XMM-Newton discovery of transient 285.4 s X-ray pulsar XMMU
  J013359.5+303634 in M33
  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 fit
with an absorbed simple power law model of photon index Gamma~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 ~1.4e37 ergs/s, 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 behavior and possible early B class
optical counterpart, XMMU J013359.5+303634 can be classified as another
extragalactic Be/X-ray binary candidate.
