Non-Steady State Accretion Disks in X-Ray Novae: Outburst Models for Nova Monocerotis 1975 and Nova Muscae 1991

We fit outbursts of two X-ray novae (Nova Monocerotis 1975=A0620-00 and Nova Muscae GS 1991=1124-683) using a time-dependent accretion disk model. The model is based on a new solution for a diffusion-type equation for the non-steady-state accretion and describes the evolution of a viscous alpha-disk in a binary system after the peak of an outburst, when matter in the disk is totally ionized. The accretion rate in the disk decreases according to a power law. We derive formulas for the accretion rate and effective temperature of the disk. The model has three free input parameters: the mass of the central object M, the turbulence parameter alpha, and the normalization parameter delta t. Results of the modeling are compared with the observed X-ray and optical B and V light curves. The resulting estimates for the turbulence parameter $\alpha$ are similar: 0.2-0.4 for A 0620-00 and 0.45-0.65 for GS 1124-683, suggesting a similar nature for the viscosity in the accretion disks around the compact objects in these sources. We also derive the distances to these systems as functions of the masses of their compact objects.Comment: 10 pages, 7 figures; style improve

Ultra-High-Energy Cosmic Ray Acceleration by Magnetic Reconnection in Newborn Accretion Induced Collapse Pulsars

We here investigate the possibility that the ultra-high energy cosmic ray (UHECR) events observed above the GZK limit are mostly protons accelerated in reconnection sites just above the magnetosphere of newborn millisecond pulsars which are originated by accretion induced collapse (AIC). We show that AIC-pulsars with surface magnetic fields $10^{12} G < B_{\star} \lesssim 10^{15}$ G and spin periods $1 ms \lesssim P_{\star} < 60 ms$, are able to accelerate particles to energies $\geq 10^{20}$ eV. Because the expected rate of AIC sources in our Galaxy is very small (\sim 10^{-5} yr^{-1}), the corresponding contribution to the flux of UHECRs is neglegible, and the total flux is given by the integrated contribution from AIC sources produced by the distribution of galaxies located within the distance which is unaffected by the GZK cutoff ($\sim 50$ Mpc). We find that the reconnection efficiency factor needs to be $\xi \gtrsim 0.1$ in order to reproduce the observed flux of UHECRs.Comment: Latex file, 16 pages, 2 figures, replaced with revised version accepted for publication in the ApJ letter

Radial dependences of physical parameters in α-disk as a consequence of two vertical structure solutions

We solve the vertical structure equations for a standard α-disk and compare the results with those obtained by an independent method (Ketsaris & Shakura 1998). On the basis of the numerical solutions for the vertical structure we obtain analytic radial dependences for the physical parameters of the disk. For a disk consisting of fully ionized hydrogen, its half-thickness is about 2.5 times larger than that obtained by the "standard" solution, averaging the vertical structure equations. Account of heavy elements leads to an additional increase of the half-thickness by approximately 25%

Analysis of accretion disc structure and stability using open code for vertical structure

Radial structure of accretion discs around compact objects is often described using analytic approximations which are derived from averaging or integrating vertical structure equations. For non-solar chemical composition, partial ionization, or for supermassive black holes, this approach is not accurate. Additionally, radial extension of `analytically-described' disc zones is not evident in many cases. We calculate vertical structure of accretion discs around compact objects, with and without external irradiation, with radiative and convective energy transport taken into account. For this, we introduce a new open Python code, allowing different equations of state (EoS) and opacity laws, including tabular values. As a result, radial structure and stability `S-curves' are calculated for specific disc parameters and chemical composition. In particular, based on more accurate power-law approximations for opacity in the disc, we supply new analytic formulas for the farthest regions of the hot disc around stellar-mass object. On calculating vertical structure of a self-irradiated disc, we calculate a self-consistent value of the irradiation parameter $C_{\rm irr}$ for stationary $\alpha$-disc. We find that, for a fixed shape of the X-ray spectrum, $C_{\rm irr}$ depends weakly on the accretion rate but changes with radius, and the dependence is driven by the conditions in the photosphere and disc opening angle. The hot zone extent depends on the ratio between irradiating and intrinsic flux: corresponding relation for $T_{\rm irr,\, crit}$ is obtained.Comment: Accepted for publication in MNRAS, 15 pages, 15 figures, 3 appendi

