An XMM-Newton view of M101 - I. The luminous X-ray source population.

We present the first results of an XMM–Newton EPIC observation of the luminous X-ray source population in the face-on supergiant spiral galaxy M101. We have studied the spectral and temporal properties of the 14 most luminous sources, all of which have intrinsic X-ray luminosities exceeding the Eddington limit for a 1.4-M⊙ neutron star, with a subset in the ultraluminous X-ray source (ULX) regime (LX≥ 1039 erg s−1). Eleven sources show evidence of short-term variability, and most vary by a factor of ∼2–4 over a baseline of 11–24 yr, providing strong evidence that these sources are accreting X-ray binary (XRB) systems. Our results demonstrate that these sources are a heterogeneous population, showing a variety of spectral shapes. Interestingly, there is no apparent spectral distinction between those sources above and below the ULX luminosity threshold. Nine sources are well fitted with either simple absorbed disc blackbody or power-law models. However, in three of the four sources best fitted with power-law models, we cannot exclude the disc blackbody fits and therefore conclude that, coupled with their high luminosities, eight out of nine single-component sources are possibly high-state XRBs. The nuclear source (XMM-10) has the only unambiguous power-law spectrum (Γ∼ 2.3), which may be evidence for the presence of a low-luminosity active galactic nucleus (LLAGN). The remaining five sources require at least two-component spectral fits, with an underlying hard component that can be modelled by a power-law continuum or, in three cases, a hot disc blackbody (Tin = 0.9–1.5 keV), plus a soft component modelled as a cool blackbody/disc blackbody/thermal plasma. We have compared the spectral shapes of nine sources covered by both this observation and an archival 100-ks Chandra observation of M101; eight show behaviour typical of Galactic XRBs (i.e. softening with increasing luminosity), the only exception being a transient source (XMM-2) which shows little change in spectral hardness despite a factor of ∼30 increase in luminosity. We find no definitive spectral signatures to indicate that these sources contain neutron star primaries, and conclude that they are likely to be stellar-mass black hole XRBs (BHXBs), with black hole masses of ∼2–23 M⊙ if accreting at the Eddington limit

Chandra monitoring observations of the ultraluminous X-ray source NGC 5204 X-1.

We report the results of a two-month campaign conducted with the Chandra X-ray observatory to monitor the ultraluminous X-ray source (ULX) NGC 5204 X-1. This was composed of a 50-ks observation, followed by ten 5-ks follow-ups spaced initially at ∼3, then at ∼10-d intervals. The ULX flux is seen to vary by factors ∼5 on time-scales of a few days, but no strong variability is seen on time-scales shorter than an hour. There is no evidence for a periodic signal in the X-ray data. An examination of the X-ray colour variations over the period of the campaign shows the ULX emission consistently becomes spectrally harder as its flux increases. The X-ray spectrum from the 50-ks observation can be fitted by a number of disparate spectral models, all of which describe a smooth continuum with, unusually for a ULX, a broad emission feature evident at 0.96 keV. The spectral variations, both within the 50-ks observation and over the course of the whole campaign, can then be explained solely by variations in the continuum component. In the context of an optically thick corona model (as found in other recent results for ULXs) the spectral variations can be explained by the heating of the corona as the luminosity of the ULX increases, consistent with the behaviour of at least one Galactic black hole system in the strongly Comptonized very high state. We find no new evidence supporting the presence of an intermediate-mass black hole in this ULX

An XMM-Newton view of M101 - II. Global X-ray source properties.

We present the global X-ray properties of the point source population in the grand-design spiral galaxy M101, as seen with XMM–Newton. 108 X-ray sources are detected within the D25 ellipse of M101, of which ∼24 are estimated to be background galaxies. Multiwavelength cross-correlations show that 20 sources are coincident with HII regions and/or supernova remnants (SNRs), seven have identified/candidate background galaxy counterparts, six are coincident with foreground stars and one has a radio counterpart. While the spectral and timing properties of the brightest sources were presented by Jenkins et al., here we apply an X-ray colour classification scheme to split the entire source population into different types, i.e. X-ray binaries (XRBs), SNRs, absorbed sources, background sources and supersoft sources (SSSs). Approximately 60 per cent of the population can be classified as XRBs, although there is source contamination from background active galactic nuclei (AGN) in this category as they have similar spectral shapes in the X-ray regime. 15 sources have X-ray colours consistent with SNRs, three of which correlate with known SNR/HII radio sources. Another two are promising new candidates for SNRs, one is unidentified, and the remainder are a mixture of foreground stars, bright soft XRBs and AGN candidates. We also detect 14 candidate SSSs, with significant detections in the softest X-ray band (0.3–1keV) only. 16 sources display short-term variability during the XMM–Newton observation, twelve of which fall into the XRB category, giving additional evidence of their accreting nature. Using archival Chandra and ROSAT High Resolution Imager data, we find that ∼40 per cent of the XMM sources show long-term variability over a baseline of up to ∼10 yr, and eight sources display potential transient behaviour between observations. Sources with significant flux variations between the XMM and Chandra observations show a mixture of softening and hardening with increasing luminosity. The spectral and timing properties of the sources coincident with M101 confirm that its X-ray source population is dominated by accreting XRBs