Swift/XRT observations of IGR J05255-0711 and IGR J16413-4046

ATel #2731; R. Landi, E. Maiorano (INAF/IASF Bologna), M. Fiocchi, A. Bazzano (INAF/IASF Rome), A. J. Bird (Univ. Southampton), N. Gehrels (NASA/GSFC), D. N. Burrows (PSU)
on 12 Jul 2010; 14:54 UT
Credential Certification: Raffaella Landi (landi@iasfbo.inaf.it)

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We report on results of X-ray follow-up observations obtained with Swift/XRT of two unidentified INTEGRAL sources listed in the 4th IBIS Survey Catalogue (Bird et al. 2010, ApJS, 186, 1): IGR J05255-0711 and IGR J16413-4046.

IGR J05255-0711
XRT pointed at the source region on 2010, April 27 and May 01 for a total exposure of 6.9 ks. Three sources compatible with the IBIS error circle were detected by XRT, and are shown as crosses over-laid on an NVSS image of the field (XRT sources are #1, #2 and #3 in the figure).

Source #1: this source, located at RA(J2000) = 05h 25m 12.4s and Dec(J2000) = -07d 09m 28.0s (6 arcsec uncertainty), is detected at 2.8 sigma c.l. in 0.3-10 keV band and disappears above 3 keV. This object has a moderately bright radio counterpart (NVSS J052513-070935) with a 20 cm flux of 185.43 mJy. No optical or infrared counterpart is found for this object. Unfortunately, because of the low statistical quality of the X-ray data, we can only infer a 2-10 keV flux of 6.7 x 10^-14 erg cm^-2 s^-1, assuming a simple power law model with the photon index fixed at 1.8.

Source #2: its location (RA(J2000) = 05h 25m 09.83s and Dec(J2000) = -07d 07m 48.38s, 4.6 arcsec uncertainty) is outside the 90% error box, but compatible with the 99% IBIS uncertainty. It is detected at 7 sigma c.l. in 0.3-10 keV and it is still visible at E > 3 keV with a 2.5 sigma c.l.. Also this objects has a moderately bright radio counterpart (NVSS J052509-070746) with a 20 cm flux of 177.22 mJy. Within the XRT uncertainty, we find a USNO-A2.0 counterpart having magnitude R ~ 18.9; this source is not listed in the 2MASS catalogue. The X-ray spectroscopy indicates a steep power law spectrum (Gamma ~2.2) and a 2-10 keV flux of 1.6 x 10^-13 erg cm^-2 s^-1.

Source #3: the source is located at RA(J2000) = 05h 25m 32.80s and Dec(J2000) = -07d 13m 06.2s (6 arcsec uncertainty) and is detected at 2.5 sigma c.l. in 0.3-10 keV, disappearing above 3 keV. No counterparts are found at radio, infrared and optical wavelengths.

These information, along with the fact that the IBIS source is detected only during a revolution (n. 478) and hence indicating a variable nature for this object, suggest that the hard XRT source could be the possible X-ray counterpart.

Further deeper X-ray observations are encouraged in order to investigate the real nature of this source.

IGR J16413-4046
XRT pointed at the source region on 2010, June 14 for 1.4 ks. Within the IBIS error circle, we find a hard (3-10 keV) X-ray source detected only at 2.5 sigma c.l., which disappears at soft energies (0.3-3 keV); it is located at RA(J2000) = 16h 41m 19.31s and Dec(J2000) = -40d 47m 31.84s (7.7 arcsec uncertainty). The XRT location is compatible with a USNO-A2.0 object having magnitude R ~15.6; this object is also listed in the 2MASS survey with magnitudes I ~13.65, H ~13.23 and K ~13.27. Unfortunately, because of the low signal-to-noise ratio of the source, we can only infer a 2-10 keV flux of 6.2 x 10^-14 erg cm^-2 s^-1, assuming a simple power law model with the photon index fixed at 1.8.

Both XRT and IBIS instrument show a weak and persistent source, nevertheless, to better investigate its nature we suggest further deeper X-ray observations.

NVSS image of IGR J05255-0711