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Chandra Observations of the M31 X-Ray Transients XMMU J004245.9+411036 and XMMU J004241.2+411821

ATel #1658; 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 13 Aug 2008; 21:47 UT
Credential Certification: Jose Luis Galache (jlgalache@cfa.harvard.edu)

Subjects: X-ray, Binary, Black Hole, Neutron Star, Transient

Referred to by ATel #: 1671, 1674, 1870

As part of our ongoing Chandra/HST M31 transient program we report on Chandra ACIS observations of M31 containing the new transient sources reported by Pietsch et al. (2008, ATel #1647). Two 5ks observations were performed on 2008-05-31.47 UT (Obs. 1) and 2008-07-15.70 UT (Obs. 2), both prior to the observation of Pietsch et al. (2008, Obs. 3).

XMMU J004245.9+411036
The source fell on a chip gap during part of Obs. 1 due to spacecraft dithering, we could therefore not obtain a reliable spectrum for it. In Obs. 2 the spectrum is best fit by an absorbed powerlaw with a fixed hydrogen column density of nH = 6.6e20 atoms/cm2 (the Galactic value towards M31) and a photon index of 2.4. The values of the 0.5-5 keV luminosities (assuming 780 kpc to M31) for this source are:

Obs. 1 (MJD 54617.5): 3.5e37 erg/s (estimated from count-rate assuming nH = 6.6e20 atoms/cm2 and Γ = 1.7)
Obs. 2 (MJD 54662.7): 1.4e37 erg/s
Obs. 3 (MJD 54665.3): 6.2e36 erg/s

This source shows a small decline in luminosity over the course of 45 days followed by a sudden dimming over 2.6 days. Due to the large gap between the first two observations it is not possible to say whether the outburst is coming to an end or whether the sudden dimming is a recurrent feature. Based on its transient nature, luminosity and its somewhat soft spectrum, we suggest that XMMU J004245.9+411036 may be a black hole transient.

XMMU J004241.2+411821
We could not obtain a reliable spectrum for this source in Obs. 2 for the same reasons outlined above. The following 0.5-5 keV luminosities were obtained when fitting the spectrum of Obs. 1 with the same model as Pietsch et al. (2008, absorbed powerlaw with fixed hydrogen column density of nH = 6.6e20 atoms/cm2 and photon index of 1.7):

Obs. 1 (MJD 54617.5): 3.1e37 erg/s
Obs. 2 (MJD 54662.7): 8.1e36 erg/s (estimated from count-rate assuming nH = 6.6e20 atoms/cm2 and Γ = 1.7)
Obs. 3 (MJD 54665.3): 4.2e37 erg/s

This source shows a possible dimming and subsequent brightening over the three observations. Due to the uncertainty in the estimated luminosity of Obs. 2 it entirely possible that this source has maintained the same luminosity over the 2 months observing window.

Further Chandra observations covering the positions of these two transients are scheduled in the upcoming months.