NICER observations of the new outburst of V4641 Sgr

ATel #13443; D. Altamirano (University of Southampton), Zaven Arzoumanian, Keith Gendreau, Peter Bult, Tod E. Strohmayer (NASA/GSFC), Jeroen Homan (Eureka Scientific & SRON)
on 4 Feb 2020; 10:28 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Diego Altamirano (d.altamirano@soton.ac.uk)

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V4641 Sgr (also known as SAX J1819.3-2525) is a black-hole X-ray binary with a dynamically confirmed black hole mass of 6.4 ± 0.4 M_sun, and a companion mass of 2.9 ± 0.4 M_sun. The binary has an orbital period of 2.8 d and is at a distance of ~6.2 kpc (Orosz et al. 2001, ApJ, 555, 489; MacDonald et al. 2014, ApJ, 784, 2). V4641 Sgr outbursts evolve differently from the typical BH-XRB outburst, and it has been suggested that the phenomenology observed is very similar to that seen from the BH-XRB V404 Cyg (e.g., Muñoz-Darias et al. 2018, MNRAS, 479, 3987).

Recently, Shaw et al. (2020; ATel #13431) and Imazoto et al. (2020; ATel #13437) reported on a new outburst of V4641 Sgr. In particular, Shaw et al. reported that MAXI has detected the outburst since early January 2020, and that given visibility constraints the outburst of V4641 Sgr probably started in December 2019. Shaw et al. (2020) also reported on a short NuSTAR DDT observation performed on 2020 January 22. This observation revealed a disk-dominated X-ray spectrum (kTin=1.33 ± 0.01 keV), indicating that the source is probably in a spectrally soft accretion state. These authors estimate the unabsorbed 2-10 keV flux to be ~8E-10 erg cm-2 s-1.

Triggered by ATel #13431, NICER performed two ~300 s observations on 2020 January 31, at 18:54 and 20:25 UT. We detect V4641 Sgr at an average of 550 cts/s (0.3-12 keV). A power-spectral analysis revealed no significant power; however, we note that we are limited in sensitivity due to the shortness of the observation. While V4641 Sgr was within 45 degrees of the Sun and formally inaccessible because of NICER's Sun-avoidance constraint, there were brief periods of time during which the shadow of the Earth (i.e., orbit night, just before sunrise) afforded NICER short glimpses. The 1-10 keV NICER 300 s averaged spectrum is satisfactorily modelled with an absorbed disk black-body component at kTin=1.48 ± 0.02 keV. We find clear residuals in the form of emission lines at ~6.8 and ~8 keV. The 1-10 keV unabsorbed flux was 1.5e-9 erg cm-2 s-1 (and 1.1e-9 erg cm-2 s-1 in the 2-10 keV range). Our best fit parameters are consistent with those of ATel #13431, however, at a factor of ~1.4 higher flux in the 2-10 keV range.

NICER will make additional short snapshot observations until V4641 Sgr comes out of the NICER Sun constraint on February 9th, and longer observations will be planned after that.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.