X‐Ray Source Population in the Elliptical Galaxy NGC 720 with Chandra

Abstract

With a Chandra ACIS-S3 observation, we detect 42 X-ray point sources inthe elliptical galaxy NGC 720, including a possible central source. Mostof these sources will be low-mass X-ray binaries, and 12 are locatedwithin 2'' of globular cluster candidates. We investigate both thehardness ratios and combined spectra of the sources. They exhibit adistribution of X-ray colors similar to those seen in other early-typegalaxies. We find that there is a population of highly absorbed sourceslocated at large distances from the center of the galaxy. The overallspatial distribution of sources is consistent with the ellipticity andposition angle of the galaxy, but the sources appear to form severalarcs. NGC 720 contains nine ultraluminous sources (ULXs;L_{X}{\gt}=10^{39} ergs s^{-1}). This number ismore than have previously been detected in an early-type galaxy butsimilar to the number seen in the Antennae merger system. The ratioL_{ULX}/L_{B} for NGC 720 is more than double the ratiofor the S0 galaxy NGC 1553 and a factor of 7 higher than for theelliptical galaxy NGC 4697, although uncertainties in the distance andthe source spectral properties could bring these ratios into agreement.The X-ray source luminosity function is also nearly as flat as thoseseen in disk and merger systems. The large number of ULXs in NGC 720 andits relatively flat X-ray luminosity function may provide evidenceagainst the association of all ULXs with young stars. We also see apossible high-luminosity break in the luminosity function at2{\times}10^{39}h^{-2}_{50} ergs s^{-1}.