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 R w , where D is a constant, the properties of the shock breakout radiation depend on R w and R d ≡ κDv sh /c, where κ is the opacity and v sh is the shock velocity. If R w > R d , the rise to maximum light begins at ∼ R d /v sh ; the duration of the rise is also ∼ R d /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 R w < R d , the rise to maximum light begins at R w /v sh ; the duration of the rise is R 2 w /v sh R d ; 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.

We report four years of radio and X-ray monitoring of the Type IIn supernova SN 2006jd at radio wavelengths with the Very Large Array, Giant Metrewave Radio Telescope and Expanded Very Large Array; at X-ray wavelengths with Chandra, XMM-Newton and Swift-XRT. We assume that the radio and X-ray emitting particles are produced by shock interaction with a dense circumstellar medium. The radio emission shows an initial rise that can be attributed to freefree absorption by cool gas mixed into the nonthermal emitting region; external free-free absorption is disfavored because of the shape of the rising light curves and the low gas column density inferred along the line of sight to the emission region. The X-ray luminosity implies a preshock circumstellar density ∼ 10 6 cm −3 at a radius r ∼ 2 × 10 16 cm, but the column density inferred from the -2photoabsorption of X-rays along the line of sight suggests a significantly lower density. The implication may be an asymmetry in the interaction. The X-ray spectrum shows Fe line emission at 6.9 keV that is stronger than is expected for the conditions in the X-ray emitting gas. We suggest that cool gas mixed into the hot gas plays a role in the line emission. Our radio and X-ray data both suggest the density profile is flatter than r −2 because of the slow evolution of the unabsorbed emission.

We report two epochs of Chandra-ACIS X-ray imaging spectroscopy of the nearby bright Type IIn supernova SN 2010jl, taken around 2 months and then a year after the explosion. The majority of the X-ray emission in both the spectra is characterized by a high temperature ( 10 keV) and is likely to be from the forward shocked region resulting from circumstellar interaction. The absorption column density in the first spectrum is high (∼ 10 24 cm −2 ), more than 3 orders of magnitude higher than the Galactic absorption column, and we attribute it to absorption by circumstellar matter. In the second epoch observation, the column density has decreased by a factor of 3, as expected for shock propagation in the circumstellar medium. The unabsorbed 0.2−10 keV luminosity at both epochs is ∼ 7 ×10 41 erg s −1 . The 6.4 keV Fe line clearly present in the first spectrum is not detected in the second spectrum. The strength of the fluorescent line is roughly that expected for the column density of circumstellar gas, provided the Fe is not highly ionized. There is also evidence for an absorbed power law component in both the spectra, which we attribute to a background ultraluminous X-ray source.

We consider a model for the low-luminosity gamma-ray burst GRB 060218 that plausibly accounts for multiwavelength observations to day 20. The model components are: (1) a long-lived (t j ∼ 3000 s) central engine and accompanying low-luminosity (L j ∼ 10 47 erg s −1 ), semirelativistic (γ ∼ 10) jet; (2) a low-mass (∼ 4 × 10 −3 M ) envelope surrounding the progenitor star; and (3) a modest amount of dust (A V ∼ 0.1 mag) in the interstellar environment. Blackbody emission from the transparency radius in a low-power jet outflow can fit the prompt thermal X-ray emission, and the nonthermal X-rays and γ-rays may be produced via Compton scattering of thermal photons from hot leptons in the jet interior or the external shocks. The later mildly relativistic phase of this outflow can produce the radio emission via synchrotron radiation from the forward shock. Meanwhile, interaction of the associated SN 2006aj with a circumstellar envelope extending to ∼ 10 13 cm can explain the early optical emission. The X-ray afterglow can be interpreted as a light echo of the prompt emission from dust at ∼ 30 pc. Our model is a plausible alternative to that of Nakar, who recently proposed shock breakout of a jet smothered by an extended envelope as the source of prompt emission. Both our results and Nakar's suggest that bursts such as GRB 060218 may originate from unusual progenitors with extended circumstellar envelopes, and that a jet is necessary to decouple the prompt emission from the supernova.

