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ATEL # 2892; Kazutaka Yamaoka (AGU), Satoshi Nakahira (AGU), and John A. Tomsick (UCB/SSL)
on 30 Sep 2010; 18:34 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Kazutaka Yamaoka (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Black Holes, Transients
We have monitored the Galactic microquasar V4641 Sgr in a weak outburst (ATel #2785 and #2832) with Swift twice a week for about a month. The recent Swift/XRT data indicates that the 2-10 keV flux has been increasing from about 1-3 mCrab to about 9 mCrab. The observed fluxes were 1.0e-10, 9.1e-11, 1.5e-10, 1.6e-10, 2.1e-10, and 2.0e-10 erg cm^-2 s^-1 in the 2-10 keV range on September 9, 12-13, 18, 21, 26. and 29, respectively. This trend is consistent with the RXTE/PCA Galactic Center Monitoring data ( http://lheawww.gsfc.nasa.gov/users/craigm/galscan/html/XTE_J1819-254.html ). Overall, the Swift/XRT and RXTE/PCA data suggest an increase in X-ray activity but with significant variability on time scales of a few days.
The XRT spectrum taken on September 29 is still soft and quite similar to the previous one with the 1-3 mCrab flux level (ATel #2832). It is well explained by a power-law with an exponential cutoff with the photon index of 0.81 +/- 0.24, the e-folding energy of 1.92 (-0.26, +0.34) keV, and the hydrogen column density NH of (0.23 +/- 0.04)e22 cm^-2 (chi^2/dof = 349/309), or a disk blackbody with the innermost temperature of 1.08 +/- 0.03 keV, and the NH of (0.16 +/- 0.02)e22 cm^-2 (chi^2/dof = 372/310).
Further Swift monitoring is planned over a week. Multi-wavelength observations are strongly encouraged. We thank the Swift team for approving and scheduling our ToO observations.