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 4.6σ level a previously proposed possible counterpart located 1 arcsec away. The X-ray spectrum is well fitted by a disc blackbody with kTin = 1.18 ± 0.02 keV and Rin(cos θ ) 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 × 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.

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