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Swift and RATAN-600 monitoring of V4641 Sgr in a weak outburst

ATEL # 2832; Kazutaka Yamaoka, Satoshi Nakahira (Aoyama Gakuin Univ.), J. A. Tomsick (UCB/SSL), and Sergei A. Trushkin (SAO RAS)
on 7 Sep 2010; 16:57 UT
Password Certification: Kazutaka Yamaoka (

Subjects: Radio, X-ray, Binaries, Black Holes, Transients
Referred to by ATEL #: 2892

Following the RXTE/PCA and Swift/XRT detection of the current outburst of the black hole candidate V4641 Sgr (ATel #2785), we have monitored this source with X-rays (Swift/XRT) and radio (RATAN-600).

The Swift XRT observations have been carried out four times twice a week starting August 24. The absorbed flux in the 2-10 keV range was almost constant at about 1-3 mCrab: 5.6e-11, 3.2e-11, 7.0e-11, and 5.3e-11 erg cm^-2 s^-1 on August 24, 28, September 1 and 5, respectively. The summed XRT spectrum is roughly explained by an absorbed powerlaw with a photon index of 2.44 +/- 0.06 with NH = (0.42 +/- 0.02)e22 cm^-2 (chi^2/dof=412/287), but the spectrum is quite soft in comparison with black hole binaries in the low/hard state (1.4-1.7). We also tried other models, and found that a power-law with an exponential cutoff (chi^2/dof = 346/286) and a disk blackbody model (chi^2/dof = 356/287) give better results than a single power-law fit. The best-fit parameters for the cutoff powerlaw are the photon index: 1.16 +/- 0.11, the e-folding energy: 2.37 (-0.43, +0.64) keV, and the hydrogen column density NH: (0.24 +/- 0.04)e22 cm^-2, and for the disk-blackbody model are the innermost temperature: 1.04 +/- 0.03 keV, and the hydrogen column density NH: (0.13 +/- 0.02)e22 cm^-2.

The RATAN-600 monitoring has also been carried out till August 31. But we did not detect V4641 Sgr during the monitoring with an upper limit of nearly 5 mJy at 4.8 GHz.

Our results indicate that the source has been active at relatively weak level of 1-3 mCrab in X-rays for about 40 days since the beginning of the outburst around July 25, 2010. Further monitoring observations with multi-wavelengths are strongly encouraged.

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