We report the detection of weak pulsations from the archetypal ultraluminous X-ray source (ULX) NGC 1313 X-2. Acceleration searches reveal sinusoidal pulsations in segments of two out of six new deep observations of this object, with a period of $sim
$ 1.5 s and a pulsed fraction of $sim$ 5%. We use Monte Carlo simulations to demonstrate that the individual detections are unlikely to originate in false Poisson noise detections given their very close frequencies; their strong similarity to other pulsations detected from ULXs also argues they are real. The presence of a large bubble nebula surrounding NGC 1313 X-2 implies an age of order 1 Myr for the accreting phase of the ULX, which implies that the neutron stars magnetic field has not been suppressed over time by accreted material, nor has the neutron star collapsed into a black hole, despite an average energy output into the nebula two orders of magnitude above Eddington. This argues that most of the accreted material has been expelled over the lifetime of the ULX, favouring physical models including strong winds and/or jets for neutron star ULXs.
We report the discovery of a new ultraluminous X-ray source (ULX) associated with a globular cluster in the elliptical galaxy NGC 4649. The X-ray source was initially detected with a luminosity below 5 x 10^38 erg/s, but in subsequent observations 7
and 11 years later it had brightened substantially to 2 - 3 x 10^39 erg/s. Over the course of six separate observations it displayed significant spectral variability, in both continuum slope and absorption column. Short-term variability in the X-ray flux was also present in at least one observation. The properties of this object appear consistent with a stellar-mass black hole accreting at super-Eddington rates (i.e. in the ultraluminous accretion state), although a highly super-Eddington neutron star cannot be excluded. The coincidence of an increase in absorption column with a possible enhancement in short-term variability in at least one observation is suggestive of a clumpy radiatively-driven wind crossing our line-of-sight to the object
The controversy over whether ultraluminous X-ray sources (ULXs) contain a new intermediate-mass class of black holes (IMBHs) remains unresolved. We present new analyses of the deepest XMM-Newton observations of ULXs that address their underlying natu
re. We examine both empirical and physical modelling of the X-ray spectra of a sample of thirteen of the highest quality ULX datasets, and find that there are anomalies in modelling ULXs as accreting IMBHs with properties simply scaled-up from Galactic black holes. Most notably, spectral curvature above 2 keV in several sources implies the presence of an optically-thick, cool corona. We also present a new analysis of a 100 ks observation of Holmberg II X-1, in which a rigorous analysis of the temporal data limits the mass of its black hole to no more than 100 solar masses. We argue that a combination of these results points towards many (though not necessarily all) ULXs containing black holes that are at most a few 10s of solar mass in size.
We present results from a 112 ks long look by XMM-Newton at the ultraluminous X-ray source (ULX) Holmberg II X-1, long thought to be the one of best candidates for the missing class of intermediate mass black holes (IMBHs). Our data comprises the fir
st high quality XMM-Newton/RGS spectrum of an ULX, and an XMM-Newton/EPIC spectrum with unprecedented signal-to-noise. A detailed timing analysis shows that any variability on time-scales of minutes to hours is very weak (< few per cent fractional rms), though larger amplitude variations on much shorter time-scales could be hidden by photon counting statistics. This result suggests that if Ho II X-1 harbours an IMBH, then we are observing this source in a highly unusual and atypical state when compared with the known variability behaviour of other accreting systems of large mass. Moreover unlike Galactic X-ray binaries, our spectral analysis indicates the possible presence of an optically-thick low temperature corona. Taken together our timing and spectral analysis suggests that the compact companion is most-likely a high luminosity analogue of black hole binary systems similar to GRS 1915+105, the Galactic microquasar, harbouring a compact object of mass no greater than 100 solar masses.
We report the results of a programme of dual-epoch Chandra ACIS-S observations of five ultraluminous X-ray sources (ULXs) in nearby spiral galaxies. All five ULXs are detected as unresolved, point-like X-ray sources by Chandra, though two have faded
below the 10^39 erg/s luminosity threshold used to first designate these sources as ULXs. Using this same criterion, we detect three further ULXs within the imaged regions of the galaxies. The ULXs appear to be related to the star forming regions of the galaxies, indicating that even in ``normal spiral galaxies the ULX population is predominantly associated with young stellar populations. A detailed study of the Chandra ACIS-S spectra of six of the ULXs shows that five are better described by a powerlaw continuum than a multi-colour disc blackbody model, though there is evidence for additional very soft components to two of the powerlaw continua. The measured photon indices in four out of five cases are consistent with the low/hard state in black hole binaries, contrary to the suggestion that powerlaw-dominated spectra of ULXs originate in the very high state. A simple interpretation of this is that we are observing accretion onto intermediate-mass black holes, though we might also be observing a spectral state unique to very high mass accretion rates in stellar-mass black hole systems. Short-term flux variability is only detected in one of two epochs for two of the ULXs, with the lack of this characteristic arguing that the X-ray emission of this sample of ULXs is not dominated by relativistically-beamed jets. The observational characteristics of this small sample suggest that ULXs are a distinctly heterogeneous source class.
