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 L_X > 5x10^40 erg/s) 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 onto 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 3x10^41 erg/s) 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 photon indexes ~ 1.7, and are potentially more variable on short timescales, with data consistent with ~ 10-20 per cent rms variability on timescales of 0.2-2 ks. 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 10^3-10^4 M_Sun. As such, this strengthens the case for these brightest ULXs as good candidates for the eventual conclusive detection of the highly elusive IMBHs. 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.
Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43~s in 2003 to 1.13~s in 2014. It has an isotropic peak luminosity of about 1000 times the Eddington limit for a NS at 17.1~Mpc. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magne…
Ultraluminous X-ray sources (ULXs) were an enigma since their discovery. ASCA showed that they are accreting black holes (BHs) with unknown mass, while Chandra & XMM data shows that the many of the lower luminosity ULXs can be explained by extreme accretion onto stellar remnant BHs. The discovery of hyperluminous X-ray sources has led to specu- lation that we may, once again, have found the elusive intermediate mass black holes, yet in the last year, a third sub-class of extreme ULXs has emerged, with their nature, as yet, un- known. Here we define and discuss the sub-classes of these fascinat…
Ultraluminous X-ray sources (ULXs), first observed ~30 years ago, have been argued as extreme stellar mass black hole binaries or a new class intermediate mass black hole. In order to settle this debate, scientists have utilised a wide range of telescopes, exploiting large sections of the electro-magnetic spectrum. Here we review some of the insight gained from these observational studies, collating an overview of our current position in ULX research.
There are two proposed explanations for ultraluminous X-ray sources (ULXs) with luminosities in excess of $10^{39}$ erg s$^{-1}$. They could be intermediate-mass black holes (more than 100-1,000, solar masses, $M_\odot$) radiating at sub-maximal (sub-Eddington) rates, as in Galactic black-hole X-ray binaries but with larger, cooler accretion disks. Alternatively, they could be stellar-mass black holes radiating at Eddington or super-Eddington rates. On its discovery, M101 ULX-1 had a luminosity of $3\times10^{39}$ erg s$^{-1}$ and a supersoft thermal disk spectrum with an exceptionally low tem…
In this proof-of-concept study we demonstrate that in a binary system mass can be transferred toward an accreting compact object at extremely high rate. If the transferred mass is efficiently converted to X-ray luminosity (with disregard of the classical Eddington limit) or if the X-rays are focused into a narrow beam then binaries can form extreme ULX sources with the X-ray luminosity of Lx>10^42 erg/s. For example, Lasota & King argued that the brightest known ULX (HLX-1) is a regular binary system with a rather low-mass compact object (a stellar-origin black hole or a neutron star). The pre…
Aims. We analyzed multi-epoch X-ray data of the Ultraluminous X-ray source (ULX) NGC 5055 X-1, with luminosity up to $2.32\times10^{40}\ \rm erg\ s^{-1}$, in order to constrain the physical parameters of the source. Methods. We performed timing and spectral analysis of Chandra and XMM-Newton observations. We used spectral models which assume the emission is from an accreting black hole system. We fit the data with a multicolor disk (MCD) combined with a powerlaw (PL) or a thermal Comptonization (NTHCOMP) component, and compared those fits with a slim disk model. Results. The lightcurves of the…
We explore the nature of ultraluminous X-ray sources through detailed investigations of their spectral shape using some of the highest quality data available in the XMM-Newton public archives. Phenomenological models allow us to characterise their spectra, while more 'physically-motivated' models enable us to explore the physical processes underlying these characteristics. These physical models imply the presence of extreme (probably super-Eddington) accretion on to stellar mass black holes.
