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ULXs – Implications for our View of the Universe
Super-Eddington flows in stellar and supermassive black holes
I will review the observations of the most luminous accretion flows, in both stellar and supermassive black holes, and
point out the discrepancies between these and our best current theoretical models for such flows. I will also talk about the jet behaviour and what this might tell us about black hole spin and its evolution with cosmic time.
Radio emission from ultraluminous X-ray sources: the near-future
In this talk I will (briefly) discuss the observations to date of radio emission from ULXs, and then go on to outline the prospects for future discoveries and observations with next-generation radio telescopes, some of which are already almost here (MeerKAT, ASKAP, en route to the SKA).
A multi-wavelength view of ULXs: the zoo of optical counterparts
Similar to what is observed in Galactic X-ray binaries, it appears that there are many different types of donor stars able to power ULXs on different timescales (i.e. persistent vs. transient). In this talk, I will review and present new results on the different "classes" of ULXs optical counterparts encountered so far, encompassing different X-ray/optical behavior. I will show that the search for X-ray heated supergiant stars is probably key into constraining the mass function of ULXs, which will help to constrain the range of BH masses in ULXs.
Broadband X-ray Spectroscopy of ULXs with XMM and NuSTAR
NuSTAR and XMM have completed a program to obtain broadband 0.3 - 79 keV spectroscopy of a sample of the most luminous nearby ULXs. I will discuss the broadband spectral properties in the context of super-Eddington accretion models.
Near-infrared observations of ULXs
Are ultraluminous X-ray sources powered by stellar or intermediate mass black holes? To answer this question we need reliable mass measurements of these systems. The best way to do this would be to measure the radial velocity curves of the companion stars and thus derive the mass functions for these black holes. This has proven to be very difficult for ULXs because the optical light from these systems is dominated by the accretion disc.
However, some ULXs may have red supergiant donor stars, that are intrinsically bright in the near-infrared and may enable us to measure their radial velocity curves in that part of the spectrum. We have conducted a survey of nearby ULXs to search for near-infrared counterparts. Of our 62 targets, 11 have a counterpart that could potentially be a red supergiant. I will present results of this survey and preliminary results of our NIR spectroscopic follow-up of several of the sources where we detected a NIR counterpart.
Edwin van der Helm
The orbital companion of HLX-1
HLX-1 is currently the best intermediate mass black hole candidate, but the observed periodic X-ray light curve requires a good explanation. One of the currently favoured models is that there is a stellar companion on an eccentric orbit that tranfers mass to the black hole at pericentre through Roche lobe overflow. While this model explains the current observations, we do not know much about the origin and stability of such a system. We have performed SPH simulations to investigate the orbital evolution of a star on such an orbit and find that it is only marginally stable, raising questions about the origin of this system.
What are ULXs and what are the implications?
ULXs, Hyperaccretion, and All That
Several lines of argument suggest that many ULXs may be stellar-mass X-ray binary systems with extremely
high mass transfer rates. I review ideas suggesting how hyperaccretion can produce high apparent luminosities.
Puzzling accretion onto a black hole in M101 ULX-1
Ultraluminous X-ray sources could be the long-sought intermediate mass black holes
as necessary for current supermassive black hole formation theories; alternatively they could be
stellar mass black holes somehow managing to radiate at near- or even super-Eddington luminosities.
The key of ULX studies is to measure the black hole mass via dynamical means, which turns out to
extremely difficult. Our dynamical mass measurement for M101 ULX-1 shows that it is a stellar mass
black hole instead of an intermediate mass black hole. This fact makes it hard to understand the supersoft
X-ray spectrum of M101 ULX-1.
What can we learn from the environment and from multi-wavelength observations of ULXs?