We present evidence, based on Chandra ACIS-S observations of the nearby spiral galaxy NGC 6946, that the extraodinary X-ray luminosity of the MF 16 supernova remnant actually arises in a black-hole X-ray binary. This conclusion is drawn from the poin
t-like nature of the X-ray source, its X-ray spectrum closely resembling the spectrum of other ultraluminous X-ray sources thought to be black-hole X-ray binary systems, and the detection of rapid hard X-ray variability from the source. We briefly discuss the nature of the hard X-ray variability, and the origin of the extreme radio and optical luminosity of MF 16 in light of this identification.
We report the results of an observation of a large diameter (110 pc) supernova remnant (SNR) found to encircle the position of the ultraluminous X-ray source (ULX) IC 342 X-1. The inferred initial energy input to the SNR is at least 2 -- 3 times grea
ter than the canonical energy for an ``ordinary supernova remnant. Two regions on the inside of the shell are bright in [O III] lambda5007 emission, possibly as the result of X-ray photoionization by the ULX. If this is the case, then the morphology of this nebulosity implies that the X-ray emission from the ULX is anisotropic. The presence of the ULX, most probably a black hole X-ray binary, within an unusually energetic supernova remnant suggests that we may be observing the aftermath of a gamma-ray burst, though other origins for the energetic nebula are discussed.
(Abridged) We report the results of a 20 ks Chandra ACIS-S observation of the interacting galaxy pair NGC 4485/4490. A total of 29 discrete X-ray sources are found coincident with NGC 4490, but only one is found within NGC 4485. The sources range in
observed X-ray luminosity from ~ 2 x 10^37 to ~ 4 x 10^39 erg/s. Extensive diffuse X-ray emission is detected coincident with the disk of NGC 4490, and in the tidal tail of NGC 4485, which appears to be thermal in nature and hence the signature of a hot ISM in both galaxies. However, the diffuse component accounts for only ~ 10% of the total X-ray luminosity of the system (2 x 10^40 erg/s, 0.5 - 8 keV), which arises predominantly in a handful of the brightest discrete sources. This diffuse emission fraction is unusually low for a galaxy pair that has many characteristics that would lead it to be classified as a starburst system, possibly as a consequence of the small gravitational potential well of the system. The discrete source population, on the other hand, is similar to that observed in other starburst systems, possessing a flat luminosity function slope of ~ -0.6 and a total of six ultraluminous X-ray sources (ULX). Five of the ULX are identified as probable black hole X-ray binary systems, and the sixth (which is coincident with a radio continuum source) is identified as an X-ray luminous supernova remnant. The ULX all lie in star-formation regions, providing further evidence of the link between the ULX phenomenon and active star formation. Importantly, this shows that even in star forming regions, the ULX population is dominated by accreting systems. We discuss the implications of this work for physical models of the nature of ULX, and in particular how it argues against the intermediate-mass black hole hypothesis.
We use archival HST/WFPC2 V and I band images to show that the optical counterpart to the ultra-luminous x-ray source NGC 5204 X-1, reported by Roberts et al., is composed of two sources separated by 0.5. We have also identified a third source as a p
ossible counterpart, which lies 0.8 from the nominal x-ray position. PSF fitting photometry yields V-band magnitudes of 20.3, 22.0 and 22.4 for the three sources. The V-I band colours are 0.6, 0.1, and -0.2, respectively (i.e. the fainter sources are bluer). We find that all V-I colours and luminosities are consistent with those expected for young stellar clusters (age <10 Myr).
We utilize the complimentary capabilities of XMM-Newton and Chandra, to conduct a detailed imaging and spectral study of the nearby galaxy NGC 4945 focussing on its nucleus and immediate surroundings (within ~1 kpc of the nucleus). A complex morpholo
gy is revealed including a predominantly hard, but partially resolved, nuclear source plus a spectrally soft, conically shaped X-ray plume, which extends 30 (500 pc) to the northwest. In NGC 4945 our direct view of the active galactic nucleus (AGN) is blocked below ~10 keV by extremely heavy line-of-sight absorption and the observed X-ray spectrum is dominated by multi-temperature thermal emission associated with the nuclear starburst and the X-ray plume. Nevertheless the signature of the AGN is present in the form of a neutral Compton reflection component and a 6.4 keV fluorescent iron Ka line. We conjecture that the site of the continuum reprocessing is the far wall of a highly inclined molecular torus, a geometry which is consistent with the presence of H2O megamaser emission in this source. The soft spectrum (~0.6 keV) and limb-brightened appearance of the X-ray plume suggest an interpretation in terms of a mass-loaded superwind emanating from the nuclear starburst.
