We present the results of two Chandra observations ( separated by 1 year ) of the population of X-ray sources in the spiral galaxy M51 ( NGC 5194 and NGC 5195 ) .
113 X-ray sources have been detected in an 8 \farcm 4 \times 8 \farcm 4 ( 20.4 \times 20.4 kpc ) region , and 84 and 12 of them project within the disks of NGC 5194 and NGC 5195 , respectively .
Nine and 28 sources have luminosities exceeding 1 \times 10 ^ { 39 } ergs s ^ { -1 } ( ultraluminous X-ray sources or ULXs ) and 1 \times 10 ^ { 38 } ergs s ^ { -1 } in the 0.5 – 8 keV band , respectively , assuming they are associated with M51 .
The number of ULXs is much higher than found in most normal spiral and elliptical galaxies .
Most of the X-ray sources and all seven of the ULXs in NGC 5194 are located in , or close to , a spiral arm , suggesting a connection with recent star formation .
The cumulative luminosity function of the X-ray sources in NGC 5194 with L ( 0.5 – 8 keV ) > 
10 ^ { 38 } ergs s ^ { -1 } is well described by a power law N ( > L ( 0.5 – 8 keV ) ) \propto L ( 0.5 – 8 keV ) ^ { - \alpha } with \alpha = 0.91 .
The X-ray spectra of most of the detected sources are consistent with a power law with a photon index between 1 and 2 , with a few sources showing harder or softer spectra .
The spectra of most ULXs are consistent with both a power law and a multicolor disk blackbody ( MCD ) model , while a power law model is preferable to a MCD model in two ULXs .
One ULX ( NGC 5194 # 69 ) shows drastic spectral steepening accompanied by a decline in luminosity by a factor of 3460 in the 2 – 10 keV band between the two observations .
This source also exhibited a possible period of 2.1 hrs in the year 2000 observation .
Another ULX ( NGC 5194 # 26 ) shows strong emission lines from highly ionized species .
The masses of the compact objects and mass accretion rates in ULXs and other X-ray sources are not well constrained by these observations .
If we adopt a MCD interpretation , their MCD parameters imply that most of the X-ray sources are stellar mass ( \sim 5 – 10 M _ { \odot } ) black holes accreting near or above the Eddington rate , although other possibilities ( intermediate-mass black holes and relativistically beamed emission ) can not be excluded .
The power law sources may , instead , represent Comptonized disk , or nonthermal , emission .
Two ULXs have very soft spectra ; MCD models require kT \approx 0.1 keV .
We discuss the possibility that this soft emission originates in an accretion disk around an intermediate-mass black hole .
We also present a study of the nucleus of , and discrete sources ( including two ULXs ) in , the companion galaxy NGC 5195 .