MeerKAT and Swift confirm renewed activity of black hole X-ray binary V4641 Sgr

ATel #16819; Noa Grollimund (Universite Paris Cite & CEA Saclay), Stephane Corbel (Universite Paris Cite & CEA Saclay & Observatoire de Paris), Rob Fender (Oxford University), Sara Motta (INAF-OAB), on behalf of the X-KAT collaboration
on 18 Sep 2024; 19:57 UT
Credential Certification: S. CORBEL (stephane.corbel@cea.fr)

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The black hole X-ray binary V4641 Sgr recently began an outburst, detected by MAXI/GSC (ATel #16804) and confirmed by INTEGRAL/JEM-X (ATel #16811). Following the report of renewed activity of the source, we conducted follow-up radio observations with the MeerKAT radio telescope, along with X-ray observations with Swift, as part of the X-KAT and SwiftKAT programmes.

Radio observations were carried out at a central frequency of 1.28 GHz, with a total bandwidth of 860 MHz. We used J1939-6342 for flux and bandpass calibration, and J1833-2103 for complex gain calibration. V4641 Sgr was observed for 15 minutes on 2024/09/16 15:05:00.4 UTC (MJD 60569.634 ± 0.005). We detect an unresolved radio source coincident with the nominal position of V4641 Sgr. By fitting a point source in the image plane, we measured a flux density of 0.17 +/- 0.04 mJy/beam.

We performed a Swift/XRT observation of V4641 Sgr in Windowed Timing mode on 2024/09/14 06:15:34 UTC (MJD 60567.26) with a total exposure of ~380 s. The source was clearly detected in the XRT image at a flux ~8.1E-10 ergs/cm2/s in the 0.6-10 keV energy band.

Multi-wavelength observations are encouraged to study the rise of the outburst. Further radio monitoring is planned, as well as weekly Swift observations of the target.

X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme contact Rob Fender. The Swift follow-up of X-ray binaries is largely performed as part of the SwiftKAT program, which provides quasi simultaneous X-ray coverage of the X-KAT targets.