Untangling the X‐Ray Emission from the Sa Galaxy NGC 1291 with Chandra

Abstract

We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa galaxy NGC 1291. The X-ray emission from the bulge resembles the X-ray emission from a subclass of elliptical and S0 galaxies with low X-ray-to-optical luminosity ratios. The X-ray emission is composed of a central pointlike nucleus, ~50 point sources that are most likely low-mass X-ray binaries (LMXBs), and diffuse gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global temperature of 0.32+0.04-0.03 keV and metallicity of 0.06+/-0.02 solar, and both quantities marginally decrease with increasing radius. The hot gas fills the hole in the H I distribution, and the softening of the spectrum of the X-ray gas with radius might indicate a thermal coupling of the hot and cold phases of the interstellar medium as previously suggested. The integrated X-ray luminosity of the LMXBs, once normalized by the optical luminosity, is a factor of 1.4 less than in the elliptical galaxy NGC 4697 or S0 galaxy NGC 1553. The difference in LX,stellar/LB between the galaxies appears to be because of a lack of very bright sources in NGC 1291. No sources above 3×1038 ergs s-1 were found in NGC 1291 when ~7 were expected from scaling from NGC 4697 and NGC 1553. The cumulative LX,stellar/LB value including only sources below 1.0×1038 ergs s-1 is remarkably similar between NGC 1291 and NGC 4697, if a recent surface brightness fluctuation-determined distance is assumed for NGC 4697. If this is a common feature of the LMXB population in early-type systems, it might be used as a distance indicator. Finally, a bright, variable [(1.6-3.1)×1039 ergs s-1] source was detected at the optical center of the galaxy. Its spectrum shows excess soft emission superposed on a highly absorbed power-law component, similar to what has been found in several other low-luminosity active galactic nuclei. However, the soft component does not vary in intensity like the hard component, indicating that the soft component is not reprocessed hard component emission.