Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACTUltraluminous X-ray sources (ULXs) with luminosities lying between ∼3 × 10 39 and 2 × 10 40 erg s −1 represent a contentious sample of objects as their brightness, together with a lack of unambiguous mass estimates for the vast majority of the central objects, leads to a degenerate scenario where the accretor could be a stellar remnant (black hole or neutron star) or intermediate-mass black hole (IMBH). Recent, high-quality observations imply that the presence of IMBHs in the majority of these objects is unlikely unless the accretion flow somehow deviates strongly from expectation based on objects with known masses. On the other hand, physically motivated models for supercritical inflows can re-create the observed X-ray spectra and their evolution, although have been lacking a robust explanation for their variability properties. In this paper, we include the effect of a partially inhomogeneous wind that imprints variability on to the X-ray emission via two distinct methods. The model is heavily dependent on both inclination to the line of sight and mass accretion rate, resulting in a series of qualitative and semiquantitative predictions. We study the time-averaged spectra and variability of a sample of well-observed ULXs, finding that the source behaviours can be explained by our model in both individual cases as well as across the entire sample, specifically in the trend of hardness-variability power. We present the covariance spectra for these sources for the first time, which shed light on the correlated variability and issues associated with modelling broad ULX spectra.Key words: accretion, accretion discs -X-rays: binaries. I N T RO D U C T I O NUltraluminous X-ray sources (ULXs) have been widely observed in the local Universe, with inferred isotropic luminosities above 10 39 erg s −1 (Roberts 2007;Feng & Soria 2011). Those below ∼3 × 10 39 erg s −1 can be readily associated with accretion on to stellar mass black holes (BHs) (∼10 M ) accreting close to or at their Eddington limit (see Sutton, Roberts & Middleton 2013, and references therein). There is now strong evidence to support this assertion, with the discovery of extremely bright ballistic jets from a ULX in M31 Middleton, Miller-Jones & Fender 2014b), which unambiguously links the flow with Eddington rate accretion (Fender, Belloni & Gallo 2004), and the first dynamical mass measurement of the compact object in a ULX, from M101 E-mail: mjm@ast.cam.ac.uk ULX-1 (Liu et al. 2013). Ob...

A deep survey of the Large Magellanic Cloud at ∼ 0.1−100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3 − 2.4 pending a flux increase by a factor > 3 − 4 over ∼ 2015 − 2035. Large-scale interstellar emission remains mostly out of reach of the survey if its > 10 GeV spectrum has a soft photon index ∼ 2.7, but degree-scale 0.1 − 10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1 − 10% of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within < 100 pc. Finally, the survey could probe the canonical velocity-averaged cross section for self-annihilation of weakly interacting massive particles for cuspy Navarro-Frenk-White profiles.

Ultraluminous X-ray sources (ULXs) are a population of extragalactic objects whose luminosity exceeds the Eddington limit for a 10 M black hole (BH). Their properties have been widely interpreted in terms of accreting stellar-mass or intermediate-mass BHs. However at least three neutron stars (NSs) have been recently identified in ULXs through the discovery of periodic pulsations. Motivated by these findings we studied the spectral properties of a sample of bright ULXs using a simple continuum model which was extensively used to fit the X-ray spectra of accreting magnetic NSs in the Galaxy. We found that such a model, consisting of a power-law with a high-energy exponential cut-off, fits very well most of the ULX spectra analyzed here, at a level comparable to that of models involving an accreting BH. On these grounds alone we suggest that other non-pulsating ULXs may host NSs. We found also that above 2 keV the spectrum of known pulsating ULXs is harder than that of the majority of the other ULXs of the sample, with only IC 342 X-1 and Ho IX X-1 displaying spectra of comparable hardness. We thus suggest that these two ULXs may host an accreting NS and encourage searches for periodic pulsations in the flux. Subject headings: stars: neutron -(stars:) pulsars: general -X-rays: individual (NGC 7793 P13, NGC 5907 X-1, IC 342 X-1, Ho IX X-1, Ho II X-1, NGC 5408 X-1, NGC 1313 X-1, NGC 1313 X-2, NGC 5204 X-1, NGC 55 ULX1, NGC 5643 ULX1, NGC 6946 X-1) arXiv:1701.03595v1 [astro-ph.HE]

We present a timing analysis of the 2015 outburst of the accreting millisecond Xray pulsar SAX J1808.4-3658, using non-simultaneous XMM-Newton and NuSTAR observations. We estimate the pulsar spin frequency and update the system orbital solution. Combining the average spin frequency from the previous observed, we confirm the long-term spin down at an average rateν SD = 1.5(2) × 10 −15 Hz s −1 . We also discuss possible corrections to the spin down rate accounting for mass accretion onto the compact object when the system is X-ray active. Finally, combining the updated ephemerides with those of the previous outbursts, we find a long-term orbital evolution compatible with a binary expansion at a mean rateṖ orb = 3.6(4) × 10 −12 s s −1 , in agreement with previously reported values. This fast evolution is incompatible with an evolution driven by angular momentum losses caused by gravitational radiation under the hypothesis of conservative mass transfer. We discuss the observed orbital expansion in terms of non-conservative mass transfer and gravitational quadrupole coupling mechanism. We find that the latter can explain, under certain conditions, small fluctuations (of the order of few seconds) of the orbital period around a global parabolic trend. At the same time, a non-conservative mass transfer is required to explain the observed fast orbital evolution, which likely reflects ejection of a large fraction of mass from the inner Lagrangian point caused by the irradiation of the donor by the magneto-dipole rotator during quiescence (radio-ejection model). This strong outflow may power tidal dissipation in the companion star and be responsible of the gravitational quadrupole change oscillations.

