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We report on the discovery and analysis of the transient X-ray pulsarCXO/XMMU J073709.13+653544 detected in the 2004 August-October Chandra and XMMobservations of the nearby spiral galaxy NGC2403. The source exhibits X-raypulsations with a period P~18 s, a nearly sinusoidal pulse shape and pulsedfraction 46-70%. The source shows a rapid decrease of the pulsation period from18.25 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 the source are hard and are well fitted with an absorbedsimple power law of photon index 0.9~1.2. The X-ray properties of the sourceand the absence of an optical/UV counterpart allow us to identify CXO/XMMUJ073709.13+653544 as accreting X-ray pulsar in NGC2403. The maximum unabsorbedluminosity of the source in the 0.3-7 keV range, ~2.6x10e38 erg/s at 3.2 Mpc,is at least 260 times higher than its quiescent luminosity. The correspondingluminosity in the 0.3-100 keV energy range could be as high as ~1.2x10e39erg/s, assuming the typical pulsar energy spectrum with high-energy cut-off at10-20 keV. The rate of decrease of the pulsation period of the source (-10e-7s/s) is one of the fastest observed among accreting pulsars. The evolution ofthe pulsation period suggests that it is dominated by the intrinsic spin-up ofthe compact object. The accretion rate implied by X-ray luminosity of CXO/XMMUJ073709.13+653544 could account for the observed spin-up rate, assuming thatthe X-ray source is powered by disk accretion onto highly magnetized neutronstar. Based on the transient behavior and overall X-ray properties of thesource, we conclude that it could be an X-ray pulsar belonging to either a Bebinary system or a low-mass system similar to GRO J1744-28.Comment: Accepted for publication in ApJ. 14 pages, 6 figures, uses emulateapj  style. Updated to match the accepted versio

ChandraandXMM‐NewtonDiscovery of Transient X‐Ray Pulsar in the Nearby Spiral Galaxy NGC 2403