[vsnet-alert 9404] V4641 Sgr May event info from ATEL, activity confirmed

[Taichi Kato]

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The Astronomer's Telegram                   http://www.astronomerstelegram.org 
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Posted: Sat Jul  7 01:30:01 EDT 2007 -- Tue Jul 10 01:30:02 EDT 2007
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ATEL #1134                                                           ATEL #1134

Title:          Recent activity of the black hole binary V4641 Sgr
Author: V.P. Goranskij (SAI, Moscow Univ.), E.A. Barsukova, and A.N.
                Burenkov (Special Astrophys. Observatory, Russia)
Queries:        goray at sao.ru
Posted: 9 Jul 2007;  10:40  UT
Subjects:       Optical, Binaries, Black Holes, Transients, Variables, Stars

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: http://jet.sao.ru/~goray/v4641/at4641.htm
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ATEL #1135                                                           ATEL #1135

Title:          Swift Observations of Rapid X-ray Flux Variations in V4641
                Sgr
Author: E. M. Cackett, J. M. Miller (University of Michigan)
Queries:        ecackett at umich.edu
Posted: 9 Jul 2007;  15:51  UT
Subjects:       X-ray, Binaries, Black Holes, Transients

Following the recent optical activity of the black hole binary V4641 Sgr
(ATEL #1134), we observed V4641 Sgr with Swift XRT over the period May
23 to 25 2007.

During the first set of observations on May 23, V4641 Sgr was observed
for ~1.8 ks, and the source is clearly detected by the XRT.  We measure
a 0.5-10 keV flux = (1.2 +/- 0.2)E-11 ergs/cm^2/s.  The spectrum is well-fit
by a simple absorbed power-law, with N_H fixed at the Galactic value (1.81E21
cm^-2), and a power-law spectral index = 1.5 +/- 0.2.

The 2nd set of observations were performed on May 24.  Unfortunately,
during the last section of observations the XRT was abnormally hot causing
a large increase in background level.  Excluding this section we have a
total good time of 1.2 ks.  However, V4641 Sgr is not detected during these
observations, with a 0.5-10 keV flux upper limit of 5E-13 ergs/cm^2/s.
Thus the source decreased in flux by at least a factor of ~25.

The final Swift observations were performed on May 25, totalling
0.9 ks.  During these observations V4641 Sgr is clearly detected again,
with a 0.5-10 keV flux = (2.1 +/- 0.2)E-11 ergs/cm^2/s.  Thus, the source
increased in flux by at least a factor of 40 from the previous day.  The
spectrum is well-fit by a simple absorbed power-law, with N_H fixed at
the Galactic value, and a power-law spectral index = 1.7 +/- 0.5.