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Compact X-ray Sources in Nearby Galaxy Nuclei

E. J. M. Colbert·R. F. Mushotzky
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Abstract

We have found compact, near-nuclear X-ray sources in 21 (54\%) of a complete sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT X-ray luminosities (0.2 $-$ 2.4 keV) of these compact X-ray sources are $\sim$10$^{37}$$-$10$^{40}$ erg~s$^{-1}$. The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is $\sim$390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep ($Γ\approx$ 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft (high) state. ASCA data from the elliptical galaxies indicate that hot (kT $\approx$ 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N$_H \approx$ 10$^{21}$ cm$^{-2}$) were found to the power-law component indicates that these objects are somehow geometrically and/or physically different from AGNs in normal active galaxies. The X-ray sources in the spiral galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be $\sim$10$^2$$-$10$^3$ M$_\odot$. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.

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E. J. M. Colbert, R. F. Mushotzky. 1998-09-11. Compact X-ray Sources in Nearby Galaxy Nuclei. https://doi.org/10.1016/s0273-1177(99)00231-8

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