Swift J055137.6-381713: a probable new X-ray transient.
ATel #8863; P. A. Evans (U. Leicester), J. P. Osborne (U. Leicester), J. A. Kennea (PSU), K. L. Page (U. Leicester),
on 24 Mar 2016; 13:33 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Phil Evans (pae9@star.le.ac.uk)
Subjects: X-ray, Request for Observations, Transient
Referred to by ATel #: 8883
In Swift-XRT observations beginning at 18:30:15UT on 2016 March 22, a
previously uncatalogued X-ray source was serendipitously detected. The
source, designated Swift J055137.6-381713, is located at:
RA (J2000) = 05h 51m 37.63s = 87.9068 deg
Dec (J2000) = -38d 17' 13.7" = 38.2871 deg
with an uncertainty of 4.1 arcsec (radius, 90% confidence)
No X-ray source near this position is found in the X-ray Master
catalogue provided by HEASARC. We derive a 3-sigma upper limit from
the Rosat all-sky survey (RASS) data of 0.031 PSPC ct/sec (0.1-2.4 keV).
The XRT spectrum can be modelled as a power-law with a photon index
of 1.2 (+/-0.3) and and absorption column N_H < 6e20 cm-2.
The light curve at present is sparsely sampled, showing no signs of
rapid variability. The peak count-rate (0.3-10 keV) is 0.055 (+/-0.014)
ct/sec which, using the above spectrum, corresponds to a 0.1-2.4 keV
PSPC rate of 0.076 (+0.04, -0.03) ct/sec, above the upper limit from the
RASS presented above. The XRT count-rate corresponds to an observed 0.3-10 keV flux
of (3.1 +/- 0.8)e-12 erg cm-2 s-1 (due to the low
absorption, the unabsorbed flux is the same).
Since the X-ray flux is above the existing upper limit, we suggest
that this source is likely a new X-ray transient, although we cannot
currently rule out that it is a slow variable which oscillates above and
below the RASS threshold.
A USNO-B1.0 star is 2.2 arcsec from the XRT position, with colours
B2=14.9, I=15.7, R1=14.5, R2=14.1, and a 2MASS star is 3.8 arcsec away
from the XRT position, with colours h=16.0, j=16.5, k=15.6. These are
not consistent with a late-type star, suggesting a coronal stellar flare is not
the origin of the X-ray source.
Further X-ray observations are planned, and we encourage
multiwavelength follow up to identify the nature of this object.
