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The giant X-ray bump of GRB 121027A observed by Swift is different from the typical X-ray flares in gamma-ray bursts. The observed structural variability in the rise and decay phases of the bump has four components. Of these four components, only the data in the bump from about 5300 to about 6100 s is of good enough quality to be analysed using the stepwise filter correlation method. A $86^{+5.9}_{-9.4} \rm s$ periodic oscillation is postulated, which is confirmed by the Lomb–Scargle method. A jet precession model is proposed to account for this variability. [ABSTRACT FROM AUTHOR]

ABSTRACT In accretion systems, outflows may have significant influence on luminosity fluctuations. In this paper, following Lyubarskii's general scheme, we revisit the power spectral density of luminosity fluctuations by taking into account the role of outflows. Our analysis is based on the assumption that the coupling between the local outflow and inflow is weak for accretion-rate fluctuations. We find that, for inflow mass accretion rate , the power spectrum of the flicker-noise component will present a power-law distribution p( f) ∝ f−(1 + 4 s/3) for advection-dominated flows. We also obtain descriptions of p( f) for both standard thin discs and neutrino-cooled discs, which show that the power-law index of a neutrino-cooled disc is generally larger than that of a photon-cooled disc. Furthermore, the relationship obtained between p( f) and s indicates the possibility of evaluating the strength of outflows through the power spectrum in X-ray binaries and gamma-ray bursts. In addition, we discuss the possible influence of outflow-inflow coupling on our results. [ABSTRACT FROM AUTHOR]

We revisit the vertical structure of neutrino-dominated accretion flows (NDAFs) in spherical coordinates with a new boundary condition based on the mechanical equilibrium. The solutions show that NDAF is significantly thick. The Bernoulli parameter and neutrino trapping are determined by the mass accretion rate and the viscosity parameter. According to the distribution of the Bernoulli parameter, the possible outflow may appear in the outer region of the disk. The neutrino trapping can essentially affect the neutrino radiation luminosity. The vertical structure of NDAF is like a 'sandwich', and the multilayer accretion may account for the flares in gamma-ray bursts. [ABSTRACT FROM AUTHOR]

ABSTRACT The stress evolution process is taken into account in the linear stability analysis of standard thin accretion discs. We find that the growth rate of thermally unstable modes can decrease significantly owing to the stress delay, which may help to understand the quasi-periodic variability of GRS 1915+105. We also discuss the possible application of stress evolution to the stability of the Shapiro-Lightman-Eardley disc. [ABSTRACT FROM AUTHOR]

By numerically solving the set of basic equations describing black hole accretion flows with low accretion rates, we show that although the dynamical structure of these flows is essentially unaffected by radiative processes in comparison with the case in which the radiation is not considered, the radiative cooling can be more important than the advective cooling in the flow's convection-dominated zone, and this result may have implications to distinguish observationally convection-dominated accretion flows from advection-dominated accretion flows. [ABSTRACT FROM AUTHOR]

This paper presents global solutions of advection-dominated accretion flows (ADAFs) around black holes. The global solution passes through two critical points, the sonic point Rs and viscous point Rv and is thus naturally divided by these points into three radial zones: the subsonic, subviscous outer zone (R > Rv), the subsonic, superviscous middle zone (Rs < R < Rv), and the supersonic, superviscous inner zone (R < Rs). We have obtained a complete picture by means of the Rs — j parameter space which sums up the situation of ADAF solution at a glance. We find three classes of global solutions: the ADAF-thin disk solution, the ADAF-thick disk solution and the α — X type adiabatic shock-included solution. We also present the unique correspondences among Rs, Rv and Rout of the ADAF-thin disk solutions, where Rout is the outer boundary of ADAF. The adiabatic shock in the accretion flow connects the supersonic and superviscous zone of an X-type solution to the subsonic and superviscous zone of an α-type soultion. Because the shock occurs between two superviscous zones, it can exist for flows with either a low or a high viscosity. [Copyright &y& Elsevier]

This paper studies the mechanism of transition from a Shakura-Sunyaev disk to an advection-dominated accretion flow in more details, taking into consideration two-temperature plasma accretion flows with bremsstrahlung and synchrotron radiation and Comptonization. The results further show that such a transition is realizable for a quite wide range of physical conditions. [Copyright &y& Elsevier]

We present a spatially 1.5-dimensional, time-dependent numerical study of accretion disks around Kerr black holes. Our study focuses on the limit-cycle behavior of thermally unstable accretion disks. We find that maximal luminosity may be a more appropriate probe of black hole spin than the cycle duration and influence radius. [ABSTRACT FROM AUTHOR]

We investigate the vertical structure of neutrino-dominated accretion flows in spherical coordinates. In our calculation, the empty funnel along the rotation axis can naturally explain the neutrino annihilable ejection. The outflow is possible due to the positive Bernoulli function, and the luminosity of neutrino annihilation is enhanced by one or two orders of magnitude. [ABSTRACT FROM AUTHOR]

We show that when the gravitational force in the vertical direction is correctly calculated, the well-known S- shaped sequence of thermal equilibrium solutions can be constructed only for small radii of black hole accretion flows, such that slim accretion discs can possibly exist only in the inner regions of these flows. [ABSTRACT FROM AUTHOR]

The standard thin disk model predicts that when the accretion rate is moderately high, the disk is radiation-pressure-dominated and thermally unstable. However, observations indicate the opposite, namely the disk is quite stable. We present an explanation in this work by taking into account the role of the magnetic field which was ignored in the previous analysis. [ABSTRACT FROM AUTHOR]

We study the fluctuations of standard thin accretion disks by linear analysis of the time-dependent energy equation together with the vertical hydrostatic equilibrium and the equation of state. We show that some of the simulation results in Hirose et al., such as the time delay, the relationship of power spectra, and the correlation between magnetic energy and radiation energy, can be understood well by our analytic results. [ABSTRACT FROM AUTHOR]

It was revealed in our previous studies that there exists a maximal possible accretion rate for slim discs with constant accretion rates because the correctly calculated vertical gravitational force can only gather some limited amount of accreted matter. Here we show that when the accretion rate is not constant and instead decreases with decreasing radius because of outflows, such that the amount of accreted matter is adjusted to be within the allowed limit, global slim disc solutions can be constructed even for the case that accretion rates at large radii apparently exceed the maximal possible value. This result further demonstrates that outflows seem to be unavoidable for accretion flows with large accretion rates at large radii. [ABSTRACT FROM AUTHOR]