X-RAY-SPECTRUM OF THE SPIRAL GALAXY M51 (NGC 5194)

From a ∼1 deg2 sky region containing the spiral galaxy M51 (NGC 5194), an X-ray flux of (1.1 ± 0.6) x 10-11 ergs s-1 cm-2 was detected with the Ginga observatory in the 2-20 keV band. The observed spectrum is very hard and bears no low-energy absorption. It can be fitted either with a power-law model of photon index α = 1.43 ± 0.08, or a thermal bremsstrahlung model of temperature kT > 28 keV. The emission, presumably coming from M51 itself, infers a 2-20 keV luminosity of (1.2 ± 0.6) x 1041 ergs s-1 at the source distance of 9.6 Mpc. It is suggested that a significant fraction of the observed X-ray emission originates in a weak Seyfert-like nucleus of M51.