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Swift XRT observations of the black hole binary V4641 Sgr

ATEL # 1796; Kazutaka Yamaoka (Aoyama Gakuin Univ.), Jeroen Homan (MIT), and Makoto Uemura (Hiroshima Univ.)
on 21 Oct 2008; 20:29 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Kazutaka Yamaoka (

Subjects: X-ray, Request for Observations, Binaries, Black Holes, Transients

In response to the recent X-ray and optical activity reported in VSNET alerts, we have performed a Swift ToO observation of V4641 Sgr on October 17 2008, for a total exposure of 2.1 ks.

The source was clearly detected in the Swift XRT image. The energy spectrum was could be fit reasonably well by an absorbed power law with a photon index of 2.1+-0.2 and a hydrogen column density of (2.9+-0.6)x10^21 cm^-2. The absorbed fluxes in 0.3-10 keV and 2-10 keV bands were 2.16e-11 erg/cm^2/s and 1.43e-11 erg/cm^2/s, respectively, corresponding to ~0.6-0.7 mCrab. Variability up to a factor of ~4 was seen on a time scale of a few hours.

The source has been active for two weeks now, but at a very low level in both X-rays and optical. Further multiwavelength observations are encouraged.

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R. E. Rutledge , Editor
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