Detection of new X-ray transient in M 31
ATel #4375; F. Hofmann, W. Pietsch, R. Sturm, J. Greiner, F. Haberl, M. Henze (Max-Planck-Institut fuer extraterrestrische Physik, MPE)
on 14 Sep 2012; 16:12 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)
Subjects: X-ray, Transient
In an observation of the M 31 central region with the X-ray telescope
(XRT) on board the Swift satellite (ObsId 0035336069, start time 2012-09-13.16
UT, exposure 2.64 ks), we detect about 60 counts from
a new X-ray transient at position
RA = 00h42m47.21 Dec = +41d14'05.5" (± 3.0", J2000, 90% confidence) after correcting
systematic offsets (Kaaret 2002, ApJ, 578, 114), which we name SWIFT
J004247.2+411405. About half the counts are detected above 1 keV excluding a
super-soft spectrum. However, a power law fit proposes a rather soft spectrum
(photon index of >4). This is confirmed by the best fitting
bremsstrahlung (kT = 0.42 keV, NH =
2.0 × 1021 cm-2) or disk black body models
(kT = 0.27 keV, NH =
1.1 × 1021 cm-2). The unabsorbed luminosity in
the 0.2-10 keV band is 4 × 1037 erg s-1.
In earlier
observations with XMM-Newton EPIC, Chandra HRC I, and Swift XRT, the
transient is not visible. The 3&sigma upper limit for a source at the
transient position derived for the last Swift
observation seven days before (ObsId 0035336068) is
6× 10-3 cts s-1.
After this Swift observation, the
source brightened at least by a factor of 4, compared to the last Chandra HRC I
observation (ObsID 13281, starting on 2012-06-01.90 UT) by at least a factor
of 40 (assuming the Swift spectrum above).
Swift observation 0035336069 was split in 5 slots spread over 19 hours.
There is no indication of strong
time variability between the slots, excluding the possibility of a flaring
Galactic star in the foreground. The timing and spectral
behavior suggests that SWIFT J004247.2+411405 is a transient low mass X-ray binary system
(likely a black hole system) detected in the soft state.
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.
