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, similar to50 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(-0.03)(+0.04) 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 L-X,L-stellar/L-B between the galaxies appears to be because of a lack of very bright sources in NGC 1291. No sources above 3x10(38) ergs s(-1) were found in NGC 1291 when similar to7 were expected from scaling from NGC 4697 and NGC 1553. The cumulative L-X,L-stellar/L-B value including only sources below 1.0x10(38) 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) x 10(39) 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.

Description
Keywords
binaries : close, galaxies : ISM, X-rays : galaxies, X-rays : ISM, X-rays : stars, COLD GAS, ELLIPTICAL GALAXIES, LENTICULAR GALAXIES, SPECTRAL PROPERTIES, GLOBULAR-CLUSTERS, ATOMIC-HYDROGEN, ROSAT, HOT, MECHANISMS, SAMPLE, Astronomy & Astrophysics
Citation
Irwin, J., Sarazin, C., Bregman, J. (2002): An Ultraluminous X-Ray Object with a 2 Hour Period in M51. The Astrophysical Journal, 570(1). DOI: 10.1086/339734