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NASA NTRS1993

An atlas of supernova remnant candidates in Messier 31

Narrow-band CCD imagery in H-alpha and forbidden SII of a large fraction of the spiral arms in the Northeast half of Messier 31 has been used to isolate a sample of 52 'forbidden-line' SNR candidates for which the integrated ratio forbidden SII:H-alpha is greater than 0.5. An atlas of images in these emission lines, red optical continuum, and 1465 MHz radio continuum is presented, together with the tabulated integral properties of these sources. Assessing the completeness of the sample yields a crude estimate of the massive supernova rate (due to stars more massive than 7 solar masses) of 1 in 80 yr. The range of measured luminosities in both H-alpha and radio continuum is fully consistent with those found for 'forbidden-line' SNR in Messier 33, the LMC, and the Galaxy. With the inclusion of our candidates the number of extragalactic SNRs (with well-known distances) now exceeds the number of known galactic SNRs.

Braun, R.↗
NASA NTRS1993

Resolved structure in M33 supernova remnants

We present WF/PC narrow band forbidden S II 6725-wavelength images of two M33 supernova remnants obtained with the Wide Field/Planetary Camera on the HST. Comparison of these images with ground-based CCD data indicates that HST has resolved the nebular structures into shells or partial shells, permitting improved measurements of the diameters of these objects. Ground-based echelle spectra obtained at Kitt Peak with the 4-m telescope show profiles with half-width zero-intensity velocities of 163 and 275 km/sec for the two objects, indicating rapid bulk motions of the emitting filaments. The morphology of the emission seen in the WF/PC images allows the non-Gaussian shapes of the integrated echelle profiles to be understood. We briefly compare the predicted WF/PC count rates with those actually realized and discuss the reason for the discrepancy.

Blair, William P.↗
NASA NTRS1993

Spectrum formation in supernovae - Numerical techniques

The study combines several novel techniques for spectrum simulation in the Eddington computer program which solves the comoving frame equation of transfer coupled with the statistical and radiative equilibrium equations. One of these is a generalization of the accelerated lambda iteration (ALI) scheme to include an approximate frequency-derivative operator. This greatly enhances the convergence rate of ALI in optically thick, high-velocity shear flows. Another is a partial linearization technique which is capable of efficiently solving a very large number of rate equations on a moderately sized computer. An expansion opacity and emissivity approximation is derived which makes it possible to determine the effect on the transfer and statistical equilibrium of a very large number of lines not explicitly represented in the frequency grid and additionally to treat line-blanketing from species not explicitly included in the rate equations. The utility of these techniques is illustrated with models of two supernovae.

Eastman, Ronald G.↗
NASA NTRS1993

The Hubble diagram in V for supernovae of Type Ia and the value of H(0) therefrom

The Hubble diagram for Type I supernovae is derived in V and is summarized from the literature in B and in m(pg). The ridge line equation of the diagram in V and the calibration of the absolute magnitudes at maximum are presented. The intrinsic (B - V) color at B maximum light is 0.09 +/- 0.04 mag. The Virgo Cluster distance is derived and found to be 23.9 +/- 2.4 Mpc. This Virgo distance gives the cosmic value of the Hubble constant to be H(0) = 47 +/- 5 km/sec per Mpc.

Sandage, Allan↗
NASA NTRS1993

A search for radiative neutrino decay from supernovae

Two supernovae have been identified in the COMPTEL data base as being the best sources to investigate for evidence of gamma-ray emission caused by radiative neutrino decay. These are SN1987a and SN1993J. A detailed simulation has shown us that we can expect a gain in sensitivity 1-3 orders of magnitude (depending on neutrino mass) over previous results. Instrument response is now being modeled using a SPARC10 computer acquired for this study. A library of simulated gamma-ray lines is being produced for COMPTEL as a by-product of this effort.

