Optical follow-up observations of V4641 Sgr
ATel #11949; A. K. H. Kong (National Tsing Hua University, Taiwan)
on 13 Aug 2018; 23:14 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)
Subjects: Optical, X-ray, Binary, Black Hole, Transient
Following the report of a weak X-ray outburst from the black hole transient V4641 Sgr (ATel #11931), we have performed optical follow-up observations since 2018 August 10. Observations were taken with the T17 0.43m telescope of iTelescope.Net in Siding Spring, Australia. We used a subset of comparison stars and the orbital solution in MacDonald et al. (2014) to derive the magnitude of V4641 Sgr and the corresponding orbital phase, respectively. The magnitudes of V4641 Sgr are listed below:
2018-08-10 at Phase 0.72 V=13.44
2018-08-11 at Phase 0.13 V=13.35
2018-08-12 at Phase 0.49 V=13.36 B=13.74
2018-08-13 at Phase 0.84 V=13.66 B=14.01
with a typical 1-sigma uncertainty of ~0.1 mag.
By comparing the quiescent light curves in MacDonald et al. (2014), it is clear that V4641 Sgr shows increased activity in optical on 2018-08-11 and 2018-08-12 with 0.3-0.5 mag enhancement. On 2018-08-13, both the B and V magnitudes are consistent with quiescence. Our observations indicate that V4641 Sgr may be in an optical active phase. Further optical and X-ray observations are encouraged to investigate the nature of this outburst. Swift observations have been approved to follow up the source.
