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

Abstract

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

Rapid variability in the synchrotron self-Compton model for blazars

“…In 1994 May the ASCA satellite revealed an X-ray¯are (Takahashi et al 1996) during an high state of TeV emission (Macomb et al 1995). Observations report an increase of a factor ,2 of the 2± 10 keV¯ux, with a doubling time-scale of ,12 h, while much lower amplitude variability is present in the IR, optical, UV and GeV (EGRET) bands.…”

“…They interpret this as due to synchrotron cooling. With the numerical code described in the previous sections we can qualitatively reproduce this behaviour, ®rst by ®tting the high and the low states (Macomb et al 1995) as equilibrium SSC spectra, and then by generating simulated light curves at all frequencies. In Fig.…”

“…In Fig. 10 we report the simultaneous spectra taken during the high and low states (data from Macomb et al 1995Macomb et al , 1996 We assumed that the rapid variability revealed by ASCA during the highly active X-ray/TeV state is due to the sum of a rapidly evolving component and a quasi-constant one, corresponding to the high-state ®t. We add to this constant emission a variable component, for which we found the following parameters:…”

“…Note that the case of 3C 279, one of the best studied blazars which showed asymmetric light curves at high energies, is probably Rapid variability of blazars 559 q 1999 RAS, MNRAS 306, 551±560 Figure 10. High and low states of Mrk 421 during the 1994 multiwavelength campaign (Macomb et al 1995); continuous lines are equilibrium SSC models. We found satisfactory ®ts with the following parameters: R 4:8´10 16 cm, B 0:08 G, injection law Qg~g À1:5 expÀg=g max with g min 1, g max 1:3´10 5 , t esc 4R=c, injected compactness , inj 9´10 À5 , beaming factor d 15:5 (low state); R 4:8´10 16 cm, B 0:04 G, Qg~g À1:7 expÀg=g max with g min 1, g max 8:5´10 5 , , inj 2:5´10 À4 , t esc 3R=c, d 15:5 (high state).…”

A theoretical unifying scheme for gamma-ray bright blazars

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