Chandra observations of the nucleus of M33

Abstract

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 kT_in= 1.18 +/- 0.02 keV and R_in(cosθ)^0.5= 57 +/- 3 km, consistent with earlier results from ASCA and BeppoSAX. The source flux is steady between 1 and 10⁴ s (<3 per cent rms variability, 0.5-10 keV) with a 0.5-10 keV luminosity of 1.5 × 10³⁹ erg s^-1. The X-ray properties and an association with a radio source are reminiscent of the galactic microquasar GRS 1915+105.