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 inclination-dependent 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.
Since their discovery, Ultraluminous X-ray sources (ULXs) have attracted attention due to their combination of extreme luminosities and extra-nuclear locations. However, they are a fairly rare phenomenon, and attempts to investigate the general properties of the population have been hindered by a relative lack of known sources. Here, we present a large catalogue of ULX candidates including 655 detections of 475 discrete sources, based on the 2XMM Serendipitous Survey. To demonstrate the potential of such a resource, we present some scientific analysis of this population, focusing on the spectral turnover seen, often at ~6 keV, in the highest quality ULX data. We also demonstrate how the recent reflection and Comptonisation interpretations of this feature may be distinguished observationally in the future, specifically using NGC 4517 ULX1, a previously unanalysed source with high quality data, as an example case.
Following the report of enhanced X-ray activity towards the direction of Terzan 1 by MAXI from May 19--30 (Atel #5096), we performed an observation with Swift/XRT on June 4th (Obs. ID: 00032852001) and found no evidence of X-ray activity in the vicinity of Terzan 1. The exposure of this observation after standard processing was 35 seconds, though the unfiltered data's exposure time is 897 seconds.
Although a great deal of progress has been made towards understanding the nature of ultraluminous X‐ray sources (ULXs) over the past decade, many basic issues remain to be settled. Chief amongst these is the mass of the accretor in these systems: what proportion of ULXs are powered by accretion onto an intermediate‐mass black hole (IMBH), and how big are these objects? Or can we explain their extraordinary X‐ray luminosities by exotic, extreme accretion rate processes onto more prosaic stellar‐mass black holes? Here we report the results of two new studies using the best available XMM‐Newton observations to probe the astrophysics of ULXs. A systematic study of ULX power spectra shows a significant proportion of luminous objects with unusually suppressed variability compared to standard black hole binaries and AGN. Furthermore, detailed spectral studies confirm that ULX spectra appear unlike any standard accretion state in the XMM‐Newton band pass. Hence we infer that many ULXs contain small black holes operating in a new, super‐Eddington "ultraluminous" accretion state. We show that ULX spectra appear to vary with accretion rate, being closest to standard states at around Eddington, with an optically thick corona becoming the characteristic feature as we reach super‐Eddington rates. At the highest rates we see evidence for the emergence of a cool photosphere, likely related to a massive outflowing wind.
We present the preliminary results of two Gemini campaigns to constrain the mass of the black hole in an ultraluminous X-ray source (ULX) via optical spectroscopy. Pilot studies of the optical counterparts of a number of ULXs revealed two candidates for further detailed study, based on the presence of a broad He II 4686 Angstrom emission line. A sequence of 10 long-slit spectra were obtained for each object, and the velocity shift of the ULX counterpart measured. Although radial velocity variations are observed, they are not sinusoidal, and no mass function is obtained. However, the broad He II line is highly variable on timescales shorter than a day. If associated with the reprocessing of X-rays in the accretion disc, its breadth implies that the disc must be close to face-on.
Ultraluminous X-ray sources (ULXs) were an enigma since their discovery. ASCA showed that they are accreting black holes (BHs) with unknown mass, while Chandra & XMM data shows that the many of the lower luminosity ULXs can be explained by extreme accretion onto stellar remnant BHs. The discovery of hyperluminous X-ray sources has led to specu- lation that we may, once again, have found the elusive intermediate mass black holes, yet in the last year, a third sub-class of extreme ULXs has emerged, with their nature, as yet, un- known. Here we define and discuss the sub-classes of these fascinating systems, exploring possible explanations for their nature, and indicating possible future steps in their analysis.
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 globular cluster 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 systematically analyze all the available X-ray spectra of disc accreting neutron stars (atolls and millisecond pulsars) from the RXTE database. We show that while these all have similar spectral evolution as a function of mass accretion rate, there are also subtle differences. There are two different types of hard/soft transition, those where the spectrum softens at all energies, leading to a diagonal track on a colour-colour diagram, and those where only the higher energy spectrum softens, giving a vertical track. The luminosity at which the transition occurs is correlated with this spectral behaviour, with the vertical transition at L/L_Edd ~0.02 while the diagonal one is at ~0.1. Superimposed on this is the well known hysteresis effect, but we show that classic, large scale hysteresis occurs only in the outbursting sources, indicating that its origin is in the dramatic rate of change of mass accretion rate during the disc instability. We show that the long term mass accretion rate correlates with the transition behaviour, and speculate that this is due to the magnetic field being able to emerge from the neutron star surface for low average mass accretion rates. While this is not strong enough to collimate the flow except in the millisecond pulsars, its presence may affect the inner accretion flow by changing the properties of the jet.
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