We study the spectral properties and accretion flow behavior of an ultraluminous X-ray source M82\,X-1 using {\it NuSTAR} observations. We use the physical two component advective flow (TCAF) model to fit the data and to derive the accretion flow properties of the source. From the model fitted parameters, we found that M82\,X-1 is harboring an intermediate mass black hole at its centre, where the mass varies from $156.04^{+13.51}_{-15.30}$ to $380.96^{28.38}_{-29.76}$ M$_\odot$. The error weighted average mass of the black hole is $273\pm43$ M$_\odot$, which accreted in nearly super-Eddington …
The origin of Ultraluminous X-ray sources (ULXs) in external galaxies whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times is mysterious. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. Here we review the ULX properties, their X-ray spectra indicate a presence of hot winds in their accretion disks supposing the supercritical accretion. However, the strongest evidences come from optical spectroscopy. The spectra of the ULX count…
Ultraluminous X-ray sources (ULXs) represent a class of binary systems that are more luminous than any black hole in our Galaxy. The nature of these objects remained unclear for a long time. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. In the last few years our understating of these objects was significantly improved, which made the model of super-Eddington accretion much preferable. Both the X-ray and optical spectra provide evidence for the strong outflows coming from supercritical ac…
We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations indicates the presence of neutron-star accretors in three ULXs and therefore apparently super-Eddington luminosities. Optical and X-ray line profiles of ULXs and the properties of associated radio and optical nebulae suggest that ULXs produce powerful outflows, also indicative of super-Eddington accretion. We discuss models of super-Eddington accretio…
The origin of the high X-ray luminosities of most ultraluminous X-ray sources (ULXs) still remains poorly understood. Most of the scenarios proposed to explain their nature assume that ULXs are powered by accretion on to a black hole (BH). In this case, the detection of compact radio emission and the location of the ULXs in the Fundamental Plane (X-ray versus radio-luminosity plane) can provide an estimate of the ULX BH mass and help address the question of their physical nature. We present the results of a high-resolution (very long baseline interferometry) radio observational campaign aimed …
I report the analysis of an ultraluminous X-ray source (ULX) located in the galaxy NGC 1291. This X-ray point source is denominated IXO6 in the Catalog of Candidate IXO (Colbert & Ptak). An Intermediate-luminosity X-ray Object (IXO) is defined as an off-nuclear, compact object with luminosity Lx [2-10keV] >= 1039 erg s-1. The cutoff Lx is defined as a value greater than the Eddington luminosity of a 1.4 Mo black hole (10 38.3 erg s-1). IXO is an early denomination of what is call now a ULX point source. The Catalog was derived from a ROSAT survey and represents 87 IXOS in 54 galaxies. IXO6 was…
We study a large sample of dwarf galaxies using archival Chandra X-ray observations, with the aim of detecting accreting intermediate-mass black holes (IMBHs). IMBHs are expected to inhabit dwarf galaxies and to produce specific signatures in terms of luminosity and X-ray spectra. We report the discovery of an X-ray source associated with an Abell 85 dwarf galaxy that fits the IMBH description. The stellar mass of the host galaxy is estimated to be 2 $\times$ 10$^8$ $M_\odot$, which makes it one of the least massive galaxies to potentially host an accreting black hole. The source is detected i…
Luminosities of ultraluminous X-ray sources (ULXs) are uncomfortably large if compared to the Eddington limit for isotropic accretion onto stellar-mass object. Most often either supercritical accretion onto stellar mass black hole or accretion onto intermediate mass black holes is invoked the high luminosities of ULXs. However, the recent discovery of coherent pulsations from M82 ULX with NuSTAR showed that another scenario implying accretion onto a magnetized neutron star is possible for ULXs. Motivated by this discovery, we re-visited the available XMM-Newton archival observations of several…
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 w…
In our recent work, we demonstrated that a novel X-ray scaling method, originally introduced for Galactic black holes, could be reliably extended to estimate the mass of supermassive black holes accreting at moderate to high level. Here, we apply this X-ray scaling method to ultraluminous X-ray sources (ULXs) to constrain their \mbh. Using 49 ULXs with multiple \xmm\ observations, we infer that ULXs host both stellar mass BHs and intermediate mass BHs. The majority of the sources of our sample seem to be consistent with the hypothesis of highly accreting massive stellar BHs with $M_{\rm BH}\si…
