Andrew D. Sutton

The ultraluminous state revisited : fractional variability and spectral shape as diagnostics of super-Eddington accretion.

Although we are nearing a consensus that most ultraluminous X-ray sources (ULXs) below 1041 erg s−1 represent stellar mass black holes accreting in a super-Eddington ‘ultraluminous’ accretion state, little is yet established of the physics of this extreme accretion mode. Here, we use a combined X-ray spectral and timing analysis of an XMM-Newton sample of ULXs to investigate this new accretion regime. We start by suggesting an empirical classification scheme that separates ULXs into three classes based on the spectral morphologies observed by Gladstone et al.: a singly peaked broadened disc class, and two-component hard ultraluminous and soft ultraluminous regimes, with the spectra of the latter two classes dominated by the harder and softer component, respectively. We find that at the lowest luminosities (LX < 3 × 1039 erg s−1) the ULX population is dominated by sources with broadened disc spectra, whilst ULXs with two-component spectra are seen almost exclusively at higher luminosities, suggestive of a distinction between ∼Eddington and super-Eddington accretion modes. We find high levels of fractional variability are limited to ULXs with soft ultraluminous spectra, and a couple of the broadened disc sources. Furthermore, the variability in these sources is strongest at high energies, suggesting it originates in the harder of the two spectral components. We argue that these properties are consistent with current models of super-Eddington emission, where a massive radiatively driven wind forms a funnel-like geometry around the central regions of the accretion flow. As the wind provides the soft spectral component this suggests that inclination is the key determinant in the observed two-component X-ray spectra, which is very strongly supported by the variability results if this originates due to clumpy material at the edge of the wind intermittently obscuring our line-of-sight to the spectrally hard central regions of the ULX. The pattern of spectral variability with luminosity in two ULXs that straddle the hard/soft ultraluminous regime boundary is consistent with the wind increasing at higher accretion rates, and thus narrowing the opening angle of the funnel. Hence, this work suggests that most ULXs can be explained as stellar mass black holes accreting at and above the Eddington limit, with their observed characteristics dominated by two variables: accretion rate and inclination.

The most extreme ultraluminous X-ray sources: evidence for intermediate-mass black holes?

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 × 10 40 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 × 10 41 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 � ∼ 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 u 3 –10 4 M� . 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 missing link: a low-mass X-ray binary in M31 seen as an ultraluminous X-ray source

A new, transient ultraluminous X-ray source (ULX) was recently discovered by Chandra in M31 with a luminosity at ∼5 × 10 39 erg s −1 . Here we analyse a series of five subsequent XMM–Newton observations. These show a steady decline in X-ray luminosity over 1.5 months, from 1.8 × 10 39 to 0.6 × 10 39 erg s −1 , giving an observed e-fold time-scale of ∼40 d. This is similar to the decay time-scales seen in multiple soft X-ray transients in our own Galaxy, supporting the interpretation of this ULX as a stellar mass black hole in a low-mass X-ray binary (LMXB), accreting at super-Eddington rates. This is further supported by the lack of detection of an O/B star in quiescence and the spectral behaviour of the XMM–Newton data being dominated by a disc-like component rather than the power law expected from a sub-Eddington intermediate-mass black hole. These data give the best sequence of high Eddington fraction spectra ever assembled due to the combination of low absorption column to M31 and well-calibrated bandpass down to 0.3 keV of XMM–Newton in full frame mode. The spectra can be roughly described by

The powerful jet of an off-nuclear intermediate-mass black hole in the spiral galaxy NGC 2276

