- ID:
- ivo://CDS.VizieR/J/MNRAS/453/1562
- Title:
- CRTS close supermassive black hole binaries
- Short Name:
- J/MNRAS/453/1562
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Hierarchical assembly models predict a population of supermassive black hole (SMBH) binaries. These are not resolvable by direct imaging but may be detectable via periodic variability (or nanohertz frequency gravitational waves). Following our detection of a 5.2-year periodic signal in the quasar PG 1302-102, we present a novel analysis of the optical variability of 243 500 known spectroscopically confirmed quasars using data from the Catalina Real-time Transient Survey (CRTS) to look for close (<0.1pc) SMBH systems. Looking for a strong Keplerian periodic signal with at least 1.5 cycles over a baseline of nine years, we find a sample of 111 candidate objects. This is in conservative agreement with theoretical predictions from models of binary SMBH populations. Simulated data sets, assuming stochastic variability, also produce no equivalent candidates implying a low likelihood of spurious detections. The periodicity seen is likely attributable to either jet precession, warped accretion discs or periodic accretion associated with a close SMBH binary system. We also consider how other SMBH binary candidates in the literature appear in CRTS data and show that none of these are equivalent to the identified objects. Finally, the distribution of objects found is consistent with that expected from a gravitational-wave-driven population. This implies that circumbinary gas is present at small orbital radii and is being perturbed by the black holes. None of the sources is expected to merge within at least the next century. This study opens a new unique window to study a population of close SMBH binaries that must exist according to our current understanding of galaxy and SMBH evolution.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/645/A102
- Title:
- Crust structure of neutron stars
- Short Name:
- J/A+A/645/A102
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The thermal evolution of neutron stars in soft X-ray transients (SXTs) is sensitive to the equation of state, nucleon superfluidity, and the composition and structure of the crust. Carrying out comparisons of the observations of their crust cooling with simulations offers a powerful tool for verifying theoretical models of dense matter. We study the effect of physics input on the thermal evolution of neutron stars in SXTs. In particular, we consider different modern models of the sources of deep crustal heating during accretion episodes and the effects brought on by impurities embedded in the crust during its formation. We simulated the thermal structure and evolution of episodically accreting neutron stars under different assumptions regarding the crust composition and on the distribution of heat sources and impurities. For the non-accreted crust, we considered the nuclear charge fluctuations that arise at crust formation. For the accreted crust, we compared different theoretical models of composition and internal heating. We also compared the results of numerical simulations to observations of the crust cooling in SXT MXB 1659-29. The non-accreted part of the inner crust of a neutron star can have a layered structure, with almost pure crystalline layers interchanged with layers composed of mixtures of different nuclei. The latter layers have relatively low thermal conductivities, which has an effect on the thermal evolution of the transients. The impurity distribution in the crust strongly depends on models of the dense matter and the crust formation scenario. The shallow heating that is needed to reach an agreement between the theory and the observations depends on characteristics of the crust and envelope.
4063. Crux region redshifts
- ID:
- ivo://CDS.VizieR/J/A+AS/127/463
- Title:
- Crux region redshifts
- Short Name:
- J/A+AS/127/463
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In our systematic optical galaxy search behind the southern Milky Way, 3760 (mostly unknown) galaxies with diameters D>0.2' were identified in the Crux region (287{deg}<~l<~318{deg}, |b|<~10{deg}, Woudt & Kraan-Korteweg 1998, in press). Prior to this investigation, only 65 of these galaxies had known redshifts. In order to map the galaxy distribution in redshift space we obtained spectra for 226 bright (B_J_<~18.0mag) objects with the 1.9m telescope of the South African Astronomical Observatory (SAAO). Redshifts could be determined for 209 objects, of which 173 have good signal-to-noise ratios. Of the 36 tentative redshifts, four are confirmed through independent values in the literature. The redshifts of three objects indicate them to be galactic in origin. One of these confirms a suspected Planetary Nebula. For 17 of the galaxies, no redshift could be determined due to poor signal-to-noise ratios. In addition, 26 redshifts have have been measured in the Hydra-Antlia region investigated earlier (Kraan-Korteweg et al., 1995, Cat. <J/A+A/297/617>), of which one is a tentative estimate. Two main structures crossing the Galactic Plane in the Crux region have now become clear. A narrow, nearby filament from (l, b)=(340{deg}, -25{deg}) to the Centaurus cluster can be traced. This filament runs almost parallel to the extension of the Hydra-Antlia clusters found earlier and is part of what we have earlier termed the ``Centaurus Wall'' extending in redshift-space between 0<=v<=6000km/s (Fairall & Paverd 1995, in Wide-Field Spedctroscopy and the Distant Universe, p. 121). The main outcome of this survey however, is the recognition of another massive extended structure between 4000<=v<=8000km/s. This broad structure, dubbed the Norma Supercluster (Woudt et al. 1997, in press), runs nearly parallel to the Galactic Plane from Vela to ACO 3627 (its centre) from where it continues to the Pavo cluster. This massive structure is believed to be associated with the Great Attractor. The survey has furthermore revealed a set of cellular structures, similar to those seen in redshift space at higher galactic latitudes, but never before seen so clearly behind the Milky Way.
