▪ Abstract Study of radio supernovae over the past 20 years includes two dozen detected objects and more than 100 upper limits. From this work it is possible to identify classes of radio properties, demonstrate conformance to and deviations from existing models, estimate the density and structure of the circumstellar material and, by inference, the evolution of the presupernova stellar wind, and reveal the last stages of stellar evolution before explosion. It is also possible to detect ionized hydrogen along the line of sight, to demonstrate binary properties of the stellar system, and to show clumpiness of the circumstellar material. More speculatively, it may be possible to provide distance estimates to radio supernovae. Over the past four years the afterglow of gamma-ray bursters has occasionally been detected in the radio, as well in other wavelengths bands. In particular, the interesting and unusual gamma-ray burst GRB980425, thought to be related to SN1998bw, is a possible link between supernovae an...

Radio emission from supernovae and gamma-ray bursters

Proper motions have been measured for 132 line-emitting filaments in the Crab Nebula on direct plates taken with the 100 and 200-inch telescopes. These motions, if assumed constant and extrapolated backwards in time, converge toward a point about 12" southeast of the double star near the center of the nebula. The filaments approach, on the average, most closely to that point or expansion center in about the year 1140, indicating that the expansion has been somewhat accelerated.

Proper motions and radial velocities were measured for an additional 126 features on the same direct plates and Mt. Wilson and Palomar spectra. These, along with other, nondynamical data, indicate a most probable distance to the object of 2.02 kpc. Two projections of the nebula perpendicular to the one seen in the plane of the sky are constructed and do not differ significantly from that presented to us.

The proper motion and radial velocity of the nebula as a whole are found and converted into galactic coordinates. This requires several assumptions of questionable validity. The nebula seems to be moving about 112 km/sec faster than the galactic rotation at its position. The proper motion of the so-called central star translates into an equally unlikely space motion, but this is also extremely uncertain. The likelihood of a physical connection between the star and the nebula is discussed.

The line-emitting filaments are shown to be distributed throughout the nebula rather than being confined to a thin outer envelope. The motions are largely radial, each filament having a velocity approximately proportional to its distance from the expansion center. The deviations from this proportionality are as large as .032"/yr or 300 km/sec and have mean values near .010"/yr or 70 km/sec. These deviations are correlated with the scatter of the filaments around the expansion center in 1140.

Photographic materials on pp. 99 and 101 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

Motions and structure of the filamentary envelope of the Crab Nebula

We compare the angular expansion velocities, determined with VLBI, with the linear expansion velocities measured from optical spectra for supernova 1993J in the galaxy M81, over the period from 7 days to ~9 yr after shock breakout, and estimate the distance to SN 1993J using the expanding shock front method (ESM) We find that the best distance estimate is obtained by fitting the angular velocity of a point halfway between the contact surface and outer shock front to the maximum observed hydrogen gas velocity We obtain a direct, geometric, distance estimate for M81 of D = 396 ± 005 ± 029 Mpc with statistical and systematic error contributions, respectively, which combine to a total standard error of ±029 Mpc The upper limit of 425 Mpc corresponds to the hydrogen gas with the highest observed velocity just reaching out to the contact surface a few days after shock breakout The lower limit of 367 Mpc corresponds to this gas reaching as far out as the forward shock for the whole observing period, which would mean that Rayleigh-Taylor fingers have grown to the forward shock already a few days after shock breakout Our distance estimate is 9% ± 13% larger than that of 363 ± 034 Mpc from the HST Key Project The radio shell and the Hα absorbing and emitting gas are similarly decelerated on average, but the latter slightly less so than the former several years after shock breakout This may indicate developing Rayleigh-Taylor fingers, extending progressively further into the shocked circumstellar medium

https://iopscience.iop.org/article/10.1086/521326/pdf

SN 1993J VLBI. IV. A Geometric Distance to M81 with the Expanding Shock Front Method

Phase-referenced VLBI observations of supernova 1993J at 24 epochs, from 50 days after shock breakout to the present, allowed us to determine the coordinates of the explosion center relative to the quasi-stationary core of the host galaxy M81 with an accuracy of 45 μas and to determine the nominal proper motion of the geometric center of the radio shell with an accuracy of 9 μas yr-1. The uncertainties correspond to 160 AU for the position and 160 km s-1 for the proper motion at the distance of the source of 3.63 Mpc. After correcting for the expected galactic proper motion of the supernova around the core of M81 using H I rotation curves, we obtain a peculiar proper motion of the radio shell center of only 320 ± 160 km s-1 to the south, which limits any possible one-sided expansion of the shell. We also find that the shell is highly circular, the outer contours in fact being circular to within 3%. Combining our proper-motion values with the degree of circular symmetry, we find that the expansion of the shock front from the explosion center is isotropic to within 5.5% in the plane of the sky. This is a more fundamental result on isotropic expansion than can be derived from the circularity of the images alone. The brightness of the radio shell, however, varies along the ridge and systematically changes with time. The degree of isotropy in the expansion of the shock front contrasts with the asymmetries and polarization found in optical spectral lines. Asymmetric density distributions in the ejecta, or more likely in the circumstellar medium, are favored to reconcile the radio and optical results. We see no sign of any disklike density distribution of the circumstellar material, with the average axis ratio of the radio shell of SN 1993J being less than 1.04.

SN 1993J VLBI. I. The Center of the Explosion and a Limit on Anisotropic Expansion

The assumption of spherical symmetry, made in most theoretical models of supernova (SN) explosions, light curves, and spectra, and in attempts to use SN explosions as cosmological distance indicators, has never been justified observationally. An observational test of this assumption is suggested here, based upon the likelihood that SN atmospheres are scattering dominated and the fact that light from an unresolved, asymmetric, scattering atmosphere is linearly polarized.

Linear Polarization of Supernova Light: A Measure of Deviation from Spherical Symmetry

Determination of the angular expansion velocity of the radiosphere of the supernova SN1979c in the galaxy Ml00 in the Virgo cluster, combined with determination of the radial expansion velocity of the photosphere, yields an estimate of the distance to the Virgo cluster centre of Dvirgo = 19+8−6 Mpc and of the Hubble constant of H0=65+35−25 km s−1 Mpc−1, where the uncertainties given are based on conservative, but reasonable estimates of all significant errors and are intended to represent 90% confidence intervals.

Hubble's constant determined using very-long baseline interferometry of a supernova

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Hubble's constant determined using very-long baseline interferometry of a supernova