X-ray studies of ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are extra-galactic, non-nuclear point sources, with X-ray luminosities brighter than 10^39 erg s^-1, in excess of the Eddington limit for 10 M_sun black holes. Recent results indicate that the majority of ULXs are stellar remnant black holes accreting material at or above the Eddington rate, rather than sub-Eddington accretion onto intermediate mass black holes. However, precisely how these ULXs accrete material at a super-Eddington rate remains an open question. This thesis focuses on the nature of these system as well as their environments, and attempts to explain physically how the sources operate in this super-critical accretion regime. This work begins with a study of the X-ray spectra of ULXs in very nearby galaxies (D < 5 Mpc). A range of physical models is used to explain the ULX spectra and to interpret the results physically. The outcomes consistently suggest that ULXs are stellar remnant black holes accreting material at or above the Eddington rate. It is demonstrated that the hard spectral component is consistent with emission from the inner radius of an advection-dominated slim accretion disc; the mass of black holes powering ULXs can be constrained from this hard emission, falling in the regime of stellar-mass black hole (~3 - 30 M_sun). Assuming that the soft spectral component represents soft thermal emission from an optically-thick outflowing wind, the size of the wind is constrained to be between ~10^4 – 10^6 R_g. We further explore the nature of ULXs by studying the X-ray spectral evolution of the individual source Holmberg IX X-1 with observed source luminosity. We find that the spectra tend to evolve from relatively flat or two-component spectra in the medium energy band, at lower luminosities, to a spectrum that is distinctly curved and disc-like at the highest luminosities. This spectral variability is consistent with the prediction of super-Eddington accretion models, in which the outflowing wind is expected to be launched from within the photospheric radius; the increase in accretion rate causes the more powerful wind to scatter a higher fraction of hard photons into the line of sight, while those that survive the passage through the wind will be Compton down-scattered to lower energies; these increase and soften the hard spectral component, resulting in a disc-like spectrum peaking at lower energy than the hard component seen at lower luminosity. Furthermore, we find observational evidence that the ULX might precess around its rotational axis, implied by a degree of degeneracy between different spectra observed at the same luminosity. Finally, we study the population of ULXs present in a sample of 17 nearby luminous infrared galaxies (LIRGs). It is found that the LIRGs possess significantly fewer ULXs per unit star formation rate than nearby normal galaxies, by a factor of about 10. We argue that part of the deficit could be due to the high metallicity environment of the host galaxies suppressing the formation of ULXs, and the lag between star formation starting and the appearance of ULXs; however, the majority of the deficit of ULXs is likely to be due to the high amount of gas and dust in the LIRGs obscuring a large fraction of ULXs

NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation

We present a study of the X-ray source NGC 2403 XMM4 (4XMM J073702.2+653934) based on 20 yr of archival observationswith XMM–Newton, Chandra, Swift, and NuSTAR. Although it has previously been classified as an ultraluminous X-ray source(ULX), we show that itsluminosity rarely, if ever, passesthe 1039 erg s−1 threshold luminosity for a ULX. It does, however, behavevery similarly to ULXs, with its 0.3–10 keV spectra well described by two thermal components, the softer of which behavesconsistently with the expectations for an advection-dominated disc (L ∝ T1.49±0.85), and we find tentative evidence for an extraspectral component above 10 keV. We also find moderately significant evidence for an absorption feature in one spectrum thatcould originate in an outflowing wind, although a cyclotron resonance scattering feature is also a possibility. Most intriguingly,we find a possible transient pulsation at ∼3.32 Hz in a short segment of one observation using an accelerated pulsation search.This evidence suggests that NGC 2403 XMM4 is displaying many of the hallmarks of super-Eddington accretion at luminositiesin the range 5–10 × 1038 erg s−1 that, when considered alongside the putative pulsation, points to the presence of a neutron staras the accreting object this system

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

Tracing the evolving X-ray reverberation lags within an individual AGN light curve

We present the Granger causality (GC) test for the X-ray reverberation analysis of Active Galactic Nuclei (AGN). If the light curves in the continuum-dominated band help predict (Granger cause) those dominated by reflection, the Granger lags that associate to the intrinsic reverberation lags can be inferred. We focus on six AGN observed by XMM-Newton, including the sources well-known to exhibit clear X-ray reverberation lags (IRAS 13224-3809 and 1H 0707-495) and those in which reverberation signatures are not well confirmed (MCG-6-30-15, IZW1, Mrk 704 and Mrk 1040). We employ the sliding-window algorithm and estimate the Granger (intrinsic) Fe-L lags along the light curve as the window moves through. This reveals the evolving lags towards the end of some individual observations, suggesting that the corona varies progressively. Occasionally, we observe two clearly separate lags that suggest an extended corona consisting of two zones while producing competing reverberation of two lags. While the GC test is purely hypothetical and might not explain true causality, our conclusion is that the lags are present and could be understood as reverberation lags. Assuming the lags changing solely with the corona, we find that the IRAS 13224-3809 corona varies between $\sim 10$-$25$ $r_{\rm g}$ and sometimes move to $\gtrsim 50$ $r_{\rm g}$. The corona of 1H 0707-495 and MCG-6-30-15 may be analogous to that of IRAS 13224-3809, while in IZw1, Mrk 704 and Mrk 1040 a more compact corona is expected.Comment: 13 pages, 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

