arXiv subjects

Roger A. Chevalier

Publications and source records attributed to Roger A. Chevalier.

At least 73 records · Page 4Linked to original sources

arXiv2005

Late Emission from the Type Ib/c SN 2001em: Overtaking the Hydrogen Envelope

The Type Ib/c supernova SN 2001em was observed to have strong radio, X-ray, and Halpha emission at an age of about 2.5 yr. Although the radio and X-ray emission have been attributed to an off-axis gamma-ray burst, we model the emission as the interaction of normal SN Ib/c ejecta with a dense, massive (3 Msun) circumstellar shell at a distance about 7 x 10^{16} cm. We investigate two models, in which the circumstellar shell has or has not been overtaken by the forward shock at the time of the X-ray observation. The circumstellar shell was presumably formed by vigorous mass loss with a rate (2-10) x 10^{-3} Msun/yr at 1000-2000 yr prior to the supernova explosion. The hydrogen envelope was completely lost, and subsequently was swept up and accelerated by the fast wind of the presupernova star up to a velocity of 30-50 km/s. Although interaction with the shell can explain most of the late emission properties of SN 2001em, we need to invoke clumping of the gas to explain the low absorption at X-ray and radio wavelengths.

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arXiv2005

Radio and X-Ray Emission as Probes of Type IIP Supernovae and Red Supergiant Mass Loss

Type IIP (plateau) supernovae are thought to come from stars with initial mass about 8-25 solar masses that end their lives as red supergiants. The expected stellar end points can be found from evolutionary calculations and the corresponding mass loss properties at this point can be estimated from typical values for Galactic stars. The mass loss densities of observed supernovae can be estimated from observations of the thermal X-ray and radio synchrotron emission that result from the interaction of the supernova with the surrounding wind. Type IIP supernovae are expected to have energy-conserving interaction during typical times of observation. Because Type IIP supernovae have an extended period of high optical luminosity, Compton cooling can affect the radio emitting electrons, giving rise to a relatively flat radio light curve in the optically thin regime. Alternatively, a high efficiency of magnetic field production results in synchrotron cooling of the radio emitting electrons. Both the X-ray and radio luminosities are sensitive to the mass loss and initial masses of the progenitor stars, although the turn-on of radio emission is probably the best estimator of circumstellar density. Both the mass loss density and the variation of density with stellar mass are consistent with expectations for the progenitor stars deduced from direct observations of recent supernovae. Current observations are consistent with mass being the only parameter; observations of a supernova in a metal poor region might show how the mass loss depends on metallicity.

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arXiv2004

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 star's radius: red supergiant stars with most of the H envelope intact (SN IIP), stars with some H but most lost (IIL, IIb), stars with all H lost (Ib, 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 pm 500 yr for 3C58, which is not consistent with an origin in SN 1181. There is no evidence that mass fallback plays a role in neutron star properties.

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arXiv2004

Hubble Space Telescope and Ground-Based Observations of SN 1993J and SN 1998S: CNO Processing in the Progenitors

Ground-based and Hubble Space Telescope observations are presented for SN 1993J and SN 1998S. SN 1998S shows strong, relatively narrow circumstellar emission lines of N III-V and C III-IV, as well as broad lines from the ejecta. Both the broad ultraviolet and optical lines in SN 1998S indicate an expansion velocity of ~7,000 km/s. The broad emission components of Ly-alpha and Mg II are strongly asymmetrical after day 72 past the explosion, and differ in shape from H-alpha. Different models based on dust extinction from dust in the ejecta or shock region, in combination with H-alpha from a circumstellar torus, are discussed. It is concluded, however, that the double-peaked line profiles are more likely to arise as a result of optical depth effects in the narrow, cool, dense shell behind the reverse shock, than in a torus-like region. The ultraviolet lines of SN 1993J are broad, with a box-like shape, coming from the ejecta and a cool dense shell. The shapes of the lines are well fitted with a shell with inner velocity ~7,000 km/s and outer velocity ~10,000 km/s. For both SN 1993J and SN 1998S a strong nitrogen enrichment is found, with N/C~12.4 in SN 1993J and N/C~6.0 in SN 1998S. From a compilation of all supernovae with determined CNO ratios, we discuss the implications of these observations for the structure of the progenitors of Type II supernovae.

