We revisit the question of the nature of ultraluminous X‐ray sources (ULXs) through a detailed investigation of their spectral shape, using the highest quality X‐ray data available in the XMM–Newton public archives (≳10 000 counts in their EPIC spectrum). We confirm that simple spectral models commonly used for the analysis and interpretation of ULXs (power‐law continuum and multicolour disc blackbody models) are inadequate in the face of such high‐quality data. Instead we find two near ubiquitous features in the spectrum: a soft excess and a rollover in the spectrum at energies above 3 keV. We investigate a range of more physical models to describe these data. Slim discs which include radiation trapping (approximated by a p‐free disc model) do not adequately fit the data, and several objects give unphysically high disc temperatures (kTin > 3 keV). Instead, disc plus Comptonized corona models fit the data well, but the derived corona is cool and optically thick (τ∼ 5–30). This is unlike the τ∼ 1 coronae seen in Galactic binaries, ruling out models where ULXs are powered by sub‐Eddington accretion on to an intermediate‐mass black hole despite many objects having apparently cool disc temperatures. We argue that these observed disc temperatures are not a good indicator of the black hole mass as the powerful, optically thick corona drains energy from the inner disc and obscures it. We estimate the intrinsic (corona‐less) disc temperature, and demonstrate that in most cases it lies in the regime of stellar mass black holes. These objects have spectra which range from those similar to the highest mass accretion rate states in Galactic binaries (a single peak at 2–3 keV) to those which clearly have two peaks, one at energies below 1 keV (from the outer, un‐Comptonized disc) and one above 3 keV (from the Comptonized, inner disc). However, a few ULXs have a significantly cooler corrected disc temperature; we suggest that these are the most extreme stellar mass black hole accretors, in which a massive wind completely envelopes the inner‐disc regions, creating a cool photosphere. We conclude that ULXs provide us with an observational template for the transition between Eddington and super‐Eddington accretion flows, with the latter occupying a new ultraluminous accretion state.

With the advent of more sensitive all-sky instruments, the transient universe is being probed in greater depth than ever before. Taking advantage of available resources, we have established a comprehensive database of black hole (and black hole candidate) X-ray binary (BHXB) activity between 1996 and 2015 as revealed by all-sky instruments, scanning surveys, and select narrow-field X-ray instruments on board the INTErnational Gamma-Ray Astrophysics Laboratory, Monitor of All-Sky X-ray Image, Rossi X-ray Timing Explorer, and Swift telescopes; the Whole-sky Alberta Time-resolved Comprehensive black-Hole Database Of the Galaxy or WATCHDOG. Over the past two decades, we have detected 132 transient outbursts, tracked and classified behavior occurring in 47 transient and 10 persistently accreting BHs, and performed a statistical study on a number of outburst properties across the Galactic population. We find that outbursts undergone by BHXBs that do not reach the thermally dominant accretion state make up a substantial fraction (∼40%) of the Galactic transient BHXB outburst sample over the past ∼20 years. Our findings suggest that this “hard-only” behavior, observed in transient and persistently accreting BHXBs, is neither a rare nor recent phenomenon and may be indicative of an underlying physical process, relatively common among binary BHs, involving the mass-transfer rate onto the BH remaining at a low level rather than increasing as the outburst evolves. We discuss how the larger number of these “hard-only” outbursts and detected outbursts in general have significant implications for both the luminosity function and mass-transfer history of the Galactic BHXB population.

We present the results from an X‐ray and optical study of a new sample of eight extreme luminosity ultraluminous X‐ray source (ULX) candidates, which were selected as the brightest ULXs (with LX > 5 × 1040 erg s−1) located within 100 Mpc identified in a cross‐correlation of the 2XMM‐DR1 and RC3 catalogues. These objects are so luminous that they are difficult to describe with current models of super‐Eddington accretion on to all but the most massive stellar remnants; hence they are amongst the most plausible candidates to host larger, intermediate‐mass black holes (IMBHs). Two objects are luminous enough in at least one observation to be classed as hyperluminous X‐ray source (HLX) candidates, including one persistent HLX in an S0 galaxy that (at 3 × 1041 erg s−1) is the second most luminous HLX yet detected. The remaining seven sources are located in spiral galaxies, and several appear to be closely associated with regions of star formation as is common for many less luminous ULXs. However, the X‐ray characteristics of these extreme ULXs appear to diverge from the less luminous objects. They are typically harder, possessing absorbed power‐law continuum spectra with Γ∼ 1.7, and are potentially more variable on short time‐scales, with data consistent with ∼10–20 per cent rms variability on time‐scales of 0.2–2 ks (albeit at low to moderate significance in many data sets). These properties appear consistent with the sub‐Eddington hard state, which given the observed luminosities of these objects suggests the presence of IMBHs with masses in the range of . As such, this strengthens the case for these brightest ULXs as good candidates for the eventual conclusive detection of the highly elusive IMBHs in the present‐day Universe. However, we caution that a combination of the highest plausible super‐Eddington accretion rates and the largest permitted stellar black hole remnants cannot be ruled out without future, improved observations.

The high stellar densities in the cores of globular clusters cause significant stellar interactions. These stellar interactions can produce close binary mass-transferring systems involving compact objects and their progeny, such as X-ray binaries and radio millisecond pulsars. Comparing the numbers of these systems and interaction rates in different clusters drives our understanding of how cluster parameters affect the production of close binaries. In this paper we estimate stellar encounter rates (Γ) for 124 Galactic globular clusters based on observational data as opposed to the methods previously employed, which assumed "King-model" profiles for all clusters. By deprojecting cluster surface brightness profiles to estimate luminosity density profiles, we treat "King-model" and "core-collapsed" clusters in the same way. In addition, we use Monte-Carlo simulations to investigate the effects of uncertainties in various observational parameters (distance, reddening, surface brightness) on Γ, producing the first catalog of GC stellar encounter rates with estimated errors. Comparing our results with published observations of likely products of stellar interactions (numbers of X-ray binaries, numbers of radio millisecond pulsars, and γ-ray luminosity) we find both clear correlations and some differences with published results.

We present a photometric survey of the optical counterparts of ultraluminous X-ray sources (ULXs) observed with the Hubble Space Telescope in nearby ( 5 Mpc) galaxies. Of the 33 ULXs with Hubble & Chandra data, 9 have no visible counterpart, placing limits on their M V of ∼ -4 to -9, enabling us to rule out O-type companions in 4 cases. The refined positions of two ULXs place them in the nucleus of their host galaxy. They are removed from our sample. Of the 22 remaining ULXs, 13 have one possible optical counterpart, while multiple are visible within the error regions of other ULXs. By calculating the number of chance coincidences, we estimate that 13±5 are the true counterparts. We attempt to constrain the nature of the companions by fitting the SED and M V to obtain candidate spectral types. We can rule out O-type companions in 20 cases, while we find that one ULX (NGC 253 ULX2) excludes all OB-type companions. Fitting with X-ray irradiated models provides constraints on the donor star mass and radius. For 7 ULXs, we are able to impose inclinationdependent upper and/or lower limits on the black holes mass, if the extinction to the assumed companion star is not larger than the Galactic column. These are NGC 55 ULX1, NGC 253 ULX1, NGC 253 ULX2, NGC 253 XMM6, Ho IX X-1, IC342 X-1 & NGC 5204 X-1. This suggests that 10 ULXs do not have O companions, while none of the 18 fitted rule out B-type companions.