Christina Kaye Lacey

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.

An X-Ray, Optical, and Radio Search for Supernova Remnants in the Nearby Sculptor Group Sd Galaxy NGC 7793

This paper is the second in a series devoted to examining the multiwavelength properties of supernova remnants (SNRs) located in nearby galaxies. We consider here the resident SNRs in the nearby Sculptor group Sd galaxy NGC 7793. Using our own Very Large Array (VLA) radio observations at 6 and 20 cm, as well as archived ROSAT X-ray data, previously published optical results, and our own Hα image, we have searched for X-ray and radio counterparts to previously known optically identified SNRs and for new previously unidentified SNRs at these two wavelength regimes. Consistent with our prior results for NGC 300, only a tiny minority of the optically identified SNRs have been found at another wavelength. The most noteworthy source in our study is N7793-S26, which is the only SNR in this galaxy that is detected at all three wavelengths (X-ray, optical, and radio). It features a long (~450 pc) filamentary morphology that is clearly seen in both the optical and the radio images. N7793-S26s radio luminosity exceeds that of the Galactic SNR Cas A, and based on equipartition calculations we determine that an energy of at least 1052 ergs is required to maintain this source. Such a result argues for the source being created by multiple supernova explosions rather than by a single supernova event. A second optically identified SNR, N7793-S11, has detectable radio emission but no detectable X-ray emission. A radio-selected sample of candidate SNRs has also been prepared by searching for coincidences between nonthermal radio sources and regions of Hα emission in this galaxy. This search has produced five new candidate radio SNRs to be added to the 28 SNRs that have already been detected by optical methods. A complementary search for new candidate X-ray SNRs has also been conducted by searching for soft-spectrum sources (kT < 1 keV) that are coincident with regions of Hα emission. That search has yielded a candidate X-ray SNR that is coincident with one (and possibly two) of the candidate radio SNRs, but considerable diffuse X-ray emission throughout the disk of NGC 7793 reduces the efficacy of the search. Like NGC 300, very little overlap in identifications is seen between the SNRs found through X-ray, optical, and radio methods, and such a result argues for the role played by distance-dependent selection effects in determining the detectability of SNRs. In addition, we find that the density of the ambient interstellar medium (ISM) surrounding SNRs significantly impacts the spectral characteristics of the SNRs in this galaxy, consistent with surveys of the SNR populations in other galaxies.

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).

Cosmic-Ray Production and the Role of Supernovae in NGC 6946

We present a study of 35 radio-selected supernova remnant (SNR) candidates in the nearby spiral galaxy NGC 6946. In this study, an optically selected sample of 27 SNRs from Matonick & Fesen is compared to our sample of radio-selected SNRs. The optically selected SNRs are found to have almost no overlap with the radio-selected sample. This dichotomy is further enhanced by the observation that the optically selected SNRs favor the interarm regions, while the radio-emitting SNRs lie predominately on the spiral arms in or near regions of high star formation. The separation of the two samples of SNRs is discussed in terms of selection effects and differences in cosmic-ray production. The optical sample of SNRs is probably biased toward those SNRs located in areas of NGC 6946 where the confusion of Hα emission by H II regions is relatively low and the SNRs are easier to identify by their emission-line signatures. The radio-selected sample is also subject to selection biases such that these SNRs favor the arms. However, the absence of radio emission from the optically selected, largely interarm SNRs and the relatively large number of radio-detected SNRs in the arms require additional explanation. The properties of the radio-selected SNRs are discussed in the context of diffusive shock acceleration theory. We find that the theory can account for the range in radio flux densities and the nondetection of the optically selected SNRs. The differences in the radio properties between the arm and interarm population of SNRs can be explained by differences in the average gas densities and magnetic fields since the latter affect both the cosmic-ray yield and the radio properties of the SNRs. The possibility that the arm and interarm SNRs arise from different stellar populations is also addressed.

A SEARCH FOR CHANDRA-DETECTED X-RAY COUNTERPARTS TO OPTICALLY IDENTIFIED AND CANDIDATE RADIO SUPERNOVA REMNANTS IN FIVE NEARBY FACE-ON SPIRAL GALAXIES

