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The Ongoing X-Ray Flare in Markarian 501

ATel #15124; Bidzina Kapanadze (Ilia State University, Tbilisi, Georgia; E. Kharadze National Astrophysical Observatory, Abastumani, Georgia)
on 17 Dec 2021; 20:31 UT
Credential Certification: Bidzina Kapanadze (bidzina_kapanadze@iliauni.edu.ge)

Subjects: X-ray, Blazar

Referred to by ATel #: 15134, 15477, 15753, 15828

In ATel#15059 (posted on 2021 November 23), we reported the onset of X-ray flare in the nearby, TeV-detected blazar Markarian 501 (z=0.034) which is a bright X-ray source and prominent with strong TeV/X-ray flares (see, e.g., Ahnen et al., 2017, A & A, 603, 31; Kapanadze et al. 2017, MNRAS, 469; 1655). Moreover, the source is undergoing variations in the baseline X-ray level on the timescales of several years, 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, finding the source in the further long-term increasing X-ray flaring trend: the 0.3-10 keV count rate has reached a value of 8.94+/-0.11 cts/s during the XRT observation performed on December 16 (by a factor of ~2 higher than the minimum rate recorded on 2021 November 10, before the onset of the current flare). On average, the spectral evolution is characterized by the "harder-when-brighter" trend. 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.