Swift Observations of Rapid X-ray Flux Variations in V4641 Sgr
E. M. Cackett, J. M. Miller (University of Michigan)
Following the recent optical activity of the black hole binary V4641 Sgr (ATEL #1134), we observed V4641 Sgr with Swift XRT over the period May 23 to 25 2007. During the first set of observations on May 23, V4641 Sgr was observed for ~1.8 ks, and the source is clearly detected by the XRT. We measure a 0.5-10 keV flux = (1.2 +/- 0.2)E-11 ergs/cm^2/s. The spectrum is well-fit by a simple absorbed power-law, with N_H fixed at the Galactic value (1.81E21 cm^-2), and a power-law spectral index = 1.5 +/- 0.2. The 2nd set of observations were performed on May 24. Unfortunately, during the last section of observations the XRT was abnormally hot causing a large increase in background level. Excluding this section we have a total good time of 1.2 ks. However, V4641 Sgr is not detected during these observations, with a 0.5-10 keV flux upper limit of 5E-13 ergs/cm^2/s. Thus the source decreased in flux by at least a factor of ~25. The final Swift observations were performed on May 25, totalling 0.9 ks. During these observations V4641 Sgr is clearly detected again, with a 0.5-10 keV flux = (2.1 +/- 0.2)E-11 ergs/cm^2/s. Thus, the source increased in flux by at least a factor of 40 from the previous day. The spectrum is well-fit by a simple absorbed power-law, with N_H fixed at the Galactic value, and a power-law spectral index = 1.7 +/- 0.5.
optical,x-ray,binary,black hole,transient