Dynamical mass measurements hold the key to answering whether ultraluminous X-ray sources (ULXs) are intermediate mass black holes (IMBHs) or stellar mass black holes with special radiation mechanisms. NGC 1313 X-2 is so far the only ULX with HST light curves, the orbital period, and the black hole's radial velocity amplitude based on the He II $\lambda4686$Å disk emission line shift of $\sim200$ km/s. We constrain its black hole mass and other parameters by fitting observations to a binary light curve code with accommodations for X-ray heating of the accretion disk and the secondary. Given th…
Here we show that the overabundance of ultra-luminous, compact X-ray sources (ULXs) associated with moderately young clusters in interacting galaxies such as the Antennae and Cartwheel can be given an alternative explanation that does not involve the presence of intermediate mass black holes (IMBHs). We argue that gas density within these systems is enhanced by the collective potential of the cluster prior to being accreted onto the individual cluster members and, as a result, the aggregate X-ray luminosity arising from the neutron star cluster members can exceed $>10^{39} {\rm erg s^{-1}}$. V…
The extreme extragalactic sources known as Ultraluminous X-ray Sources (ULX) represent a unique testing environment for compact objects population studies and the accretion process. Their nature has long been disputed. Their luminosity, well above the Eddington luminosity for a stellar-mass black hole, can imply the presence of an intermediate-mass black hole or a stellar black hole accreting above the Eddington limit. Both these interpretations are important to understand better the accretion process and the evolution of massive black holes. The last few years have seen a dramatic improvement…
Most ultraluminous X-ray sources (ULXs) are believed to be X-ray binary systems, but previous observational and theoretical studies tend to prefer a black hole rather than a neutron star accretor. The recent discovery of 1.37 s pulsations from the ULX M82 X-2 has established its nature as a magnetized neutron star. In this work we model the formation history of neutron star ULXs in an M82- or Milky Way-like galaxy, by use of both binary population synthesis and detailed binary evolution calculations. We find that the birthrate is around $10^{-4}\, \rm yr^{-1}$ for the incipient X-ray binaries …
Recent observational and theoretical results have suggested that some of ultraluminous X-ray (ULX) sources may contain neutron star (NS) accretors. However, the formation channel and properties of donor stars of NS ULXs remain uncertain. By adopting the non-conservative and rotation-dependent mass transfer model in the primordial binary evolution, we investigate the way to form pulsar ULXs like observed pulsar ULXs in a systematic way. Our simulation results indicate that pulsar ULXs with Be stars and intermediate or/and high mass donors match observed apparent luminosities, orbital periods an…
Ultraluminous X-ray sources (ULXs) are bright extragalactic sources with X-ray luminosities above 10^39 erg/s powered by accretion onto compact objects. According to the first studies performed with XMM-Newton ULXs seemed to be excellent candidates to host intermediate-mass black holes (10^2-4 solar masses). However, in the last years the interpretation of super-Eddington accretion onto stellar-mass black holes or neutron stars for most ULXs has gained a strong consensus. One critical missing piece to confirm the super-Eddington scenario was the direct detection of the massive, radiatively-dri…
Ultraluminous X-ray sources (ULXs) are accreting black holes that may contain the missing population of intermediate mass black holes or reflect super-Eddington accretion physics. Ten years of Chandra and XMM-Newton observations of ULXs, integrated by multiband studies of their counterparts, have produced a wealth of observational data and phenomenological classifications. We review the properties of their host galaxies, list popular spectral models and implications for standard and supercritical accretion physics, demonstrate how X-ray timing of these objects places constraints on their masse…
We present the analysis of the X-ray variability and spectral timing properties of the ultraluminous X-ray source (ULX) NGC 5408 X-1, one of the most variable ULXs known so far. The variability properties are used as a diagnostic of the accretion state of the source and to derive estimates of the black hole (BH) mass. The observed high level of fast X-ray variability (fractional root-mean-square variability - rms - amplitude of $\sim$ 30 percent in the hard energy band), the hardening of the fractional rms spectrum, and the properties of the QPO, all resemble those of a source in a hard-interm…
Since their discovery, Ultraluminous X-ray sources (ULXs) have attracted attention due to their combination of extreme luminosities and extra-nuclear locations. However, they are a fairly rare phenomenon, and attempts to investigate the general properties of the population have been hindered by a relative lack of known sources. Here, we present a large catalogue of ULX candidates including 655 detections of 475 discrete sources, based on the 2XMM Serendipitous Survey. To demonstrate the potential of such a resource, we present some scientific analysis of this population, focusing on the spectr…