The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: the Case of the Circumnuclear Starburst Ring in NGC 7552

Luminous infrared galaxies (LIRGs), the most extreme star-forming galaxies in the nearby (D$<$30 Mpc) Universe, show a notable X-ray emission deficiency (up to a factor of $\sim$10) compared with predictions from scaling relations of galaxy-wide high mass X-ray binary (HMXB) luminosity with star-formation rate. In the nearby ($\approx$20 Mpc) LIRG NGC 7552, the majority of the IR emission originates in a circumnuclear starburst ring, which has been resolved into several discrete knots of star formation. We present results from recent Chandra observations of NGC 7552, which reveal significant deficits in the 2-7 keV X-ray luminosities from two of the most powerful star-forming knots. We hypothesize that the expected luminous HMXB populations in these knots are either (1) obscured by very large column densities or (2) suppressed due to the knots having relatively high metallicity and/or very young ages ($\lesssim$ 5 Myr). We distinguish between these possibilities using data from recent NuSTAR observations, whose sensitivity above 10 keV is capable of uncovering heavily obscured HMXB populations, since emission at these energies is more immune to absorption effects. We find no evidence of a heavily obscured HMXB population in the central region of NGC 7552, suggesting suppressed HMXB formation. We further show that metallicity-dependent scaling relations cannot fully account for the observed deficit from the most powerful star-forming knots or the central region as a whole. Thus, we suggest that recent bursts in local star formation activity likely drive the high $L_{\rm{IR}}$ within these regions on timescales $\lesssim$ 5 Myr, shorter than the timescale required for the formation of HMXBs.Comment: Accepted for publication in ApJ (17 pages, 3 tables, 8 figures

From Elementary Particles to Black Holes (II)

บทคัดย่อ บทความวิชาการนี้เป็นบทความในส่วนที่ 2 ซึ่งมีเนื้อหาต่อเนื่องจากบทความในส่วนที่ 1 โดยมีจุดมุ่งหมายเพื่อแสดงให้เห็นถึงความสำคัญและบทบาทของฟิสิกส์พลังงานสูงในยุคปัจจุบัน รวมไปถึงความท้าทายที่นักฟิสิกส์กำลังเผชิญอยู่ บทความในส่วนที่ 2 นี้จะกล่าวถึงฟิสิกส์ของหลุมดำ โดยครึ่งแรกของบทความที่ 2 กล่าวถึงทฤษฎีและข้อขัดแย้งของหลุมดำในทฤษฎีสัมพัทธภาพกับทฤษฎีควอนตัม และหลักการการขจัดข้อขัดแย้งดังกล่าวที่มีการเสนอขึ้น ซึ่งอาจใช้เป็นวิธีการพัฒนาทฤษฎีควอมตัม-กราวิตีได้ในอนาคต ในส่วนครึ่งหลังของบทความส่วนที่ 2 ได้กล่าวถึงกระบวนการในการตรวจหาและศึกษาหลุมดำโดยใช้การสังเกตการณ์ทางดาราศาสตร์ รวมไปถึงปริศนาการเปล่งรังสีเอกซ์จากแก๊สรอบๆหลุมดำที่มีความสว่างเกินกว่าปกติ   คำสำคัญ: หลุมดำ  ระบบดาวคู่ ABSTRACT This article is the second and final part of the two-article-series review on current knowledge in high-energy physics.  The present article concerns the physics of black holes. The first part of the article deals with theoretical aspects of black holes and the apparent contradictions between quantum theory and general theory of relativities. Possible resolution, as well as its potential in the development of quantum gravity, is discussed. The second part of the article explains the astronomical methods for observing and studying black holes. It also discusses the puzzle of ultraluminous X-rays emitted from the gas clouds surrounding black holes.   Keywords: Black holes, Binary syste

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

Spectra of black hole accretion models of ultraluminous X-ray sources

© 2017 The Authors. We present general relativistic radiation magnetohydrodynamics simulations of super- Eddington accretion on a 10M ☉ black hole. We consider a range of mass accretion rates, black hole spins and magnetic field configurations. We compute the spectra and images of the models as a function of viewing angle and compare them with the observed properties of ultraluminous X-ray sources (ULXs). The models easily produce apparent luminosities in excess of 10 40 erg s -1 for pole-on observers. However, the angle-integrated radiative luminosities rarely exceed 2.5 × 10 39 erg s -1 even for mass accretion rates of tens of Eddington. The systems are thus radiatively inefficient, though they are energetically efficient when the energy output in winds and jets is also counted. The simulated models reproduce the main empirical types of spectra - disc-like, supersoft, soft, hard - observed in ultraluminous X-ray sources (ULXs). The magnetic field configuration, whether 'standard and normal evolution' (SANE) or 'magnetically arrested disc' (MAD), has a strong effect on the results. In SANE models, the X-ray spectral hardness is almost independent of accretion rate, but decreases steeply with increasing inclination. MAD models with non-spinning black holes produce significantly softer spectra at higher values of M●, even at low inclinations. MAD models with rapidly spinning black holes are unique. They are radiatively efficient (efficiency factor ~10-20 per cent), superefficient when the mechanical energy output is also included (70 per cent) and produce hard blazar-like spectra. In all models, the emission shows strong geometrical beaming, which disagrees with the more isotropic illumination favoured by observations of ULX bubbles