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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
Password Certification: Raffaella Landi (email@example.com)
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 cm-2 s-1.
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 XRT 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 firm conclusion.
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 cm-2 s-1.
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 and Dec(J2000) = -65d 02m 23.2s (4.2 arcsec uncertainty) and is associated with the ROSAT Faint Survey source 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.