We report the detection of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13. The ~0.42s 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 (>>5 sigma) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star with an observed peak luminosity of ~1e40 erg/s (assuming isotropy), well above the Eddington limit for a 1.4 M_sun accretor. This makes P13 the seco…
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…
Ultraluminous X-ray sources are extragalactic, off-nucleus, point sources in galaxies with an X-ray luminosity above 3x10^39 erg/s, thought to be powered by accretion onto a compact object. Possible explanations include accretion onto neutron stars with strong magnetic fields, stellar-mass black holes (< 20 Msolar) at or in excess of the classical Eddington limit or intermediate-mass black holes (10^3-5 Msolar). The lack of sufficient energy resolution in previous analyses has hitherto prevented an unambiguous identification of any emission or absorption lines in the X-ray band, thereby preclu…
Most Ultraluminous X-ray sources (ULXs) are thought to be powered by super-Eddington accretion onto stellar-mass compact objects. Accretors in this extreme regime are naturally expected to ionise copious amounts of plasma in their vicinity and launch powerful radiation-driven outflows from their discs. High spectral resolution X-ray observations (with RGS gratings onboard XMM-Newton) of a few ULXs with the best datasets indeed found complex line spectra and confirmed such extreme (0.1-0.3c) winds. However, a search for plasma signatures in a large ULX sample with a rigorous technique has never…
We report on XMM-Newton/Chandra/Swift/HST observations of the ultraluminous X-ray source (ULX) in NGC 247, which is found to make transitions between the supersoft ultraluminous (SSUL) regime with a spectrum dominated by a cool ($\sim 0.1$ keV) blackbody component and the soft ultraluminous (SUL) regime with comparable luminosities shared by the blackbody and power-law components. Multi-epoch observations revealed an anti-correlation between the blackbody radius and temperature, $R_{\rm bb} \propto T_{\rm bb}^{-2.8 \pm 0.3}$, ruling out a standard accretion disk as the origin of the soft X-ray…
Ultraluminous pulsars are a definite proof that persistent super-Eddington accretion occurs in nature. They support the scenario according to which most Ultraluminous X-ray Sources (ULXs) are super-Eddington accretors of stellar mass rather than sub-Eddington intermediate mass black holes. An important prediction of theories of supercritical accretion is the existence of powerful outflows of moderately ionized gas at mildly relativistic speeds. In practice, the spectral resolution of X-ray gratings such as RGS onboard XMM-Newton is required to resolve their observational signatures in ULXs. Us…
Following a serendipitous detection with the NuSTAR observatory, we present a multi-epoch spectral and temporal analysis of an extreme ultraluminous X-ray source (ULX) located in the outskirts of the Circinus galaxy, hereafter Circinus ULX5, including coordinated follow-up observations with XMM-Newton and NuSTAR. The NuSTAR data presented here represent one of the first instances of a ULX reliably detected at hard (E>10 keV) X-rays. Circinus ULX5 is variable on long timescales by at least a factor of ~5 in flux, and was caught in a historically bright state during our 2013 observations (0.3-30…
In the last few years multiwavelength observations have boosted our understanding of Ultraluminous X-ray Sources (ULXs). Yet, the most fundamental questions on ULXs still remain to be definitively answered: do they contain stellar or intermediate mass black holes? How do they form? We investigate the possibility that the black holes hosted in ULXs originate from massive (40-120 $M_\odot$) stars in low metallicity natal environments. Such black holes have a typical mass in the range $\sim 30-90 M_\odot$ and may account for the properties of bright (above $\sim 10^{40}$ erg s$^{-1}$) ULXs. More …
M82 X-1, the brightest X-ray source in the galaxy M82, has been thought to be an intermediate-mass black hole (100 to 10,000 solar masses) because of its extremely high luminosity and variability characteristics, although some models suggest that its mass may be only about 20 solar masses. The previous mass estimates were based on scaling relations that use low-frequency characteristic timescales which have large intrinsic uncertainties. For stellar-mass black holes, we know that the high-frequency quasi-periodic oscillations (100-450 hertz) in the X-ray emission that occur in a 3:2 frequency …
We present highlights from a study of a sample of 10 extreme-luminosity candidate ultraluminous X-ray sources (L_X > 5 x 10^(40) erg sec^(-1)), all at distances < 100 Mpc, identified from a cross-correlation of the RC3 catalogue of galaxies with the 2XMM catalogue. Five of the sample have also been observed by Chandra. Of the 10 sources, seven reside in the disc or arms of spiral galaxies, and the remaining three are close to large elliptical galaxies. Unlike many less luminous ultraluminous X-ray sources, temporal variability is observed on short (ks) and long (year) timescales for most sourc…
How Beaming Shapes the Demographics of Ultraluminous X-ray Sources?