We present a search for X-ray counterparts to optically identified and candidate radio supernova remnants (SNRs) in five nearby galaxies, M81, M101, NGC 2403, NGC 4736 (M94), and NGC 6946, using observations made with the Chandra X-Ray Observatory. A total of 138 optically identified SNRs and 50 candidate radio SNRs in these galaxies were sampled by these observations. Nine optically identified SNRs and 12 candidate radio SNRs are positionally coincident with Chandra-detected X-ray sources that were not already known to be time-variable or associated with X-ray binaries. We used survival statistics to determine if the properties of the optically identified SNRs with and without Chandra-detected counterparts (referred to as group A and group NotA, respectively), as well as the candidate radio SNRs with and without Chandra-detected counterparts (referred to as group B and group NotB, respectively) differ in a statistically significant manner. We find that for the SNRs in groups A and NotA, only the mean value of the diameter d differs significantly between the two groups (26 ? 4 pc compared to 62 ? 6 pc). In addition, for the SNRs in groups B and NotB, we find that only the spectral index ? differs significantly between the two groups (0.6 ? 0.1 compared to 0.9 ? 0.1). We find no correlation between unabsorbed X-ray and optical luminosities for the group A SNRs and no correlation between unabsorbed X-ray and radio luminosities for the group B SNRs: this result indicates that the interstellar medium surrounding these SNRs is inhomogeneous rather than uniform. We claim that the higher incidence of Chandra-detected counterparts for candidate radio SNRs compared to the optically identified SNRs (as noticed in previous works) illustrates the role played by ambient density in affecting searches for SNRs in nearby galaxies at multiple wavelengths. We argue that deep systematic X-ray, optical, and radio observations of other galaxies are necessary to examine the multiwavelength properties of SNRs, to explore wavelength-dependent selection effects in more detail, and to search for time variability in the emission from X-ray counterparts to optically identified SNRs and candidate radio SNRs.

A Study of Compact Radio Sources in Nearby Face-on Spiral Galaxies. I. Long-Term Evolution of M83

We present analyses of deep radio observations of M83 taken with the Very Large Array spanning 15 yr, including never-before-published observations from 1990 and 1998. We report on the evolution of 55 individual point sources, which include four of the six known historical supernovae in this galaxy. A total of 10 sources have X-ray counterparts from a Chandra survey. Each of these sources show nonthermal spectral indices, and most appear to be X-ray supernova remnants. Comparing the radio source list to surveys in the optical and X-ray, we identify three optical/X-ray supernova remnants. Nearly half of the detected radio sources in these observations are coincident with known H II regions lying in the spiral arm structures of the galaxy. We also report on changes in emission from the complex nuclear region, which has shown variability at 20 cm wavelengths. We confirm that the peak radio emission from the nucleus is not coincident with the known optical center. One lesser nuclear peak is consistent with the optical/IR nucleus. Previous dynamical studies of a dark nuclear mass indicate a possible match to other radio nuclear emission regions in M83.

Radio Continuum Imaging of the Spiral Galaxy NGC 4258

We analyze 35 resolution, high-sensitivity radio continuum images of the nearby spiral galaxy NGC 4258 at 6 and 20 cm derived from multiple observations used to monitor the radio supernova SN 1981K by Van Dyk et al. Seven bright H II regions and five supernova remnant candidates are identified. Extinctions to the H II regions are estimated for the first time from a comparison of radio flux densities to new optical fluxes derived from Hα observations by Dutil & Roy. The bright end of the H II region luminosity function is established at each wavelength. The luminosity functions are best fit by power laws consistent with the shape of previously published radio and optical luminosity functions for a number of galaxies. The supernova remnants are all about 2-3 times the radio luminosity of Cas A. In addition, the galaxys nucleus is shown to have been variable over the SN 1981K monitoring period. The spectral index (α) distribution of the anomalous radio arms is investigated and found to be relatively uniform at α = -0.65 ± 0.10.

VLA Observations of J1228+441, a Luminous Supernova Remnant in NGC 4449

The luminous, oxygen-rich supernova remnant J1228+441 is located in the irregular galaxy NGC 4449 and has been observed at radio wavelengths for 30 years. An analysis of recent Very Large Array (VLA) observations of NGC 4449, combined with VLA archive data and previously published VLA and Westerbork Synthesis Radio Telescope observations, yields light curves at 6 and 20 cm from 1972 to 2002. The light curves at all radio frequencies exhibit a marked decline in radio emission, confirming past findings. This paper presents and discusses the radio light curves and spectral index α variations from 1972 to 2002, where Sν ∝ ν-α, and compares J1228+441 with other radio supernovae. The spectral index of J1228+441 appears to have steepened in the last 5 years at higher frequencies, from α = 0.64 ± 0.02 in 1996 to α = 1.01 ± 0.02 in 2001-2002.

Supernovae at the Highest Angular Resolution

The study of supernovae (SNe) and their environments in host galaxies at the highest possible angular resolution in a number of wavelength regimes is providing vital clues to the nature of their progenitor stars.

A Chandra Observation of the Nearby Sculptor Group Sd Galaxy NGC 7793

We present the results of a Chandra observation made of the nearby spiral galaxy NGC 7793: the effective exposure time of this observation was 49094 seconds. Twenty-two discrete sources were identified at a minimum of a 3