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[vsnet-campaign-xray 99] (fwd) X-ray Transients in M31 from ATEL
(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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