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（教育部现代天文与天体物理重点实验室（南京大学））
We report on a new XMM-Newton observation of NGC 247 from December 2009. The galaxy contains a supersoft, ultraluminous X-ray source (ULX) whose spectrum consists of a thermal component with a temperature about 0.1 keV and a power-law tail with a photon index around 2.5. The thermal emission is absolutely the dominant component, contributing 96% of the total luminosity in the 0.3-10 keV band. Variability is detected at timescales of 10^2 s and longer with a ν^-1 power spectrum. These properties are consistent with black hole binaries in the thermal state and suggest the presence of an intermed…
Recently, several ultraluminous X-ray (ULX) sources were shown to host a neutron star (NS) accretor. We perform a suite of evolutionary calculations which show that, in fact, NSs are the dominant type of ULX accretor. Although black holes (BH) dominate early epochs after the star-formation burst, NSs outweigh them after a few 100 Myr and may appear as late as a few Gyr after the end of the star formation episode. If star formation is a prolonged and continuous event (i.e., not a relatively short burst), NS accretors dominate ULX population at any time in solar metallicity environment, whereas …
Ultraluminous X-ray sources (ULXs) are a population of extragalactic objects whose luminosity exceeds the Eddington limit for a 10 Msun black hole (BH). Their properties have been widely interpreted in terms of accreting stellar-mass or intermediate-mass BHs. However at least three neutron stars (NSs) have been recently identified in ULXs through the discovery of periodic pulsations. Motivated by these findings we studied the spectral properties of a sample of bright ULXs using a simple continuum model which was extensively used to fit the X-ray spectra of accreting magnetic NSs in the Galaxy.…
Recent evidence - in particular the hard X-ray spectra obtained by NuSTAR, and the large amplitude hard X-ray variability observed when ultraluminous X-ray sources (ULXs) show soft spectra - reveals that common ULX behaviour is inconsistent with known sub-Eddington accretion modes, as would be expected for an intermediate-mass black hole (IMBH). Instead, it appears that the majority of ULXs are powered by super-Eddington accretion onto stellar-mass black holes. Here, we will review work that delves deeper into ULX spectral-timing behaviour, demonstrating it remains consistent with the expectat…
Most ultraluminous X-ray sources (ULXs) display a typical set of properties not seen in Galactic stellar-mass black holes (BHs): higher luminosity Lx > 3 10^39 erg/s, unusually soft X-ray components (kT < 0.3 keV) and a characteristic downturn in their spectra above ~ 5 keV. Such puzzling properties have been interpreted either as evidence of intermediate-mass BHs, or as emission from stellar-mass BHs accreting above their Eddington limit, analogous to some Galactic BHs at peak luminosity. Recently, a very soft X-ray spectrum has been observed in a rare and transient stellar-mass BH. Here we s…
Ultra-Luminous X-ray sources are accreting black holes that might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. We analyze the best X-ray timing and spectral data from the ULX in NGC 5408 provided by XMM-Newton. The main goal is to study the broad-band noise variability of the source. We found an anti-correlation of the fractional root-mean square variability versus the intensity of the source, similar to black-hole binaries during hard states.
The origin of Ultraluminous X-ray Sources (ULXs) in external galaxies whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times is mysterious. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. Here we review the ULX properties. Their X-ray spectra indicate a presence of hot winds in their accretion disks supposing the supercritical accretion. In recent years, new surprising results were discovered in X-ray data, ULX-pulsars and high-v…
We present Spitzer Infrared Spectrograph (IRS) observations of the ultra-luminous X-ray source (ULX) NGC 6946 X-1 and its associated nebula MF 16. This ULX has very similar properties to the famous Holmberg II ULX, the first ULX to show a prominent infrared [O IV] emission line comparable to those found in AGN. This paper attempts to constrain the ULX Spectral Energy Distribution (SED) given the optical/UV photometric fluxes and high-resolution X-ray observations. Specifically, Chandra X-ray data and published Hubble optical/UV data are extrapolated to produce a model for the full optical to X…
A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1
Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（拉普拉塔国立大学天文与地球物理科学学院）；Instituto de Astrofísica de Canarias (IAC)（加那利天体物理学研究所）；Departamento de Astrofísica, Universidad de La Laguna（拉古纳大学天体物理学系）；Space Telescope Science Institute（太空望远镜科学研究所）；Instituto Argentino de Radioastronomía (CCT La Plata, CONICET（阿根廷射电天文学研究所）；CICPBA（意大利国家天体物理研究所卡利亚里天文台）；UNLP)（空间科学研究所）；INAF, Osservatorio Astronomico di Cagliari（加泰罗尼亚空间研究学院）；Institute of Space Sciences (ICE, CSIC)；Institut d’Estudis Espacials de Catalunya (IEEC)