Jet ejection by accreting black holes is a mass invariant mechanism unifying stellar and supermassive black holes (SMBHs) that should also apply for intermediate-mass black holes (IMBHs),whicharethoughttobetheseedsfromwhichSMBHsform.Wepresentthedetection of an off-nuclear IMBH of ∼5 × 10 4 Mlocated in an unusual spiral arm of the galaxy NGC 2276 based on quasi-simultaneous Chandra X-ray observations and European VLBI Network (EVN) radio observations. The IMBH, NGC2276-3c, possesses a 1.8 pc radio jet that is oriented in the same direction as large-scale (∼650 pc) radio lobes and whose emission is consistent with flat to optically thin synchrotron emission between 1.6 and 5 GHz. Its jet kinetic power (4 × 10 40 erg s −1 ) is comparable to its radiative output and its jet efficiency (≥46percent) is as large as that of SMBHs. A region of ∼300 pc along the jet devoid of young stars could provide observational evidence of jet feedback from an IMBH. The discovery confirms that the accretion physics is mass invariant and that seed IMBHs in the early Universe possibly had powerful jets that were an important source of feedback.

Diagnosing the accretion flow in ultraluminous X-ray sources using soft X-ray atomic features

The lack of unambiguous detections of atomic features in the X-ray spectra of ultraluminous X-ray sources (ULXs) has proven a hindrance in diagnosing the nature of the accretion flow. The possible association of spectral residuals at soft energies with atomic features seen in absorption and/or emission and potentially broadened by velocity dispersion could therefore hold the key to understanding much about these enigmatic sources. Here we show for the first time that such residuals are seen in several sources and appear extremely similar in shape, implying a common origin. Via simple arguments we assert that emission from extreme colliding winds, absorption in a shell of material associated with the ULX nebula and thermal plasma emission associated with star formation are all highly unlikely to provide an origin. Whilst CCD spectra lack the energy resolution necessary to directly determine the nature of the features (i.e. formed of a complex of narrow lines or intrinsically broad lines), studying the evolution of the residuals with underlying spectral shape allows for an important, indirect test for their origin. The ULX NGC 1313 X-1 provides the best opportunity to perform such a test due to the dynamic range in spectral hardness provided by archival observations. We show through highly simplified spectral modelling that the strength of the features (in either absorption or emission) appears to anticorrelate with spectral hardness, which would rule out an origin via reflection of a primary continuum and instead supports a picture of atomic transitions in a wind or nearby material associated with such an outflow.

NUSTAR AND XMM-NEWTON OBSERVATIONS OF THE EXTREME ULTRALUMINOUS X-RAY SOURCE NGC 5907 ULX1: A VANISHING ACT

We present results obtained from two broadband X-ray observations of the extreme ultraluminous X-ray source (ULX) NGC5907 ULX1, known to have a peak X-ray luminosity of ~5e40 erg/s. These XMM-Newton and NuSTAR observations, separated by only ~4 days, revealed an extreme level of short-term flux variability. In the first epoch, NGC5907 ULX1 was undetected by NuSTAR, and only weakly detected (if at all) with XMM-Newton, while in the second NGC5907 ULX1 was clearly detected at high luminosity by both missions. This implies an increase in flux of ~2 orders of magnitude or more during this ~4 day window. We argue that this is likely due to a rapid rise in the mass accretion rate, rather than to a transition from an extremely obscured to an unobscured state. During the second epoch we observed the broadband 0.3-20.0 keV X-ray luminosity to be (1.55+/-0.06)e40 erg/s, similar to the majority of the archival X-ray observations. The broadband X-ray spectrum obtained from the second epoch is inconsistent with the low/hard accretion state observed in Galactic black hole binaries, but is well modeled with a simple accretion disk model incorporating the effects of photon advection. This strongly suggests that, when bright, NGC5907 ULX1 is a high-Eddington accretor.

