Underlying particle spectrum of Mkn 421 during the huge X-ray flare in April 2013

Abstract

Context. In April 2013, the nearby TeV blazar, Mkn 421, showed one of the largest flares in X-rays in the past decade. Aims. We study all multiwavelength data available during MJD 56 392 to 56 403, with special emphasis on X-ray data to understand the underlying particle energy distribution. Methods. We studied the correlations between the UV and gamma-ray bands with the X-ray band using the z-transformed discrete correlation function. We modelled the underlying particle energy spectrum with a single population of electrons emitting synchrotron radiation, and statistically fitted the simultaneous time-resolved data from Swift-XRT and NuSTAR. Results. The flux varied rapidly in the X-ray band, with a minimum doubling timescale of 1.69 ± 0.13 h. There were no corresponding flares in UV and gamma-ray bands. The variability in UV and gamma rays was relatively modest with ∼8% and ∼16%, respectively, and no significant correlation was found with the X-ray light curve. The observed X-ray spectrum shows a clear curvature that can be fit by a log parabolic spectral form. This is best explained as originating from a log parabolic electron spectrum. However, a broken power law or a power law with an exponentially falling electron distribution cannot be ruled out either. Moreover, the excellent broadband spectrum from 0.3-79 keV allows us to make predictions of the UV flux. We find that this prediction is compatible with the observed flux during the low state in X-rays. However, during the X-ray flares, depending on the adopted model, the predicted flux is a factor of 2-50 lower than the observed one. This suggests that the X-ray flares are probably caused by a separate population that does not contribute significantly to the radiation at lower energies. Alternatively, the underlying particle spectrum can be much more complex than those explored in this work.

A time dependent approach to model X-ray and γ–ray light curves of Mrk 421 observed during the flare in February 2010

“…The effect of contamination from host galaxy of Mrk 421 on V-magnitude in our present study is obtained to be negligible. The optical emission during the flare is probably not related to that in X-ray and γ-ray energy bands and it is consistent with the results from previous studies of Mrk 421 [37,38]. In our simplistic model, we attribute the entire flare to the varying particle injection into the emission region.…”

“…These differences suggest that the optical emission may not be associated with the region responsible for emission in higher energy bands [37,38]. This claim is further strengthened by the spectral behaviour of the broadband SED during flare maximum (Figure 2) where, the synchrotron spectral component significantly underpredicts the observed optical flux.…”

Long-term study of Mkn 421 with the HAGAR Array of Telescopes

“…The most dramatic flare in the GeV band occurred during in 2012 (MJD ∼ 56 140−56 180; calendar dates 2012-08-01 to 2012-09-10), which is reflected in the radio band after a gap of one and half months (MJD ∼ 56 193, Calendar date 2012-09-23). In contrast, the largest X-ray/optical flare happened during April 2013, which has been studied in detail by various authors (Sinha et al 2015;Pian et al 2014;Paliya et al 2015). In this section, we study the flux correlations between the various frequencies and quantify the nature of the variability.…”

“…This blazar is a HBL with the synchrotron spectrum peaking in the X-ray regime. It is highly variable on all timescales and across all wavelengths from radio to TeVs, and has been studied comprehensively during its various flaring episodes by several authors (Aleksić et al 2012(Aleksić et al , 2010Acciari et al 2009;Shukla et al 2012;Isobe et al 2010;Krawczynski et al 2001;Ushio et al 2009;Tramacere et al 2009;Horan et al 2009;Lichti et al 2008;Fossati et al 2008;Albert et al 2007;Brinkmann et al 2003;Sinha et al 2015;Paliya et al 2015;Donnarumma et al 2009). Abdo et al (2011) studied the quiescent state emission from this source with the best-sampled SED to date.…”

X-ray Intraday Variability of the TeV Blazar Mrk 421 with Chandra

“…In April 2013, Mrk 421 was observed to undergo a major X-ray outburst and was comprehensively investigated by multiple observational facilities, including the NuSTAR and Swift satellites. Intensive studies of Mrk 421 probing its multiwavelength (MW) behaviour are numerous (Takahashi et al 1994;Kerrick et al 1995;Gupta et al 2004;Costa et al 2008;Pittori et al 2008;Smith et al 2008;Lico et al 2012;Gaur et al 2012b;Blasi et al 2013;Pian et al 2014;Paliya et al 2015;Sinha et al 2015;Hovatta et al 2015;Baloković et al 2016, and references therein). MW campaigns incorporating Fermi-LAT gamma-ray detections comprehensively studied Mrk 421 and produced its first ever complete γ-ray continuum during a quiescent state (Abdo et al 2014).…”