The atomic features in the X-ray spectra of the ultraluminous X-ray source (ULX) may be associated with the outflow (Middleton et al. 2015), which may provide a way to explore the physics of the ULXs. We construct a conical outflow model, and calculate the thermal X-ray Fe emission lines from the outflows. Our results show that thermal line luminosity decreases with increasing outflow velocity or/and opening angle of the outflow for a fixed kinetic power of the outflows. Assuming the kinetic power of the outflows to be comparable with the accretion power in the ULXs, we find that the equivalen…
We present results from a study of short term variability in 19 archival observations by XMM-Newton of 16 Ultraluminous X-ray Sources (ULXs). Eight observations (six sources) showed intrinsic variability with power spectra in the form of either a power law or broken power law-like continuum and in some cases quasi-periodic oscillations (QPOs). The remaining observations were used to place upper limits on the strength of possible variability hidden within. Seven observations (seven sources) yielded upper limits comparable to, or higher than, the values measured from those observations with dete…
Soft, potentially thermal spectral components observed in some ULXs can be fit with models for emission from cool, optically-thick accretion disks. If that description is correct, the low temperatures that are observed imply accretion onto "intermediate-mass" black holes. Subsequent work has found that these components may follow an inverse relationship between luminosity and temperature, implying a non-blackbody origin for this emission. We have re-analyzed numerous XMM-Newton spectra of extreme ULXs. Crucially, observations wherein the source fell on a chip gap were excluded owing to their u…
The point-like X-ray source HLX-1 is the brightest known ultraluminous X-ray source and likely the strongest intermediate-mass black hole candidate. HLX-1 is hosted in the S0 galaxy ESO 243-49, but offset with respect to the nucleus, and its optical counterpart was identified with a massive star cluster. In this paper, we study, through N-body/smoothed particle hydrodynamics simulations, the scenario where ESO 243-49 is undergoing (or just underwent) a minor merger with a gas-rich low-mass late-type galaxy. The simulations suggest that the observed star formation rate (SFR) in ESO 243-49 is a …
M82 hosts two well-known ultraluminous X-ray sources (ULXs). X-1, an intermediate-mass black hole (IMBH) candidate, and X-2, an ultraluminous X-ray pulsar (ULXP). Here we present a broadband X-ray spectral analysis of both sources based on ten observations made simultaneously with Chandra and NuSTAR. Chandra provides the high spatial resolution to resolve the crowded field in the 0.5--8 keV band, and NuSTAR provides the sensitive hard X-ray spectral data, extending the bandpass of our study above 10 keV. The observations, taken in the period 2015--2016, cover a period of flaring from X-1, allo…
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality, simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because of its high X-ray luminosity ($(2$$-$$4)\times10^{40}$ erg s$^{-1}$) and its spectral/temporal features, a closer inspection shows that its properties are more similar to those of a typical super-Eddington accreting stellar black hole and we classify it as a `broadened disc' ultraluminou…
The classical limit on the accretion luminosity of a neutron star is given by the Eddington luminosity. The advanced models of accretion onto magnetized neutron stars account for the appearance of magnetically confined accretion columns and allow the accretion luminosity to be higher than the Eddington value by a factor of tens. However, the recent discovery of pulsations from ultraluminous X-ray source (ULX) in NGC 5907 demonstrates that the accretion luminosity can exceed the Eddington value up to by a factor of 500. We propose a model explaining observational properties of ULX-1 in NGC 5907…
We present the first Spitzer Infrared Spectrograph (IRS) observations of the [O IV] 25.89 $μ$m emission line detected from the ultraluminous X-ray source (ULX) in Holmberg II. This line is a well established signature of high ionization, usually associated with AGN. Its detection suggests that the ULX has a strong impact on the surrounding gas. A Spitzer high resolution spectral map shows that the [O IV] is coincident with the X-ray position of the ULX. The ratios of the [O IV] to lower ionization lines are similar to those observed in AGN, suggesting that a strong UV and X-ray source is respo…
The X-ray spectrum of the nearest ultraluminous X-ray source, M33 X-8, obtained by Suzaku during 2010 January 11 -- 13, was closely analyzed to examine its nature. It is, by far, the only data with the highest signal statistic in 0.4 -- 10 keV range. Despite being able to reproduce the X-ray spectrum, Comptonization of the disk photons failed to give a physically meaningful solution. A modified version of the multi-color disk model, in which the dependence of the disk temperature on the radius is described as r^(-p) with p being a free parameter, can also approximate the spectrum. From this mo…
