X-Ray Flare and Extreme Intraday Flux Variability in the TeV-Detected Blazar PKS 2155-304

ATel #16182; Bidzina Kapanadze (Ilia State University, Tbilisi, Gerogia; E. Kharadze National Astrophysical Observatory, Abastumani, Georgia)
on 10 Aug 2023; 18:58 UT
Credential Certification: Bidzina Kapanadze (bidzina_kapanadze@iliauni.edu.ge)

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The TeV-detected blazar PKS 2155-304 (z=0.116) has been observed 6 times with the X-Ray Telescope (XRT) onboard the Neil Gehrels Swift Observatory since 2023 June 28 in the framework of our Target of Opportunity observations (Request Number 19004). During this campaign, the source showed an X-ray flare and has reached the 0.3-10 keV brightness level corresponding to the mean XRT count rate (CR) of 5.41+/-0.19 cts/s during the 1.5 ks XRT observation performed on August 9. Note that the comparable brightness was last observed in 2019 June. Moreover, the aforementioned observation consisted of three segments, each performed during the different Swift orbits and the count rate doubled between the first and second segments (with CR=4.55+/-0.25 cts/s and CR=11.00+/-0.37 cts/s, respectively) separated by the time interval of 46 ks and then dropped to CR=4.16+/-0.22 cts/s in about 8 ks. Based on the light-travel arguments, this extreme intraday variability imply the size of the emission zone smaller than 2.4$\times$10^{15} cm when adopting the typical Doppler factor of 10. The spectrum was well-fit with the logparabola model (when fixing the Galactic absorption to the hydrogen column density of 1.48$\times$10^20), yielding the curvature parameter $b$=0.35+/-0.13, the photon index at 1 keV $a$=2.43+/-0.04 and the unabsorbed 0.3-10 keV flux of (1.67_/-0.06)$\times$10^{-10} erg/cm^2/s. Since the source is in X-ray flaring state and a further brightening may occur, as well as enhanced multiwavelength (MWL) activity is predicted also in other spectral ranges in the framework of one-zone SSC scenario, we strongly encourage intense MWL observations with the space and ground-based instruments. 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.