SRG/eROSITA observation of PKS 0735+17

ATel #15108; S. Haemmerich (Remeis-Observatory/ECAP, FAU Erlangen), A. Zainab (Remeis-Observatory/ECAP, FAU Erlangen), A. Gokus (Remeis-Observatory/ECAP, FAU Erlangen & Uni Wuerzburg), P. Weber (Remeis-Observatory/ECAP, FAU Erlangen), I. Kreykenbohm (Remeis-Observatory/ECAP, FAU Erlangen), J. Wilms (Remeis-Observatory/ECAP, FAU Erlangen)
on 13 Dec 2021; 13:58 UT
Credential Certification: Andrea Gokus (andrea.gokus@fau.de)

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On 2021 December 8 20:02:51.1 UT IceCube detected a track-like neutrino event (IC211208A; Santander et al., GCN#31191). Optical follow-up observations revealed an increase in brightness of BL Lac PKS 0735+17, after the source had decayed in flux prior to the neutrino event (Zhirkov et al., ATel#15098, Zhirkov et al., ATel#15100). In early November 2021 optical activity had been reported as well (Savchenko et al., ATel #15021). PKS 0735+17 has also shown increased gamma-ray activity starting about a week before IC211208A (Garrappa et al., ATel #15099). A target-of-opportunity observation with the Neil Gehrels Swift observatory revealed an increased X-ray flux compared to archival data from 2009 - 2011 (Santander & Buson, ATel #15102). Radio observations revealed that the radio flux of PKS 0735+17 has increased since the beginning of 2021, and is exhibiting a long-term radio flare (Kadler et al., ATel #15105).

We report on the recent observations of the BL Lac PKS 0735+17 by the eROSITA telescope (Predehl et al. 2021, A&A 647, A1) onboard the Russian/German Spectrum-X-Gamma mission (Sunyaev et al. 2021, arxiv:2104.1326). Its position (RA = 07:38:07.32, DEC = +17:42:13.58) was scanned multiple times during the ongoing all-sky survey. The most recent scan took place between 2021 October 22 09:02 UTC and 2021 October 26 17:02 UTC, 48 days before IC211208A. During a total exposure of 146s, 33 counts were detected.

Its X-ray spectrum can be described by an absorbed power-law model, with the equivalent neutral hydrogen column density fixed to N_H=3.76x10^20 cm^-2 (HI4PI Collaboration, N. Ben Bekhti, et al., 2016, A&A 594, A116). The best fit photon index is 2.0 +/- 0.6 and the absorbed 0.3-10 keV flux is (1.1 +/- 0.4) x 10^-12 erg/cm^2/s (all following values use the same band). The average count was (0.21 +/- 0.04) counts/s. The three previous eROSITA observations of PKS 0735+17 yielded the following results:

2020 April 18/19:
Flux: (1.6 +/- 0.6) x 10^-12 erg/cm^2/s
Count rate: (0.17 +/- 0.04) counts/s

2020 October 21/22:
Flux: (1.3 +/- 0.5) x 10^-12 erg/cm^2/s
Count rate: (0.23 +/- 0.05) counts/s

2021 April 19/20:
Flux: (1.2 +/- 0.4) x 10^-12 erg/cm^2/s
Count rate: (0.24 +/- 0.04) counts/s

The spectral shape changes between the observations, however, the uncertainties are large (photon indices: 2020 April: 1.2+/-0.6, 2020 Oct: 1.7+/-0.6, 2021 April: 2.0+/-0.5). The eROSITA data confirms the findings by Swift (Santander & Buson, ATel #15102) of PKS 0735+17 being in an enhanced state in the X-rays at the moment.

Follow-up observations with other X-ray missions and multi-wavelength facilities are encouraged in order to study the source further, in order to assess the possible neutrino association and to study the currently observed X-ray enhancement on longer time scales.