Dust Echos from Gamma Ray Bursts

The deviation from the power-law decline of the optical flux observed in GRB 970228 and GRB 980326 has been used recently to argue in favor of the connection between GRBs and supernovae. We consider an alternative explanation for this phenomenon, based on the scattering of a prompt optical burst by 0.1 solar masses of dust located beyond its sublimation radius 0.1-1 pc from the burst. In both cases, the optical energy observed at the time of the first detection of the afterglow suffices to produce an echo after 20-30 days, as observed. Prompt optical monitoring of future bursts and multiband photometry of the afterglows will enable quantitative tests of simple models of dust reprocessing and a prediction of the source redshift.Comment: 4 pages including 3 postscript figures, LaTeX (emulateapj.sty; newapa.sty for bibliography definitions); accepted for publication in ApJ

The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass transfer

We show that the 1996 outburst of the X-ray binary transient system GRO J1655-40 can be explained by the standard dwarf-nova type disc instability, followed by an episode of enhanced mass transfer from the secondary if the mass transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit. We argue that irradiation of the secondary during the onset of the outburst driven by the thermal instability in the outer disc can increase the mass transfer rate above the minimum value required for stable accretion. This will then produce the period of near-constant X-ray emission seen in this system. This scenario can also explain the observed anti-correlation between the optical and X-ray fluxes. It is generally accepted that optical emission in low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays. There is also strong circumstantial evidence that in order for the outer disc to see the irradiating flux, it must be warped. Depending on the warp propagation mechanism, either a burst of mass from the secondary or viscous decay are likely to decrease the degree of warping, thereby causing the decrease in the observed optical flux while the X-ray flux remains constant or even increases, exactly as observed in GRO J1655-40. Finally, the decrease of the disc warping and, therefore, irradiation will cause the disc to become unstable once again, terminating the outburst.Comment: Astronomy and Astrophysics - in pres

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]

On The Synchrotron Self-Compton Emission from Relativistic Shocks and Its Implications for Gamma-Ray Burst Afterglows

We consider the effects of inverse Compton scattering of synchrotron photons from relativistic electrons in GRB afterglows. We compute the spectrum of the inverse Compton emission and find that it can dominate the total cooling rate of the afterglow for several months or even years after the initial explosion. We demonstrate that the presence of strong inverse Compton cooling can be deduced from the effect it has on the time-evolution of the cooling break in the synchrotron spectral component, and therefore on the optical and X-ray afterglow lightcurves. We then show how the physical interpretation of the observed characteristics of the synchrotron spectrum must be modified to take into consideration this extra source of cooling, and give a revised prescription for computing physical parameters characterizing the expanding shock wave from the observed quantities. We find that for a given set of observables (synchrotron break frequencies and fluxes) there is either no consistent physical interpretation or two of them. Finally we discuss the prospects of directly detecting the inverse Compton emission with Chandra. We argue that such a detection is possible for GRBs exploding in a reasonably dense (n>1 cm^-3) medium.Comment: 21 pages, ApJ submitte

Narrative and ethical (in)action: creating spaces of resistance with refugee-storytellers in the Calais ‘Jungle’ camp

This paper explores how a multimodal narrative methodology can open a creative, relational and safe space, in which refugee-storytellers negotiate their positioning within racialised power imbalances. Personal narratives that facilitate storytellers’ agency have a potential to empower and elicit social change. When refugees are denied their right to claim/speak/act, the act of narrating becomes a vehicle for social change. Creative workshops delivered in the Calais ‘Jungle’ refugee camp in 2016–17 enabled us to co-construct a relational space with refugee participants, based on the principle of ethical hesitancy. In this paper we argue that the relational space offered possibilities for refugee storytellers to resist and challenge the representation of refugee stories, whilst giving rise to ethically important moments. These moments provide important perspectives on how practitioners and researchers can use narrative processes in creating spaces of resistance and social change with refugee participants

Disk/corona model: The transition to ADAF

We propose a model of the accretion flow onto a black hole consisting of the accretion disk with an accreting two-temperature corona. The model is based on assumptions about the radiative and conductive energy exchange between the two phases and the pressure equilibrium. The complete model is determined by the mass, the accretion rate, and the viscosity parameter. We present the radial dependencies of parameters of such a two-phase flow, with advection in the corona and the disk/corona mass exchange due to evaporation/condensation included, and we determine the transition radius from a two-phase disk/corona accretion to a single-phase optically thin flow (ADAF) in the innermost part of the disk as a function of accretion rate. We identify the NLS1 galaxies with objects accreting at a rate close to the Eddington accretion rate. The strong variability of these objects may be related to the limit cycle behaviour expected in this luminosity range, as the disk, unstable due to the dominance by the radiation pressure, oscillates between the two stable branches: the advection-dominated optically thick branch and the evaporation branch.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

Heating and Cooling of Hot Accretion Flows by Non Local Radiation

We consider non-local effects which arise when radiation emitted at one radius of an accretion disk either heats or cools gas at other radii through Compton scattering. We discuss three situations: 1. Radiation from the inner regions of an advection-dominated flow Compton cooling gas at intermediate radii and Compton heating gas at large radii. 2. Soft radiation from an outer thin accretion disk Compton cooling a hot one- or two-temperature flow on the inside. 3. Soft radiation from an inner thin accretion disk Compton cooling hot gas in a surrounding one-temperature flow. We describe how previous results are modified by these non-local interactions. We find that Compton heating or cooling of the gas by the radiation emitted in the inner regions of a hot flow is not important. Likewise, Compton cooling by the soft photons from an outer thin disk is negligible when the transition from a cold to a hot flow occurs at a radius greater than some minimum $R_{tr,min}$. However, if the hot flow terminates at $R < R_{tr,min}$, non-local cooling is so strong that the hot gas is cooled to a thin disk configuration in a runaway process. In the case of a thin disk surrounded by a hot one-temperature flow, we find that Compton cooling by soft radiation dominates over local cooling in the hot gas for \dot{M} \gsim 10^{-3} \alpha \dot{M}_{Edd}, and R \lsim 10^4 R_{Schw}. As a result, the maximum accretion rate for which an advection-dominated one-temperature solution exists, decreases by a factor of $\sim 10$, compared to the value computed under an assumption of local energy balance.Comment: LaTeX aaspp.sty, 25 pages, and 6 figures; to appear in Ap