[vsnet-alert 6921] (fwd) X-ray Transients in M31 from ATEL

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

(fwd) X-ray Transients in M31 from ATEL

   This message is from ATEL #79:

Title:		X-ray Transients in M31
Author:		M. Garcia, A. Kong, F. Primini, J. McClintock, S. Murray,
		and R. DiStefano; Harvard-Smithsonian Center for Astrophysics
Queries:	garcia@cfa.harvard.edu
Posted:		10 Dec 2001;  20:41  UT
Subjects:	X-ray, Black Holes, Neutron Stars, Transients

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 10<sup>37</sup>  ergs/sec (emitted
over 0.3-7.0 keV, assuming a power-law spectrum with  photon index of 2,
N<sub>H</sub>=10<sup>21</sup> cm<sup>-2</sup>, 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  10<sup>36</sup> 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 10<sup>37</sup> ergs/sec, with
a  spectrum well fit with a power law of photon index 2.2 and  N<sub>H</sub>
= 3.7 x 10<sup>21</sup> cm<sup>-2</sup>.  This source  was also seen in
a 40ks ACIS-S image on Oct 4, 2001 at L<sub>X</sub> =  5.0 x 10<sup>37</sup>
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<sub>X</sub>
= 3.4 x 10<sup>37</sup> 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<sub>X</sub> = 10<sup>38</sup>  ergs/sec
in the 5ks ACIS-S image and at 3 x 10<sup>38</sup> 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<sub>X</sub> = 5 x 10<sup>35</sup> 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  10<sup>37</sup>
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 10<sup>37</sup>  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  10<sup>35</sup> ergs/sec, and on Oct 4, 2001 we clearly detected
it in  a 40 ks ACIS-S observation at 9 x 10<sup>35</sup> ergs/sec.  (The
two  observations are consistent with a steady source at L<sub>X</sub>
= 8  x 10<sup>35</sup> ergs/sec.)  Our most recent observations of Nov
19,  2001 show that it has re-brightened to L<sub>X</sub> = 4.6 x  10<sup>37</sup>
ergs/sec. The spectrum during this observation can be  well fit by a power
law with slope = 1.9 and N<sub>H</sub> = 3.3  x 10<sup>21</sup>.  The large
variability implied by the Chandra and  previous observations (factor of
 ~100) qualify this source as a  probable  transient.

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