Svoboda, Robert C.↗
NASA NTRS1993

Supernova 1987A revisited

Updates on research on SN 1987A are provided, with emphasis on the aftermath of the explosion: the interpretation of the light curve and spectrum, the physics of the envelope, and the interaction of the supernova with its circumstellar envelope. UVOIR light curves, gamma rays and hard X-rays, and dust formation are examined, and explosion dynamics is explored. Attention is given to optical and IR spectrum development, with emphasis on the photospheric spectrum, nebular spectrum, ionization, emission-line profiles, line strengths, and iron, cobalt, and nickel. The circumstellar environment is discussed, with special reference to progenitor mass loss, circumstellar ring and nebulosity, radio emission, and soft X-rays.

Mccray, Richard↗
NASA NTRS1994

The x-ray structure of the supernova remnant W49B

Comparison of x-ray and radio images of W49B and other supernova remnants (SNR) provides detailed information on the mechanisms responsible for the emission and on the evolution of the remnants. There is faint x-ray emission from all parts of W49B but most of it is concentrated near the center of the remnant, unlike the radio emission which arises in a shell near the periphery. This structure indicates that this SNR is in the adolescent phase of its lifetime.

Dickel, John R.↗
NASA NTRS1994

An X-ray study of five supernova remnants in the Carina spiral arm

The ROSAT Position Sensitive Proportional Counter (PSPC) is used to perform an exploratory study of four fields in the Carina spiral arm containing five radio supernova remnants, only one of which has previously been studied in X-rays. We present upper limits for the detection in X-rays of G298.5 - 0.3, G298.6 - 0.0, and G299.0+0.2, and report a 4 sigma detection of G296.8-0.3. In addition, we present detailed spatial and spectral analysis of the bright X-ray remnant G296.1-0.7, which has previously been studied by both the Einstein IPC and EXOSAT LE/CMA. We detect relatively slight, but statistically significant, variations in the spectrum across the remnant via spatially resolved spectral fits and a study of the spatial variation of hardness ratios. In general, the spectrum is characteristic of a thermal plasma with kT about 0.2 keV and N(sub H) about 1.5 x 10(exp 21/sq. cm). The total X-ray emitting mass is estimated to be about 250 solar mass for an optically estimated distance of 4 kpc to the remnant. At this distance, the linear dimensions of the remnant are roughly 35 - 50 pc, implying an age on the order of 20,000 yr. Assuming that X-ray and radio brightnesses are related by SIGMA(sub R) proportional to SIGMA(exp 0.69)(sub X) and that the four radio remnants have X-ray spectral characteristics similar to G296.1-0.7, we find that the column densities to these sources must be several times 10(exp 22)/sq cm in order to explain their low X-ray count rates. This column density is considerably in excess of the X-ray fitted column density to G296.1-0.7, but is comparable to the total column densities in H I measured via the 21 cm line in the directions to all five remnants. This implies that G296.1 - 0.7 is at a significantly smaller distance than the other remnants.

Hwang, Una↗
NASA NTRS1994

Supernova remnant candidates for the soft gamma-ray repeater 1900+14

Motivated by the association of two soft gamma-ray repeaters (SGRs) with supernova remnants (SNR) we have carried out radio, optical and X-ray studies of two cataloged SNRs in the large KONUS error box 11 deg x 8 min of SGR 1900+14. Our very large array (VLA) observations of SNR G43.9+1.6 do not reveal any obvious plerionic component. A radio flat-spectrum source, close to, but outside the error box was found. We suggest this to be a distant H II region foreground to the SNR. A sensitive VLA image at meter wavelengths show that the other SNR, G42.8+0.6, is an ordinary typical SNR with a shell morphology with no peculiarities such as a plerionic component. No ROSAT source with an apparent flux greater than or approximately 10(exp -13) ergs cm(exp -2) s(exp -1) is found within the two SNRs. Recently, Hurley et al. have reported a new very small error box close to G42.8+0.6. There is no radio feature within or close to the error box. However, a ROSAT source is found just outside this localization. We speculate that this is the quiescent X-ray counterpart of SGR 1900+14. We suggest that SGR 1900+14 is a neutron star that was born with high speed which has now overtaken the expanding shell of SNR G42.8+0.6. Owing to the low confining pressure, there has been no development of a synchrotron bubble which explains the absence of the radio plerion. In our picture, SGR 1900+14 is the oldest known SGR.

