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ATEL # 1134; V.P. Goranskij (SAI, Moscow Univ.), E.A. Barsukova, and A.N. Burenkov (Special Astrophys. Observatory, Russia)
on 9 Jul 2007; 10:40 UT
Password Certification: Vitaly Goranskij (email@example.com)
Subjects: Optical, Binaries, Black Holes, Transients, Variables, Stars
Referred to by ATEL #: 1135
V4641 Sgr is known as a detached binary containing a 9 Msol. black hole and an A0 type companion. Its orbital period is 2.81728 day. The light curve has a double-wave shape with two minima of unequal depth, the deepest minimum coincides with the inferior conjunction of the black hole at the epoch of T_0 = JDhel.2451764.315. Y. Maeda (vsnet-alert 9360) reported the brightening of the star up to 12.8 mag on 2007 May 20.7 UT, but P.F. Williams (vsnet-alert 9360) noted that the star was in its normal brightness of about 13.3 mag the day before and after.
Spectra of V4641 Sgr were taken using SAO 1-m telescope between May 23.972 and 24.002 UT (the orbital phase of 0.35), wavelength range 3900-7500A, resolution 8A, S/N~30-40. A0 type companion is seen with the H_alpha line more shallow than in the quiet state being filled by emission. Additionally we performed accurate CCD V and R_C photometry in 16 nights between June 5 and 25 using 60-cm telescope of Sternberg Institute's Crimean Station and SAO 1-m telescope. These observations show essential light excess over the quiet light level seen only in the orbital phases between -0.25 and +0.25 with the maximum value of 0.15 mag in the V band at the black hole inferior conjunction. The excess is absent in other orbital phases. This phenomenon has not been observed previously. We treat it as irradiation of the area of A0 star facing to the black hole. The area re-emits faint X-ray radiation of black hole in optical bands. Probably, the source of X-rays ionizes the rarefied gas around the black hole and forms the emission excess in H_alpha.
V band light curve and spectra of V4641 Sgr