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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- August 2015
- DOI:
- 10.1051/0004-6361/201526264
- arXiv:
- arXiv:1506.03629
- Bibcode:
- 2015A&A...580A.100S
- Keywords:
-
- BL Lacertae objects: individual: Mkn 421;
- galaxies: active;
- X-rays: galaxies;
- radiation mechanisms: non-thermal;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 11 pages, 7 figures, Accepted in A&