Constraining the mass of accreting black holes in ultraluminous X-ray sources with ultrafast outflows

The nature of ultraluminous X-ray sources (ULXs) – off-nuclear extragalactic sources with luminosity, assumed isotropic, ≳1039 erg s−1 – is still debated. One possibility is that ULXs are stellar black holes (BHs) accreting beyond the Eddington limit. This view has been recently reinforced by the discovery of ultrafast outflows at ∼0.1–0.2c in the high-resolution spectra of a handful of ULXs, as predicted by models of supercritical accretion discs. Under the assumption that ULXs are powered by super-Eddington accretion on to BHs, we use the properties of the observed outflows to self-consistently constrain their masses and accretion rates. We find masses ≲100 M⊙ and typical accretion rates ∼10−5 M⊙ yr−1, i.e. ≈10 times larger than the Eddington limit calculated with a radiative efficiency of 0.1. However, the emitted luminosity is only ≈10 per cent beyond the Eddington luminosity, because most of the energy released in the inner part of the accretion disc is used to accelerate the wind, which implies radiative efficiency ∼0.01. Our results are consistent with a formation model where ULXs are BH remnants of massive stars evolved in low-metallicity environments.DF acknowledges support by European Research Council (ERC) Starting Grant 638707 ‘Black holes and their host galaxies: coevolution across cosmic time’. CP and ACF acknowledge support by ERC Advanced Grant 340442 ‘Accreting black holes and cosmic feedback’

Searching for outflows in ultraluminous X-ray sources through high-resolution X-ray spectroscopy

Ultraluminous X-ray sources (ULXs) are non-nuclear point sources exceeding the Eddington luminosity of a 10 M⊙ black hole. Modern consensus for a majority of the ULX population is that they are powered by stellar-mass black holes or neutron stars accreting well above the Eddington limit. Theoretical models of super-Eddington accretion predict existence of powerful outflows of moderately ionized gas at mildly relativistic velocities. So far, these winds have been found in three systems: NGC 1313 X-1, NGC 5408 X-1 and NGC 55 ULX. In this work, we create a sample of all ULXs with usable archival high-resolution X-ray data, with 10 sources in total, in which we aim to find more signatures of outflows. We perform Gaussian line scans to find any narrow spectral signatures, and physical wind model scans where possible. We tentatively identify an outflow in NGC 5204 X-1, blueshifted to 0.34c, which produces emission features with a total significance of at least 3σ. Next we compare ULXs with similar hardness ratios. Holmberg IX X-1 shows absorption features that could be associated with a photoionized outflowing absorber, similar to that seen in NGC 1313 X-1. The spectrum of Holmberg II X-1 possesses features similar to NGC 5408 X-1 and NGC 6946 X-1 shows O VIII rest-frame emission. All other sources from the sample also show tentative evidence of spectral features in their high-resolution spectra. Further observations with the XMM–Newton and Chandra gratings will place stronger constraints. Future missions like XARM and Athena will be able to detect them at larger distances and increase our sample

X-ray lags in PDS 456 revealed by Suzaku observations

X-ray reverberation lags from the vicinity of supermassive black holes have been detected in almost 30 active galactic nuclei (AGNs). The soft lag, which is the time delay between the hard and soft X-ray light curves, is usually interpreted as the time difference between the direct and reflected emission, but is alternatively suggested to arise from the direct and scattering emission from distant clouds. By analysing the archival Suzaku observations totalling an exposure time of ∼770 ks, we discover a soft lag of 10 ± 3.4 ks at 9.58 × 10−6 Hz in the luminous quasar PDS 456, which is the longest soft lag and lowest Fourier frequency reported to date. In this study, we use the maximum likelihood method to deal with non-continuous nature of the Suzaku light curves. The result follows the mass–scaling relation for soft lags, which further supports that soft lags originate from the innermost areas of AGNs and hence are best interpreted by the reflection scenario. Spectral analysis has been performed in this work and we find no evidence of clumpy partial-covering absorbers. The spectrum can be explained by a self-consistent relativistic reflection model with warm absorbers, and spectral variations over epochs can be accounted for by the change of the continuum, and of column density and ionization states of the warm absorbers.EMC gratefully acknowledges support from the NSF through CAREER award number AST-1351222. CSR thanks NASA for support under grant NNX15AU54G. ACF acknowledges ERC Advanced Grant 340442