Vasisht, G.↗
NASA NTRS1994

Two new supernova remnants in OB associations in the Large Magellanic Cloud

We discovered two extended x-ray sources in a Roentgen Satellite (ROSAT) Positron Sensitive Proportional Counter (PSPC) observation pointed at the H II region N9 in the Large Magellanic Cloud. These two sources are in the H II regions N4D and N9. The x-ray characteristics suggest that both might be supernova remnants (SNRs). Follow-up charge coupled device (CCD) images taken with interference filters show high (S II)/H-alpha ratios in the optical nebulae of these x-ray sources, confirming the presence of high velocity shocks commonly seen in SNRs. These two sources are also detected in the radio continuum at 8.55 and 4.75 GHz; both appear nonthermal compared to nearby H II regions. The confirmation of these two SNRs demonstrates that many SNRs in or near H II regions have been overlooked in previous surveys, and that the ROSAT x-ray survey combined with an optical CCD imaging survey of the Magellanic Clouds would provide the most effective way to uncover SNRs.

Smith, R. Chris↗
NASA NTRS1994

The very luminous supernova remnant in NGC 6946

In this paper, I discuss an extremely luminous X-ray source found on the northern arm of the nearly face-on spiral galaxy NGC 6946. The source was originally discovered in an Einstein observation, but the poor spatial resolution prevented an accurate identification. Here, Roentgen Satellite (ROSAT) data provide a position to approximately 5 sec - 10 sec. The spectrum is rather soft, and moderately absorbed. The source appears to be local to NGC 6946 and a supernova remnant, based upon the positional coincidence with an optical counterpart. The luminosity, assuming a distance to NGC 6946 of 5.1 Mpc, is 2.8 x 10(exp 39) erg/s, among the highest known X-ray luminosities of a remnant. The optical counterpart is at least 100 years old.

Schlegel, Eric M.↗
NASA NTRS1994

IUE spectra and optical imaging of the oxygen-rich supernova remnant N132D

We present new optical Charge Coupled Devices (CCD) interference filter imagery and International Ultraviolet Explorer (IUE) spectroscopy for the oxygen-rich supernova remnant N132D in the Large Magellanic Cloud. The optical images show a wealth of structure, and comparison with an archival Einstein High Resolution Imager (HRI) X-ray image shows that a few optical features have X-ray counter-parts, but in general there is little correlation between X-ray and optical features. The IUE spectra at two positions show strong lines of carbon and oxygen, with lines of neon, magnesium, silicon, and helium also present and variable in relative intensities. We use optical data for N132D from Dopita & Tuohy (1984) with our UV observations to compare with shock models (both with and without thermal conduction) and X-ray photoionization model calculations. While none of the model fits is entirely satisfactory, the generally weak UV emission relative to optical disagrees with the general character of shock model predictions and indicates that photoionization is the dominant excitation mechanism for the UV/optical emission. This conclusion is similar to what was found for E0102 - 7219, the oxygen-rich remnant in the Small Magellanic Cloud. We derive rough abundances for the emitting material in N132D, compare to stellar nucleosynthesis models, and discuss the implications for its precursor. A precursor near 20 solar mass is consistent with the data.

Blair, William P.↗
NASA NTRS1994

CCD soft X-ray observations of the Puppis a supernova remnant

We present the first images and spectra of an astronomical object, other than the Sun, acquired with a charge coupled device (CCD) imaging X-ray spectrometer. During a 230 s sounding rocket observation, we have acquired moderate-resolution spectra and moderate-resolution images of a portion of the Pup A supernova remnant (SNR). Based on these data, we conclude that the X-ray spectrum of Pup A is inconsistent with any single-temperature equilibrium or nonequilibrium plasma model. We find evidence for variations in the emitting plasma on scales as small as 5.0 min and as large as 30.0 min. The spatial structure of the spectral variations in the remnant is found to be inconsistent with the standard Sedov model for the evolution of a SRN into a homogeneous interstellar medium (ISM). We suggest that the remnant is expanding into a region of the ISM having a density of approximately 1 cm(exp -3) with inhomogeneities on the order of 50%. We have found evidence for the presence of a knot of plasma enriched in neon, but require more data to be conclusive.