The effect of thermal winds on the outbursts evolution of LMXB systems

Theoretical models of accretion discs and observational data indicate that the X-ray emission from the inner parts of an accretion disc can irradiate its outer regions and induce a thermal wind, which carries away the mass and angular momentum from the disc. Our aim is to investigate the influence of the thermal wind on the outburst light curves of black hole X-ray binary systems. We carry out numerical simulations of a non-stationary disc accretion with wind using upgraded open code freddi. We assume that the wind launches only from the ionised part of the disc and may turn off if the latter shrinks fast enough. Our estimates of the viscosity parameter $\alpha$ are shifted downward compared to a scenario without a wind. Generally, correction of $\alpha$ depends on the spectral hardness of central X-rays and the disc outer radius, but unlikely to exceed a factor of 10 in the case of a black hole low-mass X-ray binary (BH LMXB). We fit 2002 outburst of BH LMXB 4U 1543-47 taking into account the thermal wind. The mass loss in the thermal wind is of order of the accretion rate on the central object at the peak of the outburst. New estimate of the viscosity parameter $\alpha$ for the accretion disc in this system is about two times lower than the previous one. Additionally, we calculate evolution of the number of hydrogen atoms towards 4U 1543-47 due to the thermal wind from the hot disc.Comment: 19 pages, 22 figures, accepted for publication in MNRA

The thickness of accretion α-disks: Theory and observations

Observations of X-ray binaries indicate substantial half-thicknesses for the accretion disks in these systems (up to h/R ≈ 0.25, where h is the disk half-thickness and R its radius), while standard α accretion disks predict appreciably smaller half-thicknesses. We study the theoretical vertical structure of such disks using two independent numerical methods, and show that their maximum half-thicknesses in the subcritical regime cannot exceed h/R ≈ 0.1. We consider various reasons for the apparent increase in the disk thickness, the most probable of which is the presence of matter above the disk in the form of a hot corona that scatters hard radiation from the central source and inner parts of the disk. As a result, the observed thickness of the disk and the illumination of its outer parts effectively increase. This mechanism can also explain both the optical-to-X-ray flux ratio in these systems and the observed parameters of eclipsing X-ray binaries. © 2007 Pleiades Publishing, Ltd

Light curve modeling for time-dependent accretion disks in X-ray novae with general relativity effects taken into account

We present a method of modeling X-ray and optical light curves of time-dependent accretion α-disks in X-ray novae. The model is based on an analytic solution for α-disk evolution after the outburst maximum. The method involves relativistic effects near a Kerr black hole and self-irradiation of an accretion disk. The method is applied to the outbursts of X-ray Nova Monocerotis 1975 (A 0620-00) and X-ray Nova Muscae 1991 (GRS 1124-68). Recently, narrow limits were obtained for the masses and the distances of these binaries (11 ± 2M⊙, 1.1 kpc for A 0620-00 and 7 ± 0.6M⊙, 5.1 kpc for GRS 1124-68, Gelino et al. (2001a,b)). This allows us to limit other model parameters: Kerr parameter (0.5-0.7 and ∼ 0.95 respectively for A 0620-00 and GRS 1124-68) and α-parameter (0.6-0.8 and 0.4-0.5). The inner radius of the accretion disk in GRS 1124-68 has to be few times larger than that of the last marginally stable orbit. Modeled disks in both systems should be thicker than in the standard theory and should thermalize a significant part of the incident X-ray flux (≳ 20%). Possible reasons for these results are discussed