We present X-ray grating spectra of the recurrent nova RS Ophiuchi during its 2006 outburst, obtained with XMMNewton and Chandra. For the first month after optical maximum, the X-ray spectrum was hard and dominated by emission lines of H-like and He-like ions. The X-ray luminosity was 2:4 ; 10 36 ergs s À1 in the 0.33Y10 keV range. The spectra indicate a collisionally dominated plasma with a broad range of temperatures and an energy-dependent velocity structure. During an observation obtained in week 4, a soft X-ray flare occurred in which a new system of soft, higher velocity emission lines appeared in the spectrum. Then, during weeks 6Y10, the supersoft continuum of the hot white dwarf atmosphere was the dominant emission component. The X-ray luminosity reached at least 9 ; 10 37 ergs s À1 in the 0.2Y1 keV range, while the intrinsic nebular absorption decreased by a factor of 5 since the first observation. Preliminary model fitting indicates a white dwarf temperature of $800,000 K, and a mass of at least 1.2 M . Therefore, RS Oph may be an important Type Ia supernova progenitor. We show that the data are consistent with mass loss ending before day 54 of the outburst, and nuclear burning ending around day 69. A rapid decay in X-ray luminosity followed after week 10. The X-ray luminosity 5, 7, and 8 months after optical maximum dropped by more than 2 orders of magnitude. The spectra do not appear to be consistent with emission from an accretion disk.

We report results from a spectral and timing analysis of M82 X‐1, one of the brightest known ultraluminous X‐ray sources. Data from a new 105‐ks XMM–Newton observation of M82 X‐1, performed in 2004 April, and of archival RossiXTE observations are presented. A very soft thermal component is present in the XMM spectrum. Although it is not possible to rule out a residual contamination from the host galaxy, modelling it with a standard accretion disc would imply a black hole (BH) mass of ≈103 M⊙. An emission line was also detected at an energy typical for fluorescent Fe emission. The power density spectrum of the XMM observation shows a variable Quasi‐Periodic Oscillation (QPO) at frequency of 113 mHz with properties similar to those discovered by Strohmayer and Mushotzky. The QPO was also found in seven archival RXTE observations, that include those analysed by Strohmayer and Mushotzky, and Fiorito and Titarchuk. A comparison of the properties of this QPO with those of the various types of QPOs observed in Galactic black hole candidates strongly suggests an association with the type‐C, low‐frequency QPOs. Scaling the frequency inversely to the BH mass, the observed QPO frequency range (from 50 to 166 mHz) would yield a BH mass anywhere in the interval few tens to 1000  M⊙.

We present new optical and Chandra observations of the field containing the ultraluminous X-ray source NGC 1313 X-2. On an ESO 3.6 m image, the Chandra error box embraces a R = 21.6 point-like object and excludes a previously proposed optical counterpart. The resulting X-ray/optical flux ratio of NGC 1313 X-2 is ∼ 500. The value of f X /f opt , the X-ray variability history and the spectral distribution derived from a re-analysis of the ROSAT , ASCA and XMM data indicate a luminous X-ray binary in NGC 1313 as a likely explanation for NGC 1313 X-2. If the X-ray soft component observed in the XMM EPIC spectrum originates from an accretion disk, the inferred mass of the compact remnant is ≈ 100M ⊙ , making it an intermediate mass black hole. The derived optical luminosity (L ≈ 10 5 L ⊙ ) is consistent with that of a ≈ 15 − 20M ⊙ companion. The properties of the environment of NGC 1313 X-2 are briefly discussed.