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November 2007 Chandra Observations of X-Ray Transients in M31

ATEL # 1328; J.L. Galache, M.R. Garcia, M.A.P. Torres, S.S. Murray (CfA), D. Steeghs (U. of Warwick/CfA) and B.F. Williams (U. of Washington)
on 14 Dec 2007; 18:22 UT
Password Certification: Jose Luis Galache (jlgalache@cfa.harvard.edu)

Subjects: X-ray, Binaries, Black Holes, Transients

As part of our ongoing Chandra/HST M31 transient program we report on two active sources recently detected in two Chandra ACIS observations.

Source 1
We present two 5ks ACIS-I observations of the ongoing outburst of the M31 transient source CXOM31 J004217.0+411508 (r3-46). Using an absorbed powerlaw model with photon index = 1.7 and nH = 6.6e20 atoms/cm2 at a distance of 780 kpc (as reported by Henze et al. 2007, ATEL #1307), we derive the following luminosities in the 0.2-10 keV range (observations with an asterisk are from ATEL #1307):

2007-11-03.18 UT: 3.1e37 erg/s
*2007-11-07.64 : 1.9e37 erg/s
*2007-11-17.76 UT: 1.5e37 erg/s
*2007-11-24.21 UT: 1.6e37 erg/s
2007-11-27.16 UT: 2.1e37 erg/s

These values indicate that the source has been active at an almost constant luminosity for at least 3 weeks.


Source 2
A new transient X-ray source in the M31 globular cluster Bol 194 was reported by Haberl et al. (2007, ATel #1296) at Lx ~ 9e37 erg/s, in an observation made on 2007-11-07.64 UT. This source was detected by our team ~4 days earlier (on 2007-11-03.18 UT) during a 5ks observation with Chandra ACIS-S at a net count rate of 2.1e-2 cts/s. These values are consistent with the HRC-I count rate for the ranges of kT and nH provided by Haberl et al. (2007, ATel #1306). A further 5ks observation with Chandra ACIS-S ~24 days later (on 2007-11-27.16 UT) did not detect the source. We establish the 95% upper limit for the detection with ACIS-S to be ~6e-4 cnt/s, corresponding to Lx ~ 2.5e35 erg/s, assuming the same spectral parameters. We note that 3 days prior (2007-11-24.34 UT) the source was still detected in a ToO by Swift at Lx ~ (4-30)e37 erg/s (ATel #1306). Such behaviour is not uncommon in M31 super soft sources.

Given the low counts in our initial detection a detailed spectral analysis is not possible. Using the method described in Prestwich et al. (2003, ApJ 595, 719) we find its hardness ratios to be HR1 = -1.036 and HR2 = 0.077. These values classify the transient as a super soft source, in agreement with the X-ray spectrum reported in ATel #1306.


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