- ID:
- ivo://CDS.VizieR/J/A+A/506/1501
- Title:
- CrVIII radiative and excitation rates
- Short Name:
- J/A+A/506/1501
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this paper we report on calculations of energy levels, radiative rates, oscillator strengths, line strengths, and effective collision strengths for transitions among the lowest 362 levels of the (1s^2^2s^2^2p^6^) 3s^2^3p^5^, 3s3p^6^, 3s^2^3p^4^3d, 3s3p^5^3d, 3s^2^3p^3^3d^2^, 3s3p^4^3d^2^, 3p^6^3d, and 3s^2^3p^4^4l configurations of Cr VIII.
- ID:
- ivo://CDS.VizieR/J/A+A/382/184
- Title:
- Crystalline silicates around evolved stars
- Short Name:
- J/A+A/382/184
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This is the first paper in a series of three where we present the first comprehensive inventory of solid state emission bands observed in a sample of 17 oxygen-rich circumstellar dust shells surrounding evolved stars. The data were taken with the Short and Long Wavelength Spectrographs on board of the Infrared Space Observatory (ISO) and cover the 2.4 to 195{mu}m wavelength range. The spectra show the presence of broad 10 and 18{mu}m bands that can be attributed to amorphous silicates. In addition, at least 49 narrow bands are found whose position and width indicate they can be attributed to crystalline silicates. Almost all of these bands were not known before ISO.
- ID:
- ivo://CDS.VizieR/J/A+A/640/A77
- Title:
- Crystallization of a neutron star inner crust
- Short Name:
- J/A+A/640/A77
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The possible presence of amorphous and heterogeneous phases in the inner crust of a neutron star is expected to reduce the electrical conductivity of the crust, with potentially important consequences on the magneto-thermal evolution of the star. In cooling simulations, the disorder is quantified by an impurity parameter which is often taken as a free parameter. We aim to give a quantitative prediction of the impurity parameter as a function of the density in the crust, performing microscopic calculations including up-to-date microphysics of the crust. A multi-component approach is developed at finite temperature using a compressible liquid drop description of the ions with an improved energy functional based on recent microscopic nuclear models and optimized on extended Thomas-Fermi calculations. Thermodynamic consistency is ensured by adding a rearrangement term and deviations from the linear mixing rule are included in the liquid phase. The impurity parameter is consistently calculated at the crystallization temperature as determined in the one-component plasma approximation for the different functionals. Our calculations show that at the crystallization temperature the composition of the inner crust is dominated by nuclei with charge number around Z~40, while the range of the Z distribution varies from about 20 near the neutron drip to about 40 closer to the crust-core transition. This reflects on the behavior of the impurity parameter that monotonically increases with density up to around 40 in the deeper regions of the inner crust. Our study shows that the contribution of impurities is non-negligible, thus potentially having an impact on the transport properties in the neutron-star crust. The obtained values of the impurity parameter represent a lower limit; larger values are expected in the presence of non-spherical geometries and/or fast cooling dynamics.
