INTEGRAL, NICER and Swift confirm the re-brightening of V4641 Sgr

ATel #14971; Motta, S. (INAF-OAB), Kuulkers, E. (ESA/ESTEC), Sanchez-Fernandez, C., Kajava, J. J. E. (ESA/ESAC), Gendreau, K., Arzoumanian, Z. (NASA/GSFC), Belloni, T. (INAF-OAB), Sanna, A. (University of Cagliari), Ferrigno, C. (ISDC/University of Geneva)
on 14 Oct 2021; 08:27 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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V4641 Sgr (SAX J1819.3–2525 = XTE J1819-254) is a nearby (~6.6 kpc) black hole X-ray binary discovered in 1999, when it underwent a bright and short outburst (see, e.g., Wijnands & van der Klis 2000, ApJ 528, L93). V4641 Sgr remained in quiescence ever since, save from sporadic mild re-brightenings. On 12 October 2021 the MAXI alert system detected flares from a position consistent with that of V4641 Sgr (Atel #14968), which suggested that the source might be entering a new outburst phase.

In a pointed Swift/XRT observation starting on 14 October at 01:27 UT (exposure ~960 sec), we detected V4641 Sgr at a position of RA, Dec (J2000.0): 18h 19m 21.6s, -25d 24m 25.0s (estimated 90% uncertainty radius: 3.5 arcsec). This is ~0.8 arcsec away from the source nominal optical position. We therefore confirm that the new transient detected by MAXI is indeed V4641 Sgr, possibly entering a new outburst. The 0.6-10 keV flux is ~6.9E-10 erg/cm2/s during the observation. The X-ray spectrum can be preliminarily described by an absorbed power-law with Gamma ~1.7 and NH ~ 4.3E+21/cm2.

INTEGRAL pointed at V4641 Sgr from 13 October 17:32 to 14 October 08:41. Preliminary analysis shows that the source is detected at approximately 18 mCrab in the JEM-X 3-10 keV band, which confirms the Swift results.

NICER pointed at the target from 13 October 15:40 to 16:10 (UT) and detected it at a flux ~5.2E-10 ergs/cm2/s in the 0.6 to 10 keV band, consistent with what was seen by Swift and INTEGRAL. The NICER spectrum shows a clear emission line at ~6.8 keV, which we tentatively associate with a blue-shifted Iron K-alpha line. No significant variability was observed in the ~550s observation.

Multi-wavelength observations are strongly encouraged.

We thank the INTEGRAL, NICER and Swift teams for promptly scheduling the observations.