XMM-Newton View of the Ultraluminous X-Ray Sources in M51

摘要

We present results based on XMM-Newton observations of the nearby spiral galaxy M51 (NGC 5194 and NGC 5195). We confirm the presence of the seven known ultraluminous X-ray sources (ULXs) with luminosities exceeding the Eddington luminosity for a 10 M☉ black hole, a low-luminosity active galactic nucleus (LLAGN) with 2-10 keV luminosity of 1.6 × 1039 ergs s-1, and soft thermal extended emission from NGC 5194 detected with Chandra. In addition, we also detected a new ULX with luminosity of ~1039 ergs s-1. We have studied the spectral and temporal properties of the LLAGN and eight ULXs in NGC 5194 and an ULX in NGC 5195. Two ULXs in NGC 5194 show evidence for short-term variability, and all but two ULXs vary on long timescales (over a baseline of ~2.5 yr), providing strong evidence that these are accreting sources. One ULX in NGC 5194, source 69, shows possible periodic behavior in its X-ray flux. We derive a period of 5925 ± 200 s at a confidence level of 95% on the basis of three cycles. This period is lower than the period of 7620 ± 500 s derived from a Chandra observation in 2000. The higher effective area of XMM-Newton enables us to identify multiple components in the spectra of ULXs. Most ULXs require at least two components, a power law and a soft X-ray excess component that is modeled by an optically thin plasma or a multicolor disk blackbody (MCD). However, the soft excess emissions inferred from all ULXs except source 69 are unlikely to be physically associated with the ULXs, as their strengths are comparable to that of the surrounding diffuse emission. The soft excess emission of source 69 is well described either by a two-temperature MEKAL plasma or a single-temperature MEKAL plasma (kT ~ 690 eV) and an MCD (kT ~ 170 eV). The MCD component suggests a cooler accretion disk compared to those in Galactic X-ray binaries, consistent with those expected for intermediate-mass black holes (IMBHs). An iron Kα line (EW ~ 700 eV) or K absorption edge at ~7.1 keV is present in the EPIC pn spectrum of source 26. The spectrum of the ULX in NGC 5195, source 12, is consistent with a simple power law. The LLAGN in NGC 5194 shows an extremely flat hard X-ray power law (Γ ~ 0.7), a narrow iron Kα line at 6.4 keV (EW ~ 3 keV), and strong soft X-ray excess emission. The full-band spectrum is well described by a two-component MEKAL plasma and reflection from cold material such as a putative torus.