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 \sim 18 s and a nearly sinusoidal pulse shape and pulsed fraction 46-70 % during the first three observations .
A detailed timing analysis reveals a rapid decrease of the pulsation period from 18.25 s on Aug. 9 to 17.93 s on Sep. 12 and possibly 17.56 s on Oct. 3 , 2004 .
The X-ray spectra of CXOU J073709.1+653544 are hard and are well fitted with an absorbed simple power law of photon index \Gamma \sim 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 ^ { th } magnitude allow us to identify CXOU J073709.1+653544 as accreting X-ray pulsar in NGC 2403 .
The maximum unabsorbed luminosity of the source in the 0.3-7 keV range , { L } _ { X } \sim 2.6 \times 10 ^ { 38 } ergs s ^ { -1 } at 3.2 Mpc , is at least 260 times higher than its quiescent luminosity , and exceeds the isotropic Eddington limit for a 1.4 { M } _ { \odot } compact object accreting hydrogen-rich material .
The corresponding estimated luminosity in the 0.3-100 keV energy range could be as high as \sim 1.2 \times 10 ^ { 39 } ergs s ^ { -1 } , assuming the typical pulsar energy spectrum with high-energy cut-off at 10-20 keV .
The rate of decrease of the pulsation period of the source ( \dot { P } \sim - 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 highly magnetized neutron star .
Based on the transient behavior and overall X-ray properties of the source , we conclude that 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 .