XMM-NEWTON OBSERVATION OF DIFFUSE GAS AND LOW-MASS X-RAY BINARIES IN THE ELLIPTICAL GALAXY NGC 4649 (M60)

摘要

We present an XMM-Newton X-ray observation of the X-ray-bright E2 elliptical galaxy NGC 4649. In addition to bright diffuse emission, we resolve 158 discrete sources, ～50 of which are likely to be LMXBs associated with NGC 4649. We find evidence for variability in three sources between this observation and a previous Chandra observation. Additionally, we detect five sources that were not detected with Chandra despite its better detection limit, suggesting that these sources have since brightened. The total X-ray spectrum of the resolved sources is well fit by a hard power law, while the diffuse spectrum requires a hard and a soft component, presumably due to the relatively soft diffuse gas and the harder unresolved sources. A deprojection of the diffuse emission revealed a radial temperature gradient that is hot in the center, drops to a minimum at about 20″-50″ (1.6-4.1 kpc), and rises again in the outer regions. The diffuse emission appears to require a two-temperature model with heavy-element abundance ratios that differ from the solar values. We have verified the existence of faint radial features extending out from the core of NGC 4649 that had previously been seen with Chandra. The fingers are morphologically similar to radial features seen in hydrodynamic simulations of cooling flows in elliptical galaxies, and although their other properties do not match the predictions of the particular simulations used, we conclude that the radial fingers might be due to convective motions of hot outflowing gas and cooler inflowing gas. We also find evidence for a longer, previously undetected filament that extends to the northeastern edge of NGC 4649. The diffuse gas in the region of the filament appears to have a lower temperature and may also have a higher abundance as compared to nearby regions. There also appears to be an excess of X-ray sources along the filament, although the excess is not statistically significant. We conclude that the filament may be the result of a tidal interaction, possibly with NGC 4647, although more work is necessary to verify this conclusion.