Swift/XRT follow-up of unidentified INTEGRAL sources (3)
ATel #1538; R. Landi, N. Masetti, A. Malizia (INAF/IASF Bologna), M. Del Santo, A. Tarana (IASF/INAF Roma), A. J. Bird, A. J. Dean (Univ. Southampton), P. Caraveo, F. Senziani (INAF/IASF Milano)
on 20 May 2008; 14:25 UT
Credential Certification: Raffaella Landi (firstname.lastname@example.org)
Subjects: X-ray, Gamma Ray, Request for Observations
Referred to by ATel #: 1540
We report the results of a series of X-ray follow-up observations performed with Swift/XRT of three
unidentified INTEGRAL sources
belonging to the 3rd IBIS Survey Catalogue (Bird et al. 2007, ApJS, 170, 175).
Within the IBIS uncertainty XRT detects an X-ray source, with a statistical significance of
3.3 sigma, located at
RA(J2000) = 09h 25m 47.61s and Dec(J2000) = +69d 27m 51.5s (6.4 arcsec uncertainty).
This object is associated with the 2MASS extended object 2MASX J09254750+6927532, which has a
counterpart in the USNO-B1.0 catalogue with magnitude R = 12.41-13.08.
The X-ray data only provide an estimate of the 2-10 keV flux of 6 x 10-13 erg
The broad-band (0.3-10 keV) XRT image reveals several sources within the IBIS error circle. In order to
search for the likely counterpart of the INTEGRAL object, we extracted the XRT image in the hard X-ray
domain (3-10 keV), finding that only two sources contribute to the hard X-ray emission: Eta Carinae and the
Wolf-Rayet star W25 (HD 93162), also located in the Carina Nebula. While above 3 keV Eta Carinae is the
dominant source over the XRT band, below this energy W25 is brighter. Furthermore, extrapolation of the
spectrum of Eta Carinae and W25 to the 20-100 kev energy band indicates a higher flux in the first case and
a significantly lower flux in the second case with respect to the flux measured by INTEGRAL/IBIS. We
therefore conclude that Eta Carinae is the likely counterpart of the IBIS source (see also Leyder, Walter
& Rauw 2008, A&A 477, L29), although the strong variability of both objects prevents us from reaching a
We find one source within the IBIS uncertainty with a statistical significance of 3.3 sigma.
This object is detected at RA(J2000) = 11h 09m 47.7s and Dec(J2000) = -64d 52m 51.4s (6
arcsec uncertainty) and coincides with a ROSAT Faint Survey object (1RXS J110944.4-645256). Within the
XRT uncertainty we find three USNO-B1.0 objects, two of which also belong to the 2MASS survey.
The X-ray spectrum of this source is very soft with a 2-10 keV flux of ~7 x 10-14 erg
We notice that at about 30 arcsec outside the 90% IBIS error circle
there is another source
detected with a statistical significance of 10.3 sigma, which is located at RA(J2000) = 11h 09m 26.27s
Dec(J2000) = -65d 02m 23.2s (4.2 arcsec uncertainty) and is associated with the ROSAT Faint Survey
1RXS J110927.4-650245. There is no USNO-B1.0 or 2MASS counterpart to this object. The X-ray data are well
described by an unabsorbed power law with a photon index of 1.2 and a 2-10 keV flux of
2.7 x 10-12 erg cm-2 s-1.
Due to its hard X-ray spectrum and higher flux, this source is the most likely counterpart of the
INTEGRAL object even if it nominally lies outside the 90% IBIS error circle of IGR J11098-6457.