X-ray and UV behaviour of IGR J17528-2022 (V6598 Sgr)

ATel #16172; Roberto Nesci (INAF/IAPS) and Mariateresa Fiocchi (INAF/IAPS)
on 4 Aug 2023; 07:51 UT
Credential Certification: Roberto Nesci (roberto.nesci@inaf.it)

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IGR J17528-2022 is a LMXB which was studied in quiescence by Hare et al. (2021, ApJ 914,85) and classified as an IP. Its X-ray spectrum was well fitted by an absorbed, partially covered, thermal bremsstrahlung model plus Fe emission at 6.4 keV. The optical counterpart is at r=19.16 (PanSTARRS DR1); a brighter star (r=14.92) is located at just 5.5 arcsec distance. In the Gaia DR3 IGR J17529-2022 has a negative parallax, so its distance is unknown but surely large, and is rather red (BP=19.82, RP=17.60). The star had a strong optical outburst on 2023-07-15 discovered by Andrew Pearce and classified as a classical Nova in ATel #16135. A detailed follow-up was made by Munari et al. (ATel #16141) who claim a similarity with two other Novae, U Sco and V2672 Oph (Nova Oph 2009). It was also detected in Gamma rays by Fermi-LAT (ATel #16151). Its optical light curve is very well sampled in the AAVSO database: after a fast decay, the star had a flatter decrease down to about R=12.8, V=14.0, B=15.2 on 2023-08-03. In archival Swift-UVOT observations IGR J17529-2022 was not detected, while the nearby star was seen at U=16.5, W1=18.0, W~20.0. IGR J17528-2022 was re-observed by Swift-XRT and UVOT on 2023-07-17T15:50, two days after the optical flare, for 720 s: no X-ray emission was detected, at variance with the quiescent phase, while it was clearly detected by UVOT at W1=15.63, M2=19.14 and W2=16.99. We observed the source on 2023-08-03T07:35 as a ToO with Swift-XRT and UVOT-W1 for 1620 s. Aperture photometry was made with IRAF/apphot with a 2 arcsec radius to exclude the nearby star; 23 stars in the field were used for comparison. IGR J17528-2022 was well detected at W1=17.11 +/-0.07. In X-ray the source was detected with (6.0+/-2.2)E-3 c/s; for comparison, during quiescence it was at 2.48E-2 c/s in the Swift-XRT image of 2019-05-06. So we argue that the physical processes producing the X-ray emission have been blocked by the Nova explosion and are just starting to work again, 17 days after the Nova event. This is at mild variance with the case of the recurrent Nova U Sco, which was again detected in X-ray nine days after the optical outburst. Further follow-up is necessary to see when the X-ray emission will return to the pre-eruption level. We thank the Swift Team for the quick time allocation. We acknowledge the use of public data from the Swift data archive and the use of the AAVSO public database.