INVERSE COMPTON X-RAY EMISSION FROM TeV BLAZAR MRK 421 DURING A HISTORICAL LOW-FLUX STATE OBSERVED WITH NuSTAR

Abstract

We report on the detection of excess hard X-ray emission from the TeV BL Lac object Mrk 421 during the historical low-flux state of the source in January 2013. NuSTAR observations were conducted four times between MJD 56294 and MJD 56312 with a total exposure of 80.9 ksec. The source flux in the 3 − 40 keV range was nearly constant except for MJD 56307, when the average flux level increased by a factor of three. Throughout the exposure, the X-ray spectra of Mrk 421 were well represented by a steep power-law model with a photon index of Γ ≃ 3.1, although a significant excess was noted above 20 keV in the MJD 56302 data when the source was in its faintest state. Moreover, Mrk 421 was detected at more than the 4σ level in the 40 − 79 keV count maps for both MJD 56307 and MJD 56302 but not during the remaining two observations. The detected excess hard X-ray emissions connect smoothly with the extrapolation of the high-energy γ-ray continuum of the blazar constrained by Fermi-LAT during the source quiescence. These findings indicate that, while the overall X-ray spectrum of Mrk 421 is dominated by the highest-energy tail of the synchrotron continuum, the variable excess hard X-ray emission above 20 keV (on the timescale of a week) is related to the inverse Compton emission component. We discuss the resulting constraints on the variability and spectral properties of the low-energy segment of the electron energy distribution in the source.

On the Origin of the Hard X-Ray Excess of High-Synchrotron-Peaked BL Lac Object Mrk 421

“…During the faintest state in MJD 56302, its synchrotron component peaks at too low a frequency to make it similar to ISP/LSP. Very recently, Kataoka & Stawarz (2016) reported the detection of a significant excess above 20 keV through NuSTAR observations at this faintest epoch (see also the BeppoSAX observation; e.g., Fossati et al 1999Fossati et al , 2000.…”

“…Electrons around γ min will IC synchrotron photons and produce the hard X-ray excess around 20 KeV. We collect this simultaneous broadband SED from Baloković et al (2016) and Kataoka & Stawarz (2016), which are replotted in Figure 1. The synchrotron and SSC components peak at ν p sy ≈ 10 16 and ν p SSC ≈ 10 25 Hz, respectively, which correspond to emissions of electrons with energy around ≈ γ 0 m e c 2 .…”

“…For the first time, Kataoka & Stawarz (2016) reported a clear detection of a hard X-ray excess (i.e., a concave X-ray spectrum) above 20 keV of the HSP BL Lac object Mrk 421 during the low state (see also PKS 2155-304; Madejski et al 2016). In this paper, we investigate the possibilities for the origin of this hard X-ray excess.…”

“…The origin of the hard X-Ray excess Kataoka & Stawarz (2016) compared the spectra of the hard X-ray excess with the simultaneous 2 Fermi/LAT power-law γ-ray spectra (the photon index Γ = 1.78 ± 0.02; Baloković et al 2016). They found that this excess follows the extension of the γ-ray spectra well; therefore, they argued that this hard X-ray excess must be the low-energy part of the SSC emissions of the same electron population that is producing the Fermi/LAT γ-ray.…”

Signature of inverse Compton emission from blazars

“…3C 66A (Donato et al 2005, Wierzcholska & Wagner 2016; S5 0716+714 (Giommi et al 1999;Tagliaferri et al2003;Donato et al 2005;Ferrero et al 2006;Wierzcholska & Siejkowski 2015); ON 231 (Tagliaferri et al 2000); BL Lacertae (Tanihata et al 2000;Ravasio et al 2002); AO 0235+164 (Raiteri et al 2006); OQ 530 (Tagliaferri et al 2003); 4C +21.35 (Wierzcholska & Wagner 2016). For the well known HBL Mrk 421, Kataoka & Stawarz (2016) found that the X-ray observations from NuStar Satellite is dominated by the highest-energy tail of the synchrotron continuum till 20 keV and the variable excess hard X-ray emission is related to inverse Compton emission in its very low state. All these sources are TeV blazars and have shown variability in γ-bands (Abdo et al 2010).…”