Optical observations of V4641 Sgr during X-ray brightening in October 2021

ATel #15008; V. P. Goranskij, A. V. Zharova (SAI, Moscow University), E. A. Barsukova, A. N. Burenkov (Special Astrophysical Observatory, Russian Academy of Sciences)
on 28 Oct 2021; 11:14 UT
Credential Certification: Vitaly Goranskij (goray@sai.msu.ru)

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V4641 Sgr was discovered in the outburst on 1978 Jun 9 with 12.1 mag B in maximum (Goranskij, Astron. Zirkular No.1024, 1978). However, then it was misidentified with the nearby star GM Sgr by GCVS team. First photoelectric UBV observations of V4641 Sgr were taken on 1986 Oct 4 (IBVS 3464, 1994), and first spectrum was taken with the Russian 6-m telescope BTA on 1996 Aug 21, the star was classified as A0III (Astron. Rep. V.47, 740, 2003). On 1999 Feb 20-22, V4641 Sgr was rediscovered as a weak X-ray source named SAX J1819.3-2525 or XTE J1819-254. Later the star was classified as a black hole binary. Recent X-ray brightening was detected by NICER, SWIFT and INTEGRAL on 2021 Oct 4, and its flux maximum of 20 mCrab was registered on Oct 12 (ATel #14992, 14971,14968). We performed photometry and spectroscopy in spite of incomfortable observing conditions: low location in the sky, bad transparency near the horizon and visibility time limited by about 40 min. The first photometric set was taken with the 60 cm Zeiss telescope of the SAI Crimean Station and a FLI CCD starting on 2021 Oct 17.635 and finishing on 17.672 UT (the orbital phase was 0.63 relative to the formula Min I = JD hel 2459410.4136 + 2.81728 E). The summary image gives brightness of 13.22 V, what is 0.5 mag brighter than quiet level in this phase. Another set was taken with the 1 m Zeiss telescope of the Special Astrophysical Observatory and CCD UBV(RI)c photometer starting on 2021 Oct 20.653 and finishing on 20.681 UT (phase 0.70). Measuring of 11 frames gives V = 13.094 +/-0.009, what is 0.4 mag brighter than in the quiet state. Also we have the following magnitudes in other filters: U = 13.69, B = 13.78, Rc = 12.93, Ic = 12.72. Additionally, we obtained spectra with the same 1 m telescope and UAGS spectrograph covered wavelength range between 5755 and 7160 A. The first one was taken between 2021 Oct 21.779 and 21.807 UT (phase 0.05), the second one - between Oct 22.774 and 22.806 UT (phase 0.40). Spectral resolution is 2.0 A, and dispersion is 0.69 A/pxl. Both spectra are weak and noisy, S/N relation is 4 and 8, accordingly. We smoothed the spectra with the moving average method using 5 A average interval. Both spectra show H-alpha line profile filled with emission component with a peak displaced to the red side from the center of the absorption profile by 100 and 150 km/s, correspondingly. In the second spectrum, the emission line He I 5875 A is visible with the peak at Vr(hel) = +180 km/s. The radial velocity of the mass center of V4641 Sgr system is known as 108 +/-3 km/s. The light curve and spectra are shown in the internet link.

The light curve and fragments of spectra