Shock Model Of Gamma-ray Bursts Research Articles

Insights into tidal disruption of stars from PS1-10jh

Was PS1-10jh (Gezari et al. 2012), an optical/UV transient discovered by the Pan-STARRS Medium Deep Survey, the tidal disruption of a star by a massive black hole? We address two aspects of the problem: the composition of the putative disrupted object (using the spectroscopic data), and the energetics of the observed gas and radiation (using the photometric data). We perform photoionization calculations and compare with the observed lower limit of the line ratio L(He II 4686)/L(Halpha) > 5 to argue that this event was not the disruption of a solar-type star, and instead was likely the disruption of a helium core (as first proposed by Gezari et al. 2012). Disruption of such a dense object requires a relatively small central BH, M_BH <~ 2 x 10^5 M_sun. We use the photometric data to infer that PS1-10jh comprised an outflow of ~ 0.01 M_sun of gas, escaping from the BH at ~1000 km/s, and we propose that this outflow was driven primarily by radiation pressure trapped by Thomson and resonance line scattering. The large ratio of radiated energy to kinetic energy, E_rad/E_K ~ 10^4, together with the large value of E_rad ~ 2 x 10^(51) erg, suggests that the outflow was shocked at large radius (perhaps similar to super-luminous supernovae or the internal shock model for gamma-ray bursts). We describe puzzles in the physics of PS1-10jh, and discuss how this event may help us understand future tidal disruptions and super-Eddington accretion events as well.

A MORPHOLOGICAL ANALYSIS OF GAMMA-RAY BURST EARLY-OPTICAL AFTERGLOWS

Within the framework of the external shock model of gamma-ray bursts (GRBs) afterglows, we perform a morphological analysis of the early optical lightcurves to directly constrain model parameters. We define four morphological types, i.e. the reverse shock dominated cases with/without the emergence of the forward shock peak (Type I/ Type II), and the forward shock dominated cases without/with $\nu_m$ crossing the band (Type III/IV). We systematically investigate all the Swift GRBs that have optical detection earlier than 500 s and find 3/63 Type I bursts (4.8%), 12/63 Type II bursts (19.0%), 30/63 Type III bursts (47.6%), 8/63 Type IV bursts (12.7%) and 10/63 Type III/IV bursts (15.9%). We perform Monte Carlo simulations to constrain model parameters in order to reproduce the observations. We find that the favored value of the magnetic equipartition parameter in the forward shock ($\epsilon_B^f$) ranges from $10^{-6}$ to $10^{-2}$, and the reverse-to-forward ratio of $\epsilon_B$ ($R_B$) is about 100. The preferred electron equipartition parameter $\epsilon_e^{r,f}$ value is 0.01, which is smaller than the commonly assumed value, e.g., 0.1. This could mitigate the so- called "efficiency problem" for the internal shock model, if $\epsilon_e$ during the prompt emission phase (in the internal shocks) is large (say, $\sim 0.1$). The preferred $R_B$ value is in agreement with the results in previous works that indicates a moderately magnetized baryonic jet for GRBs.

GAMMA-RAY BURST SPECTRUM WITH DECAYING MAGNETIC FIELD

In the internal shock model for gamma-ray bursts (GRBs), the synchrotron spectrum from the fast cooling electrons in a homogeneous downstream magnetic field (MF) is too soft to produce the low-energy slope of GRB spectra. However the magnetic field may decay downstream with distance from the shock front. Here we show that the synchrotron spectrum becomes harder if electrons undergo synchrotron and inverse-Compton cooling in a decaying MF. To reconcile this with the typical GRB spectrum with low energy slope $\nu F_\nu\propto\nu$, it is required that the postshock MF decay time is comparable to the cooling time of the bulk electrons (corresponding to a MF decaying length typically of $\sim10^5$ skin depths); that the inverse-Compton cooling should dominate synchrotron cooling after the MF decay time; and/or that the MF decays with comoving time roughly as $B\propto t^{-1.5}$. An internal shock synchrotron model with a decaying MF can account for the majority of GRBs with low energy slopes not harder than $\nu^{4/3}$.

