Enhanced X-Ray Activity of the southern TeV-Detected Blazar PKS 2155-304
ATel #14865; Bidzina Kapanadze (Ilia State University, Tbilisi, Gerogia; E. Kharadze National Astrophysical Observatory, Abastumani, Georgia)
on 18 Aug 2021; 19:46 UT
Credential Certification: Bidzina Kapanadze (bidzina_kapanadze@iliauni.edu.ge)
The southern TeV-detected blazar PKS 2155-304 (z=0.116) is a bright X-ray source and prominent with its very strong TeV/X-ray flaring behaviour (see, e.g., Aharonian et al. 2009, A & A, 502, 749; Abramowski et al. 2012, A & A, 539; Kapanadze et al. 2014, MNRAS, 444; 1076). Consequently, this object represents one of the frequent targets of X-Ray Telescope onboard Neil Gehrels Swift Observatory (Swift-XRT) in the southern hemisphere (262 observations with 405 ks of the total exposure time since 2005 November 17). During this monitoring, the 0.3-10 keV count rate showed an overall variability by a factor of 28 and maximum value of 11.4+/-0.2 cts/s, and the source underwent a very strong X-ray flare along with the unprecedented TeV outburst in 2006 July (along with the strong or moderate flares observed during next years; see https://www.swift.psu.edu/monitorin g/source.php?source=PKS2155-304). The flux varied on timescales from a few months down to very fast fluctuations within the 1-ks exposure (see, e.g., Kapanadze et al. 2014, MNRAS, 444, 1076). The spectra generally were curved and well-fitted with the log-parabolic model, yielding the ranges of the photon index at 1 keV $a$=2.05--2.75, curvature parameter $b$=0.13--0.80, the synchrotron SED peak location $E_p$=0.02--0.89 keV. Moreover, the source exhibits the signatures of the longer-term variations in the baseline X-ray level with timescales of several years, showing its lowest historical 0.3-10 keV count rate of ~0.4 cts/s in 2020 September.
The Swift monitoring of PKS 2155-304 have been resumed in the framework of our Target of Opportunity (ToO) request Number 16167, and the today's Swift-XRT pointing to the source detected a significantly higher X-ray brightness, a factor of ~5 higher compared to the previous observation performed on 2020 September 26 with the 0.3-10 keV count rate of 2.0+/-0.1 cts/s. In the framework of one-zone SSC models, a flaring activity of PKS 2155-304 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.
