Search for counterparts of newly discovered INTEGRAL/IBIS sources

ATel #4166; R. Landi, L. Bassani, N. Masetti (INAF - IASF Bologna), A. Bazzano, P. Ubertini (INAF - IAPS Rome), A. J. Bird, M. Goossens (University of Southampton)
on 12 Jun 2012; 15:45 UT
Credential Certification: Raffaella Landi (landi@iasfbo.inaf.it)

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With respect to the recent INTEGRAL/IBIS 9-year Galactic Hard X-ray Survey (Krivonos et al. 2012, arXiv:1205.3941), we used archival Swift/XRT data to find likely counterparts for two hard X-ray sources newly detected by INTEGRAL: SWIFT J0958.0-4208 and IGR J22534+6243.

SWIFT J0958.0-4208
This source is also reported in the Swift/BAT 58-month catalogue (http://heasarc.nasa.gov/docs/swift/results/bs58mon/). Within the hard X-ray positional error circles we find a ROSAT bright source 1RXS J095750.4-420801. XRT pointed at the region surrounding SWIFT J0958.0-4208 for a total exposure of ~21.3 ks. There are two X-ray objects within the IBIS/BAT error circles:

Source #1: This is the brightest of the two detections and the closest to the ROSAT source, although the relative positional uncertainties do not intersect. It is seen at 31 and 23 sigma confidence level (c.l.) in the 0.3-10 keV energy range and above 3 keV, respectively. The XRT position is RA(J2000) = 09h 57m 50.66s and Dec(J2000) = -42d 08m 37.5s (3.6 arcsec uncertainty, at 90% c.l.). There are three USNO-B1.0 objects falling within the XRT positional uncertainty and one 2MASS source (2MASS J09575064-4208355), with magnitudes J ~15.4, H ~14.9, and K ~14.9.

The X-ray data are well modelled using a black body component (kT ~0.1 keV) and a power law having a flat (Gamma ~0.77) spectrum (N_H(Gal) = 1.65 x 10²¹ cm^-2). The observed 2-10 keV flux is ~4.7 x 10^-12 erg cm^-2 s^-1.

Source #2: This second source is detected at 10.4 and 4.8 sigma c.l. in the 0.3-10 keV energy range and above 3 keV, respectively. It is located at RA(J2000) = 09h 57m 40.71s and Dec(J2000) = -42d 08m 44.6s (4.0 arcsec uncertainty, at 90% c.l.). Within the XRT positional error, we find a USNO-B1.0 source (USNO-B1.0 0478-0250012) with magnitudes B ~19.7 and R ~17.6-19.3.

The X-ray spectrum is fitted with a power law (N_H(Gal) = 1.66 x 10²¹ cm^-2) having photon index Gamma ~1.5 and an observed 2-10 keV flux of ~2.8 x 10^-13 erg cm^-2 s^-1.

Although source #1 is probably the more likely counterpart to SWIFT J0958.0-4208, we cannot exclude at this stage contamination from the second object.

IGR J22534+6243
In this case, there is an X-ray source compatible with the 90% IBIS error circle, serendipitously detected during the Swift/XRT pointing of the gamma ray burst GRB060421. It has coordinates RA(J2000) = 22h 53m 55.23s and Dec(J2000) = +62d 43m 38.0s (3.5 arcsec uncertainty, at 90% c.l.), and is detected at 45 and 35 sigma c.l. in the 0.3-10 keV energy range and above 3 keV, respectively. It is associated with the ROSAT faint source 1RXS J225352.8+624354, which is still unidentified. Within the XRT positional uncertainty we find a USNO-B1 object (USNO-B1 1527-0428738) having magnitudes R ~13 and B~ 15.5, which is also listed in the 2MASS catalogue (2MASS J22535512+6243368), with magnitudes J ~11.6, H ~11.0, and K ~10.5.

The X-ray spectrum is well modelled with an absorbed (N_H(Gal) = 8.96 x 10²¹ cm^-2, N_H(intr) ~1.1 x 10²² cm^-2) power law (Gamma ~1.4) having an observed 2-10 keV flux of ~3.4 x 10^-12 erg cm^-2 s^-1.

Given the location of both sources on the Galactic plane, their X-ray spectral properties, and the lack of any evident radio counterparts we favour a Galactic nature for both objects.

We acknowledge the use of public data from the Swift data archive.