Optical observations of the black hole X-ray binary V4641 Sgr during the 2024 observing season

ATel #16916; Goranskij, V. P. (SAI, Moscow University; SAO RAS); Barsukova, E. A., Fatkhullin, T. A. (SAO RAS)
on 21 Nov 2024; 18:07 UT
Credential Certification: Vitaly Goranskij (goray@sai.msu.ru)

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Renewed X-ray activity from V4641 Sgr was reported by MAXI/GSC team on Sep. 5 and 6 (ATel#16804), with the brightening likely beginning on Sep. 3. It was confirmed by INTEGRAL on Sep. 7 and 8 (ATel#16811), and by Swift/XRT on Sep. 14 (ATel#16819). MeerKAT detected a radio source at this position on Sep. 16, and observed a radio flare on Oct. 6 (ATel#16819, ATel#16852). In the optical range, we observed this object on 12 nights between Jul. 11 and Oct. 31 using SAO 1-m Zeiss telescope and UBVRI photometers. Additionally, we obtained two spectra with the UAGS spectrograph on Jun. 14 and Aug. 13. In the quiescent state, V4641 Sgr exhibits a two-wave-shaped light curve in the V band, characterized by the light elements of Min I = JD hel. 2459410.4136 + 2.81727E. The V magnitudes are as follows: Min I 14.03, Max I 13.48, Min II 13.79 and Max II 13.46. In the active states, the star displays an excess brightness compared to the quiescent light curve. Our observations indicate that V4641 Sgr remained active throughout the season of 2024, with an average excess of 0.3 mag in the V filter. The highest excess, of 0.47 mag (V=13.306), occurred on Jul. 11.00 UT, while the lowest excess of 0.07 mag (V=13.660), was observed on Sep. 7.9 UT. Furthermore, the final observation of 2023 season, taken on 2023 Oct. 7.01 UT also recorded the star in an active state, with the excess of 0.27 mag (V = 13.76). This suggests that renewed activity in optics may have begun between Aug. 21 and Oct. 7, 2023. Photometric monitoring during this period did not reveal rapid variability, with only orbital trends being evident. The spectra were obtained in the wavelength range of 4160-6900 A, with a resolution of 3.7 A and a signal-to-noise S/N of approximately 10. The spectrum is dominated by the A0 companion. The equivalent widths of the Balmer lines are nearly constant, with Halpha having EW of about 7.3 A, and Hbeta about 9.1 A. No emission component is detected in the profiles of Halpha line.

The V-band light curve of V4641