XMM-Newton discovery of transient X-ray pulsar in NGC 1313

Abstract

We report on the discovery and analysis of the transient X-ray pulsar XMMU J031747.5-663010 detected in the 2004 November 23 XMM-Newton observation of the spiral galaxy NGC 1313. The X-ray source exhibits pulsations with a period P ~ 765.6 s and a nearly sinusoidal pulse shape and pulsed fraction ~38 per cent in the 0.3-7 keV energy range. The X-ray spectrum of XMMU J031747.5-663010 is hard and well fitted with an absorbed simple power law of photon index Γ ~ 1.5 in the 0.3-7 keV energy band. The X-ray properties of the source and the absence of an optical/ultraviolet counterpart brighter than 20 mag allow us to identify XMMU J031747.5-663010 as an accreting X-ray pulsar located in NGC 1313. The estimated absorbed 0.3-7 keV luminosity of the source L_X ~ 1.6 × 10³⁹ergs^-1, makes it one of the brightest X-ray pulsars known. Based on the relatively long pulse period and transient behaviour of the source, we classify it as a Be binary X-ray pulsar candidate. XMMU J031747.5-663010 is the second X-ray pulsar detected outside the local group, after transient 18 s pulsating source CXOU J073709.1+653544 discovered in the nearby spiral galaxy NGC 2403.