- ID:
- ivo://CDS.VizieR/J/A+A/633/A149
- Title:
- Crystallization of neutron star outer crust
- Short Name:
- J/A+A/633/A149
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The interior of a neutron star is usually assumed to be made of cold catalyzed matter. However, the outer layers are unlikely to remain in full thermodynamic equilibrium during the formation of the star and its subsequent cooling, especially after crystallization occurs. We study the cooling and the equilibrium composition of the outer layers of a non-accreting neutron star down to crystallization. Here the impurity parameter, generally taken as a free parameter in cooling simulations, is calculated self-consistently using a microscopic nuclear model for which a unified equation of state has recently been determined. We follow the evolution of the nuclear distributions of the multi-component Coulomb liquid plasma fully self-consistently, adapting a general formalism originally developed for the description of supernova cores. We calculate the impurity parameter at the crystallization temperature as determined in the one-component plasma approximation. Our analysis shows that the sharp changes in composition obtained in the one-component plasma approximation are smoothed out when a full nuclear distribution is allowed. The Coulomb coupling parameter at melting is found to be reasonably close to the canonical value of 175, except for specific values of the pressure for which supercooling occurs in the one-component plasma approximation. Our multi-component treatment leads to non-monotonic variations of the impurity parameter with pressure. Its values can change by several orders of magnitude reaching about 50, suggesting that the crust may be composed of an alternation of pure (highly conductive) and impure (highly resistive) layers. The results presented here complement the recent unified equation of state obtained within the same nuclear model. Our self-consistent approach to hot dense multi-component plasma shows that the presence of impurities in the outer crust of a neutron star is non- negligible and may have a sizeable impact on transport properties. In turn, this may have important implications not only for the cooling of neutron stars, but also for their magneto-rotational evolution.
4068. CS 29497-004 abundances
- ID:
- ivo://CDS.VizieR/J/A+A/607/A91
- Title:
- CS 29497-004 abundances
- Short Name:
- J/A+A/607/A91
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report an abundance analysis for the highly r-process-enhanced (r-II) star CS 29497-004, a very metal-poor giant with solar system Teff=5013K and [Fe/H]=-2.85, whose nature was initially discovered in the course of the HERES project. Our analysis is based on high signal-to-noise ratio, high-resolution (R~75000) VLT/UVES spectra and MARCS model atmospheres under the assumption of local thermodynamic equilibrium, and obtains abundance measurements for a total of 46 elements, 31 of which are neutron-capture elements. As is the case for the other ~25 r-II stars currently known, the heavy-element abundance pattern of CS 29497-004 well-matches a scaled solar system second peak r-process-element abundance pattern. We confirm our previous detection of Th, and demonstrate that this star does not exhibit an "actinide boost". Uranium is also detected (log{epsilon}(U)=-2.20+/-0.30), albeit with a large measurement error that hampers its use as a precision cosmochronometer. Combining the various elemental chronometer pairs that are available for this star, we derive a mean age of 12.2+/-3.7Gyr using the theoretical production ratios from published waiting-point approximation models. We further explore the high-entropy wind model (Farouqi et al., 2010ApJ...712.1359F) production ratios arising from different neutron richness of the ejecta (Ye), and derive an age of 13.7+/-4.4Gyr for a best-fitting Ye=0.447. The U/Th nuclei-chronometer is confirmed to be the most resilient to theoretical production ratios and yields an age of 16.5+/-6.6Gyr. Lead (Pb) is also tentatively detected in CS 29497-004, at a level compatible with a scaled solar r-process, or with the theoretical expectations for a pure r-process in this star.
- ID:
- ivo://CDS.VizieR/J/MNRAS/373/1531
- Title:
- 6C** sample of steep-spectrum radio sources
- Short Name:
- J/MNRAS/373/1531
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present basic observational data on the 6C** sample. This is a new sample of radio sources drawn from the 151-MHz 6C survey, which was filtered with radio criteria chosen to optimize the chances of finding radio galaxies at z>4. The filtering criteria are a steep-spectral index and a small angular size. The final sample consists of 68 sources from a region of sky covering 0.421sr. We present Very Large Array radio maps, and the results of K-band imaging and optical spectroscopy.
- ID:
- ivo://CDS.VizieR/J/MNRAS/375/1349
- Title:
- 6C** sample of steep-spectrum radio sources. II
- Short Name:
- J/MNRAS/375/1349
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We use the 6C** sample to investigate the comoving space density of powerful, steep-spectrum radio sources. This sample, consisting of 68 objects, has virtually complete K-band photometry and spectroscopic redshifts for 32 per cent of the sources. In order to find its complete redshift distribution, we develop a method of redshift estimation based on the Kz diagram of the 3CRR, 6CE, 6C* and 7CRS radio galaxies. Based on this method, we derive redshift probability density functions for all the optically identified sources in the 6C** sample. Using a combination of spectroscopic and estimated redshifts, we select the most radio luminous sources in the sample. Their redshift distribution is then compared with the predictions of the radio luminosity function of Jarvis et al. (2001MNRAS.327..907J). We find that, within the uncertainties associated with the estimation method, the data are consistent with a constant comoving space density of steep-spectrum radio sources beyond z>2.5, and rule out a steep decline.