Type IIn and related supernovae show evidence for an interaction with a dense circumstellar medium that produces most of the supernova luminosity. Xray emission from shock heated gas is crucial for the energetics of the interaction and can provide diagnostics on the shock interaction. Provided that the shock is at an optical depth τ w c/v s in the wind, where c is the speed of light and v s is the shock velocity, a viscous shock is expected that heats the gas to a high temperature. For τ w 1, the shock wave is in the cooling regime; inverse Compton cooling dominates bremsstrahlung at higher densities and shock velocities. Although τ w 1, the optical depth through the emission zone is 1 so that inverse Compton effects do not give rise to significant X-ray emission. The electrons may not reach energy equipartition with the protons at higher shock velocities. As X-rays move out through the cool wind, the higher energy photons are lost to Compton degradation. If bremsstrahlung dominates the cooling and Compton losses are small, the energetic radiation can completely photoionize the preshock gas. However, inverse Compton cooling in the hot region and Compton degradation in the wind reduce the ionizing flux, so that complete photoionization is not obtained and photoabsorption by the wind further reduces the escaping X-ray flux. We conjecture that the combination of these effects led to the low observed X-ray flux from the optically luminous SN 2006gy.

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The shock breakout emission is the first light that emerges from a supernova. In the spherical case it is characterized by a brief UV flash. In an axisymmetric, non-spherical prolate explosion, the shock first breaches the surface along the symmetry axis, then peels around to larger angles, producing a breakout light curve which may differ substantially from the spherically symmetric case. We study the emergence of a non-relativistic, bipolar shock from a spherical star, and estimate the basic properties of the associated bolometric shock breakout signal. We identify four possible classes of breakout light curves, depending on the degree of asphericity. Compared to spherical breakouts, we find that the main distinguishing features of significantly aspherical breakouts are 1) a longer and fainter initial breakout flash and 2) an extended phase of slowly-declining, or even rising, emission which is produced as ejecta flung out by the oblique breakout expand and cool. We find that the breakout flash has a maximum duration of roughly ∼R*/vbo where R* is the stellar radius and vbo is the velocity of the fastest-moving ejecta. For a standard Wolf–Rayet progenitor, the duration of the X-ray flash seen in SN 2008D exceeds this limit, and the same holds true for the prompt X-ray emission of low-luminosity GRBs such as GRB 060218. This suggests that these events cannot be explained by an aspherical explosion within a typical Wolf–Rayet star, implying that they originate from non-standard progenitors with larger breakout radii.

Observations of both gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) point to the idea that some relativistic jets are suffocated by their environment before we observe them. In these 'choked' jets, all the jet's kinetic energy is transferred into a hot and narrow cocoon of near-uniform pressure. We consider the evolution of an elongated, axisymmetric cocoon formed by a choked jet as it expands into a cold power-law ambient medium ρ ∝ R −α , in the case where the shock is decelerating (α < 3). The evolution proceeds in three stages, with two breaks in behaviour: the first occurs once the outflow has doubled its initial width, and the second once it has doubled its initial height. Using the Kompaneets approximation, we derive analytical formulae for the shape of the cocoon shock, and obtain approximate expressions for the height and width of the outflow versus time in each of the three dynamical regimes. The asymptotic behaviour is different for flat (α ≤ 2) and steep (2 < α < 3) density profiles. Comparing the analytical model to numerical simulations, we find agreement to within ∼ 15 per cent out to 45 degrees from the axis, but discrepancies of a factor of 2-3 near the equator. The shape of the cocoon shock can be measured directly in AGNs, and is also expected to affect the early light from failed GRB jets. Observational constraints on the shock geometry provide a useful diagnostic of the jet properties, even long after jet activity ceases.