XMM–Newton discovery of transient 285.4 s X-ray pulsar XMMU J013359.5+303634 in M33

Abstract

I report on the discovery and analysis of the first transient X-ray pulsar detected in the Local Group galaxy M33. The 2010 July–August deep XMM–Newton observations of M33 fields revealed a new bright X-ray source XMMU J013359.5+303634 exhibiting pulsations with a period P ∼ 285.4 s and pulsed fraction ∼47 per cent in the 0.3–10 keV energy range. The pulse phase averaged spectrum of XMMU J013359.5+303634 is typical of X-ray pulsars and can be fitted with an absorbed simple power-law model of photon index Γ ∼ 1.2 in the 0.3–10 keV energy band. The search for an optical counterpart did not yield any stellar object brighter than 20 mag, suggesting that XMMU J013359.5+303634 is not a Galactic foreground object and almost certainly belongs to M33. Assuming the distance of 817 kpc, the maximum observed luminosity of the source in the 0.3–10 keV energy range is ∼1.4 × 1037 erg s−1, at least 20 times higher than quiescent luminosity. The brightest optical object inside the error circle of XMMU J013359.5+303634 has a visual magnitude of 20.9 and properties consistent with being an early B V star when placed at a distance of M33. Based on the X-ray pulsations and spectrum, transient behaviour and possible early B class optical counterpart, XMMU J013359.5+303634 can be classified as another extragalactic Be/X-ray binary candidate.