Chandra observations of the nucleus of M33

The nearby galaxy M33 hosts the most luminous steady X-ray source in the Local Group. The high spatial resolution of Chandra allows us to confirm that this ultra-luminous X-ray source is within the nucleus and rule out at the 4.6sigma level a previously proposed possible counterpart located 1 arcsec away. The X-ray spectrum is well fitted by a disc blackbody with kT(in) = 1.18 +/- 0.02 keV and R-in(cos theta)(0.5) = 57 +/- 3 km, consistent with earlier results from ASCA and BeppoSAX. The source flux is steady between 1 and 10(4) s (<3 per cent rms variability, 0.5-10 keV) with a 0.5-10 keV luminosity of 1.5 x 10(39) erg s(-1). The X-ray properties and an association with a radio source are reminiscent of the galactic microquasar GRS 1915+105.