State/Spectral Transition of the Ultraluminous Supersoft X-ray Source in M101
ATel #222; A. K.H. Kong and R. Di Stefano (Harvard-Smithsonian Center for Astrophysics)
on 28 Jan 2004; 11:12 UT
Credential Certification: Albert Kong (akong@cfa.harvard.edu)
Subjects: X-ray, Binary, Black Hole
Referred to by ATel #: 305, 374
We analyzed recent archival Chandra observations of the spiral galaxy, M101, taken between 2004 Jan 19 and 2004 Jan 24. The ultraluminous supersoft X-ray source (CXOU J140332.3+542103) reported by Mukai et al. (2003) and Di Stefano and Kong (2003) was detected in 2 of 3 recent Chandra ACIS-S observations. In the first 56 ksec observation taken on 2004 Jan 19, 27 counts were detected in 0.1-7 keV, with 19 counts below 1.1 keV. The count distribution indicates that the source spectrum is not supersoft (Di Stefano and Kong 2003). By assuming a power-law spectrum with N_H=1e21 cm^-2 and photon index of 2, the unabsorbed 0.3-7 keV luminosity is 2.3e37 erg/s (d=6.7 Mpc). This luminosity is nearly a factor of 200 fainter than the very first Chandra observation taken on 2000 March 26 (Mukai et al. 2003; Di Stefano and Kong 2003). The source was not detected in the second short (3 ksec) observation taken on 2004 Jan 21, with a 3-sigma detection limit of 1.9e38 erg/s. During the third 22 ksec observation taken on 2004 Jan 24, the source was seen at 3.6e37 erg/s, assuming the same power-law spectral model. The source was also harder in the third observation, with 10 out of 17 counts from below 1.1 keV. The recent Chandra observations indicate that the source is currently in transition, possibly to the low/hard state.
[1] Mukai, K. et al. 2003, ApJ, 582, 184
[2] Di Stefano, R. and Kong, A.K.H. 2003, ApJ, 592, 884
