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Register a new userThe nature of the ultraluminous X-ray sources inside galaxies and their relation to local QSOs
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It is suggested that many of the ultraluminous compact x-ray sources now being found in the main bodies of galaxies, particularly those that are active, like M82, NGC 3628 and others, are local QSOs, or BL Lac objects, with high intrinsic redshifts in the process of being ejected from those galaxies. Evidence in support of this hypothesis is summarized.
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It is shown that all of the 32 point X-ray sources which lie within about 10 of the centre of nearby galaxies, and which have so far been optically identified are high redshift objects - AGN or QSOs. Thus the surface density of these QSOs p similar or equal to 0.1 per square arc minute. Some of them were originally discovered as X-ray sources and classified as ultraluminous X-ray sources (ULXs), nearly all of which lie near the centers of active galaxies. We demonstrate that this concentration around galactic nuclei is of high statistical significance: the probabiliy that p that they are accidental lies in the range one in a thousand to one in ten thousand, and apparently this excess cannot be accounted for by microlensing.
The advanced capabilities of the Chandra and XMM-Newton observatories mean that, for the first time, the detailed study of the brightest point-like X-ray sources in nearby galaxies outside of the local group is a realistic aim. Here, we present the results of a Chandra ACIS-S study of two of the nearest and brightest sources in the rare ultraluminous (L(X) > 10^39 erg s^-1) X-ray source (ULX) class, NGC 5204 X-1 and NGC 4559 X-1. When considered with new optical integral field spectroscopy data this provides powerful diagnostics as to the nature of these sources, in particular suggesting that NGC 5204 X-1 is a high-mass X-ray binary, and showing new evidence linking it to the Galactic microquasar phenomenon. We also find that both ULX appear to be located in cavities in emission-line gas nebulae that surround the sources. In addition, we present the results of a Chandra observation of the interacting galaxies NGC 4485/NGC 4490, a pair of late-type spiral galaxies that, remarkably, contain a total of six ULX. We identify one as a supernovae, and the remainder as probable black hole X-ray binaries. All six are located in star formation regions, underlining the emerging link between ULX and active star formation activity.
There exists a significant population of broad line, z~2 QSOs which have heavily absorbed X-ray spectra. Follow up observations in the submillimetre show that these QSOs are embedded in ultraluminous starburst galaxies, unlike most unabsorbed QSOs at the same redshifts and luminosities. Here we present X-ray spectra from XMM-Newton for a sample of 5 such X-ray absorbed QSOs that have been detected at submillimetre wavelengths. We also present spectra in the restframe ultraviolet from ground based telescopes. All 5 QSOs are found to exhibit strong C IV absorption lines in their ultraviolet spectra with equivalent width > 5 Angstroms. The X-ray spectra are inconsistent with the hypothesis that these objects show normal QSO continua absorbed by low-ionization gas. Instead, the spectra can be modelled successfully with ionized absorbers, or with cold absorbers if they posess unusually flat X-ray continuum shapes and unusual optical to X-ray spectral energy distributions. We show that the ionized absorber model provides the simplest, most self-consistent explanation for their observed properties. We estimate that the fraction of radiated power that is converted into kinetic luminosity of the outflowing winds is typically ~4 per cent, in agreement with recent estimates for the kinetic feedback from QSOs required to produce the M - sigma relation, and consistent with the hypothesis that the X-ray absorbed QSOs represent the transition phase between obscured accretion and the luminous QSO phase in the evolution of massive galaxies.
Using the Chandra Source Catalog 2.0 and a newly compiled catalogue of galaxies in the local Universe, we deliver a census of ultraluminous X-ray source (ULX) populations in nearby galaxies. We find 629 ULX candidates in 309 galaxies with distance smaller than 40,Mpc. The foreground/background contamination is ${sim}20%$. The ULX populations in bona-fide star-forming galaxies scale on average with star-formation rate (SFR) and stellar mass ($M_star$) such that the number of ULXs per galaxy is $0.45^{+0.06}_{-0.09}timesfrac{rm SFR}{rm M_odot,yr^{-1}}{+}3.3^{+3.8}_{-3.2}timesfrac{M_star}{rm M_odot}$. The scaling depends strongly on the morphological type. This analysis shows that early spiral galaxies contain an additional population of ULXs that scales with $M_star$. We also confirm the strong anti-correlation of the ULX rate with the host galaxys metallicity. In the case of early-type galaxies we find that there is a non-linear dependence of the number of ULXs with $M_star$, which is interpreted as the result of star-formation history differences. Taking into account age and metallicity effects, we find that the predictions from X-ray binary population synthesis models are consistent with the observed ULX rates in early-type galaxies, as well as, spiral/irregular galaxies.
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 accretion and their relation to the observed behaviors of ULXs. We review the evidence for intermediate mass black holes in ULXs. We consider the implications of ULXs for super-Eddington accretion in active galactic nuclei, heating of the early universe, and the origin of the black hole binary recently detected via gravitational waves.
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