Hard X-ray lags in active galactic nuclei: Testing the distant reverberation hypothesis with NGC 6814

We present an X-ray spectral and temporal analysis of the variable active galaxy NGC 6814, observed with Suzaku during November 2011. Remarkably, the X-ray spectrum shows no evidence for the soft excess commonly observed amongst other active galaxies, despite its relatively low level of obscuration, and is dominated across the whole Suzaku bandpass by the intrinsic powerlaw-like continuum. Despite this, we clearly detect the presence of a low frequency hard lag of ~1600s between the 0.5-2.0 and 2.0-5.0 keV energy bands at greater than 6-sigma significance, similar to those reported in the literature for a variety of other AGN. At these energies, any additional emission from e.g. a very weak, undetected soft excess, or from distant reflection must contribute less than 3% of the observed countrates (at 90% confidence). Given the lack of any significant continuum emission component other than the powerlaw, we can rule out models that invoke distant reprocessing for the observed lag behavior, which must instead be associated with this continuum emission. These results are fully consistent with a propagating fluctuation origin for the low frequency hard lags, and with the interpretation of the high frequency soft lags - a common feature seen in the highest quality AGN data with strong soft excesses - as reverberation from the inner accretion disk

Super-Eddington accretion on to the neutron star NGC7793 P13: Broad-band X-ray spectroscopy and ultraluminous X-ray sources

We present a detailed, broad-band X-ray spectral analysis of the ultraluminous X-ray source (ULX) pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the XMM–Newton, NuSTAR and Chandra observatories. The broad-band XMM–Newton+NuSTAR spectrum of P13 is qualitatively similar to the rest of the ULX sample with broad-band coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures ∼0.5 and 1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be comparable, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron star's magnetic field soon after its onset, implying a limit of B ≲ 6 × 1012 G for the dipolar component of the central neutron star's magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual ‘off’ states. These data require both a hard power-law component, suggesting residual accretion on to the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system

Ultrafast outflows disappear in high-radiation fields

Ultrafast outflows (UFOs) are the most extreme winds launched by active galactic nuclei (AGN) due to their mildly-relativistic speeds (~0.1-0.3c) and are thought to significantly contribute to galactic evolution via AGN feedback. Their nature and launching mechanism are however not well understood. Recently, we have discovered the presence of a variable UFO in the narrow-line Seyfert 1 IRAS 13224-3809. The UFO varies in response to the brightness of the source. In this work we perform flux-resolved X-ray spectroscopy to study the variability of the UFO and found that the ionisation parameter is correlated with the luminosity. In the brightest states the gas is almost completely ionised by the powerful radiation field and the UFO is hardly detected. This agrees with our recent results obtained with principal component analysis. We might have found the tip of the iceberg: the high ionisation of the outflowing gas may explain why it is commonly difficult to detect UFOs in AGN and possibly suggest that we may underestimate their actual feedback. We have also found a tentative correlation between the outflow velocity and the luminosity, which is expected from theoretical predictions of radiation-pressure driven winds. This trend is rather marginal due to the Fe XXV-XXVI degeneracy. Further work is needed to break such degeneracy through time-resolved spectroscopy

A rapidly spinning supermassive black hole at the centre of NGC 1365

Broad X-ray emission lines from neutral and partially ionized iron observed in active galaxies have been interpreted as fluorescence produced by the reflection of hard X-rays off the inner edge of an accretion disk. In this model, line broadening and distortion result from rapid rotation and relativistic effects near the black hole, the line shape being sensitive to its spin. Alternative models in which the distortions result from absorption by intervening structures provide an equally good description of the data, and there has been no general agreement on which is correct. Recent claims that the black hole (2 × 10^6 solar masses) at the centre of the galaxy NGC 1365 is rotating at close to its maximum possible speed rest on the assumption of relativistic reflection. Here we report X-ray observations of NGC 1365 that reveal the relativistic disk features through broadened Fe-line emission and an associated Compton scattering excess of 10–30 kiloelectronvolts. Using temporal and spectral analyses, we disentangle continuum changes due to time-variable absorption from reflection, which we find arises from a region within 2.5 gravitational radii of the rapidly spinning black hole. Absorption-dominated models that do not include relativistic disk reflection can be ruled out both statistically and on physical grounds

Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13

We report the detection of coherent pulsations from the ultraluminous X-ray source (ULX) NGC 7793 P13. The ≈0.42 s nearly sinusoidal pulsations were initially discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in 2016. We subsequently also found pulsations in archival XMM-Newton data taken in 2013 and 2014. The significant (≫5σ) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star, and given the observed peak luminosity of ≈10⁴⁰ erg s⁻¹ (assuming isotropy), it is well above the Eddington limit for a 1.4 M⨀ accretor. This makes P13 the second ULX known to be powered by an accreting neutron star. The pulse period varies between epochs, with a slow but persistent spin-up over the 2013–2016 period. This spin-up indicates a magnetic field of B ≈ 1.5 × 10¹² G, typical of many Galactic accreting pulsars. The most likely explanation for the extreme luminosity is a high degree of beaming; however, this is difficult to reconcile with the sinusoidal pulse profile.We would like the thank the referee for the helpful comments. M.J.M. acknowledges support from an STFC Ernest Rutherford fellowship and D.B. acknowledges financial support from the French Space Agency (CNES). This research has made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL, and has utilized the nustardas software package, jointly developed by the ASDC (Italy) and Caltech (USA). This research has also made use of data obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester, and also made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. This research has made use of a collection of ISIS functions (ISISscripts) provided by ECAP/Remeis observatory and MIT (http://www.sternwarte.uni-erlangen.de/isis/). Facilities: NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, XMM - , Swift -

XMM–Newton campaign on ultraluminous X-ray source NGC 1313 X-1: wind versus state variability

Most ultraluminous X-ray sources (ULXs) are thought to be powered by neutron stars and black holes accreting beyond the Eddington limit. If the compact object is a black hole or a neutron star with a magnetic field ≲1012 G, the accretion disc is expected to thicken and launch powerful winds driven by radiation pressure. Evidence of such winds has been found in ULXs through the high-resolution spectrometers onboardXMM–Newton, but several unknowns remain, such as the geometry and launching mechanism of these winds. In order to better understand ULX winds and their link to the accretion regime, we have undertaken a major campaign with XMM–Newton to study the ULX NGC 1313 X-1, which is known to exhibit strong emission and absorption features from a mildly relativistic wind. The new observations show clear changes in the wind with a significantly weakened fast component (0.2c) and the rise of a new wind phase which is cooler and slower (0.06–0.08c). We also detect for the first time variability in the emission lines which indicates an origin within the accretion disc or in the wind. We describe the variability of the wind in the framework of variable super-Eddington accretion rate and discuss a possible geometry for the accretion disc

THE RHYTHM OF FAIRALL 9. I. OBSERVING THE SPECTRAL VARIABILITY WITH XMM-NEWTON AND NuSTAR

© 2016. The American Astronomical Society. All rights reserved. We present a multi-epoch X-ray spectral analysis of the Seyfert 1 galaxy Fairall 9. Our analysis shows that Fairall 9 displays unique spectral variability in that its ratio residuals to a simple absorbed power law in the 0.5-10 keV band remain constant with time in spite of large variations in flux. This behavior implies an unchanging source geometry and the same emission processes continuously at work at the timescale probed. With the constraints from NuSTAR on the broad-band spectral shape, it is clear that the soft excess in this source is a superposition of two different processes, one being blurred ionized reflection in the innermost parts of the accretion disk, and the other a continuum component such as a spatially distinct Comptonizing region. Alternatively, a more complex primary Comptonization component together with blurred ionized reflection could be responsible