We select a sample of nearby Ultraluminous X-ray sources with long XMM-Newton observations and analyse all the available XMM-Newton data using both X-ray spectral fitting techniques and hardness-intensity diagrams. The sample includes IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1, NGC 1313 X-1, NGC 1313 X-2 and NGC 253 X-1. We found that, although a common reference model can be used to describe the X-ray spectra, the sources show different spectral evolutions, phenomenologically described in terms of variations in the properties of a soft component and a high energy tail. Variations at low energies are accounted for (mostly) by changes in the normalization of the soft component and/or in the column density to the source, while variations in the high energy tail by changes in the parameters of an optically thick corona. This spectral variability is rather well characterized on a colour-colour and hardness-intensity diagram in terms of suitably defined hardness ratios. We suggest the existence of a variability pattern on the hardness-intensity diagram and we interpret it in terms of the switch between a near-Eddington and a super-Eddington accretion regime. The transition between the two regimes seems to be driven mostly by changes in the contribution of the soft component, which can be explained in terms of the increasing importance of wind emission. The analysis is complemented by an investigation of the shortterm time variability of all the sources. In general, no clear correlation between the spectral and temporal properties is found. viduals (IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1 and NGC 253 X-1)

We present a systematic analysis of the X-ray spectra of NGC 1313 X-1 and NGC 1313 X-2, using three years of XMM-Newton observations. We fitted the continuum with a Comptonization model plus a multicolour blackbody disc, which describes the effects of an accretion disc plus a corona. We checked the consistency of this spectral model on the basis of the variability patterns of its spectral parameters. We found that the two sources show different spectral states. We tentatively interpret the observed behaviour of NGC 1313 X-1 and X-2 within the framework of near-Eddington and/or super-Eddington accretion. We also attempted to determine the chemical abundances in the local environment of NGC 1313 X-1 and X-2 from the EPIC and RGS spectra. The results appear to indicate subsolar metallicity for both sources

Context. The source X1822-371 is a low-mass X-ray binary system (LMXB) viewed at a high inclination angle. It hosts a neutron star with a spin period of ∼0.59 s, and recently, the spin period derivative was estimated to be (−2.43 ± 0.05) × 10 −12 s/s. Aims. Our aim is to address the origin of the large residuals below 0.8 keV previously observed in the XMM/EPIC-pn spectrum of X1822-371. Methods. We analyse all available X-ray observations of X1822-371 made with XMM-Newton, Chandra, Suzaku and INTEGRAL satellites. The observations were not simultaneous. The Suzaku and INTEGRAL broad band energy coverage allows us to constrain the spectral shape of the continuum emission well. We use the model already proposed for this source, consisting of a Comptonised component absorbed by interstellar matter and partially absorbed by local neutral matter, and we added a Gaussian feature in absorption at ∼0.7 keV. This addition significantly improves the fit and flattens the residuals between 0.6 and 0.8 keV.Results. We interpret the Gaussian feature in absorption as a cyclotron resonant scattering feature (CRSF) produced close to the neutron star surface and derive the magnetic field strength at the surface of the neutron star, (8.8 ± 0.3) × 10 10 G for a radius of 10 km. We derive the pulse period in the EPIC-pn data to be 0.5928850(6) s and estimate that the spin period derivative of X1822-371 is (−2.55 ± 0.03) × 10 −12 s/s using all available pulse period measurements. Assuming that the intrinsic luminosity of X1822-371 is at the Eddington limit and using the values of spin period and spin period derivative of the source, we constrain the neutron star and companion star masses. We find the neutron star and the companion star masses to be 1.69 ± 0.13 M and 0.46 ± 0.02 M , respectively, for a neutron star radius of 10 km. Conclusions. In a self-consistent scenario in which X1822-371 is spinning-up and accretes at the Eddington limit, we estimate that the magnetic field of the neutron star is (8.8 ± 0.3) × 10 10 G for a neutron star radius of 10 km. If our interpretation is correct, the Gaussian absorption feature near 0.7 keV is the very first detection of a CRSF below 1 keV in a LMXB.

We report on the timing analysis of the 2015 outburst of the intermittent accreting millisecond X-ray pulsar SAX J1748.9−2021 observed on March 4 by the X-ray satellite XMM-Newton. By phase-connecting the time of arrivals of the observed pulses, we derived the best-fit orbital solution for the 2015 outburst. We investigated the energy pulse profile dependence finding that the pulse fractional amplitude increases with energy while no significant time lags are detected. Moreover, we investigated the previous outbursts from this source, finding previously undetected pulsations in some intervals during the 2010 outburst of the source. Comparing the updated set of orbital parameters, in particular the value of the time of passage from the ascending node, with the orbital solutions reported from the previous outbursts, we estimated for the first time the orbital period derivative corresponding withṖ orb = (1.1 ± 0.3) × 10 −10 s/s. We note that this value is significant at 3.5σ confidence level, because of significant fluctuations with respect to the parabolic trend and more observations are needed in order to confirm the finding. Assuming the reliability of the result, we suggest that the large value of the orbital-period derivative can be explained as a result of an highly non-conservative mass transfer driven by emission of gravitational waves, which implies the ejection of matter from a region close to the inner Lagrangian point. We also discuss possible alternative explanations.