X-ray Transients in M31

ATel #79; M. Garcia, A. Kong, F. Primini, J. McClintock, S. Murray, and R. DiStefano; Harvard-Smithsonian Center for Astrophysics
on 10 Dec 2001; 20:41 UT
Credential Certification: Michael R. Garcia (garcia@cfa.harvard.edu)

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On Nov 19, 2001, The Chandra X-ray Observatory surveyed M31 with 5 overlapping very short (1ks) HRC exposures and a single short (5ks) ACIS-S exposure. Preliminary analysis allows us to report the following concerning possible X-ray transients within M31.

A new source CXOU J004428.4+415628 (RA=00:44:28.4, DEC=+41:56:28.1, J2000), near the NE spiral arm, is seen in two overlapping HRC exposures. The source has a luminosity of 1.7 x 10³⁷ ergs/sec (emitted over 0.3-7.0 keV, assuming a power-law spectrum with photon index of 2, N_H=10²¹ cm^-2, and a distance of 780 kpc) and a rather large error radius of 40'' due to it's large off-axis angle. Nearer to the nucleus two additional new sources are found. The second new source CXO J004251.2+411639 (RA=00:42:51.295, DEC=+41:16:39.88, J2000, error radius 1'' set by absolute aspect uncertainty) was detected at a luminosity of 1.4 x 10³⁶ ergs/sec. The third new source, CXOU J004239.5+411614 (RA=00:42:39.526, DEC=+41:16:14.46, J2000, error radius 1'') was detected at a luminosity of 3.3 x 10³⁷ ergs/sec, with a spectrum well fit with a power law of photon index 2.2 and N_H = 3.7 x 10²¹ cm^-2. This source was also seen in a 40ks ACIS-S image on Oct 4, 2001 at L_X = 5.0 x 10³⁷ ergs/sec, but was not found in earlier Chandra images, nor was it found by the ROSAT PSPC (Supper etal 2001, A&A 373, 63) or HRI (Primini, Forman & Jones 1993, ApJ 410, 615), but it is within 2'' of source #43 (L_X = 3.4 x 10³⁷ ergs/sec) in the Einstein survey of Trinchieri and Fabbiano (1991 ApJ 382, 82). Our new Chandra detection could therefore represent a recurrence of this transient.

The bright Chandra transient described in ATEL 76 (Kong etal 2001), CXOU J004305.5+411703, is seen at L_X = 10³⁸ ergs/sec in the 5ks ACIS-S image and at 3 x 10³⁸ ergs/sec in the 1ks HRC image, suggesting high variability. The second Chandra transient described in ATEL 76, CXOU J004242.1+411424, is at or below our detection limit of L_X = 5 x 10³⁵ ergs/sec on Nov 19, 2001 (assuming the spectral parameters in ATEL 76).

CXOM31 J004309.7+411901, at RA=00:43:09.791, DEC=+41:19:01.22 (J2000, error radius ~1'') was previously seen at a luminosity of 0.7 x 10³⁷ ergs/sec in observations during 2000 and early 2001 (Kong etal 2001 in prep). It was also detected by the Einstein Survey (#82), the ROSAT HRI Survey (#75), and the ROSAT PSPC Survey (RX J0043.1+4118) at luminosities ranging from 1 to 8 x 10³⁷ ergs/sec. Supper etal (2001) noted variability between PSPC exposures of a factor of 5. Surprisingly, two recent observations show this persistent source in a very dim state: On Aug 31, 2001 we marginally detected the source in a 5 ks ACIS-I observation at 7 x 10³⁵ ergs/sec, and on Oct 4, 2001 we clearly detected it in a 40 ks ACIS-S observation at 9 x 10³⁵ ergs/sec. (The two observations are consistent with a steady source at L_X = 8 x 10³⁵ ergs/sec.) Our most recent observations of Nov 19, 2001 show that it has re-brightened to L_X = 4.6 x 10³⁷ ergs/sec. The spectrum during this observation can be well fit by a power law with slope = 1.9 and N_H = 3.3 x 10²¹. The large variability implied by the Chandra and previous observations (factor of ~100) qualify this source as a probable transient.