The Hard X-Ray Emission of NGC 3628 and the Contribution of Low-Luminosity Spiral Galaxies to the Cosmic X-Ray Background
Abstract
We discuss the results of ASCA and ROSA T observations of the spiral galaxy NGC 3628, which has been previously classified as a starburst. However, X-ray variability establishes the origin of the nuclear emission in a single, compact source, but the luminosity of ∼3.4 × 1040 ergs s-1 is ∼270 times too large for sub-Eddington accretion onto a 1 Msun stellar object. Although the ASCA spectrum can be described by a flat power law with a photon index F 1.2, the low statistical quality of the data may conceal a more complex spectrum. However, the observed spectrum appears to be much flatter than those of prototypical starburst galaxies, such as NGC 253. The spectrum is harder than Galactic low-mass X-ray binary or black-hole candidate spectra but compatible with high-mass X-ray binary spectra. If the nuclear source is a low-luminosity active galactic nucleus (AGN), the X-ray properties have more in common with Seyfert 1-1.5 galaxies with moderate absorption (∼ 1022 cm-2) than with heavily obscured Seyfert 2 galaxies. We also present the first X-ray spectral measurements of the off-nuclear source in NGC 3628 and find it has a steeper spectrum, with Γ > 2.0 and an ASCA luminosity of about half that of the nuclear source. In the light of this and other new ASCA observations, we revisit the general problem of the contribution of low-luminosity spiral galaxies to the cosmic X-ray background (CXB) and assess their effect on the parameters of unified AGN models of the CXB as a function of their local luminosity density and X-ray spectral shape.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 1995
- DOI:
- 10.1086/176600
- Bibcode:
- 1995ApJ...455..508Y
- Keywords:
-
- COSMOLOGY: DIFFUSE RADIATION;
- GALAXIES: SPIRAL;
- GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3628;
- GALAXIES: STARBURST;
- X-RAYS: GALAXIES