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 in the elliptical galaxy NGC 720, including a possible central source. Most of these sources will be low-mass X-ray binaries, and 12 are located within 2'' of globular cluster candidates. We investigate both the hardness ratios and combined spectra of the sources. They exhibit a distribution of X-ray colors similar to those seen in other early-type galaxies. We find that there is a population of highly absorbed sources located at large distances from the center of the galaxy. The overall spatial distribution of sources is consistent with the ellipticity and position angle of the galaxy, but the sources appear to form several arcs. NGC 720 contains nine ultraluminous sources (ULXs; L[subscript X] ≥ 10[superscript 39] ergs s[superscript -1]). This number is more than have previously been detected in an early-type galaxy but similar to the number seen in the Antennae merger system. The ratio L[subscript ULX][superscript IL][subscript B] for NGC 720 is more than double the ratio for the S0 galaxy NGC 1553 and a factor of 7 higher than for the elliptical galaxy NGC 4697, although uncertainties in the distance and the source spectral properties could bring these ratios into agreement. The X-ray source luminosity function is also nearly as flat as those seen in disk and merger systems. The large number of ULXs in NGC 720 and its relatively flat X-ray luminosity function may provide evidence against the association of all ULXs with young stars. We also see a possible high-luminosity break in the luminosity function at 2 × 10[superscript 39]h[-2 over 50] ergs s[superscript -1].