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 Physics , 2002, Abstract: We derive the X-ray luminosity function (XLF) of the X-ray source population detected in the Chandra observation of NGC4038/39 (the Antennae). We explicitly include photon counting and spectral parameter uncertainties in our calculations. The cumulative XLF is well represented by a flat power law ($\alpha=-0.47$), similar to those describing the XLFs of other star-forming systems (e.g. M82, the disk of M81), but different from those of early type galaxies. This result associates the X-ray source population in the Antennae with young High Mass X-ray Binaries. In comparison with less actively star-forming galaxies, the XLF of the Antennae has a highly significant excess of sources with luminosities above 10^{39} erg\s (Ultra Luminous Sources; ULXs). We discuss the nature of these sources, based on the XLF and on their general spectral properties, as well as their optical counterparts discussed in Paper III. We conclude that the majority of the ULXs cannot be intermediate mass black-holes (M > 10-1000 \msun) binaries, unless they are linked to the remnants of massive Population III stars (the Madau & Rees model). Instead, their spatial and multiwavelength properties can be well explained by beamed emission as a consequence of supercritical accretion. Binaries with a neutron star or moderate mass black-hole (up to 20\msun), and B2 to A type star companions would be consistent with our data. In the beaming scenario, the XLF should exibit caracteristic breaks that will be visible in future deeper observations of the Antennae.
 Xin-Lin Zhou Physics , 2014, DOI: 10.1016/j.newast.2014.11.002 Abstract: Ultra-luminous X-ray sources (ULXs) are off-nuclear X-ray sources in nearby galaxies with X-ray luminosities $\geq$ 10$^{39}$ erg s$^{-1}$. The measurement of the black hole (BH) masses of ULXs is a long-standing problem. Here we estimate BH masses in a sample of ULXs with XMM-Newton observations using two different mass indicators, the X-ray photon index and X-ray variability amplitude based on the correlations established for active galactic nuclei (AGNs). The BH masses estimated from the two methods are compared and discussed. We find that some extreme high-luminosity ($L_{\rm X} >5\times10^{40}$ erg s$^{-1}$) ULXs contain the BH of 10$^{4}$-10$^{5}$ $M_\odot$. The results from X-ray variability amplitude are in conflict with those from X-ray photon indices for ULXs with lower luminosities. This suggests that these ULXs generally accrete at rates different from those of X-ray luminous AGNs, or they have different power spectral densities of X-ray variability. We conclude that most of ULXs accrete at super-Eddington rate, thus harbor stellar-mass BH.
 Physics , 2013, Abstract: The nature of ultra-luminous X-ray sources (ULXs), which are off-nuclear extragalactic X-ray sources that exceed the Eddington luminosity for a stellar-mass black hole, is still largely unknown. They might be black hole X-ray binaries in a super-Eddington accretion state, possibly with significant beaming of their emission, or they might harbor a black hole of intermediate mass (10^2 to 10^5 solar masses). Due to the enormous amount of energy radiated, ULXs can have strong interactions with their environment, particularly if the emission is not beamed and if they host a massive black hole. We present early results of a project that uses archival Herschel infrared observations of galaxies hosting bright ULXs in order to constrain the nature of the environment surrounding the ULXs and possible interactions. We already observe a spatial correlation between ULXs and dense clouds of cold material, that will be quantified in subsequent work. Those observations will allow us to test the similarities with the environment of Galactic high mass X-ray binaries. This project will also shed light on the nature of the host galaxies, and the possible factors that could favor the presence of a ULX in a galaxy.
 Physics , 2002, DOI: 10.1086/341862 Abstract: We report the optical identifications of two X-ray luminous point sources in the spiral galaxy NGC4565 based on archive data of Chandra and the Hubble Space Telescope. The central X-ray point source, RXJ1236.3+2559, is found to be the nucleus of NGC4565 with an X-ray luminosity of Lx= 4.3 10^{39} erg. We show that its multi-waveband properties are consistent with it being a low-luminosity active galactic nucleus. A faint optical counterpart with B=25.1 and I=24.0 was identified for the off-nucleus X-ray point source, RXJ1236.2+2558. Its extinction corrected B magnitude is estimated to be 24.5. The X-ray to optical flux ratio (fx/fB) is about 540. From the optical and X-ray properties we argue that RXJ1236.2+2558 is an ultra-luminous X-ray compact source with Lx=6.5 10^{39} erg. The source is probably located in a faint globular cluster at the outer edge of NGC4565's bulge.
 Aristotle Socrates Physics , 2011, DOI: 10.1088/2041-8205/756/1/L1 Abstract: The photon spectral energy distribution of the powerful transient Sw J1644+57 resembles those of the brightest Ultra-Luminous X-ray sources (ULXs). The transient nature of Sw J1644+57 is likely the result of a tidal disruption of a star by a super-massive black hole. The stellar disk generates accretion power at super-Eddington rates and the observational properties of Sw J1644+57 indicate -- in analogy with ULXs -- that the accretion flow maintains a high level of radiative efficiency with a corresponding super-Eddington luminosity. Due to its similarity to ULXs, this powerful transient may be thought of as an Ultra-Luminous X-ray event (ULX-E). Observational tests for this ULX-E model are proposed as well.
