Coronal height constraint in IRAS 13224-3809 and 1H 0707-495 by the random forest regressor

We develop a random forest regressor (RFR) machine learning model to trace the coronal evolution in two highly variable active galactic nuclei (AGNs) IRAS 13224-3809 and 1H 0707-495 observed with XMM-Newton, by probing the X-ray reverberation features imprinted on their power spectral density (PSD) profiles. Simulated PSDs in the form of a power-law, with similar frequency range and bins to the observed data, are produced. Then, they are convolved with relativistic disc-response functions from a lamp-post source before being used to train and test the model to predict the coronal height. We remove some bins that are dominated by Poisson noise and find that the model can tolerate the frequency-bin removal up to $\sim 10$ bins to maintain a prediction accuracy of $R^{2} > 0.9$. The black hole mass and inclination should be fixed so that the accuracy in predicting the source height is still $> 0.9$. The accuracy also increases with the reflection fraction. The corona heights for both AGN are then predicted using the RFR model developed from the simulated PSDs whose frequency range and bins are specifically adjusted to match those from each individual observation. The model suggests that their corona varies between $\sim~5 - 18~r_{\rm g}$, with $R^{2} > 0.9$ for all observations. Such high accuracy can still be obtained if the difference between the true mass and the trained value is $\lesssim 10\%$. Finally, the model supports the height-changing corona under the light-bending scenario where the height is correlated to source luminosity in both IRAS 13224-3809 and 1H 0707-495.Comment: 9 Figures, 1 Table, Accepted for publication in MNRA

A deficit of ultraluminous X-ray sources in luminous infrared galaxies

We present results from a Chandra study of ultraluminous X-ray sources (ULXs) in a sample of 17 nearby (DL < 60 Mpc) luminous infrared galaxies (LIRGs), selected to have star formation rates (SFRs) in excess of 7 M⊙ yr−1 and low foreground Galactic column densities (NH ≲ 5 × 1020 cm−2). A total of 53 ULXs were detected and we confirm that this is a complete catalogue of ULXs for the LIRG sample. We examine the evolution of ULX spectra with luminosity in these galaxies by stacking the spectra of individual objects in three luminosity bins, finding a distinct change in spectral index at luminosity ∼2 × 1039 erg s−1. This may be a change in spectrum as 10 M⊙ black holes transit from an ∼ Eddington to a super-Eddington accretion regime, and is supported by a plausible detection of partially ionized absorption imprinted on the spectrum of the luminous ULX (LX ≈ 5 × 1039 erg s−1) CXOU J024238.9-000055 in NGC 1068, consistent with the highly ionized massive wind that we would expect to see driven by a super-Eddington accretion flow. This sample shows a large deficit in the number of ULXs detected per unit SFR (0.2 versus 2 ULXs, per M⊙ yr−1) compared to the detection rate in nearby (DL < 14.5 Mpc) normal star-forming galaxies. This deficit also manifests itself as a lower differential X-ray luminosity function normalization for the LIRG sample than for samples of other star-forming galaxies. We show that it is unlikely that this deficit is a purely observational effect. Part of this deficit might be attributable to the high metallicity of the LIRGs impeding the production efficiency of ULXs and/or a lag between the star formation starting and the production of ULXs; however, we argue that the evidence – including very low NULX/LFIR, and an even lower ULX incidence in the central regions of the LIRGs – shows that the main culprit for this deficit is likely to be the high column of gas and dust in these galaxies, that fuels the high SFR but also acts to obscure many ULXs from our view

Thai national telescope studies of ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are extra-galactic, non-nuclear sources with X-ray luminosity in excess of 10^39 erg s^–1. It has been thought that the majority of ULX populations are stellar-mass objects accreting matter at a super-Eddington rate. Although ULX studies are often focused in the X-ray regime, this work studied the ULXs in the optical regime, identified as the ULX counterparts (CTPs). The optical variability of nine CTPs were observed using the 2.4-m Thai National Telescope. Out of the nine ULXs, we detected three ULXs exhibiting strong variability up to ~1 magnitude, suggesting that the CTP light does not come from the donor star's emission. The paper discusses the physical origins of the variability which potentially explain the observed light curves

