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.
Highlights
Active galactic nuclei (AGN) are highly energetic astrophysical objects that are powered by actively accreting supermassive black holes (SMBH; MBH ≥ 106 Msun ) at their centres, and produce unique observational features over the entire electromagnetic (EM) spectrum [1]
The results of our intraday variability (IDV) analyses are given in Table 2, where dashes “–” indicate that the sample variances < mean square errors, so no real value of Fvar can be computed for the light curves (LCs)
We investigated the spectral variability of 3–79 keV X-ray emission from the TeV blazar Mrk 501 using a model-independent hardness-ratio analysis
Summary
Active galactic nuclei (AGN) are highly energetic astrophysical objects that are powered by actively accreting supermassive black holes (SMBH; MBH ≥ 106 Msun ) at their centres, and produce unique observational features over the entire electromagnetic (EM) spectrum [1]. In a recent X-ray variability study of Mrk 501 with Swift, the source showed the most extreme X-ray flare activity in March–October 2014 during its 11.5 yr long monitoring period [18] During this epoch, several short-term X-ray flares were detected with their amplitude varied by factors of 1.9–4.7 on weekly timescales. High flux variability on such small timescales has been one of the most puzzling issues in the field of blazar astronomy as it requires very large energy outputs within small regions These emitting regions often lie near to SMBH. The motivation of this work is to examine all the NuSTAR light curves of the TeV blazar Mrk 501 for intraday flux and spectral variability, and to investigate the nature of its 3–79 keV X-ray spectra.
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