Using the Bulge of M31 as a Template for the Integrated X-Ray Emission from Low-Mass X-Ray Binaries
Abstract
We have performed joint ASCA+ROSAT PSPC spectral fitting on the inner 5' of the bulge of M31. We find that single-component spectral models provide an inadequate fit to the spectrum, in contrast to previous studies by Einstein and Ginga. Although the 2-10 keV spectrum can be fitted adequately with a bremsstrahlung model with kT=7.4+/-0.3 keV, an additional soft component with kT=0.38+/-0.03 keV is required to fit the spectrum below 2 keV. This soft component comprises 38%+/-6% of the total emission in the 0.1-2.0 keV band, and possibly more depending on the absorption value used in the fit. Since previous spatial studies of the bulge of M31 indicate that less than 25% of the X-ray emission from the bulge in this band is from a diffuse gaseous component, this implies that stellar sources, namely, low-mass X-ray binaries (LMXBs), are responsible for some if not all of the soft component. The spectral properties of M31 are very similar to those of the X-ray-faint early-type galaxy NGC 4382. This supports the claim that the unexplained soft X-ray emission seen previously in these galaxies also emanates from LMXBs.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 1999
- DOI:
- 10.1086/308081
- arXiv:
- arXiv:astro-ph/9907153
- Bibcode:
- 1999ApJ...527..125I
- Keywords:
-
- STARS: BINARIES: CLOSE;
- GALAXIES: ELLIPTICAL AND LENTICULAR;
- CD;
- GALAXIES: INDIVIDUAL (M31);
- GALAXIES: ISM;
- X-RAYS: GALAXIES;
- X-RAYS: STARS;
- Stars: Binaries: Close;
- Galaxies: Elliptical and Lenticular;
- cD;
- galaxies: individual (M31);
- Galaxies: ISM;
- X-Rays: Galaxies;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 7 pages, 2 embedded Postscript figures, uses emulateapj.sty, accepted by Astrophysical Journal