E. Pian

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.

BeppoSAX Observations of Unprecedented Synchrotron Activity in the BL Lacertae Object Markarian 501

The BL Lacertae object Markarian 501, one of only three extragalactic sources (with Mrk 421 and 1ES 2344+514) so far detected at TeV energies, was observed with the BeppoSAX satellite in 1997 April 7, 11, and 16 during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA, and CAT Cherenkov telescopes. Over the whole 0.1-200 keV range, the spectrum was exceptionally hard (??1, with F? ? ???), indicating that the X-ray power output peaked at (or above) ~100 keV. This represents a shift of at least 2 orders of magnitude with respect to previous observations of Mrk 501, a behavior never seen before in this or any other blazar. The overall X-ray spectrum hardens with increasing intensity, and at each epoch it is softer at larger energies. The correlated variability from soft X-rays to the TeV band points to models in which the same population of relativistic electrons produces the X-ray continuum via synchrotron radiation and the TeV emission by inverse Compton scattering of the synchrotron photons or other seed photons. For the first time in any blazar, the synchrotron power is observed to peak at hard X-ray energies. The large shift of the synchrotron peak frequency with respect to previous observations of Mrk 501 implies that intrinsic changes in the relativistic electron spectrum caused the increase in emitted power. Due to the very high electron energies, the inverse Compton process is limited by the Klein-Nishina regime. This implies a quasi-linear (as opposed to quadratic) relation of the variability amplitude in the TeV and hard X-ray ranges (for the synchrotron self-Compton model) and an increase of the inverse Compton peak frequency smaller than that of the synchrotron peak frequency.

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.

X-Ray Emission of Markarian 421: New Clues from Its Spectral Evolution. II. Spectral Analysis and Physical Constraints

Mrk 421 was repeatedly observed with BeppoSAX in 1997 and 1998. The source showed a very rich phenomenology with remarkable spectral variability. This is the second of two papers presenting the results of a thorough temporal and spectral analysis of all the data available to us, focusing in particular on the flare of April 1998, which was simultaneously observed also at TeV energies. The spectral analysis and correlations are presented in this paper, while the data reduction and timing analysis are the content of the companion paper. The spectral evolution during the flare has been followed over few ks intervals, allowing us to detect for the first time the peak of the synchrotron component shifting to higher energies during the rising phase and then receding. This spectral analysis nicely confirms the delay of the flare at the higher energies, which in Paper I we quantified as a hard lag of a few ks. Furthermore, at the highest energies, evidence is found of variations of the inverse Compton component. The spectral and temporal information obtained challenge the simplest models currently adopted for the (synchrotron) emission and, most important, provide clues on the particle acceleration process. A scenario accounting for all the observational constraints is discussed, where electrons are injected at progressively higher energies during the development of the flare and the achromatic decay is ascribed to the source light crossing time exceeding the particle cooling timescales.

Theoretical Implications from the Spectral Evolution of Markarian 501 Observed with BeppoSAX

We present new BeppoSAX observations of the TeV-emitting blazar Mrk 501 performed in 1999 June and a homogeneous reanalysis of all the BeppoSAX observations during the period from 1997 April to 1999 June. Recently published TeV spectra by the Cerenko Array at Themis, which are quasi-simultaneous with the 1997 BeppoSAX pointings, are used to constrain further the physical parameters of the jet in 1997 April with detailed models of the X-ray and TeV components. During the 1997-1999 period, the peak frequency of the synchrotron emission moved from about 100 keV in 1997 April to 0.5 keV in 1999 June. The shift in the peak frequency appears to be correlated with the long-term luminosity decrease. We interpret the results in the framework of a homogeneous synchrotron self-Compton model, discussing the evolution of the physical parameters of the emitting region under simple assumptions for the variability model. We also compute the expected variations of the TeV flux under the same assumptions.

SIMULTANEOUS X-RAY AND TeV OBSERVATIONS OF A RAPID FLARE FROM MARKARIAN 421

Mrk 421 was observed for about 2 days with BeppoSAX in 1998 April as part of a worldwide multiwavelength campaign. A large, well-defined flare was observed in X-rays. The same flare was observed simultaneously at TeV energies by the Whipple Observatory gamma-ray telescope. These data provide (1) the first evidence that the X-ray and TeV intensities are well correlated on timescales of hours and (2) the first exactly simultaneous X-ray and TeV spectra. The results imply that the X-ray and TeV photons derive from the same region and from the same population of relativistic electrons. The physical parameters deduced from a homogeneous synchrotron self-Compton model for the spectral energy distribution yield electron cooling times close to the observed variability timescales.

Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission

The γ-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT, and EGRET for 3 weeks between 1992 December and 1993 January. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during this multiwavelength campaign. A maximum UV variation of ~50% is detected, while during the same period the X-ray flux varied by no more than 13%. At the lowest UV flux level, the average spectrum in the 1230-2700 A interval is unusually flat for this object (αUV ~ 1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or it could be due to the presence of a thermal component at ~20,000 K, possibly associated with an accretion disk. The presence of an accretion disk in this blazar object, likely observable only in very low states and otherwise hidden by the beamed, variable synchrotron component, would be consistent with the scenario in which the seed photons for the inverse Compton mechanism producing the γ-rays are external to the relativistic jet. We further discuss the long-term correlation of the UV flux with the X-ray and γ-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X-ray and γ-ray fluxes are generally well correlated with the UV flux, with approximately square root and quadratic dependences, respectively.

The 1993 multiwavelength campaign on 3C 279: the radio to gamma-ray energy distribution in low state

Simultaneous observations of 3C 279 at radio, millimeter, near-infrared, optical, ultraviolet (with IUE) and X-ray (with ROSAT) wavelengths were obtained in 1992 December-1993 January, during a three week pointing at the source by the Compton Gamma Ray Observatory. The blazar was in a quiescent or low state during this period. Comparing the multiwavelength energy distribution to that from 1991 June, when 3C 279 was in its brightest recorded gamma-ray state, we find the following: (1) 3C 279 faded dramatically at all frequencies above 10(exp 14) Hz, while the flux variations at low frequencies (radio to millimeter wavelengths) were minor. (2) The near-infrared-optical-ultraviolet spectral shape was softer (steeper) in the quiescent state, and the X-ray spectra also appear softer, although the spectral indix measured by ROSAT refer to a lower energy band than that measured earlier with Ginga. (3) The ratio of the gamma-ray luminosity to that across all other frequencies decreased from a value of approximately equal to 10 in the flaring state to a value approximately equal to 1 in the quiescent state. These findings imply that the production of gamma-rays is closely related to the optical-ultraviolet continuum, in agreement with models where gamma-rays are produced through inverse Compton (IC) scattering by relativistic electrons emitting the synchrotron continuum. The observed nonlinear relation between the synchrotron and IC requires both a change in the electron spectrum and an associated change in the seed photons.

Four Years of Monitoring Blazar PKS 2155–304 with BeppoSAX: Probing the Dynamics of the Jet

PKS 2155-304 is one of the brightest blazars in the X-ray band. It was repeatedly monitored with BeppoSAX during three long campaigns of about 2 days each in November of 1996, 1997, and 1999. The source underwent different states of intensity and was clearly variable with successive flares detected. This paper presents temporal and spectral analysis to study the X-ray variability trends for a blazar. The variability shows larger amplitude and shorter timescale at higher energies. The power spectral densities have steep power-law slopes of ~2-3, indicating shot-noise variability. Structure function analysis reveals the existence of a typical timescale characteristic of the half-duration of the flares. From the cross-correlation analysis we find that the values of soft lags, i.e., delays of soft (0.1-1.5 keV) photons with respect to hard (3.5-10 keV) ones, differ from flare to flare, ranging from a few hundred seconds to about 1 hr. There is a suggestion that the flares with shorter duration show smaller soft lags. The soft lags are also energy-dependent, with longer lags of lower energy emission with respect to the emission in the 4-10 keV range. The time-resolved X-ray spectral fits with a curved model show that peak energies of the synchrotron component are located in the very soft X-ray range or even below the BeppoSAX lower energy limit, 0.1 keV. A correlation between peak energies and fluxes is marginal. Spectral evolution during some flares shows clockwise loops in the spectral index-flux plane, confirming the soft lags indicated by the cross-correlation analysis. Two flares, however, show evidence that spectral evolution follows opposite tracks in the soft- and hard-energy bands, respectively. The rich phenomenology is interpreted in the context of a model in which relativistic electrons are accelerated through internal shocks taking place in the jets. The most important parameter turns out to be the initial time interval between the two shells ejected from the central engine to produce the flare, which may determine the structure of the shock and, in turn, the physical quantities of the emitting region used to reproduce the observed trends of the X-ray variability.

X-Ray Emission of Markarian 421: New Clues from Its Spectral Evolution. I. Temporal Analysis

Mrk 421 was repeatedly observed with BeppoSAX in 1997 and 1998. This is the first of two papers where we present the results of a thorough temporal and spectral analysis of all the data available to us, focusing in particular on the flare of 1998 April, which was simultaneously observed also at TeV energies. Here we focus on the time analysis, while the spectral analysis and physical interpretation are presented in the companion paper. The detailed study of the flare in different energy bands reveals very important new results: (1) hard photons lag the soft ones by 2-3 ks—a behavior opposite to what is normally found in high-energy peak BL Lac X-ray spectra; (2) the flare light curve is symmetric in the softest X-ray band, while it becomes increasingly asymmetric at higher energies, with the decay being progressively slower than the rise; and (3) the flux decay of the flare can be intrinsically achromatic if a stationary underlying emission component is present. The temporal and spectral information obtained challenge the simplest models currently adopted for the (synchrotron) emission and most importantly provide clues on the particle acceleration process.