X-Ray Flare in TeV-Detected Blazar 1ES 1218+304

ATel #15969; Bidzina Kapanadze (Ilia State University, Tbilisi, Gerogia; National Astrophysical Observatory, Abastumani, Georgia)
on 31 Mar 2023; 09:33 UT
Credential Certification: Bidzina Kapanadze (bidzina_kapanadze@iliauni.edu.ge)

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The TeV-detected BL Lac object 1ES 1218+304 (z=0.184) was observed 131 times by Swift between 2005 October 30 and 2023 March 31 with a total exposure of 151 ks. These observations revealed a strong X-ray variability in the 0.3-10 keV count rate between 0.47+/-0.02 cts/s and 7.18+/-0.18 cts/s with more than 10 flares by a factor of 2-4 during the densely-sampled Swift campaigns (see https://www.swift.psu.edu/monitoring/source.php?source=1ES1218+304). The last Swift Target-of-Opportunity (TOO) campaign has been carried out during 2023 January 27 -- March 31 on the basis of our Request Number 18361 (10 observations with a net exposure of about 15 ks). This monitoring revealed a long-term gradual brightening by ~40% to the 0.3-10 keV count rate CR=3.65+/-0.11 cts/s (corresponding to the unabsorbed 0.3-10 keV flux of (5.82+/-0.18)x10^{-11} erg/cm^2/s). The spectrum is well-fitted with a simple powerlaw, yielding a hard 0.3-10 keV photon index $\Gamma$=1.78+/-0.03. Note that the hard power-law spectrum can be obtained via the relativistic magnetic reconnection, which, on its turn, is relativistically magnetized (i.e. the magnetic field energy is higher than the matter rest energy; see Sironi & Spitkovsky 2014, Astrophys. J., 783, L21). Since the source is in X-ray flaring state and 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.