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arXiv2004

Pulsar Wind Nebulae: Theoretical Aspects and Observational Constraints

Of the known pulsar wind nebulae, 8 are good candidates for being in the early stage of evolution where the wind nebula is interacting with the freely expanding supernova ejecta. Several of these have been identified with historical supernovae. Although the identification of SN 1181 with 3C 58 has been thought to be relatively secure, the large size of the nebula, the amount of swept up mass, and the internal energy indicate a larger age. For G11.2-0.3, the nebular size and internal energy are consistent with the identification with the possible supernova of 386. Although the Crab Nebula appears to have approximate energy equipartition between particles and the magnetic field, the nebulae 3C 58 and MSH 15-52 appear to be particle dominated. The low magnetic field is consistent with models in which the nebulae are created by a shocked pulsar wind.

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arXiv2003

The Diversity of Gamma-Ray Bursts and the Surroundings of Massive Stars

The finding of a Type Ic supernova connected with GRB 030329 showed a massive star origin for this burst, supporting evidence for this association in previous bursts with lightcurve bumps at the appropriate time for a supernova. Here, we explore the possibility that all long bursts have massive star progenitors, interacting with either the freely expanding wind of the progenitor or the shocked wind. We present models for the afterglows of GRB 020405 and GRB 021211, which are a challenge to wind interaction models. Considering sources for which wind interaction models are acceptable, a range of wind densities is required, from values typical of Galactic Wolf-Rayet stars to values ~100 times smaller. The reason for the low densities is unclear, but may involve low progenitor masses and/or low metallicities. If mass is a factor, a low density event should be associated with a low mass supernova. The interpretation of bursts apparently interacting with constant density media as interaction with a shocked wind requires both a range of mass loss densities and a range of external pressures. The highest pressures, p/k > 10^8 cm^{-3} K, may be due to an extreme starburst environment, which would imply that the burst is superposed on an active star forming region. Although the range of observed events can be accomodated by the shocked wind theory, special circumstances are necessary to bring this about. Finally, we consider the high velocity, high ionization absorption features observed in some afterglow spectra. If the features are circumstellar, the presence of the burst in a starburst region may be important for the formation of clumps near the burst.

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arXiv2003

Pulsar Wind Nebulae and Their Supernovae

Young supernova remnants that contain pulsar wind nebulae provide diagnostics for both the inner part of the supernova and the interaction with the surrounding medium, providing an opportunity to relate these objects to supernova types. Among observed young nebulae, there is evidence for a range of supernova types, including Type IIP (Crab Nebula and SN 1054) and Type IIb/IIn/IIL (G292.0+1.8).

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arXiv2003

Supernova Remnant and Pulsar Wind Nebula Interactions

I review several topics in the structure of supernova remnants. Hydrodynamic instabilities in young remnants may give rise to the cellular structure that is sometimes observed, although structure in the ejecta might also play a role. The presence of ejecta close to the forward shock front of a young remnant can be the result of ejecta clumps or the dynamical effects of cosmic rays. Slower moving ejecta clumps can affect the outer shock structure of older remnants such as Vela. Young remnants typically show a circular structure, but often have a one-sided asymmetry; the likely reasons are an asymmetric circumstellar medium, or pulsar velocities in the case of pulsar wind nebulae. In older remnants, asymmetric pulsar wind nebulae can result from asymmetric reverse shock flows and/or pulsar velocities.

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arXiv2003

The Surroundings of Gamma-Ray Bursts: Constraints on Progenitors

The association of a supernova with a gamma-ray burst (GRB 030329) implies a massive star progenitor, which is expected to have an environment formed by pre-burst stellar winds. Although some sources are consistent with the expected wind environment, many are not, being better fit by a uniform density environment. One possibility is that this is a shocked wind, close to the burst because of a high interstellar pressure and a low mass loss density. Alternatively, there is more than one kind of burst progenitor, some of which interact directly with the interstellar medium. Another proposed environment is a pulsar wind bubble that has expanded inside a supernova, which requires that the supernova precede the burst.

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arXiv2003

Cassiopeia A and its Clumpy Presupernova Wind

The observed shock wave positions and expansion in Cas A can be interpreted in a model of supernova interaction with a freely expanding stellar wind with a mass loss rate of ~3e-5 Msun/yr for a wind velocity of 10 km/s. The wind was probably still being lost at the time of the supernova, which may have been of Type IIn or IIb. The wind may play a role in the formation of very fast knots observed in Cas A. In this model, the quasi-stationary flocculi (QSFs) represent clumps in the wind, with a density contrast of several 1000 compared to the smooth wind. The outer, unshocked clumpy wind is photoionized by radiation from the supernova, and is observed as a patchy HII region around Cas A. This gas has a lower density than the QSFs and is heated by nonradiative shocks driven by the blast wave. Denser clumps have recombined and are observed as HI compact absorption features towards Cas A.

