A new outburst of V4641 Sgr

ATel #13431; A. W. Shaw (U. Nevada, Reno), J. A. Tomsick (UC Berkeley/SSL), A. Bahramian (ICRAR/Curtin), P. Gandhi (U. Southampton), C. O. Heinke (U. Alberta), R. M. Plotkin (U. Nevada, Reno)
on 31 Jan 2020; 08:57 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Aarran Shaw (aarrans@unr.edu)

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MAXI has been detecting increased X-ray activity from the black hole transient V4641 Sgr (also known as SAX J1819.3-2525) since 2020 January 7, with a typical average count rate of ~0.1 photon cm^-2 s^-1 in the 2-20 keV band. The source has been detected by MAXI since it emerged from its Sun constraint, so the outburst may have started whilst the source was obscured by the Sun. V4641 Sgr has a history of short, weak outbursts since it was discovered in a bright outburst in 1999 (e.g. ATels #2785, #5803, #7858 and #11931).

V4641 Sgr is currently Sun constrained for a number of telescopes, including almost all X-ray observatories. However, we obtained a short NuSTAR DDT observation on 2020 January 22, which revealed a disk-dominated X-ray spectrum with a best-fit inner disk temperature kT_in=1.33±0.01 keV, indicating that the source is probably in a soft accretion state. In addition we detect strong emission lines at 6.8 and 8.05 keV. Though the NuSTAR energy range is 3-78 keV, we estimate the unabsorbed 2-10 keV flux to be ~8E-10 erg cm^-2 s^-1.

We encourage any possible follow-up observations, particularly at radio frequencies. V4641 Sgr becomes visible to Swift on 2020 February 10, and we will request DDT observations in order to monitor the outburst.

We thank the NuSTAR SOC team for the quick scheduling of DDT observations.