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ATEL # 2785; Kazutaka Yamaoka and Satoshi Nakahira (Aoyama Gakuin Univ.)
on 11 Aug 2010; 15:27 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Kazutaka Yamaoka (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Black Holes
Referred to by ATEL #: 2832, 2892
A significant X-ray increase of V4641 Sgr (= XTE J1819-254) has been detected by RXTE/PCA in its Galactic Bulge monitoring program since July 25, 2010. The PCA count rate has remained almost constant at about 20-30 counts s^-1, corresponding to about 2-3 mCrab. The PCA light curve is publicly available at the following URL: http://lheawww.gsfc.nasa.gov/users/craigm/galscan/html/XTE_J1819-254.htmlFollowing the RXTE/PCA detection of V4641 Sgr, we carried out the Swift XRT ToO observation with an exposure of 740 sec on August 8. The source was clearly detected, and the Swift XRT spectrum was fit by an absorbed power law with the photon index of 1.21 +/- 0.21 fixing the hydrogen column density (N_H) at 1.81x10^21 cm^-2 toward the source direction (Kalberla et al. 2005; chi^2/d.o.f = 19.2/16). The observed flux in the 2-10 keV range was (2.4 +/- 0.5) x10^-11 erg cm^-2 s^-1, corresponding to about 1.1 mCrab. The RXTE/PCA and Swift/XRT observations indicate that the source is currently in a weak X-ray outburst, which shows a similar level to the 2008 outburst (ATel #1796). However, this weak outburst may evolve into much larger one. Multi-wavelength observations are strongly encouraged.