A key to disentangle the puzzle of ultraluminous X-ray sources (ULXs) is to investigate their environment, by means of multi-wavelength observations. In this talk, I review what we know (and what we do not know) about the environment and about multi-wavelength observations of ULXs. I address this issue from both an observational and theoretical perspective. I focus on the link between star formation rate and ULXs in late-type galaxies. Recent observations indicate that there is a connection between ULXs and star forming regions, especially young stellar associations and star clusters. In several cases (e.g. M82 X-1 and a conspicuous group of ULXs in the Antennae galaxies), the ULX is displaced by a few tens of parsecs with respect to the closest young star cluster. Then, I discuss the main observational hints for an inverse correlation between the occurrence of ULXs and the metallicity of the environment. The extremely metal poor galaxies are a very important test bed for this correlation. When at least one ULX counterpart is known, optical and/or radio observations provide another important piece of information. Finally, I compare different models aimed to explain the link between ULXs and their environment.
Searching for jet radio emission from intermediate-mass black holes
Relativistic jets were first discovered in the nuclei of galaxies. The later finding of collimated jets arising from the compact counterpart of X-ray binaries suggested that jets should be present not only in supermassive and stellar-mass black holes (BHs), but also in intermediate-mass BHs (IMBHs).
According to evolutionary models of BH growth, IMBHs are the initial seed of supermassive BHs, which subsequently grow either through hierarchical BH merging or gas accretion. IMBHs with masses up to 1000 Msun could be formed from very young and massive stars via super-Eddington accretion or from the core collapse of young and massive stars, while < 1E6 Msun BHs could result from the direct collapse of pre-galactic gas discs. These processes should have left a population of IMBHs in the haloes of galaxies, where the presence of wandering BHs is also expected after tidal stripping of merging satellite galaxies.
The most probable IMBH candidates are the most extreme ultraluminous X-ray sources
(ULXs), whose X-ray luminosities in excess of 5E40 erg/s can difficulty be explained by stellar
evolution models or super-Eddington accretion. However,
observational evidence of IMBHs is scarce and no steady jet emission has ever been detected in an IMBH.
I will present the results of a program aimed at studying radio emission in ULXs and clarifying the nature of these sources.
The radio observations reveal compact radio emission from two ULXs, which become potential IMBH candidates, as well as the first detection of possible steady jet emission from an IMBH. With a total size of ~600 pc, this source could be the largest non-nuclear extragalactic jet ever discovered.
Theoretical ideas for the formation and feeding of IMBHs Cole Miller, University of Maryland
Models for the formation of black holes of hundreds to thousands of solar masses typically involve either very low-metallicity stars or interactions in dense stellar environments. Models for the feeding of such intermediate-mass black holes, especially in the context of ultraluminous X-ray sources, usually invoke Roche lobe overflow from a massive or giant companion. However, studies over the last several years have added both new challenges and new opportunities for these models. I will survey some of the new ideas and constraints related to formation and feeding of IMBHs.
Super-Eddington Accretion Flows and Outflows; a model for ULXs
A central source of the ULXs is a still debated issue. The large luminosity of ULXs indicates the intermediate mass black holes and/or super-Eddington accretion. We show the results of our numerical simulations of the super-Eddington accretion onto stellar mass black holes. The observed luminosity highly exceeds the Eddington luminosity for a face-on observer via the mildly collimated radiative flux. The strong radiation force induces the powerful outflows with large covering factor. Hot matter above the disk produces the high-energy X-ray photons via Comptonization. Our results can explain the large luminosity, X-ray spectra, and nebular emission of some ULXs.
Non-nuclear sources at non-X-ray wavelengths MW Pakull
I will summarize our knowledge about ULXs gained from observations at optical, infrared and radio wavelengths. The main emphasis is on their impact onto the interstellar medium via photoionisation and in the form of winds and jets. The study of the counterparts proper will be covered by Fabien Grisé.
