Roger A. Chevalier

The afterglow of GRB 050709 and the nature of the short-hard gamma-ray bursts.

The final chapter in the long-standing mystery of the γ-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

Detection of Circumstellar Material in a Normal Type Ia Supernova

Type Ia supernovae are important cosmological distance indicators. Each of these bright supernovae supposedly results from the thermonuclear explosion of a white dwarf star that, after accreting material from a companion star, exceeds some mass limit, but the true nature of the progenitor star system remains controversial. Here we report the spectroscopic detection of circumstellar material in a normal type Ia supernova explosion. The expansion velocities, densities, and dimensions of the circumstellar envelope indicate that this material was ejected from the progenitor system. In particular, the relatively low expansion velocities suggest that the white dwarf was accreting material from a companion star that was in the red-giant phase at the time of the explosion.

Neutron star accretion in a supernova

An estimate can be made of the effect of a central mass on an explosion through the study of self-similar blast waves in a medium having an inverse-square-of-r density profile, as is approximately the case in the inner core of the progenitor of SN 1987A; its parameters are such that the nucleon density is substantially constant within the shock front. In SN explosions with a massive envelopes, the core material is decelerated by its interaction with the envelope, and a reverse shock wave propagates back to the center. 42 refs.

A novel explosive process is required for the gamma-ray burst GRB 060614.

Over the past decade, our physical understanding of γ-ray bursts (GRBs) has progressed rapidly, thanks to the discovery and observation of their long-lived afterglow emission. Long-duration (≳2 s) GRBs are associated with the explosive deaths of massive stars (‘collapsars’, ref. 1), which produce accompanying supernovae; the short-duration (≲2 s) GRBs have a different origin, which has been argued to be the merger of two compact objects. Here we report optical observations of GRB 060614 (duration ∼100 s, ref. 10) that rule out the presence of an associated supernova. This would seem to require a new explosive process: either a massive collapsar that powers a GRB without any associated supernova, or a new type of ‘engine’, as long-lived as the collapsar but without a massive star. We also show that the properties of the host galaxy (redshift z = 0.125) distinguish it from other long-duration GRB hosts and suggest that an entirely new type of GRB progenitor may be required.

Gravitational collapse of an isothermal sphere

We investigate the spherical gravitational collapse of isothermal spheres using numerical hydrodynamics. The initial configuration is close to hydrostatic equilibrium. In the initial density profile has a finite core radius (i.e., it is not singular), supersonic velocities develop during the initial collapse. At the time of central core formation, when the central density diverges, the central inflow velocity approaches −3.3 times the sound speed and the central density approaches an r −2 profile. These conditions are similar to those found in the self-similar solution of Larson and Penston, but occur only at the center and not at all radii as in the self-similar solution at core formation

PULSAR WIND NEBULAE IN EVOLVED SUPERNOVA REMNANTS

For pulsars similar to the one in the Crab Nebula, most of the energy input to the surrounding wind nebula occurs on a timescale 103 yr; during this time, the nebula expands into freely expanding supernova ejecta. On a timescale ~104 yr, the interaction of the supernova with the surrounding medium drives a reverse shock front toward the center of the remnant, where it crushes the pulsar wind nebula (PWN). We have carried out one- and two-dimensional, two-fluid simulations of the crushing and reexpansion phases of a PWN. We show that (1) these phases are subject to Rayleigh-Taylor instabilities that result in the mixing of thermal and nonthermal fluids, and (2) asymmetries in the surrounding interstellar medium give rise to asymmetries in the position of the PWN relative to the pulsar and explosion site. These effects are expected to be observable in the radio emission from evolved PWN because of the long lifetimes of radio-emitting electrons. The scenario can explain the chaotic and asymmetric appearance of the Vela X PWN relative to the Vela pulsar without recourse to a directed flow from the vicinity of the pulsar. The displacement of the radio nebulae in G327.1-1.1, MSH 15-56 (G326.3-1.8), G0.9+0.1, and W44 relative to the X-ray nebulae may be due to this mechanism. On timescales much greater than the nebular crushing time, the initial PWN may be mixed with thermal gas and become unobservable, so that even the radio emission is dominated by recently injected particles.

