Matthew Middleton

A periodicity of ∼ 1 hour in X-ray emission from the active galaxy RE J1034+396

Active galactic nuclei and quasars are thought to be scaled-up versions of Galactic black hole binaries, powered by accretion onto supermassive black holes with masses of 106–109 , as opposed to the ∼10  in binaries (here is the solar mass). One example of the similarities between these two types of systems is the characteristic rapid X-ray variability seen from the accretion flow. The power spectrum of this variability in black hole binaries consists of a broad noise with multiple quasi-periodic oscillations superimposed on it. Although the broad noise component has been observed in many active galactic nuclei, there have hitherto been no significant detections of quasi-periodic oscillations. Here we report the discovery of an ∼1-hour X-ray periodicity in a bright active galaxy, RE J1034+396. The signal is highly statistically significant (at the 5.6σ level) and very coherent, with quality factor Q > 16. The X-ray modulation arises from the direct vicinity of the black hole.

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.

Bright radio emission from an ultraluminous stellar-mass microquasar in M 31

A subset of ultraluminous X-ray sources (those with luminosities of less than 1040 erg s−1; ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of ∼5–20, probably by means of an accretion disk. The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 1039 erg s−1. The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.

Black hole spin in GRS 1915+105

Microquasars are galactic black hole binary systems with radio jets which can sometimes be spatially resolved to show superluminal motion. The first and best known of this class of objects is GRS 1915+105, the brightest accreting source in our Galaxy. There is persistent speculation that strong jet emission could be linked to black hole spin. If so, the high spin should also be evident in accretion disc spectra. We search the RXTE archive to find disc-dominated X-ray spectra from this object, as these are the only ones which can give reliable spin determinations by this method. Finding these is complicated by the rapid, unique limit cycle variability, but we are able to identify such spectra by going to the shortest possible time resolution (16 s). We fit them with a simple multicolour disc blackbody (DISKBB), and with the best current model which include full radiative transfer as well as relativistic effects (BHSPEC). Both these models show that the spin is intermediate, neither zero nor maximal. BHSPEC, the most physical model, gives a value for the dimensionless spin of a* ∼0.7 for a distance of 12.5 kpc and inclination of 66°. This, together with the range of spins 0.1 < a· < 0.8 derived using this method for other black holes, suggests that jet emission is probably fundamentally powered by gravity rather than spin, and implies that high-to-maximal spin is not a pre-requisite for powerful relativistic jets.

Challenging times: a re-analysis of NGC 5408 X-1

The ultraluminous X-ray source, NGC 5408 X-1, is one of only three such objects to show a quasi-periodic oscillation (QPO) in its power spectrum. Previous analysis of this signal identified it with the well-studied type-C low-frequency QPO (LFQPO) seen in black hole binaries (BHBs), implying an intermediate mass black hole (IMBH). However, in BHBs this QPO has a centroid frequency which scales tightly with the position of the low-frequency break in the broad-band power spectrum. We use this relation to predict the frequency of the power spectral break in NGC 5408 X-1, and show that this is inconsistent with the break frequencies in both available, archival XMM–Newton observations. Thus the broad-band power spectral shape does not support this identification of the QPO. The energy spectra also do not support an IMBH interpretation. They can be fit by a twocomponent model, best described by soft thermal emission at low energies, together with lowtemperature, optically thick Comptonization producing a tail which dominates above 2 keV. The parameters of the tail are unlike those seen in any of the sub-Eddington BHB spectral states. The energy-dependent variability supports this deconvolution, as it is consistent with the soft thermal component below 2 keV diluting extreme variability of the high-energy tail. The only objects with similar spectra which have similar amounts of variability are the BHB, GRS 1915+105 and some extreme Narrow-Line Seyfert 1s. This suggests that NGC 5408 X-1 is in a similar super-Eddington state, placing a natural limit on the mass of 100 M � .I ts QPO could then be similar to the ultra-LFQPO seen occasionally in GRS 1915+105, consistent with a large stellar mass black hole. We suggest a model geometry which may explain the spectra and variability of highly super-Eddington sources.

RE J1034+396: the origin of the soft X-ray excess and quasi-periodic oscillation

The X-ray quasi-periodic oscillation (QPO) seen in RE J1034+396 is so far unique amongst active galactic nuclei (AGN). Here, we look at another unique feature of RE J1034+396, namely its huge soft X-ray excess, to see if this is related in any way to the detection of the QPO. We show that all potential models considered for the soft energy excess can fit the 0.3–10 keV X-ray spectrum, but the energy dependence of the rapid variability (which is dominated by the QPO) strongly supports a spectral decomposition where the soft excess is from low-temperature Comptonization of the disc emission and remains mostly constant, while the rapid variability is produced by the power-law tail changing in normalization. The presence of the QPO in the tail rather than in the disc is a common feature in black hole binaries (BHBs), but low-temperature Comptonization of the disc spectrum is not generally seen in these systems. The main exception to this is GRS 1915+105, the only BHB which routinely shows super-Eddington luminosities. We speculate that the super-Eddington accretion rates lead to a change in disc structure, and that this also triggers the X-ray QPO.

