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Register a new userA census of ultraluminous X-ray sources in the local Universe
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Added by
Konstantinos Kovlakas
Publication date
2020
fields
Physics
and research's language is
English
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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.
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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.
Using Chandra observations we have identified a sample of seven off-nuclear X-ray sources, in the redshift range z=0.072-0.283, located within optically bright galaxies in the COSMOS Survey. Using the multi-wavelength coverage available in the COSMOS field, we study the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from H_alpha measurements and their stellar masses using SED fitting techniques with the aim to compute the probability to have an off-nuclear source based on the host galaxy properties. We divide the host galaxies in different morphological classes using the available ACS/HST imaging. We find that our ULXs candidates are located in regions of the SFR versus M$_star$ plane where one or more off-nuclear detectable sources are expected. From a morphological analysis of the ACS imaging and the use of rest-frame colours, we find that our ULXs are hosted both in late and early type galaxies. Finally, we find that the fraction of galaxies hosting a ULX ranges from ~0.5% to ~0.2% going from L[0.5-2 keV]=3 x 10^39 erg s^-1 to L[0.5-2 keV]= 2 x 10^40 erg s^-1.
We study spectral variability of 11 ultraluminous X-ray sources (ULX) using archived XMM-Newton and Chandra observations. We use three models to describe the observed spectra: a power-law, a multi-colour disc (MCD) and a combination of these two models. We find that 7 ULXs show a correlation between the luminosity Lx and the photon index Gamma. Furthermore, 4 out of these 7 ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an Lx-Gamma anti-correlation. The spectra of 4 ULXs in the sample can be adequately fitted with a MCD model. We compare these sources to known black hole binaries (BHB) and find that they follow similar paths in their luminosity-temperature diagrams. Finally we show that the `soft excess reported for many of these ULXs at about 0.2 keV seems to roughly follow a trend Lsoft propto T^{-3.5} when modelled with a power-law plus a `cool MCD model. This is contrary to the L propto T^4 relation that is expected from theory and what is seen for many accreting BHBs. The observed trend could instead arise from disc emission beamed by an outflowing wind around a about 10 solar mass black hole.
We have carried out a systematic X-ray spectral analysis of a sample of low luminosity quasars (LLQSO) to investigate the nature of the central engines of these sources. The optically-selected LLQSO sample consists of close, known bright active galactic nuclei (AGN) which serves as an important link between the powerful quasars at higher redshift and local Seyfert galaxies. We find that although the bolometric luminosities of the LLQSOs are lower than those of the higher redshift quasars by almost an order of magnitude, their distribution of the Eddington rate $lambda_{rm Edd}$ is similar. We detect a strong anti-correlation between $alpha_{rm OX}$ and $L_{2500 rm AA}$, as has also been detected in several other quasar studies with large sample sizes, indicating that as the UV luminosity of the source increases, the X-ray luminosity decreases. We do not detect any significant neutral obscuration ($N_{rm H} ge10^{22}, rm cm^{-2}$) in the X-ray spectra of the LLQSOs, and hence rule out obscuration as a possible cause for their lower luminosity. We conclude that the central engines of the LLQSOs function similarly to those of the higher redshift quasars, and the difference is possibly because of the fact that the LLQSOs have lower black hole masses. We do not find any correlation between the molecular gas in the host galaxies and accretion states of the AGN. This indicates that the presence of molecular gas in the host galaxies of the LLQSOs does not significantly influence the instantaneous accretion rates of their SMBHs.
We present the Census of the Local Universe (CLU) narrow-band survey to search for emission-line (ha) galaxies. CLU-ha~has imaged $approx$3$pi$ of the sky (26,470~deg$^2$) with 4 narrow-band filters that probe a distance out to 200~Mpc. We have obtained spectroscopic follow-up for galaxy candidates in 14 preliminary fields (101.6~deg$^2$) to characterize the limits and completeness of the survey. In these preliminary fields, CLU can identify emission lines down to an ha~flux limit of $10^{-14}$~$rm{erg~s^{-1}~cm^{-2}}$ at 90% completeness, and recovers 83% (67%) of the ha~flux from catalogued galaxies in our search volume at the $Sigma$=2.5 ($Sigma$=5) color excess levels. The contamination from galaxies with no emission lines is 61% (12%) for $Sigma$=2.5 ($Sigma$=5). Also, in the regions of overlap between our preliminary fields and previous emission-line surveys, we recover the majority of the galaxies found in previous surveys and identify an additional $approx$300 galaxies. In total, we find 90 galaxies with no previous distance information, several of which are interesting objects: 7 blue compact dwarfs, 1 green pea, and a Seyfert galaxy; we also identified a known planetary nebula. These objects show that the CLU-ha~survey can be a discovery machine for objects in our own Galaxy and extreme galaxies out to intermediate redshifts. However, the majority of the CLU-ha~galaxies identified in this work show properties consistent with normal star-forming galaxies. CLU-ha~galaxies with new redshifts will be added to existing galaxy catalogs to focus the search for the electromagnetic counterpart to gravitational wave events.
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