[vsnet-outburst 24619] V4641 Sgr outburst (Atel #13431)

[Keisuke Isogai]

I forward the Atel about a new outburst of V4641 Sgr.
Unfortunately, it is difficult to observe due to the solar conjunction.
In the case of Japan, the object will rise to 10 degrees at 6 a.m.
The coordinates are 18:19:21.63 -25:24:25.8

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ATEL #13431							     ATEL #13431

Title:	A new outburst of V4641 Sgr
Author:	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)
Queries:	aarrans at unr.edu
Posted:	31 Jan 2020; 08:57 UT
Subjects:X-ray, Black Hole, Transient

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<sup>-2</sup> s<sup>-1</sup>
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<sub>in</sub>=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<sup>-2</sup> s<sup>-1</sup>.

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.
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