SRG/eROSITA observations of PKS 0903-57
ATel #15135; S. Haemmerich (Remeis-Observatory/ECAP, FAU Erlangen), A. Gokus (Remeis-Observatory/ECAP, FAU Erlangen & Uni Wuerzburg), I. Kreykenbohm (Remeis-Observatory/ECAP, FAU Erlangen), P. Weber (Remeis-Observatory/ECAP, FAU Erlangen), J. Wilms (Remeis-Observatory/ECAP, FAU Erlangen), A. Zainab (Remeis-Observatory/ECAP, FAU Erlangen)
on 23 Dec 2021; 16:27 UT
Credential Certification: Andrea Gokus (andrea.gokus@fau.de)
Subjects: X-ray, AGN, Blazar
On 2021 November 19 and 20, the blazar PKS 0903-57 was reported to exhibit enhanced gamma-ray activity (Ciprini et al., ATel #15057). The source seems to have been in an enhanced state since October 2021. In the past, five flares have been reported by the Fermi/LAT collaboration. Swift follow-up observations (Mereu et al., ATel #15058) revealed that the blazar also showed increased flux in the X-ray regime. Between 2021 December 03 and 05 AGILE again reported increased activity in the gamma rays (C. Pittori et al.,ATel #15086). The source was followed up by Swift between 2021 November 28 and December 06. These findings suggest a long-term X-ray flaring of the source.
Here we report on the recent observations of PKS 0903-57 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). The position of the source (RA = 09:04:53.70, DEC = -57:35:05.86) has been scanned multiple times during the ongoing all-sky survey.
The most recent observation took place between 2021 December 14 19:15 UTC and 2021 December 17 03:15 UTC. The total exposure was 448s, in which 360 counts were detected.
Describing the X-ray spectrum by an absorbed power-law model, with the equivalent neutral hydrogen column density fixed to N_H=2.6x10^21 cm^-2 (HI4PI Collaboration, N. Ben Bekhti, et al., 2016, A&A 594, A116) we find a best fit photon index of 1.7 +/- 0.2 and an absorbed 0.3-10 keV flux of (6.5 +/- 0.6) x 10^-12 erg/cm^2/s (all following values use the same band). The count rate was (0.80 +/- 0.05) counts/s.
The four previous eROSITA observations yielded the following results:
2019 December 09-11:
Flux: (2.1 +/- 0.4) x 10^-12 erg/cm^2/s
Count rate: (0.33 +/- 0.04) counts/s
photon index: 2.1+/-0.4
2020 June 07-09:
Flux: (3.1 +/- 0.4) x 10^-12 erg/cm^2/s
Count rate: (0.39 +/- 0.03) counts/s
photon index: 1.72+/-0.27
2020 December 11-13:
Flux: (4.2 +/- 0.7) x 10^-12 erg/cm^2/s
Count rate: (0.31 +/- 0.03) counts/s
photon index: 1.6+/-0.4
2021 June 12-14:
Flux: (5.3 +/- 0.9) x 10^-12 erg/cm^2/s
Count rate: (0.39 +/- 0.04) counts/s
photon index: 1.6+/-0.4
The eROSITA measurements in combination with the Swift results suggest that the blazar has increased in X-ray flux significantly over the past two years, therefore supporting the suggestion that the blazar is in an X-ray flaring state. The results also suggest that the spectral shape has changed, as the spectrum seems to become harder before the reported flaring activity. However, the values agree with each other within the uncertainties to some extent.
Further monitoring of the source, especially with X-ray facilities, is encouraged to study the long term behavior of PKS 0903-57.
