NuSTAR View of TeV Blazar Mrk 501

Abstract

We report the results of flux and spectral variability studies of all seven Nuclear Spectroscopic Telescope Array (NuSTAR) observations of TeV γ − ray emitting blazar Markarian (or Mrk) 501. We found strong evidence of intraday variability in 3–79 keV X-ray light curves (LCs) of Mrk 501 during four out of these seven observations. We examined spectral variability using a model-independent hardness-ratio analysis and found a general “harder-when-brighter” behaviour in two observations. We also investigated the nature of 3–79 keV X-ray spectra of TeV blazar Mrk 501 and found that five out of seven spectra are well described by the curved log-parabola models with photon indices (at 10 keV) α ∼ 2.12–2.32 and a curvature β ∼ 0.15–0.28. The two other spectra are somewhat better represented by simple power-law models with photon indices 2.70 and 2.75. We briefly discuss available physical models to explain our results.