arXiv subjects

Aleksander Sadowski

Publications and source records attributed to Aleksander Sadowski.

43 records · Page 3Linked to original sources

arXiv2011

Measuring Black Hole Spin by the Continuum-Fitting Method: Effect of Deviations from the Novikov-Thorne Disc Model

The X-ray spectra of accretion discs of eight stellar-mass black holes have been analyzed to date using the thermal continuum fitting method, and the spectral fits have been used to estimate the spin parameters of the black holes. However, the underlying model used in this method of estimating spin is the general relativistic thin-disc model of Novikov & Thorne, which is only valid for razor-thin discs. We therefore expect errors in the measured values of spin due to inadequacies in the theoretical model. We investigate this issue by computing spectra of numerically calculated models of thin accretion discs around black holes, obtained via three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations. We apply the continuum fitting method to these computed spectra to estimate the black hole spins and check how closely the values match the actual spin used in the GRMHD simulations. We find that the error in the dimensionless spin parameter is up to about 0.2 for a non-spinning black hole, depending on the inclination. For black holes with spins of 0.7, 0.9 and 0.98, the errors are up to about 0.1, 0.03 and 0.01 respectively. These errors are comparable to or smaller than those arising from current levels of observational uncertainty. Furthermore, we estimate that the GRMHD simulated discs from which these error estimates are obtained correspond to effective disc luminosities of about 0.4-0.7 Eddington, and that the errors will be smaller for discs with luminosities of 0.3 Eddington or less, which are used in the continuum-fitting method. We thus conclude that use of the Novikov-Thorne thin-disc model does not presently limit the accuracy of the continuum-fitting method of measuring black hole spin.

astro-ph.HE↗

arXiv2011

Relativistic slim disks with vertical structure

We report on a scheme for incorporating vertical radiative energy transport into a fully relativistic, Kerr-metric model of optically thick, advective, transonic alpha disks. Our code couples the radial and vertical equations of the accretion disk. The flux was computed in the diffusion approximation, and convection is included in the mixing-length approximation. We present the detailed structure of this "two-dimensional" slim-disk model for alpha=0.01. We then calculated the emergent spectra integrated over the disk surface. The values of surface density, radial velocity, and the photospheric height for these models differ by 20%-30% from those obtained in the polytropic, height-averaged slim disk model considered previously. However, the emission profiles and the resulting spectra are quite similar for both types of models. The effective optical depth of the slim disk becomes lower than unity for high values of the alpha parameter and for high accretion rates.

astro-ph.HE↗

arXiv2010

Leaving the ISCO: the inner edge of a black-hole accretion disk at various luminosities

The "radiation inner edge" of an accretion disk is defined as the inner boundary of the region from which most of the luminosity emerges. Similarly, the "reflection edge" is the smallest radius capable of producing a significant X-ray reflection of the fluorescent iron line. For black hole accretion disks with very sub-Eddington luminosities these and all other "inner edges" locate at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner edge of the black hole accretion disk. However, even for moderate luminosities, there is no such unique inner edge as differently defined edges locate at different places. Several of them are significantly closer to the black hole than ISCO. The differences grow with the increasing luminosity. For nearly Eddington luminosities, they are so huge that the notion of the inner edge losses all practical significance.

astro-ph.HE↗

arXiv2010

Launching of jets by cold, magnetized disks in Kerr Metric

We confirm recent discovery by Cao that in the vicinity of fast rotating black holes jets can be launched centrifugally by cold, magnetized disks even for nearly vertically shaped magnetic flux surfaces. Outflows produced under such extreme conditions are investigated via studying kinematics of test particles in the force-free magnetosphere approximation. Implications of a possibility of magneto-centrifugal launching of very well collimated central outflows around the fast rotating black holes are discussed in the general context of the jet formation scenarios in AGNs.

astro-ph.HE↗

arXiv2009

Slim disks around Kerr black holes revisited

We investigate stationary slim accretion disks around Kerr black holes. We construct a new numerical method based on the relaxation technique. We systematically cover the whole parameter space relevant to stellar mass X-ray binaries. We also notice some non-monotonic features in the disk structure, overlooked in previous studies.

astro-ph.HE↗

arXiv2008

The Total Merger Rate of Compact Object Binaries In The Local Universe

Using a population synthesis approach, we compute the total merger rate in the local Universe for double neutron stars, double black holes, and black hole -- neutron star binaries. These compact binaries are the prime source candidates for gravitational-wave detection by LIGO and VIRGO. We account for mergers originating from field populations and from dense stellar clusters. For both populations we use the same treatment of stellar evolution. Our results indicate that the merger rates of double neutron stars and black hole -- neutron star binaries are strongly dominated by field populations, while merging black hole binaries are formed much more effectively in dense stellar clusters. The overall merger rate of double compact objects depends sensitively on the (largely unknown) initial mass fraction contained in dense clusters f_cl. For f_cl < 0.0001, the Advanced LIGO detection rate will be dominated by field populations of double neutron star mergers, with a small but significant number of detections ~20 yr^-1. However for a higher mass fraction in clusters, f_cl > 0.001, the detection rate will be dominated by numerous mergers of double black holes originating from dense clusters, and it will be considerably higher, ~25 - 300 yr^-1. In addition, we show that, once mergers of double black holes are detected, it is easy to differentiate between systems formed in the field and in dense clusters, since the chirp mass distributions are strikingly different. Finally, we point out that there may exist a population of merging black hole binaries in intergalactic space.

astro-ph↗

arXiv2004

A Comprehensive Study of Young Black Hole Populations

We present theoretical models of black hole (BH) populations in young stellar environments, such as starbursts and young star clusters. Using a population synthesis approach we compute the formation rates and characteristic properties of single and binary BHs for various representative ages and choices of parameters. We find that most of the BHs (typically 80% for an initial 50% binary fraction) are single, but with many originating from primordial binaries (which either merged into a single massive star or were disrupted following a supernova explosion). A smaller but significant fraction (typically 20%) of the BHs remain in binary systems. Main-sequence stars are the most frequent BH companions, but massive BH--BH binaries are the next most numerous group. The most massive BHs found in our simulations reach 80 Msun, and are formed through mergers of massive binary components. If formed in a dense star cluster such a massive stellar BH may become the seed for growth to a more massive (``intermediate-mass'') BH. Although we do not include dynamical interactions, our results provide realistic initial conditions for N-body simulations of dense star clusters (e.g., globular clusters) including primordial BHs.

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