X-ray Transients in M31

ATel #82; M. R. Garcia, A. K.H. Kong, J. E. McClintock, F. A. Primini, P. Kaaret and S. S. Murray (Harvard-Smithsonian Center for Astrophysics)
on 8 Feb 2002; 21:59 UT
Credential Certification: Michael R. Garcia (garcia@cfa.harvard.edu)

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Chandra observations on Jan 16, 2002 surveyed M31 with a series of 5 short (1ks) HRC images. Preliminary analysis yields the following info on new transients and highly variable sources:

A new source CXOU J004307.0+411809 (RA=00h43m07s.099, DEC=+41^o18'09".95, +/- 1", J2000) is discovered at a luminosity of 2.7 x 10³⁷ ergs/sec (emitted over 0.3-7.0 keV, assuming a power-law spectrum with photon index=2 and N_H = 10²¹ cm^-2 at a distance of 780 kpc). It was also present (but unnoticed) at lower luminosities in several earlier exposures. The first of these was a 40ks ACIS-S exposure on October 5, 2001, when it had a luminosity of 5 x 10³⁵ ergs/sec. The second was a 50ks HRC-I exposure on November 1, 2001, when it had a luminosity of 2 x 10³⁶ ergs/sec. However, the source was not detected during 1999-2001 in the 40ks exposure merged from snapshots taken during 1999 through 2001 (Kong atel astrop-ph/0202065), nor is it seen in either of the XMM exposures taken during June 2000 and December 2000. These non-detections imply upper limits of ~2 x 10³⁵ ergs/sec (Chandra) and ~6 x 10³⁵ ergs/sec (XMM).

The supersoft transient discovered in XMM observations on Jan 5, 2002 (XMMU J004414.0+412204, Trudolyubov etal 2002, IAUC 7798 ) is also found as a new source in an ACIS-S image from Jan 8, 2002 as CXOU J004414.3+412205 (RA=00h44m14s.3, DEC=+41^o22'05" +/- 3"). We confirm the supersoft nature of the transient and measure a thermal temperature of 40 +/- 10 eV and a detected luminosity of ~5 x 10³⁶ ergs/sec (0.3-1.5 keV). This source may be marginally detected in the HRC images from Jan 16, 2002 at a similar luminosity. This source is not within the FOV of the merged snapshots mentioned above, but it's non detection in the ROSAT PSPC survey (Supper etal 2001) indicates a previous upper limit of ~5 x 10³⁵ ergs/sec.

The bright Chandra transient described in ATEL 76 (Kong etal 2001), CXOU J004305.5+411703, is much fainter. This was seen at 3 x 10³⁸ ergs/sec (0.3-7.0 keV) in an HRC image from Nov 19, 2001 (ATEL 79, Garcia etal 2002), but is 10x fainter at 1.3 x 10³⁷ ergs/sec in the Jan 16, 2002 HRC data. The luminosity indicated by the Jan 8, 2000 ACIS-I exposure is 2.5 x 10³⁷ ergs/sec. The spectrum on Jan 8 is well fit by a power law with photon index=2.7 +/- 0.3 and N_H = 2 +/-1 x 10²¹cm^-2.