SRG/eROSITA observation of TXS 0646-176 during its gamma-ray flare

ATel #14991; S. Haemmerich (Remeis-Observatory/ECAP), A. Gokus (Remeis-Observatory/ECAP & Uni Wuerzburg), I. Kreykenbohm (Remeis-Observatory/ECAP), A. Rau (MPE Garching), J. Wilms (Remeis-Observatory/ECAP)
on 21 Oct 2021; 16:18 UT
Credential Certification: Andrea Gokus (andrea.gokus@fau.de)

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The eROSITA telescope (Predehl et al. 2021, A&A 647, A1) on-board of Spectrum-X-Gamma (Sunyaev et al. 2021, arxiv:2104.1326) scanned the position of flaring FSRQ TXS 0646-176 (RA = 06:48:28.66, DEC = -17:44:04.07) as part of its fourth ongoing all-sky survey (eRASS4) during the state of enhanced gamma-ray activity detected by Fermi/LAT (La Mura, ATel#14980) and AGILE Pittori et al. , ATel#14981). The source was observed during eight consecutive passages, spaced 4h apart, between 2021 October 14 17:27 UTC and 2021 October 15 21:27 UTC, with a total exposure time of 273s. Therefore, the eROSITA data covers the same time frame as the gamma-ray flare. The instrument detected ~230 photons.

The X-ray spectrum of TXS 0646-176, during the flaring state is best described with an absorbed power-law (red. cash stat: 0.91). Fixing the equivalent neutral hydrogen column density along the line of sight to the value of N_H = 2.37 x 10^21 cm^(-2) (HI4PI Collaboration, N. Ben Bekhti, L. Floer, et al., 2016, A&A 594, A116), we find a photon-index of 1.26+/-0.23 and an observed absorbed flux (0.3-10.0 keV) of (9.8 +/- 1.2) x 10^-12 erg/cm^2/s. Between the individual scans the count rate remained stable with an average of (0.83 +/- 0.06) counts/s.

The X-ray properties are similar to the ones reported for a Swift/XRT observation (ATel #14987) that was conducted on 2021 October 18. Therefore, it seems that the blazar stayed in an enhanced X-ray state for a few days and that the X-ray properties did not change significantly over this time span.

TXS 0646-176 was also detected in the first three eROSITA all-sky surveys, during which the flux was always lower than in the current state.
The previous fluxes (0.3-10.0 keV) are:
2020 April 13: (3.2 +/- 0.8) x 10^-12 erg/cm^2/s
2020 October 16: (2.7 +/- 0.8) x 10^-12 erg/cm^2/s
2021 April 12/13: (5.6 +/- 0.9) x 10^-12 erg/cm^2/s
During all these observations no significant difference in spectral shape was seen (photon indices: eRASS1: 1.2 +/- 0.5, eRASS2: 1.2 +/- 0.6, eRASS3: 1.1 +/- 0.4).

Further multi-wavelength follow-up observations, especially by other X-ray facilities, are encouraged in order to study the long term behavior of the source after the flare.