Kurt W. Weiler

X-Ray, Optical, and Radio Observations of the Type II Supernovae 1999em and 1998S

Observations of the Type II-P (plateau) supernova SN 1999em and Type IIn (narrow emission line) SN 1998S have enabled estimation of the profile of the SN ejecta, the structure of the circumstellar medium (CSM) established by the pre-SN stellar wind, and the nature of the shock interaction. SN 1999em is the first Type II-P detected at both X-ray and radio wavelengths. It is the least radio luminous and one of the least X-ray luminous SNe ever detected (except for the unusual and very close SN 1987A). The Chandra X-ray data indicate nonradiative interaction of SN ejecta with a power-law density profile (ρ r-n, with n ~ 7) for a pre-SN wind with a low mass-loss rate of ~2 × 10-6 M☉ yr-1 for a wind velocity of 10 km s-1, in agreement with radio mass-loss rate estimates. The Chandra data show an unexpected, temporary rise in the 0.4-2.0 keV X-ray flux at ~100 days after explosion. SN 1998S, at an age of more than 3 yr, is still bright in X-rays and is increasing in flux density at centimeter radio wavelengths. Spectral fits to the Chandra data show that many heavy elements (Ne, Al, Si, S, Ar, and Fe) are overabundant with respect to solar values. We compare the observed elemental abundances and abundance ratios to theoretical calculations and find that our data are consistent with a progenitor mass of approximately 15-20 M☉ if the heavy-element ejecta are radially mixed out to a high velocity. If the X-ray emission is from the reverse shock wave region, the supernova density profile must be moderately flat at a velocity ~104 km s-1, the shock front is nonradiative at the time of the observations, and the mass-loss rate is (1-2) × 10-4 M☉ yr-1 for a presupernova wind velocity of 10 km s-1. This result is also supported by modeling of the radio emission, which implies that SN 1998S is surrounded by a clumpy or filamentary CSM established by a high mass-loss rate, ~2 × 10-4 M☉ yr-1, from the presupernova star.

The LWA1 Radio Telescope

LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instruments “all dipoles” modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (≈ 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and four independently steerable beams utilizing digital “true time delay” beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.

SN 1988Z : the most distant radio supernova

We present observations of the early radio emission from the unusual supernova SN 1988Z in MCG +03-28-022 made with the Very Large Array at 20, 6, 3.6, and 2 cm from 1989 December, 1 year after optical discovery, through 1992 December. At the redshift z=0.022 of the parent galaxy, SN 1988Z is the most distant radio supernova ever discovered. With a 6 cm maximum flux density of 1.90 mJy, SN 1988Z is ∼1.2 times more luminous than the unusually powerful radio supernova SN 1986J in NGC 891, making SN 1988Z also one of the most luminous radio supernovae ever discovered

Deceleration in the Expansion of SN 1993J

A rarity among supernova, SN 1993J in M81 can be studied with high spatial resolution. Its radio power and distance permit VLBI observations to monitor the expansion of its angular structure. This radio structure was previously revealed to be shell-like and to be undergoing a self-similar expansion at a constant rate. From VLBI observations at wavelengths of 3.6 and 6 cm in the period 6-42 months after explosion, we have discovered that the expansion is decelerating. Our measurement of this deceleration yields estimates of the density profiles of the supernova ejecta and circumstellar material in standard supernova explosion models.

Modulations in the radio light curve of the Type IIb supernova 2001ig: evidence for a Wolf–Rayet binary progenitor?

We describe the radio evolution of supernova (SN) 2001ig in NGC 7424, from 700 d of multifrequency monitoring with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA). We find that deviations of the radio light curves at each frequency from the standard ‘minishell’ model are consistent with density modulations in the circumstellar medium (CSM), which seem to recur with a period near 150 d. One possibility is that these are due to enhanced mass loss from thermal pulses in an asymptotic giant branch star progenitor. A more likely scenario, however, is that the progenitor was a Wolf–Rayet (WR) star, whose stellar wind collided with that from a massive hot companion on an eccentric 100-d orbit, leading to a regular build-up of CSM material on the required time and spatial scales. Recent observations of ‘dusty pinwheels’ in WR binary systems lend credibility to this model. Since such binary systems are also thought to provide the necessary conditions for envelope stripping which would cause the WR star to appear as a Type Ib/c SN event rather than a Type II, these radio observations of SN 2001ig may provide the key to linking Type Ib/c SNe to Type IIb events, and even to some types of gamma-ray bursts.

Long-Term Radio Monitoring of SN 1993J

We present our extensive observations of the radio emission from supernova (SN) 1993J, in M81 (NGC 3031), made with the Very Large Array, at 90, 20, 6, 3.6, 2, 1.2, and 0.7 cm, as well as numerous measurements from other telescopes and at other wavelengths. The combined data set constitutes probably the most detailed set of measurements ever established for any SN outside of the Local Group in any wavelength range. The radio emission evolves regularly in both time and frequency, and the usual interpretation in terms of shock interaction with a circumstellar medium (CSM) formed by a pre-supernova stellar wind describes the observations rather well. However, (1) The highest frequency measurements at 85-110 GHz at early times (<40 days) are not well fitted by the parameterization which describes the centimeter wavelength measurements. (2) At midcentimeter wavelengths there is often deviation from the fitted radio light curves. (3) At a time ~3100 days after shock breakout, the decline rate of the radio emission steepens from (t^(+β)) β ~ − 0.7 to –2.7 without change in the spectral index (ν^(+α); α ~ − 0.81); however, this decline is best described not as a power-law, but as an exponential decay with an e-folding time of ~1100 days. (4) The best overall fit to all of the data is a model including both nonthermal synchrotron self-absorption (SSA) and thermal free-free absorbing (FFA) components at early times, evolving to a constant spectral index, optically thin decline rate until the break. (5) The radio and X-ray light curves display quite similar behavior and both suggest a sudden increase in the supernova progenitor mass-loss rate occurred at ~8000 yr prior to shock breakout.