For some years, highly ionized regions have been detected around several ULXs and these can be used to estimate their luminosities, irrespective of their possibly non-isotropic photon emission. Sometimes very extended bubble-like structures are observed that expand highly supersonically. Such shock-ionized nebulae are thought to be inflated by the mechanical power of the ULXs which is comparable to their high X-ray luminosities. The energies involved are typically two orders of magnitude higher than those of typical supernovae (remnants). The optical/radio emission from “ULX bubbles” can be detected long after the central source is extinguished, and from sources that are largely hidden from our line of sight. In this context I will finally discuss X-ray quiet bubbles and their relation to the Galactic microquasar SS433.
ALMA and implications for studying ULXs
Broadband radio emission from ULXs can be dominated by a surrounding bubble nebulae rather than from relativistic jets. High radio frequency observations will help constrain the spectral index of the nebulae to determine if the mechanism is synchrotron acceleration or more like Bremsstrahlung from a cooling shock front. With ALMA’s excellent sensitivity and resolution no only can could we determine spectra shape of the nebulae, but we should be able to resolve the compact emission from the ‘cores’ of ULXs. By determining the spectral break frequency of the core emission, we should be able to determine on what spatial scales the jets are launched, if there is any radio contribution from an RIAF disk and better constrain the disk-jet relationship.
BH masses in ULXs: 3 competing scenarios
Indirect estimates of black hole masses in ULXs can be grouped
into three scenarios: ordinary stellar-mass (up to 20 solar masses),
heavy stellar-mass (up to 80 solar masses) and intermediate-mass
black holes (from 100 to a few 1000 solar masses). The majority view
has fluctuated between these scenarios over the years. In the last few
years, the consensus seems to be swinging to the lower-mass side (and
correspondingly higher, super-critical accretion rates), except
for a handful of strong intermediate-mass candidates. Direct (kinematic)
mass constraints are within reach, and the first evidence points
to stellar-mass black holes, too.
Observational evidence for super-Eddington accretion in ULXs
It seems increasingly likely that most ULXs are powered by super-Eddington accretion on to stellar remnant black holes. In this talk I will discuss some of the key evidence for this, mainly focusing on results coming from X-ray observations. I will also present results from a recent X-ray spectral and timing analysis, which indicates that inclination is key in determining the observed properties of bright ULXs, and place this study in to the context of some of the recent literature.
Numerical Study of Jets and Outflows from Super Critical Accretion Disks
Black hole accretion disks are known to be one of the most energetic
system in high-energy astrophysical phenomena. Relativistic jets and
outflows are observed in some black hole binaries, but its acceleration
mechanism is poorly understood. Theoretically, two mechanisms are proposed,
MHD and radiation processes. The former is well studied using numerical
simulations. When the accretion rate is comparable or higher than the Eddington
value, not only the MHD process, but also the radiation process would operate
in jet acceleration and should be consistently taken into account. In this case
the latter process can be important. But most of numerical studies concentrates
on the case of the low mass accretion rate and there are few studies including
radiation effects consistently.
In this study, we perform 2.5 dimensional relativistic radiation magnetohydrodynamic
simulations to study a jet formation and its acceleration from super Eddington
accretion disks. We found that the jet is accelerated up to mildly relativistic speed
by the radiation force and is collimated by the Lorentz force. The terminal velocity
is determined by the force balance between the radiation force and its radiation
drag force. We will discuss details of acceleration mechanisms.
Searching for the elusive intermediate mass black holes
Intermediate mass black holes (IMBHs, ~1e2-1e5 solar masses) have been proposed to play a role in a number of astrophysical scenarios, including the formation of supermassive black holes through mergers and accretion. Some IMBHs may avoid such mergers or may even be ejected during merger interactions. IMBHs may reside in different locations, including globular clusters, low mass galaxies or in the brightest ULXs, the hyperluminous X-ray sources (HLXs). During this presentation I will present the evidence for an IMBH in the brightest ULX known to date, ESO 243-49 HLX-1. I will also discuss other searches to locate these elusive objects in order to constrain where these objects reside, how they accrete and the population of IMBH at the formation of the supermassive black holes and how many remain today.