SN1998bw/GRB980425 and Radio Supernovae

The unusual supernova SN1998bw, which is thought to be related to the gamma-ray burster GRB980425, is a possible link between the two classes of objects. Analyzing the extensive radio emission data avaliable for SN1998bw, we are able to describe its time evolution within the well established framework available for the analysis of radio emission from supernovae. This then allows description of a number of physical properties of the object. The radio emission can best be explained as interaction of a mildly relativistic (Gamma about 1.6) shock with a dense pre-explosion stellar wind established circumstellar medium (CSM) which is highly structured both azimuthally, in clumps or filaments, and radially, with two observed density enhancements separated by about 3e17 cm. With assumptions as to pre-explosion stellar wind conditions, it is possible to estimate that the progenitor to SN 1998bw had a mass loss rate of about 3.5e-5 solar masses per yr with at least two approximately 30% increases in mass-loss rate; the most recent extending from about 1,600 - 4,700 yr before explosion and the oldest known having occurred, with possibly comparable length, about 12,000 yr before explosion. Because of its unusual characteristics for a Type Ib/c SN, the relation of SN1998bw to GRB980425 is strengthened with consequent improvement in our understanding of these poorly understood objects.Comment: 24 pages, 2 figures, to appear in Astrophysical Journa

Radio Detection of SN 1986E in NGC 4302

Radio observations of SN 1986E have shown a clear detection of emission at 6 cm wavelength about 8 months after optical discovery. Combined with a number of new upper limits and a study of the possible models, it appears that SN 1986E was probably a fairly normal Type IIL supernova, somewhat similar to SN 1980K, with radio emission at roughly expected levels. This detection continues the correlation between radio detection and late time optical emission.Comment: 14 pages, LaTeX (AASTeX), 2 PostScript figures, to appear in ApJ (Letters

Radio emission from SNe and young SNRs

Study of radio supernovae (RSNe), the earliest stages of supernova remnant (SNR) formation, over the past 20 years includes two dozen detected objects and more than 100 upper limits. From this work we are able 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 RSNe

Applying Tafkaa For Atmospheric Correction of Aviris Over Coral Ecosystems In The Hawaiian Islands

Growing concern over the health of coastal ecosystems, particularly coral reefs, has produced increased interest in remote sensing as a tool for the management and monitoring of these valuable natural resources. Hyperspectral capabilities show promising results in this regard, but as yet remain somewhat hindered by the technical and physical issues concerning the intervening water layer. One such issue is the ability to atmospherically correct images over shallow aquatic areas, where complications arise due to varying effects from specular reflection, wind blown surface waves, and reflectance from the benthic substrate. Tafkaa, an atmospheric correction algorithm under development at the U.S. Naval Research Laboratory, addresses these variables and provides a viable approach to the atmospheric correction issue. Using imagery from the Advanced Visible InfraRed Imaging Spectrometer (AVIRIS) over two shallow coral ecosystems in the Hawai ian Islands, French Frigate Shoals and Kane ohe Bay, we first demonstrate how land-based atmospheric corrections can be limited in such an environment. We then discuss the input requirements and underlying algorithm concepts of Tafkaa and conclude with examples illustrating the improved performance of Tafkaa using the same AVIRIS images

Radio Emission from SN 2001gd in NGC 5033

We present the results of monitoring the radio emission from the Type IIb supernova SN 2001gd between 2002 February 8 and 2002 October 28. Most of the data were obtained using the Very Large Array at the five wavelengths of $\lambda \lambda$1.3 cm (22.4 GHz), 2.0 cm (14.9 GHz), 3.6 cm (8.44 GHz), 6.2 cm (4.86 GHz), and 21 cm (1.4 GHz). Observations were also made with Giant Meterwave Radio Telescope at $\lambda$21 cm (1.4 GHz). The object was discovered optically well after maximum light, making any determination of the early radio evolution difficult. However, subsequent observations indicate that the radio emission has evolved regularly in both time and frequency and is well described by the SN shock/circumstellar medium interaction model.Comment: 16 pages 2 figures to appear in Astrophysical Journa

Radio Observations of SN 1980K: Evidence for Rapid Presupernova Evolution

New observations of SN 1980K made with the VLA at 20 and 6 cm from 1994 April through 1996 October show that the supernova (SN) has undergone a significant change in its radio emission evolution, dropping by a factor of ~2 below the flux density S \propto t^{-0.73} power-law decline with time t observed earlier. However, although S at all observed frequencies has decreased significantly, its current spectral index of \alpha= -0.42\pm0.15 (S \propto \nu^{+\alpha}) is consistent with the previous spectral index of \alpha=-0.60_{-0.07}^{+0.04}. It is suggested that this decrease in emission may be due to the SN shock entering a new region of the circumstellar material which has a lower density than that expected for a constant speed (w), constant mass-loss rate (Mdot) wind from the progenitor. If such an interpretation is correct, the difference in wind and shock speeds appears to indicate a significant evolution in the mass-loss history of the SN progenitor ~10^4 years before explosion, with a change in circumstellar density (\propto Mdot/w) occurring over a time span of \lesssim 4 kyr. Such features could be explained in terms of a fast ``blue-loop'' evolutionary phase of a relatively massive pre-SN progenitor star. If so, we may, for the first time, provide a stringent constraint on the mass of the SN progenitor based solely on the SN's radio emission.Comment: 22 pages, 3 figures, to appear in Ap

Detection of Pre-Shock Dense Circumstellar Material of SN 1978K

The supernova SN 1978K has been noted for its lack of emission lines broader than a few thousand km/s since its discovery in 1990. Modeling of the radio spectrum of the peculiar SN 1978K indicates the existence of HII absorption along the line of sight. To determine the nature of this absorbing region, we have obtained a high-dispersion spectrum of SN 1978K at the wavelength range 6530--6610 \AA. The spectrum shows not only the moderately broad H-alpha emission of the supernova ejecta but also narrow nebular H-alpha and [N II] emission. The high [N II]6583/H-alpha ratio, 0.8-1.3, suggests that this radio absorbing region is a stellar ejecta nebula. The expansion velocity and emission measure of the nebula are consistent with those seen in ejecta nebulae of luminous blue variables. Previous low-dispersion spectra have detected a strong [N II]5755 line, indicating an electron density of 3-12x10^5 cm^{-3}. We argue that this stellar ejecta nebula is probably part of the pre-shock dense circumstellar envelope of SN 1978K. We further suggest that SN 1997ab may represent a young version of SN 1978K.Comment: 12 pages, 1 figure, accepted for publication in the ApJ Letter