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GCN Circular 36293

LIGO/Virgo/KAGRA S240422ed: SRG/eROSITA archival upper limits for Swift/XRT candidates
2024-04-24T23:09:41Z (3 months ago)
Arne Rau at MPE <>
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Zhu Liu, Arne Rau, Chandreyee Maitra (all MPE), Lorenzo Ducci (IAAT), Mirko Krumpe, Axel Schwope (both AIP):

We report constraints on the historical X-ray emission based on archival X-ray observations from the eROSITA X-ray telescope (Predehl et al. A&A 647, A1, 2021) on board the SRG mission for the three potential electromagnetic counterparts of the LIGO/Virgo/KAGRA candidate NSBH event S240422ed (GCN 36236) detected by Swift/XRT (GCN 36278).

The three sources were within the Field-of-View of eROSITA during each of the first four eROSITA all-sky surveys (eRASS) conducted between 2019 December and 2021 December. The three sources were not detected in individual eRASS scans or in the combined eRASS data (hereafter referred to as eRASS:4). Assuming a power-law model with a photon index of 1.7 and a column density of 3.0E20 cm^-2, we derived the following X-ray flux upper limits using the eRASS:4 data (total exposure uncorrected for vignetting is approximately 800s):

Name           | 0.3-10.0 keV  | 0.2-2.3 keV  | Total Exp
               |  ergs/cm^2/s  | ergs/cm^2/s  | s
S240422ed_X101 | < 1.2e-13     | < 5.2e-14    | 816
S240422ed_X61  | < 7.9e-14     | < 3.4e-14    | 881
S240422ed_X190 | < 8.2e-14     | < 3.6E-14    | 853

The Galactic neutral hydrogen column density (HI4PI map, N. Ben Bekhti et al., 2016, A&A 594, A116)) for these three sources are much higher than 3E20 cm^-2, we thus also derived the 3 sigma absorbed X-ray flux upper limits with the column density fixed at the value estimated from the HI4PI map:

Name           | 0.3-10.0 keV  | 0.2-2.3 keV  | NH     | Total Exp
               | ergs/cm^2/s   | ergs/cm^2/s  | cm**-2 | s
S240422ed_X101 | < 1.8E-13     | < 5.5E-14    | 2.0E21 | 816
S240422ed_X61  | < 1.8E-13     | < 4.1E-14    | 5.0E21 | 881
S240422ed_X190 | < 1.3E-13     | < 3.8E-14    | 2.1E21 | 853

All the Swift/XRT sources show significant X-ray flux increases compared to the eROSITA upper limits, suggesting that they are indeed X-ray transients.
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