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arXiv2003

Wind-Interaction Models for the Early Afterglows of Gamma-Ray Bursts: The Case of GRB 021004

Wind-interaction models for gamma-ray burst afterglows predict that the optical emission from the reverse shock drops below that from the forward shock within 100s of seconds of the burst. The typical frequency $ν_m$ of the synchrotron emission from the forward shock passes through the optical band typically on a timescale of minutes to hours. Before the passage of $ν_m$, the optical flux evolves as $t^{-1/4}$ and after the passage, the decay steepens to $t^{-(3p-2)/4}$, where $p$ is the exponent for the assumed power-law energy distribution of nonthermal electrons and is typically $\sim 2$. The steepening in the slope of temporal decay should be readily identifiable in the early afterglow light curves. We propose that such a steepening was observed in the R-band light curve of GRB 021004 around day 0.1. Available data at several radio frequencies are consistent with this interpretation, as are the X-ray observations around day~1. The early evolution of GRB 021004 contrasts with that of GRB 990123, which can be described by emission from interaction with a constant density medium.

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arXiv2003

Circumstellar Interaction Around Supernovae

Circumstellar interaction has been observed around all types of massive star supernovae, especially at radio and X-ray wavelengths. The interaction shells in Type Ib/c supernovae appear to be moving rapidly, although SN 1998bw remains the only case for which the evidence points to relativistic expansion. Type IIP supernovae have relatively low mass loss rates, consistent with moderate mass single stars at the ends of their lives. Type IIb and some IIL and IIn supernovae can have strong mass loss leading up to the explosion, which occurs in a star with little H on it. The mass loss may be driven by the luminosity of the late burning stages.

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arXiv2002

Modelling the Hubble Space Telescope Ultraviolet and Optical Spectrum of Spot 1 on the Circumstellar Ring of SN 1987A

We report and interpret HST/STIS long-slit observations of the optical and ultraviolet (1150 - 10270 Angstrom) emission-line spectra of the rapidly brightening Spot 1 on the equatorial ring of SN 1987A between 1997 September and 1999 October (days 3869 -- 4606 after outburst). The emission is caused by radiative shocks created where the supernova blast wave strikes dense gas protruding inward from the equatorial ring. We measure and tabulate line identifications, fluxes and, in some cases, line widths and shifts. We compute flux correction factors to account for substantial interstellar line absorption of several emission lines. Nebular analysis shows that optical emission lines come from a region of cool (T_e ~ 10^4 K) and dense (n_e ~ 10^6 cm^-3) gas in the compressed photoionized layer behind the radiative shock. The observed line widths indicate that only shocks with shock velocities V_s < 250 km/s have become radiative, while line ratios indicate that much of the emission must have come from yet slower (V_s < 135 k/ms) shocks. We are able to fit the UV fluxes with an idealized radiative shock model consisting of two shocks (V_s = 135 and 250 km/s). The observed UV flux increase with time can be explained by the increase in shock surface areas as the blast wave overtakes more of the protrusion. The observed flux ratios of optical to highly-ionized UV lines are greater by a factor of ~ 2 -- 3 than predictions from the radiative shock models and we discuss the possible causes. We also present models for the observed H-alpha line widths and profiles, which suggests that a chaotic flow exists in the photoionized regions of these shocks. We discuss what can be learned with future observations of all the spots present on the equatorial ring.

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arXiv2002

Interactions between Pulsars, Pulsar Nebulae and Supernova Remnants

The Crab Nebula is likely to be expanding into freely expanding supernova ejecta, although the energy in the ejecta may be less than is typical for a Type II supernova. Pulsar nebulae much younger than the Crab have not been found and could have different properties. The search for such nebulae through ultraviolet/optical line emission in core collapse supernovae, or through their X-ray emission (which could show strong absorption) is warranted. Neutron stars have now been found in many young supernova remnants. There is not a clear link between neutron star and remnant type, although there is an indication that normal pulsars avoid the O-rich remnants. In the later phases of a supernova remnant, the pulsar wind nebula is crushed by the reverse shock front. Recent simulations show that this process is unstable, which can lead to mixing of the thermal and relativistic gases, and that the pulsar nebula is easily displaced from the pulsar, which can explain the position of the Vela pulsar relative to the Vela X radio nebula.

