AnXMM-Newtonview of M101 - I. The luminous X-ray source population

Abstract

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.