 Physics , 2015, DOI: 10.1088/0004-637X/812/2/166 Abstract: Lyman Break Analogs (LBA) are local proxies of high-redshift Lyman Break Galaxies (LBG). Studies of nearby starbursts have shown that Lyman continuum and line emission are absorbed by dust and that the Lyman-alpha is resonantly scattered by neutral hydrogen. A source of feedback is required to prevent scattering and allow the Lyman-alpha emission to escape. There are two X-ray point sources embedded in the Lyman Break Analog (LBA) galaxy Haro 11. Haro 11 X-1 is an extremely luminous (L$_{X} \sim 10^{41}$ ergs s$^{-1}$), spatially compact source with a hard X-ray spectrum. Haro 11 X-1 is similar to the extreme Black Hole Binary (BHB) M82 X-1. The hard X-ray spectrum indicates Haro 11 X-1 may be a Black Hole Binary (BHB) in a low accretion state. The very high X-ray luminosity suggests an intermediate mass black hole that could be the seed for formation of a supermassive black hole. Source Haro 11 X-2 has an X-ray luminosity L$_{X} \sim 5\times10^{40}$ ergs s$^{-1}$ and a soft X-ray spectrum. This strongly suggests that Haro 11 X-2 is an X-ray binary in the ultra luminous state. Haro 11 X-2 is coincident with the star forming knot that is the source of the Lyman-alpha emission, raising the possibility that strong winds from X-ray binaries play an important part in injecting mechanical power into the Interstellar Medium (ISM), thus blowing away neutral material from the starburst region and allowing the Lyman-alpha to escape. We suggest that feedback from X-ray binaries may play a significant role in allowing Lyman-alpha emission to escape from galaxies in the early universe.
 Physics , 2002, DOI: 10.1086/368174 Abstract: We report the first results of the Chandra temporal monitoring of the ultra-luminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/39). Observations at four different epochs, covering time scales of 2 years to 2 months, show variability in seven out of nine ULXs, confirming that they are likely to be accreting compact X-ray binaries (XRBs). The seven variable ULXs exhibit a variety of temporal and spectral behaviors: one has harder X-ray colors with decreasing luminosity, similar to the black hole binary Cyg X-1, but four other ULXs show the opposite behavior. We suggest that the latter may be black-hole binaries accreting at very high rates.
 Physics , 2000, DOI: 10.1086/319125 Abstract: The Ultra Luminous X-ray Sources (ULXs) are unique in exhibiting moderately bright X-ray luminosities, $L_{\rm x} \sim 10^{38-40} {\rm erg~s^{-1}}$, and relatively high blackbody temperatures, $\Tin \sim 1.0-2.0 {\rm keV}$. From the constraint that $L_{\rm x}$ cannot exceed the Eddington luminosity, $L_{\rm E}$, we require relatively high black-hole masses, $M\sim 10-100 M_\odot$, however, for such large masses the standard disk theory predicts lower blackbody temperatures, $\Tin < 1.0$ keV. To understand a cause of this puzzling fact, we carefully calculate the accretion flow structure shining at $\sim L_{\rm E}$, fully taking into account the advective energy transport in the optically thick regime and the transonic nature of the flow. Our calculations show that at high accretion rate ($\dot M \ga 30~L_{\rm E}/c^2$) an apparently compact region with a size of $\Rin \simeq (1-3)\rg$ (with $\rg$ being Schwarzschild radius) is shining with a blackbody temperature of $\Tin \simeq 1.8 (M/10M_\odot)^{-1/4}$ keV even for the case of a non-rotating black hole. Further, $\Rin$ decreases as $\dot M$ increases, on the contrary to the canonical belief that the inner edge of the disk is fixed at the radius of the marginally stable last circular orbit. Accordingly, the loci of a constant black-hole mass on the "H-R diagram" (representing the relation between $L_{\rm x}$ and $\Tin$ both on the logarithmic scales) are not straight but bent towards the lower $M$ direction in the frame of the standard-disk relation.
 Physics , 2014, DOI: 10.1017/pasa.2014.1 Abstract: Many upcoming surveys, particularly in the radio and optical domains, are designed to probe either the temporal and/or the spatial variability of a range of astronomical objects. In the light of these high resolution surveys, we review the subject of ultra-luminous X-ray (ULX) sources, which are thought to be accreting black holes for the most part. We also discuss the sub-class of ULXs known as the hyper-luminous X-ray sources, which may be accreting intermediate mass black holes. We focus on some of the open questions that will be addressed with the new facilities, such as the mass of the black hole in ULXs, their temporal variability and the nature of the state changes, their surrounding nebulae and the nature of the region in which ULXs reside.
 Physics , 2001, DOI: 10.1086/321418 Abstract: Using the X-ray data taken with ASCA, a detailed analysis was made of intensity and spectral variations of three ultra-luminous extra-galactic compact X-ray sources (ULXs); IC 342 source 1, M81 X-6, and NGC 1313 source B, all exhibiting X-ray luminosity in the range 10^{39}-1.5x10^{40} erg s^{-1}. As already reported, IC 342 source 1 showed short-term X-ray intensity variability by a factor of 2.0 on a typical time scale of 10 ks. M81 X-6 varied by a factor of 1.6 across seven observations spanning 3 years, while NGC 1313 source B varied by a factor of 2.5 between two observations conducted in 1993 July and 1995 November. The ASCA spectra of these sources, acquired on these occasions, were all described successfully as optically-thick emission from standard accretion disks around black holes. This confirms previous ASCA works which explained ULXs as mass-accreting massive black-hole binaries. In all three sources, the disk color temperature was uncomfortably high at T_{in}=1.0-2.0 keV, and was found to vary in proportion to the square-root of the source flux. The apparent accretion-disk radius is hence inferred to change as inversely proportional to T_{in}. This suggests a significant effect of advection in the accretion disk. However, even taking this effect fully into account, the too high values of T_{in} of ULXs cannot be explained. Further invoking the rapid black-hole rotation may give a solution to this issue.
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