Swift XRT confirms the new outburst of the X-ray transient XTE J1856+053
ATel #7278; A. Sanna (UNICA), D. Altamirano (Southampton), A. Rushton (Southampton), P. Jonker (SRON)
on 24 Mar 2015; 10:39 UT
Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)
Subjects: X-ray, Black Hole, Transient
Triggered by the X-ray enhancement reported by MAXI/GSC on March the
16th at a position compatible with the black hole candidate XTE J1856+053 (Atel #7233), a 1.5 ks
observation with Swift/XRT was promptly carried out. Swift/XRT operating
in Photon Counting mode detected a single bright X-ray source.
The XRT location is (J2000):
RA=284.1781
DEC=5.3077
with an error radius of 3.5'' (90% c.l.)
This position is consistent with that of the black hole candidate XTE
J1856+053 observed in outburst in 2007 (Steeghs et al. Atel #1026).
Approximately 4132 source counts were detected during the 1476 seconds-long observation.
This implies that pile-up is affecting the source spectral properties. To extract the spectrum we used the XRT on-line tool at http://www.swift.ac.uk (see Evans et al.
2009, MNRAS, 397, 1177), that applies standard pile-up corrections. We fitted the spectrum using two different
models: an absorbed diskbb model with N_H=2.7+/-0.4 10^22 cm^-2 and
T_in=1.0+/-0.1 keV and an absorbed powerlaw model with N_H=4.9+/-0.6
10^22 cm^-2 and photon index 3.5+/-0.3. Neither of them can well fit the
data. The spectral fit improves when using an absorbed two-component
model (phabs(diskbb+powerlaw)), however, the resulting
component parameters are not well constrained, probably due to a combination of effects.
For instance, the relatively low number of source counts, the possible presence
of residual effects caused by pile-up, and the possible presence of un-modelled
spectral features (e.g. a broadend Fe-K line) could lead to this.
The estimated observed 0.3-10 keV
X-ray flux is 3.9E-10 erg/cm^2/s suggesting that XTE J1856+053 entered
in a new outburst phase.
More observations have been requested to follow up the outburst
evolution and assess the source state.
