X‐Ray Source Population in the Elliptical Galaxy NGC 720 withChandra

With a Chandra ACIS-S3 observation, we detect 42 X-ray point sources in theelliptical galaxy NGC 720, including a possible central source. Most of thesesources will be low-mass X-ray binaries (LMXBs), and 12 are located within 2"of globular cluster candidates. We investigate both the hardness ratios andcombined spectra of the sources. They exhibit a distribution of X-ray colorssimilar to those seen in other early-type galaxies. We find that there is apopulation of highly absorbed sources located at large distances from thecenter of the galaxy. The overall spatial distribution of sources is consistentwith the ellipticity and position angle of the galaxy, but the sources appearto form several arcs. NGC 720 contains nine ultraluminous sources (L_x >= 10^39ergs/s). This number is more than have previously been detected in anearly-type galaxy but similar to the number seen in the Antennae merger system.The ratio L_ULX/L_B for NGC 720 is more than double the ratio for the S0 galaxyNGC 1553 and a factor of seven higher than for the elliptical galaxy NGC 4697,although uncertainties in the distance and the source spectral properties couldbring these ratios into agreement. The X-ray source luminosity function is alsonearly as flat as those seen in disk and merger systems. The large number ofULXs in NGC 720 and its relatively flat XLF may provide evidence against theassociation of all ULXs with young stars. We also see a possible highluminosity break in the luminosity function at 2x10^39 h_50^-2 ergs/s.Comment: 28 pages, 7 figures, accepted for publication in Ap