Photosphere-internal shock model of gamma-ray bursts: case studies of Fermi/LAT bursts

Radially inhomogeneous gamma-ray burst (GRB) jets release variable photospheric emission and can have internal shocks occurring above the photosphere. We generically formulate a photospheric emission model of GRBs including Compton up-scattered photospheric (UP) emission off the electrons (and positrons) in the internal shocks, and find that the photospheric emission may correspond to the traditional (band) component at ≲1 MeV and the UP emission to the high-energy emission observed by Fermi/LAT for some GRBs at ≳10 MeV. The two components can be separate from each other in the spectrum in some cases or can mimic a smooth broad-band spectrum in other cases. We apply our formulation to the well-studied long and short LAT GRBs, GRB 080916C, GRB 090902B and GRB 090510, and typically find reasonable parameters for fitting the time-binned spectra, although fine-tuning of several parameters is required. The observed delays of the high-energy emission with respect to the MeV emission which are large compared to the variability times are unlikely to be due to simple kinematic effects of a non-evolving jet. These delays may instead be attributed to the temporal evolution of the physical parameters of the jet, and thus the delay time-scales could provide a potential tool for investigating the structures of GRB jets themselves and their progenitors. The difference of the delay time-scales of long and short GRBs inferred from the Fermi data might be due to the differences in the progenitors of long and short GRBs. Some other properties and consequences of this model are discussed, including temporal correlations among the prompt optical, the soft X-ray and the distinct high-energy component as well as the band component.

Particle acceleration at multiple internal relativistic shocks

Relativistic shocks provide an efficient method for high-energy particle acceleration in many astrophysical sources. Multiple shock systems are even more effective and of importance, for example, in the internal shock model of gamma-ray bursts. We investigate the reacceleration of pre-existing energetic particles at such relativistic internal shocks by the first order Fermi process of pitch angle scattering. We use a well established eigenfunction method to calculate the resulting spectra for infinitely thin shocks. Implications for GRBs and relativistic jets are discussed.

Electron acceleration in relativistic GRB shocks

The shock model of gamma-ray bursts (GRBs) has two free parameters: B and e. It has been shown that B should range between few x 10(-3) and few x 10(-4). However, how to calculate the value of e has remained an outstanding theoretical problem for over a decade. Here, we demonstrate that the Weibel theory inevitably predicts that epsilone approximately radical epsilonB. The GRB afterglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. This e-B relation can potentially be used to reduce the number of free parameters in afterglow models.

Electron Acceleration in Relativistic Gamma-Ray Burst Shocks

The shock model of gamma-ray bursts (GRBs) contains two equipartition parameters: the magnetic energy density and the kinetic energy density of the electrons relative to the total energy density of the shock, "epsilon_B" and "epsilon_e", respectively. These are free parameters within the model. Whereas the Weibel shock theory and numerical simulations fix "epsilon_B" at the level of ~few times(10^{-3}...10^{-4}), no understanding of "epsilon_e" exists so far. Here we demonstrate that it inevitably follows from the theory that "epsilon_e"~(epsilon_B)^(1/2). The GRB afteglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. The "epsilon_e"-"epsilon_B" relation can potentially be used to reduce the number of free parameters in afterglow models.

Steep Slopes and Preferred Breaks in Gamma‐Ray Burst Spectra: The Role of Photospheres and Comptonization

The role of a photospheric component and of pair breakdown is examined in the internal shock model of gamma-ray bursts. We discuss some of the mechanisms by which they would produce anomalously steep low energy slopes, X-ray excesses and preferred energy breaks. Sub-relativistic comptonization should dominate in high comoving luminosity bursts with high baryon load, while synchrotron radiation dominates the power law component in bursts which have lower comoving luminosity or have moderate to low baryon loads. A photosphere leading to steep low energy spectral slopes should be prominent in the lowest baryon load

The External Shock Model of Gamma-Ray Bursts: Three Predictions and a Paradox Resolved

In the external shock model, gamma-ray burst (GRB) emissions are produced by the energization and deceleration of a thin relativistic blast wave that interacts with the circumburst medium (CBM). We study the physical properties of an analytic function that describes temporally evolving GRB spectra in the limit of a smooth CBM with density n(x)∝x−η, where x is the radial coordinate. The hard-to-soft spectral evolution and the intensity-hardness correlation of GRB peaks are reproduced. We predict that (1) GRB peaks are aligned at high photon energies and lag at low energies according to a simple rule, that (2) temporal indices at the leading edge of a GRB peak display a well-defined shift with photon energy, and that (3) the change in the spectral index values between the leading and trailing edges of a GRB peak decreases at higher photon energies. The reason that GRBs are usually detected with νFν peaks in the 50 keV to several MeV range for detectors triggering on peak flux over a fixed time interval is shown to be a consequence of the inverse correlation of the peak flux and the duration of the radiation emitted by decelerating blast waves.

Short-Timescale Variability in the External Shock Model of Gamma-Ray Bursts

We have developed a computer model to calculate gamma ray burst (GRB) light curves and efficiencies from the interaction of a single, thin blast wave with clouds in the external medium. Large amplitude, spiky variability occurs when the clouds have radii r ~ 10% require a large number of small clouds, each with sufficiently large column densities to extract most of the available blast-wave energy in the region of interaction. For such efficiencies, we find that erratic variability persists in the simulated GRB light curves. If GRB sources are surrounded by clouds with such properties, then short timescale variability of GRBs is possible in the external shock model.