Hot One-Temperature Accretion Flows Around Black Holes

We describe hot, optically-thin solutions for one-temperature accretion disks around black holes. We include cooling by synchrotron, bremsstrahlung, and Comptonization. Our solutions are thermally and viscously stable, with gas temperatures on the order of $T \sim 10^9-10^{10.7}$K. The thermal stability is a direct result of the inclusion of synchrotron cooling. The new solution branch is related to the advection-dominated solution for a two-temperature gas described by Narayan \& Yi (1995b). It is present only for mass accretion rates less than some critical $\dot{M}_{crit}$ which depends on the radius $R$ and viscosity parameter $\alpha$. The solutions are advection-dominated for extremely low values of $\dot{M}$. However, for a range of intermediate accretion rates, the new solutions are both hot ($T \sim 10^{10}$K) and cooling-dominated. Because of this new feature, one-temperature solutions are significantly more luminous than the corresponding two temperature solutions. The radial profile of the new solutions is unusual. The inner parts of the flow are cooling-dominated and have a disk-like geometry, while the outer parts are fully advection-dominated and nearly quasi-spherical.Comment: 24 pages tex file and 7 postscript figures all included in one compressed tar file. Accepted for publication in ApJ. In case of problems, write to [email protected]

Multiple magnon modes in the Co3_3Sn2_2S2_2 Weyl semimetal candidate

We experimentally investigate electron transport in kagome-lattice ferromagnet Co$_3$Sn$_2$S$_2$, which is regarded as a time-reversal symmetry broken Weyl semimetal candidate. We demonstrate $dV/dI(I)$ curves with pronounced asymmetric $dV/dI$ spikes, similar to those attributed to current-induced spin-wave excitations in ferromagnetic multilayers. In contrast to multilayers, we observe several $dV/dI$ spikes' sequences at low, $\approx$10$^4$ A/cm$^2$, current densities for a thick single-crystal Co$_3$Sn$_2$S$_2$ flake in the regime of fully spin-polarized bulk. The spikes at low current densities can be attributed to novel magnon branches in magnetic Weyl semimetals, which are predicted due to the coupling between two magnetic moments mediated by Weyl fermions. Presence of spin-transfer effects at low current densities in Co$_3$Sn$_2$S$_2$ makes the material attractive for applications in spintronics.Comment: final versio

Utilization of Information and Communication Technology (ICT) and library operations in state university libraries in south-south Nigeria

This study examined utilization of information and communication technology (ICT) and library job operations in state university libraries in South-South Nigeria. Two hypotheses were formulated to guide the study. Survey research design was used. The population for the study constituted 143 librarians (professionals and paraprofessionals). A sample of 93 librarians was selected using purposive sampling technique. The instrument was validated by experts. The research data was analyzed with descriptive (mean and standard deviation) and inferential statistics (One-Way ANOVA). All hypotheses were tested at 0.05 level of significance. The findings revealed that there was a significant influence of online databases services on library job operations, (f=4.447*, P<.05, df=2,728, F-critical=3.00) and there was a significant influence of CD-ROM services on library job operations, (f=2.671*, P<.05, df=2,728, Fcritical= 3.00).From the result of the study, it was recommended among others that librarians should utilize more ICT tools to facilitate their job operations in the library.Keywords: Utilization of ICT, Online databases Service, CD-ROM Service, Library job operations, State University, Libraries, South-South Nigeri

The change from accretion via a thin disk to a coronal flow: dependence on the viscosity of the hot gas

We study the transition from the geometrically thin disk to the hot coronal flow for accretion onto black holes. The efficiency of evaporation determines the truncation of the geometrically thin disk as a function of the black hole mass and the mass flow rate in the outer disk. The physics of the evaporation was already described in detail in earlier work (Meyer et al. 2000b). We show now that the value of the viscosity parameter for the coronal gas has a strong influence on the evaporation efficiency. For smaller values of the viscosity evaporation is less efficient. For a given mass flow rate from outside the geometrically thin disk then extends farther inward. Spectral transitions between soft and hard states are then expected for different mass flow rates in the outer disk. The physics is the same for the cases of stellar and supermassive black holes systems.Comment: 6 pages, 1 figure, accepted for publication in A&