[vsnet-alert 12102] V4641 Sgr active

[Taichi Kato]

   V4641 Sgr is reported to be active (see ATEL below)

Recent observations:

  YYYYMMDD(UT)   mag  observer
  20100704.542   134  (Rod Stubbings)
  20100704.615   133  (Rod Stubbings)
  20100705.360   134  (Rod Stubbings)
  20100706.428   133  (Rod Stubbings)
  20100706.500   133  (Rod Stubbings)
  20100708.442   134  (Rod Stubbings)
  20100709.378   133  (Rod Stubbings)
  20100710.628   132  (Rod Stubbings)
  20100711.398   133  (Rod Stubbings)
  20100719.460   134  (Rod Stubbings)
  20100720.584  13.45V  (Hiroyuki Maehara)
  20100730.465   133  (Rod Stubbings)
  20100730.496   133  (Rod Stubbings)
  20100807.497  13.50V  (Hiroshi Itoh)
  20100808.406   134  (Rod Stubbings)

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The Astronomer's Telegram                   http://www.astronomerstelegram.org 
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ATEL #2785							     ATEL #2785

Title:		RXTE/PCA and Swift/XRT detect an outburst of the black hole
		binary V4641 Sgr
Author:	Kazutaka Yamaoka and Satoshi Nakahira (Aoyama Gakuin Univ.)
Queries:	yamaoka at phys.aoyama.ac.jp
Posted:	11 Aug 2010;  15:27  UT
Subjects:	X-ray, Binaries, Black Holes

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.html
Following 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.  

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Password Certification:  Kazutaka Yamaoka (yamaoka at phys.aoyama.ac.jp)
http://www.astronomerstelegram.org/?read=2785
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