Swift archival observations of the field around the new INTEGRAL source IGR J19294+1816
ATel #1998; J. Rodriguez (CEA, France), M. Tuerler (ISDC, Switzerland), S. Chaty (CEA & Univ. Paris Diderot, France), J. A. Tomsick (SSL, Berkeley, USA)
on 31 Mar 2009; 11:31 UT
Credential Certification: Jerome Rodriguez (firstname.lastname@example.org)
Subjects: Binary, Black Hole, Neutron Star, Transient
Referred to by ATel #: 2002
Further to the discovery of IGR J19294+1816 by INTEGRAL (ATel #1997)
we analysed Swift archival observations of the field around this new
source, performed on 2007 december 9 and 13.
The Swift/XRT observations reveal a bright source at 4.0 arcmin from the
best INTEGRAL position, outside the 68% error circle given in ATel #1997
still within the 90% confidence limit of IGR J19294+1816.
The best XRT position is RA= 19h 29m 55.9s Dec=+18deg 18' 39"(Â±3.5" at 90%).
We note that the Swift pointings were aimed at a source named
Swift J1929.8+1821 whose name indicates a position consistent with
that of the source seen by the XRT.
This position is inconsistent with that of the Einstein source 2E 1927.5+1805.
No source is seen at the position of the latter (that is close to the border of the
field of view) although a slight extended excess may be present.
There is a single 2MASS point source lying in the XRT error box:
2MASS J19295591+1818382 with infrared magnitudes
J=14.56 Â±0.03, H=12.99 Â±0.02 and Ks=12.11Â±0.02.
This source has no counterpart in the USNO-B1.0 catalogue.
The 0.5-10 keV Swift spectrum is well fitted with an absorbed power law
(chi square 1.0 for 49 d.o.f.), with the following parameters
N_H=3.7 (-0.7/+0.6)e22 cm-2, Gamma=0.8 (+/-0.2), and
a 2-10 keV unabsorbed flux of 3.6e-11 erg/s/cm-2.
A single absorbed black body also gives a good representation of
the spectrum (chi square=0.9 for 49 d.o.f.) with N_H=1.9 (+/-0.3)e22 cm-2,
kT_bb=2.1 (+/-0.2) keV, and an unabsorbed 2-10 keV flux of 3.0e-11 erg/s/cm.
The size of the emitting black body (as returned from the fit) is
R=(0.46 * D_10) km where D_10 is the distance to the source in units of 10 kpc.
A rapid timing analysis of the Swift data reveals the presence of a feature at
8.04 (-0.05 +0.02)e-2 Hz in the power spectrum.
The width of this feature is <1.7e-3 Hz (at 90%) which may indicate
that it is a coherent pulsation. The signal is however noisy and
a folded light curve at the period of this pulsation (12.43 s) does not
reveal any particularity, although a constant does not provide
a good representation of it.
We conclude that IGR J19294+1816 is more likely associated to the swift source
(Swift J1929.8+1821), and not to the Einstein source, although we cannot completely
rule this out.
The X-ray spectrum of the source (fitted with either models) together with the possible
presence of a pulsation at 12.4s would tend to indicate the presence of an accreting
pulsar in an X-ray binary. We note that if the timing feature is not a pulsation, but a
QPO, an X-ray binary is also a plausible explanation.
Clearly more observations are needed to confirm the nature of this source.