Observations of the Nucleus of M 33

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 4.6σ level a previously proposed possible counterpart located 1 away. The X-ray spectrum is well fitted by a disc blackbody with kTin=1.18±0.02 keV and Rin(cos θ)=57±3 km, consistent with earlier results from ASCA and BeppoSAX. The source flux is steady between 1 and 10 s (<3% rms variability, 0.510 keV) with a 0.5-10 keV luminosity of 1.5 × 10 erg s. The X-ray properties and an association with a radio source are reminiscent of the galactic microquasar GRS 1915+105.