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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.