Central Bureau for Astronomical Telegrams
IAUC 7440: V4641 Sgr
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Circular No. 7440
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/iau/cbat.html ISSN 0081-0304
Phone 617-495-7244/7440/7444 (for emergency use only)
V4641 SAGITTARII
J. A. Orosz, Utrecht University; E. Kuulkers, Space Research
Organization Netherlands; M. van der Klis, University of Amsterdam;
J. E. McClintock, Harvard-Smithsonian Center for Astrophysics; R. K.
Jain and C. D. Bailyn, Yale University; and R. A. Remillard,
Massachusetts Institute of Technology, report: "We have observed
V4641 Sgr, the optical counterpart of the fast x-ray transient and
superluminal radio source SAX J1819.3-2525 (IAUC 7119, 7253, 7254,
7256, 7265, 7271, 7277), on June 2-6 using the first 8.2-m
telescope (+ FORS1) at the European Southern Observatory, Paranal.
A preliminary analysis of the spectra indicates that the optical
spectrum resembles that of an A2V star, with no evidence of
emission lines from an accretion disk. We find an orbital period
of 2.866 +/- 0.043 days, a velocity semiamplitude of 204.7 +/- 3.7
km/s, and an epoch of maximum velocity of HJD 2451701.658 +/- 0.012.
The resulting minimum mass of the compact object is 2.55 +/- 0.14
solar masses. Goranskij (1990, IBVS 3464) published the light
curve of V4641 Sgr obtained from 345 plates taken with the Crimean
0.40-m astrograph between about 1960 and 1990. Goranskij recently
reanalyzed these data and made them available via the World Wide
Web (see
http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/obs26000/msg00925.html).
We computed a Lomb-Scargle periodogram (cf. Scargle 1982, Ap.J. 263,
835, and cited references) of the data posted by Goranskij,
excluding uncertain measurements and data from 1978 June, when the
source apparently had another flare. There is significant power at
a period of 1.40865 +/- 0.000015 days. The data folded on twice
this period, 2.8173 +/- 0.00013 days, resembles an ellipsoidal
light curve with two maxima of roughly equal height and two minima
of unequal depth per orbital cycle. The epoch of minimum
brightness is HJD 2447707.454 (IBVS 3464). The photometric phase
at the time of maximum velocity is 0.741 +/- 0.066, consistent with
the expected value of 0.75 for an ellipsoidal light curve. The
radius of the Roche-lobe-filling A2 star depends essentially only
on its assumed mass. For an A2 star near the main sequence, M_2
about 2.3 solar masses and the resulting radius is then about 5.1
solar radii. The implied distance is about 6100 pc, assuming an
effective temperature of 9000 K, an apparent magnitude of V = 14,
and a color excess of E(B-V) = 0.24 (IAUC 7276), where the visual
extinction is assumed to be 3.1E(B-V). Our current radial velocity
curve is somewhat sparsely sampled, so we encourage additional
spectroscopic observations."
(C) Copyright 2000 CBAT
2000 June 19 (7440) Daniel W. E. Green
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