The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1

We present a new analysis of X-ray spectra of the archetypal ultraluminous X-ray source (ULX) Holmberg IX X-1 obtained by the Swift, XMM–Newton and NuSTAR observatories. This ULX is a persistent source, with a typical luminosity of ∼1040 erg s−1, that varied by a factor of 4–5 over eight years. We find that its spectra tend to evolve from relatively flat or two-component spectra in the medium energy band (1–6 keV), at lower luminosities, to a spectrum that is distinctly curved and disc-like at the highest luminosities, with the peak energy in the curved spectrum tending to decrease with increased luminosity. We argue that the spectral evolution of the ULX can be explained by super-Eddington accretion models, where in this case we view the ULX down the evacuated funnel along its rotation axis, bounded by its massive radiatively driven wind. The spectral changes then originate in enhanced geometric beaming as the accretion rate increases and wind funnel narrows, causing the scattered flux from the central regions of the supercritical flow to brighten faster than the isotropic thermal emission from the wind, and so the curved hard spectral component to dominate at the highest luminosities. The wind also Compton down-scatters photons at the edge of the funnel, resulting in the peak energy of the spectrum decreasing. We also confirm that Holmberg IX X-1 displays spectral degeneracy with luminosity, and suggest that the observed differences are naturally explained by precession of the black hole rotation axis for the suggested wind geometry

Mapping the X-Ray Corona Evolution of IRAS 13224-3809 with the Power Spectral Density

Abstract We develop the power spectral density (PSD) model to explain the nature of the X-ray variability in IRAS 13224–3809, including the full effects of the X-ray reverberation due to the lamppost source. We utilize 16 XMM-Newton observations individually as well as group them into three different luminosity bins: low, medium, and high. The soft (0.3–1 keV) and hard (1.2–5 keV) PSD spectra are extracted and simultaneously fitted with the model. We find that the corona height changes from h ∼ 3 r g during the lowest luminosity state to ∼25 r g during the highest luminosity state. This provides further evidence that the source height from the reverberation data is significantly larger than what is constrained by the spectral analysis. Furthermore, as the corona height increases, the energy spectrum tends to be softer while the observed fractional excess variance, F var, reduces. We find that the PSD normalization is strongly correlated with F var and moderately correlated with the PSD bending index. Therefore, the normalization is dependent on the accretion rate that controls the intrinsic shape of the PSD. While the intrinsic variability of the disk is manifested by the reverberation signals, the disk and corona may evolve independently. Our results suggest that, as the source height increases, the disk itself generates less overall variability power but more high-frequency variability resulting in the PSD spectrum that flattens out (i.e., the inner disk becomes more active). Using the luminosity-bin data, the hint of the Lorentzian component is seen, with the peak appearing at lower frequencies with increasing luminosity.</jats:p

Evolution of the truncated disc and inner hot-flow of GX 339-4

Aims. We study the changes in geometry of the truncated disc and the inner hot-flow of GX 339-4 by analyzing the Power SpectralDensity (PSD) extracted from six XMM-Newton observations taken at the very end of an outburst. Methods. A theoretical model of the PSD of GX 339-4 in the 0.3-0.7 keV (thermal reverberation dominated) and 0.7-1.5 keV (disc continuum dominated) energy bands is developed. The model assumes the standard accretion disc to be truncated at a specific radius, inside of which are two distinct hot-flow zones: one spectrally soft and the other spectrally hard. The effects of disc-fluctuations and thermal reverberation are taken into account. Results. This model successfully produces the traditional bumpy PSD profiles and provides good fits to the GX 339-4 data. The truncation radius is found to increase from $r_{\rm trc}$ $\sim$ 10 to 55 $r_{\rm g}$ as the source luminosity decreases, strongly confirming that the truncation radius can be characterized as a function of luminosity. Keeping in mind the large uncertainty in previous measurements of the truncation radius, our values are larger than some obtained from spectroscopic analysis, but smaller than those implied from reverberation lag analysis. Furthermore, the size of two inner hot-flow zones which are spectrally hard and spectrally soft are also growing from $\sim$ 5 to 27 $r_{\rm g}$ and from $\sim$ 3 to 26 $r_{\rm g}$, respectively, as the flux decreases. We find that the radial range of inner hard zone is always larger than the range of the soft hot-flow zone, but by a comparatively small factor of $\sim$ 1.1-2.2Comment: 10 pages, 8 figures, 2 tables, accepted for publication in A&