Violent Hard X-ray Variability of Mrk 421 Observed by NuSTAR in 2013 April

• Published in: arXiv.org
• Main Authors: Paliya, Vaidehi S, Bottcher, Markus, Diltz, Chris, Stalin, C S, Sahayanathan, S, Ravikumar, C D
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 26.08.2015
• Subjects: Black holes; Energy dissipation; Event horizon; Field strength; Fluxes; Physics - High Energy Astrophysical Phenomena; Spectroscopic telescopes; Telescopes; X ray telescopes
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1508.06399

The well studied blazar Markarian 421 (Mrk 421, \(z\)=0.031) was the subject of an intensive multi-wavelength campaign when it flared in 2013 April. The recorded X-ray and very high energy (VHE, E\(>\)100 GeV) \(\gamma\)-ray fluxes are the highest ever measured from this object. At the peak of the activity, it was monitored by the hard X-ray focusing telescope {\it Nuclear Spectroscopic Telescope Array} ({\it NuSTAR}) and {\it Swift} X-Ray Telescope (XRT). In this work, we present a detailed variability analysis of {\it NuSTAR} and {\it Swift}-XRT observations of Mrk 421 during this flaring episode. We obtained the shortest flux doubling time of 14.01\(\pm\)5.03 minutes, which is the shortest hard X-ray (3\(-\)79 keV) variability ever recorded from Mrk 421 and is on the order of the light crossing time of the black hole's event horizon. A pattern of extremely fast variability events superposed on slowly varying flares is found in most of the {\it NuSTAR} observations. We suggest that these peculiar variability patterns may be explained by magnetic energy dissipation and reconnection in a fast moving compact emission region within the jet. Based on the fast variability, we derive a lower limit on the magnetic field strength of \(B \ge 0.73 \delta_1^{-2/3} \, \nu_{19}^{1/3}\)~G, where \(\delta_1\) is the Doppler factor in units of 10, and \(\nu_{19}\) is the characteristic X-ray synchrotron frequency in units of \(10^{19}\)~Hz.