The Onset of X-Ray Flare in the TeV-Detected Blazar Markarian 501

ATel #15059; Bidzina Kapanadze (Ilia State University, Tbilisi, Gerogia; E. Kharadze National Astrophysical Observatory, Abastumani, Georgia)
on 23 Nov 2021; 21:36 UT
Credential Certification: Bidzina Kapanadze (bidzina_kapanadze@iliauni.edu.ge)

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The TeV-detected blazar Markarian 501 (z=0.034) is a bright X-ray source and prominent with very strong TeV/X-ray flares (see, e.g., Ahnen et al., 2017, A & A, 603, 31; Kapanadze et al. 2017, MNRAS, 469; 1655). Consequently, this object represents one of the frequent targets of X-Ray Telescope onboard Neil Gehrels Swift Observatory (Swift-XRT; 810 observations with 851 ks of the total exposure time since 2005 February 25). Apparently, the source has undergone a nearly decade-long variations in the baseline X-ray level, and since 2021 March, we observe a significantly enhanced X-ray activity of the source compared to the previous years (and comparable to the flaring behaviour shown in 2014 March-October; see https://www.swift.psu.edu/monitorin g/source.php?source=Mrk501 and Kapanadze B. et al. 2017), superimposed by shorter-term X-ray flares. Currently, Mrk 501 is observed with Swift-XRT in the framework of out Target of Opportunity (TOO) request Number #16518. After a drop by a factor of ~2.5 in the 0.3-10 keV brightness to 4.61+/-0.09 cts/s (on November 10), the source showing a gradual brightening by ~50% during the XRT pointing performed on November 22. In the framework of one-zone SSC models, a flaring activity of Mrk 501 is also expected in the UV-radio and gamma-ray parts of the spectrum, and intensive multiwavelength observations of the source are strongly encouraged for discerning the underlying emission mechanisms and instable processes. XRT is one of the Swift instruments along with Burst Alert Telescope (BAT) and UV/Optical Telescope (UVOT). It is a JET-X Wolter I type telescope, developed jointly by Pennsylvania State University, Brera Astronomical Observatory (OAB) and University of Leicester. Thanks to the unique characteristics, good photon statistics and low background counts of this instrument (in combination with EEV CCD2 detector), we can investigate a flux variability on different time-scales from minutes to years, obtain high-quality spectra for the majority of the observations, derive different spectral parameters, and study their timing behaviour in the 0.3-10 keV range of the electromagnetic spectrum. The Swift Satellite is operated by Pennsylvania State University.