A bright ultraluminous X-ray source in NGC 5907

We present a multimission X-ray analysis of a bright (peak observed 0.3–10 keV luminosity of ∼6 × 10 40 erg s −1 ), but relatively highly absorbed ultraluminous X-ray source (ULX) in the edge-on spiral galaxy NGC 5907. The ULX is spectrally hard in X-rays (� ∼ 1.2–1.7, when fitted with an absorbed power law), and has a previously reported hard spectral break consistent with it being in the ultraluminous accretion state. It is also relatively highly absorbed for a ULX, with a column of ∼0.4–0.9 × 10 22 atom cm −2 in addition to the line-of-sight column in our Galaxy. Although its X-ray spectra are well represented by accretion disc models, its variability characteristics argue against this interpretation. The ULX spectra instead appear dominated by a cool, optically thick Comptonizing corona. We discuss how the measured 9 per cent rms variability and a hardening of the spectrum as its flux diminishes might be reconciled with the effects of a very massive, radiatively driven wind and subtle changes in the corona, respectively. We speculate that the cool disc-like spectral component thought to be produced by the wind in other ULXs may be missing from the observed spectrum due to a combination of a low temperature (∼0.1 keV), and the high column to the ULX. We find no evidence, other than its extreme X-ray luminosity, for the presence of an intermediate mass black hole (MsBHs, ∼10 2 –10 4 M� ) in this object. Rather, the observations can be consistently explained

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

In recent work with high-resolution reflection grating spectrometers (RGS) aboard XMM–Newton, Pinto et al. have discovered that two bright and archetypal ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement with theoretical predictions of high accretion rates. It has been proposed that such winds can become optically thick enough to block and reprocess the disc X-ray photons almost entirely, making the source appear as a soft thermal emitter or ultraluminous supersoft X-ray source (ULS). To test this hypothesis, we have studied a ULX where the wind is strong enough to cause significant absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55 ULX shows a wealth of emission and absorption lines blueshifted by significant fractions of the light speed (0.01–0.20)c indicating the presence of a powerful wind. The wind has a complex dynamical structure with the ionization state increasing with the outflow velocity, which may indicate launching from different regions of the accretion disc. The comparison with other ULXs such as NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at higher inclination. The wind partly absorbs the source flux above 1 keV, generating a spectral drop similar to that observed in ULSs. The softening of the spectrum at lower (∼ Eddington) luminosities and the detection of a soft lag agree with the scenario of wind clumps crossing the line of sight, partly absorbing and reprocessing the hard X-rays from the innermost region.

The hyperluminous X-ray source candidate in IC 4320: another HLX bites the dust

The known members of the class of hyperluminous X-ray sources (HLXs) are few in number, yet they are of great interest as they are regarded as the likeliest intermediate-mass black hole (IMBH) candidates amongst the wider population of ultraluminous X-ray sources (ULXs). Here we report optical photometry and spectroscopy of an HLX candidate associated with the galaxy IC 4320, that reveal it is a background AGN. We discuss the implications of the exclusion of this object from the small number of well-studied HLXs, that appears to accentuate the difference in characteristics between the good IMBH candidate ESO 243-49 HLX-1 and the small handful of other HLXs.

Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

Although attempts have been made to constrain the stellar types of optical counterparts to ultraluminous X-ray sources (ULXs), the detection of optical variability instead suggests that they may be dominated by reprocessed emission from X-rays which irradiate the outer accretion disc. Here, we report results from a combined X-ray and optical spectral study of a sample of ULXs, which were selected for having broadened disc-like X-ray spectra and known optical counterparts. We simultaneously fit optical and X-ray data from ULXs with a new spectral model of emission from an irradiated, colour-temperature-corrected accretion disc around a black hole, with a central Comptonizing corona. We find that the ULXs require reprocessing fractions of ∼10 −3 , which is similar to sub-Eddington thermal dominant state black hole binaries (BHBs), but less than has been reported for ULXs with soft ultraluminous X-ray spectra. We suggest that the reprocessing fraction may be due to the opposing effects of self-shielding in a geometrically thick supercritical accretion disc and reflection from far above the central black hole by optically thin material ejected in a natal super-Eddington wind. Then, the higher reprocessing fractions reported for ULXs with wind-dominated X-ray spectra may be due to enhanced scattering on to the outer disc via the stronger wind in these objects. Alternatively, the accretion discs in these ULXs may not be particularly geometrically thick, rather they may be similar in this regard to the thermal dominant state BHBs.