An absorption origin for the soft excess in Seyfert 1 active galactic nuclei

The soft excess seen in the X-ray spectra of many active galactic nuclei (AGN) can be well modelled by reflection from a partially ionized accretion disc. However, this often requires extreme parameters, both in terms of the underlying space-time and the reflection geometry, and requires that the disc is far from hydrostatic equilibrium. An alternative model uses similarly partially ionized, velocity smeared material but from an accretion disc wind seen in absorption. We explicitly compare these two models for the origin of the soft excess using XMM-Newton data for PG QSOs and narrow-line Seyfert 1 galaxies (NLS 1 s). We find that while reflection and absorption give comparably good fits to the data, the absorption model allows a much clearer correspondence with the stellar mass black holes. All the objects are high mass accretion rate AGN, so should be analogous to the high/soft and very high states in black hole binaries. The intrinsic spectral indices derived from the absorption model are all consistent with a one-to-one mapping between spectral state and AGN type, with the NLS1s having softer spectra corresponding to the very high state, while the broad-line AGN have r = 2 as expected for the high/soft state. By contrast, a few AGN have intrinsically hard spectra with the reflection model. While this supports an absorption interpretation of the soft excess, we note that the required Gaussian velocity dispersion of ≥0.2c (corresponding to an outflow velocity ≥0.4c) is too fast for a radiatively driven accretion disc wind and instead requires that the material is entrained in a magnetic outflow (jet). We also use the simultaneous optical monitor data to derive the ratio of disc to total accretion power which is another tracer of spectral state in X-ray binaries. This does not always show that the disc in NLS 1s contributes less than 80 per cent of the total power, as expected for a very high state. We suggest that this is an artefact of the standard disc models used to determine the disc luminosity in our fits. The disc seen in the very high state of black hole binaries is often observed to be distorted from the standard shape, and a similar effect in NLS Is could recover the one-to-one mapping between black hole binary spectral state and AGN type.

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

A new way to measure supermassive black hole spin in accretion disc-dominated active galaxies

We show that disc continuum fitting can be used to constrain black hole spin in a subclass of narrow-line Seyfert 1 (NLS1) active galactic nuclei as their low mass and high mass accretion rate means that the disc peaks at energies just below the soft X-ray bandpass. We apply the technique to the NLS1 PG1244+026, where the optical/UV/X-ray spectrum is consistent with being dominated by a standard disc component. This gives a best estimate for black hole spin which is low, with a firm upper limit of a* <0.86. This contrasts with the recent X-ray determinations of (close to) maximal black hole spin in other NLS1 based on relativistic smearing of the iron profile. While our data on PG1244+026 do not have sufficient statistics at high energy to give a good measure of black hole spin from the iron line profile, cosmological simulations predict that black holes with similar masses have similar growth histories and so should have similar spins. This suggests that there is a problem either in our understanding of disc spectra, or/and X-ray reflection or/and the evolution of black hole spin.

X-Ray Outflows and Super-Eddington Accretion in the Ultraluminous X-Ray Source Holmberg IX X-1

Studies of X-ray continuum emission and flux variability have not conclusively revealed the nature of ultraluminous X-ray sources (ULXs) at the high-luminosity end of the distribution (those with L_X ≥ 10^(40) erg s^(–1)). These are of particular interest because the luminosity requires either super-Eddington accretion onto a black hole of mass ~10 M☉ or more standard accretion onto an intermediate-mass black hole. Super-Eddington accretion models predict strong outflowing winds, making atomic absorption lines a key diagnostic of the nature of extreme ULXs. To search for such features, we have undertaken a long, 500 ks observing campaign on Holmberg IX X-1 with Suzaku. This is the most sensitive data set in the iron K bandpass for a bright, isolated ULX to date, yet we find no statistically significant atomic features in either emission or absorption; any undetected narrow features must have equivalent widths less than 15-20 eV at 99% confidence. These limits are far below the ≳150 eV lines expected if observed trends between mass inflow and outflow rates extend into the super-Eddington regime and in fact rule out the line strengths observed from disk winds in a variety of sub-Eddington black holes. We therefore cannot be viewing the central regions of Holmberg IX X-1 through any substantial column of material, ruling out models of spherical super-Eddington accretion. If Holmberg IX X-1 is a super-Eddington source, any associated outflow must have an anisotropic geometry. Finally, the lack of iron emission suggests that the stellar companion cannot be launching a strong wind and that Holmberg IX X-1 must primarily accrete via Roche-lobe overflow.