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ATEL # 2074; A.K.H. Kong (NTHU, Taiwan), J.L. Galache, M.R. Garcia, R. Di Stefano, F. Primini, J. Liu, B. Patel (CfA), W. Pietsch (MPE), M. Orio (INAF Padova and UW Madison), T. Nelson (UW Madison)
on 9 Jun 2009; 21:21 UT
Password Certification: Albert Kong (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Black Holes, Transients
We report recent observations of M31 taken with Chandra/ACIS-I on 2009-05-29.1 UT and with Swift/XRT on 2009-06-07.08 UT. In the 4ks Chandra/ACIS-I image, we found a faint transient located at R.A.(J2000)=00h42m26.7s, Decl.(J2000)=+41:13:46.5 with a 90% absolute error of 0.6 arcsec. It is therefore identified as CXOM31 J004226.7+411346. The unbinned energy spectrum can be fit (using C statistic) with an absorbed power-law model (Gamma=2.1, N_H fixed at 7e20 cm^-2). All quoted errors are 90% confidence. The luminosity in the 0.3-8 keV (assuming a distance of 780 kpc) is 7e36 erg/s. In the 11ks Swift/XRT image, the transient was not detected and we set a 3-sigma upper limit at 5e36 erg/s (0.3-8 keV), indicating that the source is fading.
The transient, CXOM31 J004251.5+411701, discovered with Chandra on 2009 March 11 (ATel #1978) is still visible in the recent Chandra and Swift images. In the Chandra image, the energy spectrum can be described by an absorbed power-law with Gamma=2.7+/-0.5 and N_H=(3+/-2)e21 cm^-2. The 0.3-8 keV luminosity is 1.4e38 erg/s. Comparing with the March observation, the source has increased its brightness by a factor of ~6 and the spectrum has become softer. Then in the Swift image taken 9 days later, the spectrum is consistent with the Chandra observation with Gamma=2.4+/-0.6 and N_H=(2.1+/-1.5)e21 cm^-2. The 0.3-8 keV luminosity is 8e37 erg/s. It is possible that the source is undergoing a state transition from a low/hard state to a thermal-dominant state, typical for a black hole transient.
We thank the Swift PI, Neil Gehrels, the Swift science team, and the Swift mission operations team for their support of these observations.