Implications of Variability Patterns observed in TeV Blazars on the Structure of the Inner Jet

The recent long look X-ray observations of TeV blazars have revealed many important new features concerning their time variability. In this paper, we suggest a physical interpretation for those features based on the framework of the internal and external shock scenarios. We present a simplified model applicable to TeV blazars, and investigate through simulations how each of the model parameters would affect to the observed light curve or spectrum. In particular, we show that the internal shock scenario naturally leads to all the observed variability properties including the structure function, but for it to be applicable, the fractional fluctuation of the initial bulk Lorentz factors must be small, with sigma_gamma / gamma_average < 0.01. This implies very low dynamical efficiency of the internal shock scenario. We also suggest that several observational quantities -- such as the characteristic time scale, the relative amplitude of flares as compared to the steady (``offset'') component, and the slope of the structure function -- can be used to probe the inner jet. The results are applied to the TeV blazar Mrk421, and this, within the context of the model, leads to the determination of several physical parameters: the ejection of a shell with average thickness of ~1E13 cm occurs on average every 10 minutes, and the shells collide ~1E17 cm away from the central source.Comment: 12 pages, 13 figures, to appear in Ap

Evolution of the Synchrotron Spectrum in Mrk421 during the 1998 Campaign

The uninterrupted 7-day ASCA observations of the TeV blazar Mrk421 in 1998 have clearly revealed that X-ray flares occur repeatedly. In this paper, we present the results of the time-resolved spectral analysis of the combined data taken by ASCA, RXTE, BeppoSAX, and EUVE. In this object - and in many other TeV blazars - the precise measurement of the shape of the X-ray spectrum, which reflects the high energy portion of the synchrotron component, is crucial in determining the high energy cutoff of the accelerated electrons in the jet. Thanks to the simultaneous broadband coverage, we measured the 0.1-25 keV spectrum resolved on time scales as short as several hours, providing a great opportunity to investigate the detailed spectral evolution at the flares. By analyzing the time subdivided observations, we parameterize the evolution of the synchrotron peak, where the radiation power dominates, by fitting the combined spectra with a quadratic form (where the vFv flux at the energy E obeys log vFv(E)=log(vFv_peak) - const x (log E - log Epeak)^2). In this case, we show that there is an overall trend that the peak energy Ep and peak flux vFvp both increase or decrease together. The relation of the two parameters is best described as Ep \propto vFvp^{0.7} for the 1998 campaign. Similar results were derived for the 1997 observation, while the relation gave a smaller index when included both 1997 and 1998 data. On the other hand, we show that this relation, and also the detailed spectral variations, differ from flare to flare within the 1998 campaign. We suggest that the observed features are consistent with the idea that flares are due to a appearance of a new spectral component. With the availability of the simultaneous TeV data, we also show that there exists a clear correlation between the synchrotron peak flux and the TeV flux.Comment: 12 pages, 9 figures, to appear in Ap

Variability Time Scales of TeV Blazars Observed in the ASCA Continuous Long-Look X-ray Monitoring

Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501 and PKS 2155-304 all show continuous strong X-ray flaring. Despite the relatively faint intensity states in 2 of the 3 sources, there was no identifiable quiescent period in any of the observations. Structure function analysis shows that all blazars have a characteristic time scale of ~ a day, comparable to the recurrence time and to the time scale of the stronger flares. On the other hand, examination of these flares in more detail reveals that each of the strong flares is not a smooth increase and decrease, but exhibits substructures of shorter flares having time scales of ~10 ks. We verify via simulations that in order to explain the observed structure function, these shorter flares ("shots") are unlikely to be fully random, but in some way are correlated with each other. The energy dependent cross-correlation analysis shows that inter-band lags are not universal in TeV blazars. This is important since in the past, only positive detections of lags were reported. In this work, we determine that the sign of a lag may differ from flare to flare; significant lags of both signs were detected from several flares, while no significant lag was detected from others. However, we also argue that the nature of the underlying component can affect these values. The facts that all flares are nearly symmetric and that fast variability shorter than the characteristic time scale is strongly suppressed, support the scenario where the light crossing time dominates the variability time scales of the day-scale flares.Comment: 29 pages, 12 figures, accepted for publication in Ap

Rapid Synchrotron Flares from BL Lacertae Detected by ASCA and RXTE

We report the variable X-ray emission from BL Lacertae detected in the ASCA ToO observation conducted during the EGRET and RXTE pointings, coincident with the 1997 July outburst. The source showed a historically high state of X-ray, optical, and gamma-ray emission, with its 2-10 keV flux peaking at ~3.3 E-11 erg/cm2/s. This is more than 3 times higher than the value measured by ASCA in 1995. We detected two rapid flares which occured only in the soft X-ray band, while the hard X-ray flux also increased, but decayed with a much longer time scale. Together with the requirement of a very steep and varying power law dominating the soft X-ray band in addition to the hard power law, we suggest that both the high energy end of the synchrotron spectrum and the hard inverse Compton spectrum were visible in this source during the outburst. We discuss the possible origins of the observed variability time scales, and interpret the short time scales of the soft X-ray variability as reflecting the size of the emission region.Comment: 20 pages, 4 figures, accepted for publication in the Astrophysical Journa

Accepted for publication in the Astrophysical Journal Variability Time Scales of TeV Blazars Observed in the ASCA Continuous Long-Look X–ray Monitoring

Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501 and PKS 2155–304 all show continuous strong X–ray flaring. Despite the relatively faint intensity states in 2 of the 3 sources, there was no identifiable quiescent period in any of the observations. Structure function analysis shows that all blazars have a characteristic time scale of ∼ a day, comparable to the recurrence time and to the time scale of the stronger flares. On the other hand, examination of these flares in more detail reveals that each of the strong flares is not a smooth increase and decrease, but exhibits substructures of shorter flares having time scales of ∼10 ks. We verify via simulations that in order to explain the observed structure function, these shorter flares (“shots”) are unlikely to be fully random, but in some way are correlated with each other. The energy dependent crosscorrelation analysis shows that inter-band lags are not universal in TeV blazars. This is important since in the past, only positive detections of lags were reported. In this work, we determine that the sign of a lag may differ from flare to flare; significant lags of bot