Chandra Observation of Diffuse Gas and LMXBs in the Elliptical Galaxy NGC 4649 (M60)

摘要

We present a Chandra X-ray observation of the X-ray bright E2 ellipticalgalaxy NGC4649. In addition to bright diffuse emission, we resolve 165 discretesources, most of which are presumably low-mass X-ray binaries (LMXBs). As foundin previous studies, the luminosity function of the resolved sources iswell-fit by a broken power-law. In NGC4697 and NGC1553, the break luminositywas comparable to the Eddington luminosity of a $1.4 M_{odot}$ neutron star.One possible interpretation of this result is that those sources withluminosities above the break are accreting black holes and those below aremainly accreting neutron stars. The total X-ray spectrum of the resolvedsources is well-fit by a hard power-law, while the diffuse spectrum requires ahard and a soft component, presumably due to the relatively soft diffuse gasand the harder unresolved sources. We also find evidence for structure in thediffuse emission near the center of NGC4649. Specifically, there appear to bebright "fingers" of emission extending from the center of the galaxy and a 5arcsec long bar at the center of the galaxy. The fingers are morphologicallysimilar to radial features seen in two-dimensional hydrodynamic simulations ofcooling flows in elliptical galaxies, and although their other properties donot match the predictions of the particular simulations used we conclude thatthe radial fingers might be due to convective motions of hot outflowing gas andcooler inflowing gas. The bar is coincident with the central extended radiosource; we conclude that the bar may be caused by weak shocks in the diffusegas from an undetected low-luminosity active galactic nucleus (AGN).