The 10 year radio light curves for SN 1979C

New observations of the radio supernova SN 1979C made with the VLA at λλ20, 6, and 2 cm from 1985 March through 1990 December, augmenting previous observations which began only 8 days after optical maximum in 1979 April and extended through 1984 November, are presented. It is found that the mini-shell model of Chevalier still provides the best representation of these more complete light curves at all three wavelengths. Shorter period fluctuations in the observations are real and are probably due to emission efficiency variations caused by structure in the presupernova stellar wind density.

RADIO EMISSION FROM SN 1988Z AND VERY MASSIVE STAR EVOLUTION

We present observations of the radio emission from the unusual supernova SN 1988Z in MCG +03-28-022 made with the Very Large Array at 20, 6, 3.6, and 2 cm, including new observations from 1989 December 21, 385 days after the optically estimated explosion date, through 2001 January 25, 4438 days after explosion. At a redshift z = 0.022 for the parent galaxy (~100 Mpc for H0 = 65 km s-1 Mpc-1), SN 1988Z is the most distant radio supernova ever detected. With a 6 cm maximum flux density of 1.8 mJy, SN 1988Z is ~20% more luminous than the unusually powerful radio supernova SN 1986J in NGC 891 and only ~3 times less radio luminous at 6 cm peak than the extraordinary SN 1998bw, the presumed counterpart to GRB 980425. Our analysis and model fitting of the radio light curves for SN 1988Z indicate that it can be well described by a model involving the supernova blast wave interacting with a high-density circumstellar cocoon, which consists almost entirely of clumps or filaments. SN 1988Z is unusual, however, in that around age 1750 days, the flux density begins to decline much more rapidly than expected from the model fit to the early data, without a change in the absorption parameters. We interpret this steepening of the radio flux density decline rate as due to a change in the number density of the clumps in the circumstellar material (CSM) without a change in the average properties of a clump. If one assumes that the blast wave is traveling through the CSM at ~2,000 times faster than the CSM was established (20,000 km s-1 vs. 10 km s-1), then this steepening of the emission decline rate represents a change in the presupernova stellar wind properties ~10,000 yr before explosion, a characteristic timescale also seen in other radio supernovae. Further analysis of the radio light curves for SN 1988Z implies that the SN progenitor star likely had a zero-age main sequence mass of ~20-30 M?. We propose that SNe such as SN 1986J, SN 1988Z, and SN 1998bw with very massive star progenitors and associated massive wind ( 10-4 M? yr-1) have very highly clumped, wind-established CSM and unusually high blast-wave velocities (greater than 20,000 km s-1).

Multi-Wavelength Properties of the Type IIb SN 2008ax

We present the UV, optical, X-ray, and radio properties of the Type IIb SN 2008ax discovered in NGC 4490. The observations in the UV are one of the earliest of a Type IIb supernova (SN). On approximately day 4 after the explosion, a dramatic upturn in the u and uvw1 (λ_c = 2600 A) light curves occurred after an initial rapid decline which is attributed to adiabatic cooling after the initial shock breakout. This rapid decline and upturn is reminiscent of the Type IIb SN 1993J on day 6 after the explosion. Optical/near-IR spectra taken around the peak reveal prominent Hα, He I, and Ca II absorption lines. A fading X-ray source is also located at the position of SN 2008ax, implying an interaction of the SN shock with the surrounding circumstellar material and a mass-loss rate of the progenitor of M (overdot) = (9 ± 3) × 10^(−6) M_☉ yr^(−1). The unusual time evolution (14 days) of the 6 cm peak radio luminosity provides further evidence that the mass-loss rate is low. Combining the UV, optical, X-ray, and radio data with models of helium exploding stars implies the progenitor of SN 2008ax was an unmixed star in an interacting binary. Modeling of the SN light curve suggests a kinetic energy (E_k) of 0.5 × 10^(51) erg, an ejecta mass (M_(ej)) of 2.9 M_☉, and a nickel mass (M_(Ni)) of 0.06 M_☉.

Radio Imaging of Two Supernova Remnants Containing Pulsars

Abstract : The supernova remnants G 5.4 - 1.2 and G 8.7 - 0.1 each have a 15,000 yr old pulsar projected along their outside edges. If these are true pulsar-supernova remnant associations then the implied pulsar transverse motions for PSR1757-24 and PSR 1800-21 are excessively large (1500-2500 km s(exp -1). We present new radio observations made at the VLA in the continuum at 327 MHz and the H I line at 1420 MHz to address this issue. For G 5.4-1.2 we better constrain the true extent of the remnant and its shape. We also derive an H I absorption distance and the spectral index distribution across the bright western side of the remnant. All the available evidence suggests that G 5.4-1.2 and PSR 1757-24 are associated. Our deep 327 MHz image of G 8.7 - 0.1 reveals faint extensions of the remnant but no new emission is seen near PSR 1800 - 21. A possible new supernova remnant is also discovered north of G 8.7 - 0.1. Several difficulties are presented for the proposed association between G 8.7 - 0.1 and PSR 1800 - 21, the most serious of which are the lack of a pulsar-powered nebula and the discrepant distance estimates for the pulsar and supernova remnant. We conclude that PSR 1800 - 21 is a foreground object, unrelated to G 8.7-0.1.