Synchrotron Self-Absorption in Radio Supernovae

The radio emission from supernovae has been modeled as synchrotron emission from the interaction between the supernova and a presupernova stellar wind. The emission shows a late power-law decline and early rise due to a low-frequency absorption process, which, in some cases (SN 1979C and SN 1980K), is well modeled as free-free absorption by the external wind. However, the flux rises in many radio supernovae (e.g., SN 1987A and Type Ib and Type Ic supernovae) have not been sufficiently well observed to define the absorption mechanism. The assumption of synchrotron self-absorption yields an approximate radius of the emission region at the time of peak flux. If another mechanism is dominant, the radius must be even larger. The large radii implied for SN 1987A and the observed Type Ib and Type Ic supernovae make external free-free absorption unlikely, given what else is known about these systems, so that synchrotron self-absorption is the probable absorption mechanism. Synchrotron self-absorption may play a role in the radio emission from SN 1981K (type unknown) and SN 1978K (Type IIn). Model supernova light curves with synchrotron self-absorption are presented. A combination of synchrotron self-absorption and external free-free absorption may be relevant to SN 1993J. The radii implied for the Type II supernovae SN 1979C and SN 1980K in the synchrotron self-absorption model are smaller than the radii expected for circumstellar interaction and are thus consistent with the free-free absorption mechanism.

A relativistic type Ibc supernova without a detected γ-ray burst

Long duration γ-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of γ-rays and a long-lived radio afterglow. Until now, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected because of limited satellite sensitivity or beaming of the collimated emission away from our line of sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for type Ibc supernovae with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. A comparison with our radio survey of type Ibc supernovae reveals that the fraction harbouring central engines is low, about one per cent, measured independently from, but consistent with, the inferred rate of nearby GRBs. Independently, a second mildly relativistic supernova has been reported.

SHOCK BREAKOUT IN DENSE MASS LOSS: LUMINOUS SUPERNOVAE

We examine the case where a circumstellar medium around a supernova is sufficiently opaque that a radiation-dominated shock propagates in the circumstellar region. The initial propagation of the shock front into the circumstellar region can be approximated by a self-similar solution that determines the radiative energy in a shocked shell; the eventual escape of this energy gives the maximum luminosity of the supernova. If the circumstellar density is described by ρ = Dr –2 out to a radius Rw , where D is a constant, the properties of the shock breakout radiation depend on Rw and Rd ≡ κDv sh/c, where κ is the opacity and v sh is the shock velocity. If Rw >Rd , the rise to maximum light begins at ~Rd /v sh; the duration of the rise is also ~Rd /v sh; the outer parts of the opaque medium are extended and at low velocity at the time of peak luminosity; and a dense shell forms whose continued interaction with the dense mass loss gives a characteristic flatter portion of the declining light curve. If Rw < Rd , the rise to maximum light begins at Rw /v sh; the duration of the rise is R 2 w /v sh Rd ; the outer parts of the opaque medium are not extended and are accelerated to high velocity by radiation pressure at the time of maximum luminosity; and a dense shell forms but does not affect the light curve near maximum. We argue that SN 2006gy is an example of the first kind of event, while SN 2010gx and related supernovae are examples of the second.

Young Core-Collapse Supernova Remnants and Their Supernovae

Massive star supernovae can be divided into four categories, depending on the amount of mass loss from the progenitor star and the stars radius: red supergiant stars with most of the H envelope intact (SN IIP), stars with some H but most lost (IIL and IIb), stars with all H lost (Ib and Ic), and blue supergiant stars with a massive H envelope (SN 1987A-like). Various aspects of the immediate aftermath of the supernova are expected to develop in different ways, depending on the supernova category: mixing in the supernova, fallback on the central compact object, expansion of any pulsar wind nebula, interaction with circumstellar matter, and photoionization by shock breakout radiation. The observed properties of young supernova remnants allow many of them to be placed in one of the supernova categories; all the categories are represented except for the SN 1987A-like type. Of the remnants with central pulsars, the pulsar properties do not appear to be related to the supernova category. There is no evidence that the supernova categories form a mass sequence, as would be expected in a single-star scenario for the evolution. Models for young pulsar wind nebulae expanding into supernova ejecta indicate initial pulsar periods of 10-100 ms and approximate equipartition between particle and magnetic energies. Ages are obtained for pulsar nebulae, including an age of 2400 ± 500 yr for 3C 58, which is not consistent with an origin in SN 1181. There is no evidence that mass fallback plays a role in neutron star properties.