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IGR J00370+6122 - A new high-mass X-ray binary

ATel #281; P. R. den Hartog (1), L. M. Kuiper (1), R. H.D. Corbet (2), J. J.M. in 't Zand (1), W. Hermsen (1, 3), J. Vink(1), R. Remillard (4), M. van der Klis(3) (1 SRON Utrecht; 2 NASA/GSFC and USRA; 3 University of Amsterdam; 4 MIT)
on 18 May 2004; 12:53 UT
Credential Certification: P.R. den Hartog (Hartog@sron.nl)

Subjects: X-ray, Gamma Ray, Binary, Neutron Star, Variables

Referred to by ATel #: 282, 285, 312, 939

We report the discovery of a new variable hard X-ray source, IGR J00370+6122, in a 1.2 Ms INTEGRAL observation of the Cassiopeia region in Dec 2003. The source was found twice while analyzing the data in stretches of ~200 ks. It has a maximum significance of 11.2 sigma in an INTEGRAL IBIS/ISGRI map for the 20-60 keV energy band between MJD 53000.29 and 53002.86. The source position is R.A. = 0h 37m 05.4s, Decl. = +61o 22' 13" (J2000) with an estimated 2' accuracy. The spectrum (20-70 keV) is consistent with a power-law model with a photon index 2.7 +/- 0.3. Archival searches yielded a ROSAT X-ray counterpart (1RXS J003709.6+612131), located at 0.86' from the INTEGRAL position, and a ~10th magnitude (B and V) B1Ib supergiant, BD +60 73. The supergiant is within the 17" error radius of the ROSAT source. Newly derived data from the RXTE ASM (1.5-12 keV) show a mean flux of 1 mcrab (after subtracting a 1 mcrab systematic bias) and a periodicity of 15.665 +/- 0.006 days. The folded light curve shows a quiescent level below 1 mcrab and a single peak with peak flux of 3.3 mcrab. The epoch of maximum flux during the INTEGRAL and contemporaneous ASM observations is MJD 53001.7 +/- 0.3. The BeppoSAX Wide Field Cameras detected IGR J00370+6122 on 4 occasions between Jan 28, 1997, and Jul 12, 1999 with one-day averaged fluxes between 3.0+/-0.7 and 14.9+/-2.1 mcrab (2-28 keV). The epochs of all detections fall within 0.1 in orbital phase from the expected epochs of maximum flux as determined from the ASM ephemeris. The candidate optical counterpart and the X-ray periodicity suggest IGR J00370+6122 to be a supergiant high-mass X-ray binary with an orbital period of 15.665 days.