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 (jrodriguez@cea.fr)

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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, but still within the 90% confidence limit of IGR J19294+1816. The best XRT position is RA= 19h 29m 55.9s Dec=+18deg 18' 39"(±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 ±0.03, H=12.99 ±0.02 and Ks=12.11±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.