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ATEL # 969; J.L. Galache, M.R. Garcia, M.A.P. Torres, D. Steeghs S.S. Murray, F.A. Primini (CfA) and B.F. Williams (U. of Washington)
on 20 Dec 2006; 15:13 UT
Password Certification: Jose Luis Galache (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Black Holes, Neutron Stars, Transients
We report on 3 Chandra ACIS-I observations containing the two M31 transient X-ray sources announced by Haberl et al. (2006, ATel #881) and followed up by Pietsch et al. (2006, ATel #899). These observations are part of our ongoing Chandra/HST M31 transient program
XMMU J004215.8+411924: This High Mass X-ray Binary was first observed with Chandra for 5 ks on 2006-07-31.02 UT (10 days before its discovery with XMM). The X-ray spectrum in the 0.9-6 keV interval is well described by an absorbed power-law model with photon index ~ 1.80 and an absorption column density of nH = 4.42e21 atoms/cm2. The observed 0.5-10 keV luminosity is 1.1e38 erg/s (for a distance to M31 of 780 kpc). These values are similar to those reported in ATel #881, suggesting that XMMU J004215.8+411924 remained in a stable outburst state for at least 10 days (but no more than 20 days (ATel #899)). This outburst behaviour and spectral characteristics are typical of Be/X-ray binaries and we agree with Haberl et al.'s classification for this source. However, we do not detect significant pulsations in the 7-1000 s range (at a limit pulse fraction of 64%). A second 5 ks observation of M31 on 2006-09-24.76 shows that since the second Swift observation (ATel #899) the source remained inactive with a luminosity below the detection limit of Lx ~ 4e35 erg/s.
SWIFT J004217.4+411532: This black hole candidate was observed for 5 ks on 2006-09-24.76 (23 days after its discovery with Swift and 13 days after the follow-up observation). Fitting an absorbed power-law to the spectrum yields similar results to those of Haberl et al. However, the spectrum is very soft and a more physically consistent fit is obtained when modeling with an absorbed disk blackbody with Tin = 0.55 keV and nH fixed at the Galactic value towards M31 (6.66e20 atoms/cm2). The presence of a soft component and the high X-ray luminosity (8.7e37 erg/s in the 0.5-10 keV range) are consistent with the source being a black hole transient in a thermal (high-soft) state. A further 29 ks observation was carried out on 2006-12-04.88 UT, finding the source dimmer, Lx = 1.7e37 erg/s in the 0.5-10 keV range, and with Tin = 0.35 keV (nH was again fixed at the Galactic value).