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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
Password Certification: P.R. den Hartog (Hartog@sron.nl)

Subjects: X-ray, Gamma Ray, Binaries, Neutron Stars, 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.