Chandra and XMM-Newton Discovery of Transient X-Ray Pulsar in the Nearby Spiral Galaxy NGC 2403

We report on the discovery and analysis of the transient X-ray pulsar CXOU J073709.1+653544 detected in the 2004 August-October Chandra and XMM-Newton observations of the nearby spiral galaxy NGC 2403. The X-ray source exhibits X-ray pulsations with a period P~18 s and a nearly sinusoidal pulse shape and pulsed fraction 46%-70% during the first three observations. The observed pulsation period decreased rapidly from 18.25 s on August 9 to 17.93 s on September 12 and possibly 17.56 s on 2004 October 3. The X-ray spectra of CXOU J073709.1+653544 are hard and are well fitted with an absorbed simple power law of photon index Γ~0.9-1.2 in the 0.3-7 keV energy band. The X-ray properties of the source and the absence of an optical/UV counterpart brighter than 20 mag allow us to identify CXOU J073709.1+653544 as an accreting X-ray pulsar in NGC 2403. The maximum unabsorbed luminosity of the source in the 0.3-7 keV range, L_X~2.6×10³⁸ ergs s^-1 at 3.2 Mpc, is at least 260 times higher than its quiescent luminosity. The estimated source luminosity in the 0.3-100 keV energy range could be as high as ~1.2×10³⁹ ergs s^-1. The rate of decrease of the pulsation period of the source (P˙~-10^-7 s s^-1) is one of the fastest observed among accreting pulsars. The evolution of the pulsation period suggests that it is dominated by the intrinsic spin-up of the compact object, which is almost certainly a neutron star. The X-ray luminosity of CXOU J073709.1+653544 is high enough to account for the observed spin-up rate, assuming that the X-ray source is powered by disk accretion onto a highly magnetized neutron star. Based on the transient behavior and overall X-ray properties of the source, it could be an X-ray pulsar belonging to either a Be binary system or a low-mass system similar to the transient Galactic bursting pulsar GRO J1744-28.