Optical-to-X-ray Flare in Markarian 501

ATel #15134; Bidzina Kapanadze (Ilia State University, Tbilisi, Georgia; E. Kharadze National Astrophysical Observatory, Abastumani, Georgia)
on 22 Dec 2021; 22:10 UT
Credential Certification: Bidzina Kapanadze (bidzina_kapanadze@iliauni.edu.ge)

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In ATel#15124 (posted on 2021 December 17), we reported the X-ray flare in the nearby, TeV-detected blazar Markarian 501 (z=0.034) which 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 Kapanadze et al. 2017, MNRAS, 469; 1655). Currently, Mrk 501 is monitored by Neil Gehrels Swift Observatory in the framework of our Target of Opportunity (TOO) request Number #16518. The Swift-XRT visit to the source on December 22 revealed a further increase in the 0.3-10 keV count rate, amounting to 13.31+/-0.13 cts/s (derived from our thorough analysis of the quick-look data published on https://swift.gsfc.nasa.gov/cgi-bin/sdc/ql) and corresponding to the unabsorbed 0.3-10 keV flux of 5.25e-10 erg/cm^2/s), which is ~50 per cent higher compared to the level recorded during the previous X-ray on December 16 (and reported in In ATel#15124). The spectrum is very hard, corresponding to the photon index at 1 keV $a$=1.76+/-0.2 which can be related to the hadronic synchrotron contribution to the 0.3-10 keV energy range (see, e.g. Kapanadze et al. 2017). In the UVOT bands, the source shows elevated optical and UV brightness levels correspond to V=13.15+/-0.03 mag, B=13.82+/-0.04 mag, U=13.45+/-0.03 mag, UVW1=13.64+/-0.04 mag, UVM2=13.54+/-0.04 mag, UVW2=13.54+/-0.03 mag. Consequently, the intense 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.