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arXiv2001

Bullets in a Core Collapse Supernova Remnant: The Vela Remnant

We use two-dimensional hydrodynamical simulations to investigate the properties of dense ejecta clumps (bullets) in a core collapse supernova remnant, motivated by the observation of protrusions probably caused by clumps in the Vela supernova remnant. The ejecta, with an inner flat and an outer steep power law density distribution, were assumed to freely expand into an ambient medium with a constant density, $\sim 0.1$ H atoms cm$^{-3}$ for the case of Vela. At an age of $10^4$ yr, the reverse shock front is expected to have moved back to the center of the remnant. Ejecta clumps with an initial density contrast $χ\sim 100$ relative to their surroundings are found to be rapidly fragmented and decelerated. In order to cause a pronounced protrusion on the blast wave, as observed in the Vela remnant, $χ\sim 1000$ may be required. In this case, the clump should be near the inflection point in the ejecta density profile, at an ejecta velocity $\sim 3000 \kms $. These results apply to moderately large clumps; smaller clumps would require an even larger density contrast. Clumps can create ring structure in the shell of the Vela remnant and we investigate the possibility that RX J0852--4622, an apparent supernova remnant superposed on Vela, is actually part of the Vela shell. Radio observations support this picture, but the possible presence of a compact object argues against it. The Ni bubble effect or compression in a pulsar wind nebula are possible mechanisms to produce the clumping.

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arXiv2001

Supernova Interaction with a Circumstellar Medium

The explosion of a core collapse supernova drives a powerful shock front into the wind from the progenitor star. A layer of shocked circumstellar gas and ejecta develops that is subject to hydrodynamic instabilities. The hot gas can be observed directly by its X-ray emission, some of which is absorbed and re-radiated at lower frequencies by the ejecta and the circumstellar gas. Synchrotron radiation from relativistic electrons accelerated at the shock fronts provides information on the mass loss density if free-free absorption dominates at early times or the size of the emitting region if synchrotron self-absorption dominates. Analysis of the interaction leads to information on the density and structure of the ejecta and the circumstellar medium, and the abundances in these media. The emphasis here is on the physical processes related to the interaction.

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arXiv2001

Interaction Models for GRB Afterglows

We review interaction models for GRB afterglows, with an eye on constraining the nature of their progenitors and the geometry of explosion. Evidence is presented for two types of progenitors. The radio afterglow of GRB 980425/SN 1998bw and the multi-frequency observations of GRB 970508 can be fitted by a blast wave expanding into a wind-type medium, pointing to a massive star progenitor. The broadband afterglow data of GRBs 990123 and 990510 are better modeled by a jet expanding into a constant-density medium, implying a compact star merger origin. Among other well observed GRBs, the jet model appears to be more widely applicable than the wind model, although some cases are ambiguous. The model fits often require a deviation of the energy distribution of the radiating electrons from the commonly assumed single power-law form, particularly for sources with a rapid decline and/or pronounced steepening in the optical light curves. Transition to non-relativistic evolution has been suggested as an alternative explanation for the light curve steepening, although to produce steepening on the order of days or less would require very high ambient densities, which are generally difficult to reconcile with radio observations. Major open issues include the hydrodynamics of jet-ambient medium interaction, a self-consistent determination of the electron energy distribution, and the effects of pair production on the early afterglows which are expected to be particularly large for the wind interaction model.

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arXiv2001

Hubble Space Telescope WFPC-2 Imaging of Cassiopeia A

The young SNR Cassiopeia A was imaged with WFPC-2 through four filters selected to capture the complete velocity range of the remnant's main shell in several important emission lines. Primary lines detected were [O III] 4959,5007, [N II] 6583, [S II] 6716,6731 + [O II] 7319,7330 + [O I] 6300,6364, and [S III] 9069,9532. About 3/4th of the remnant's main shell was imaged in all four filters. Considerable detail is observed in the reverse-shocked ejecta with typical knot scale lengths of 0.2"-0.4" (1 - 2 x 10^16 cm). Both bright and faint emission features appear highly clumped. Large differences in [S III] and [O III] line intensities indicating chemical abundance differences are also seen, particularly in knots located along the bright northern limb and near the base of the northeast jet. A line of curved overlapping filament in the remnant's northwestern rim appears to mark the location of the remnant's reverse shock front in this region. Finger-like ejecta structures elsewhere suggest cases where the reverse shock front is encountering the remnant's clumped ejecta. Narrow-band [N II] images of the remnant's circumstellar knots ("QSFs") reveal them to be 0.1"-0.6" thick knots and filaments, often with diffuse edges facing away from the center of expansion. Three color composite images of the whole remnant and certain sections along with individual filter enlargements of selected regions of the bright optical shell are presented and discussed.

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