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The discovery of a new hard X-ray source in the Norma region by INTEGRAL: IGR J16293-4603

ATel #1774; L. Kuiper (SRON), P. G. Jonker (SRON, CfA), M. A.P. Torres (CfA), A. Rest (CTIO/Harvard University), S. Keek (SRON)
on 10 Oct 2008; 12:18 UT
Credential Certification: Lucien Kuiper (L.M.Kuiper@sron.nl)

Subjects: Optical, X-ray, Gamma Ray, AGN, Binary, Black Hole, Neutron Star

A deep INTEGRAL mosaic of the Norma region centered on PSR J1617-5055 combining IBIS ISGRI data collected over the period March 2, 2003 - Feb. 24, 2006 (INTEGRAL Revolutions 46-411) revealed a new hard X-ray source. We name this source IGR J16293-4603, at R.A. = 16 29 21.4 and Dec = -46 03 50 (J2000). The 3 sigma uncertainty on this position is about 3 arcmin. Fitting the 20-300 keV time averaged flux measurements with a power-law model gives a 20-70 keV flux of (1.12 +/-0.14 )E-11 erg/cm2/s and a photon index of 1.85 +/- 0.30. We observed the IBIS ISGRI error circle on Jan. 24, 2008 with the Chandra ACIS-I instrument for about 1.1 ks. We detected only one source (201 source counts) at RA = 16 29 12.87, DEC = -46 02 50.9 (J2000; error 0.6 arcsec 90% confidence) well within the 2-sigma location confidence contour of IGR J16293-4603.

An absorbed power-law fit to the 0.3-7 keV band spectral data yields a very hard photon index of 1.0 +/- 0.4 with an unabsorbed 0.8-7 keV flux of 5.14E-12 erg/cm2/s and absorbing Hydrogen column density of NH= (0.7 +/- 0.5)E22 cm-2. The soft and hard X-ray spectral properties and the positional coincidence strongly suggest that the Chandra and INTEGRAL source are related.

Optical follow-up observations were obtained with the LDSS3 imaging spectrograph mounted at the 6.5m Magellan telescope on 2008-06-24 01:13 - 01:36 UT. We acquired 180s images (one in the g' and i' bands, three in the r'-band) under seeing conditions of about 1.0 arcsec. Absolute photometric calibration was obtained using observations of a field containing stars calibrated in SDSS. The astrometric scale was defined using 2MASS sources in the field of IGR J16293-4603. The optical images reveal one extended object at the Chandra position. This object is consistent with being at least three unresolved point-like sources. The brightest of these sources falls within the Chandra error circle. PSF-fitting gives g' = 23.38 +/- 0.08, r'= 21.09 +/- 0.02 and i' = 19.45 +/- 0.02 for this optical counterpart candidate.

Current data is consistent with the source being an AGN or a peculiar galactic X-ray binary. We encourage optical follow-up spectroscopy of the optical counterpart candidate to reveal the nature of this source.