Markarian 421 is one of the brightest BL-Lac objects in gamma-rays in the northern hemisphere. Because of its brightness, the source has been the focus of several coordinated multi-wavelength (MWL) campaigns designed to study the physical processes responsible for the non-thermal continuum emission. The blazar monitoring program of VERITAS recently received a ToO during a strong flaring event by Markarian 421 in February 2010. The source was seen at flux level of approximately 8 Crab units and exhibited spectral evolution and variability features. A multi-wavelength campaign with other MWL partners was udndertaken. Results on past and recent flaring events are presented.
Blazars are a subclass of active galactic nuclei (AGNs) presenting rapid variability and non-thermal emission across nearly the entire electromagnetic spectrum, implying that the observed photons originate within highly relativistic jets oriented very close to the observers line of sight [1]. Various models have been proposed to account for the broadband spectral energy distributions (SEDs) observed in very high energy (VHE: E > 100 GeV) blazars, which typically display two major components peaking at different energies: the lower-energy peak (10 13 Hz E peak 10 19 Hz) is due to synchrotron emission by highly relativistic electron and positrons, and the higher-energy component is due to inverse-Compton (IC) scattering of the synchrotron photons by these relativistic e + e - [2,3]. Hadronic interactions producing neutral pions which decay into photons [4], and synchrotron emission from protons [5] are also possible scenarios for the highenergy component of blazars. Observationally, blazars undergo both major outbursts on long time scales and rapid flares on short time scales, most prominently at keV and TeV energies. During some outbursts, both of the SED peaks have been observed to shift toward higher energies in a generally correlated manner [6]. The correlation of the variabilities at keV and TeV energies (or lack thereof) during such outbursts has aided in refining the emission models. In addition, rapid, sub-hour flaring activity is interesting as it provides direct constraints on the size of the emission region. BL-Lac objects are a particular class of blazars with the featureless non-thermal continuum dominating over the discrete emission. Markarian 421 (Mrk 421; 1101+384), at a redshift of z = 0.031, is a high-frequency peaked (HBL: E peak 10 17 Hz) BL-Lac object that historically shows intense and rapid flaring episodes. Its flaring activity is particularly interesting for two reasons: 1) its relativistic high flux facilitates the characterization if its spectral and temporal features; 2) frequent flaring activity permits the time evolution of the low and high portions of the SED to be studied. Therefore Markarian 421 has been the focus of coordinated MWL observational campaigns, from optical to the γ-ray energy band. These campaigns are triggered by the observation of flares by any of the MWL partners monitoring the source. Here we report results from past and recent observation of Markarian 421 flaring episodes.
The VERITAS detector is an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes located in southern Arizona [7]. Designed to detect emission from astrophysical objects in the energy range from 100 GeV to greater than 30 TeV, VERITAS has an energy resolution of ∼15% and an angular resolution (68% containment) of ∼0.1 • per event at 1 TeV. A source with a flux of 1% of the Crab Nebula flux is detected in 25 hours of observations. The field of view of the VERITAS telescopes is 3.5 • . For more details on the VERITAS instrument and the imaging atmospheric-Cherenkov technique, see [8]. During this campaign, quasi-simultaneus MWL data in radio, optical and X-ray have been taken. Figure 1 shows the combined lighcurve of this long-term MWL campaign.
A detailed analysis of the MWL data over this campaign is presented in [13]. Flux variability is found in all bands except in radio. In particular, the X-ray and VHE energy bands are found to be often correlated. Such correlation implies that the particle population responsible for the synchrotron and IC component are the same. Although this correlation is seen as a general trend, it does not necessarily hold true at the level of individual flares. On the other hand, optical/TeV correlation is not found, suggesting that the optical emission might not be dominated by the optical synchrotron component from the jet. The broadband SED is well described by a single zone SSC model and no evidence for flux variability on the time scale of a minute is found.
Our Two phases of activity appear in the X-ray and TeV bands (phases 1 and 2, grey vertical lines). Phase 1: Maximum of hard X-ray flux (Swift/BAT, red line). Phase 2: Good X-ray/TeV coverage, X-ray observations partly triggered by VERITAS. Figure from [13] narios that could explain the observed variability patterns include the possibility of an inhomogeneous emission region or hadronic origin of the VHE emission. It is interesting to note that Markarian 421 during this particular flares is clearly behaving very differently here from its typical flaring periods (e.g. [6]), where the X-ray and VHE variabilities are seen to be strongly correlated. Spectral hysteresis patterns are also observed in the X-ray data during the 2006 flare. Although spectral hysteresis has been previously observed is other blazars too, the phenomenon is not fully understood yet. A possible explanation is that spectral hysteresis is produced by the combined effect of three different typical time scales [18]: the
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