded that observed in the soft X-rays, with the spectral evolution tracking a ii clockwise loop ˇˇ in the —ux versus photon index plane. Ascribing the energy-dependent variability to diUerential synchrotron cooling of relativistic elec- trons, we estimate the magnetic —eld B in the emission region. We tested two diUerent methods of com- paring the time series in various X-ray bands: (1) —tting the light curves to a Gaussian function and searching for the time shift of the peak of the —are, and (2) calculating the discrete correlation function. Both methods yielded a consistent solution of B D 0.1 G. We also found that the —are amplitude becomes larger as the photon energy increases, while the duration of the —are stays roughly constant throughout the ASCA energy band (0.7¨7.5 keV). In the framework of the time-dependent synchrotron self-Compton model in a homogeneous region, we consider a —are where the maximum Lorentz factor of the injected electrons increases uniformly throughout the emission volume. The temporal evolu- (c max ) tion of spectra as well as the light curves were reproduced with the physical parameters self-consistently determined from seven observables. We obtained B D 0.1¨0.2 G and a region size R D 10~2 pc for rela- tivistic beaming with a Doppler factor of d D 20¨30. We discuss the signi—cance of light-travel time eUects. Subject headings: BL Lacertae objects: individual (PKS 2155(304) ¨ gamma rays: theory ¨ radiation mechanisms: nonthermalX-rays: galaxies

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155?304 in 1994 May. III. Probing the Inner Jet through Multiwavelength Correlations

In 1994 May, the BL Lac object PKS 2155-304 was observed continuously for ~10 days with the International Ultraviolet Explorer and the Extreme Ultraviolet Explorer and for 2 days with ASCA, as well as with ROSAT and with ground-based radio, infrared, and optical telescopes. The light curves show a well-defined X-ray flare followed by a broader, lower amplitude extreme-ultraviolet flare ~1 day later and a broad, low-amplitude UV flare ~2 days later. X-ray fluxes obtained at three well-separated times the preceding week indicate at least one previous flare of comparable amplitude or perhaps ongoing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp changes in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of ~2 in about half a day and decayed roughly as fast. In contrast, the subsequent UV flare had an amplitude of only ~35% and lasted longer than 2 days. Assuming that the X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous, relativistic jet. Because of the high quality of the data, we can rule out that the observed flares were caused by either a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t hours) injection of high-energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions. This kind of situation is expected to occur behind a shock front and/or in an inhomogeneous jet. The present light curves are in sharp contrast to the multiwavelength variability observed in 1991 November, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hr. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes.

Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare

A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.

Complex Spectral Variability from Intensive Multiwavelength Monitoring of Markarian 421 in 1998

We conducted a multifrequency campaign for the TeV blazar Markarian 421 in 1998 April. The campaign started from a pronounced high-amplitude flare recorded by BeppoSAX and Whipple; the ASCA observation started 3 days later. In the X-ray data, we detected multiple flares, occurring on timescales of about 1 day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, the Extreme Ultraviolet Explorer, and the Rossi X-Ray Timing Explorer indicates that the variability amplitudes in the low-energy synchrotron component are larger at higher photon energies. In TeV γ-rays, large intraday variations—which were correlated with the X-ray flux—were observed when results from three Cerenkov telescopes were combined. The rms variability of TeV γ-rays was similar to that observed in hard X-rays, above 10 keV. The X-ray light curve reveals flares that are almost symmetric for most cases, implying that the dominant timescale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of 7 days indicates that the characteristic timescale is ~0.5 days. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are real, these imply that the particle acceleration and X-ray cooling timescales are similar.

Correlated hard X-ray and ultraviolet variability in NGC 5548

Contemporaneous X-ray (from the Ginga satellite) and ultraviolet (IUE) observations of NGC 5548 in 1989-1990 show that the 1350 A and 2-10 keV flux both varied by a factor of 2.6 in a highly correlated fashion. Adding an additional simultaneous IUE-Exosat observation from 1984 significantly weakens the correlation, since the UV flux was 3.5 times larger then than its average 1990 value, while the X-ray flux was only 70 percent higher. In 1990 the UV and X-ray flux variations appear to be simultaneous to within no more than six days. These results, together with the simultaneity of the variations at 5000 and 1350 A, are incompatible with the standard geometrically thin accretion model. They may be explained, however, if part of the UV and optical radiation originates from reprocessing of X-rays emitted above the disk. Thus essentially all the UV flux in 1990 would be due to reprocessing. But the large UV outburst in 1984 probably represents a genuine increase in the disk throughput due to instability in the disk itself. The similar relationship between UV and X-ray emission in NGC 4151 suggests that this scheme may be applicable to Seyfert galaxies as a class.