New outburst of X-ray transient in M 31 GC Bol 128
ATel #4359; F. Hofmann, W. Pietsch, J. Greiner, F. Haberl, M. Henze (Max-Planck-Institut fuer extraterrestrische Physik, MPE)
on 7 Sep 2012; 17:22 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)
Subjects: X-ray, Binary, Transient
Monitoring observations of the M 31 central region with the X-ray telescope
(XRT) on board the Swift satellite show a new outburst of an X-ray transient.
The position of the transient was derived from the brightest observation after correcting
systematic offsets (using the catalog of Kaaret 2002, ApJ, 578, 114)
to RA = 00h42m47.96s, Dec =
+41d11'14.9" (± 5", J2000, 90% confidence level). The transient is first
detected in observation 0035336064, starting on 2012-08-12.48 UT. In earlier
observations in 2012 with XMM-Newton EPIC, Chandra HRC I, and Swift XRT, the
transient is not visible. We only give a 3&sigma upper limit for the last Swift
observation seven days before the outburst. After this Swift observation, the
source brightened at least by a factor of 3. Compared to the last Chandra HRC I
upper limit (ObsID 13281, starting on 2012-06-01.90 UT) the transient brightened
by at least a factor
of 40. Absorption corrected luminosities in the 0.5-8.0 keV
band have been calculated using a foreground column density of
NH = 1.1 × 1021 cm-2, assuming a power
law spectrum with a photon index of 1.61 and a distance of 780 kpc to M 31 (see
below).
Swift ObsID Exposure MJD Count rate Luminosity
[s] [d] [10-2 ct/s] [1037 erg/s]
0035336063 2043 56144.13 < 0.59 < 1.91
0035336064 904 56151.48 0.91 ± 0.4 2.95 ± 1.29
0035336065 2058 56159.90 0.55 ± 0.2 1.77 ± 0.68
0035336066 959 56167.58 1.32 ± 0.5 4.27 ± 1.49
0035336067 1558 56171.58 1.29 ± 0.4 4.17 ± 1.20
0035336068 1566 56175.54 1.93 ± 0.4 6.26 ± 1.39
In ObsID 0035336068 we detect about 30 counts from the source extending to
energies of about 5 keV, indicating a hard X-ray spectrum. The source position
correlates well within the errors with GC candidate Bol 128 from
the Revised Bologna Catalog of M 31 globular clusters (Galleti et. al. 2004, A&A, 416, 917).
Previous X-ray detections of a transient in Bol 128 are reported for 1990,
1995 and 2004 (see Stiele et. al. 2008, A&A, 480, 599). A hard X-ray spectrum was derived
from XMM-Newton observations with parameters used for the luminosity estimates above. The detections in 1990 and 1995
were obtained from ROSAT HRI data (1RXH J004247.8+411107,
source number 51 in Primini et. al. 1993, ApJ, 410, 615). The outburst in 2004
is covered by XMM-Newton EPIC and Chandra ACIS observations (see also Voss &
Gilfanov 2007, A&A, 468, 49). We interpret our detection as the fourth detected outburst
of a low mass X-ray binary in Bol 128. The maximum luminosity during the 2012 outburst
maximum is similar as in previous ones. The recurrence time of outbursts is about five years.
We would like to thank the Swift Team for making these observations possible, in
particular N. Gehrels, the duty scientists as well as the science planners.