Berthiaume, G. D.↗
NASA NTRS1994

The delayed-detonation model of a type Ia supernovae. 1: The deflagration phase

The nature of the 'delayed detonation' mechanism of Khokhlov for the explosion of Type Ia supernovae is investigated by using two-dimensional numerical hydrodynamics simulations. A new algorithm is used to treat the deflagration front. Assuming that it propagates locally at the laminar flame speed, the deflagration is insufficient to unbind the star. Expansion shuts of the flame; much of this small production of iron group nuclei occurs at lower densities, which reduces the electron-capture problem. The burning front does become wrinkled, but the wavelength of the instability is much larger than the computational grid size and is resolved; this is consistent with previous analysis. Because the degenerate star has an adiabatic exponent only slightly above 4/3, the energy released by deflagration drives a pulsation of large amplitude. During the first expansion phase, adiabatic cooling shuts off the burning, and a Rayleigh-Taylor instability then gives mixing of high-entropy ashes with low-entropy fuel. During the first contraction phase, compressional heating reignites the material. This paper deals with the deflagration phase, from the onset of burning, through expansion and quenching of the flame, to the first contraction.

Arnett, David↗
NASA NTRS1994

SN 1994I in M51 and the nature of type Ibc supernovae

Early spectra of SN 1994I in M51 (NGC 5194) are presented along with arguments that is a member of the class of helium-poor Type Ic supernovae. The issue of H and He in the spectra of Type Ib and Ic events is reexamined with the conclusion that Type Ib eject substantially less H than even transition events like SN 1993J and that Type Ic eject substantially less He than Type Ib and their optical spectra are consistent with no H or He. Type Ic show and absorption of the Si II lambda 6355 blend that characterizes Type Ia. This feature requires only a solar abundance of Si. Some Type Ic show an absorption that is probably C II lambda 6580. IR spectra are presented of SN 1990W which show the line of He I lambda 10830. The strength of this feature and the rather slow decay of the late time light curve suggest that this event could be better classified as a Type Ib. Even if trace abundances of H are present in Type Ib and of He in Type Ic the spectral differences still imply a distinctly different evolution for the progenitors of Type Ib from Type Ic and events like SN 1993J.

Wheeler, J. C.↗
NASA NTRS1994

Evidence for Ni-56 yields Co-56 yields Fe-56 decay in type Ia supernovae

In the prevailing picture of Type Ia supernovae (SN Ia), their explosive burning produces Ni-56, and the radioactive decay chain Ni-56 yields Co-56 yields Fe-56 powers the subsequent emission. We test a central feature of this theory by measuring the relative strengths of a (Co III) emission feature near 5900 A and a (Fe III) emission feature near 4700 A. We measure 38 spectra from 13 SN Ia ranging from 48 to 310 days after maximum light. When we compare the observations with a simple multilevel calculation, we find that the observed Fe/Co flux ratio evolves as expected when the Fe-56/Co-56 abundance ratio follows from Ni-56 yields Co-56 yields Fe-56 decay. From this agreement, we conclude that the cobalt and iron atoms we observe through SN Ia emission lines are produced by the radioactive decay of Ni-56, just as predicted by a wide range of models for SN Ia explosions.

Kuchner, Marc J.↗
NASA NTRS1994

The nature of the compact X-ray source in the supernova remnant G27.4+0.0

High-resolution X-ray imaging data obtained with ROSAT is used to constrain the nature of the central compact source in the supernova remnant G27.4+0.0. Diffuse emission is seen from throughout the approximately 4 min diameter radio shell, while the central source remains unresolved at approximately 3 sec. We combine archival data from the Einstein HRI, IPC, and MPC with the ROSAT HRI data to define the X-ray spectra of the diffuse and point-like emission. The bulk of the shell radiation is consistent with that of a approximately 10(exp 7) K plasma, although a higher temperature component is also suggested by the data; coupled with the remnant's size and distance, we derive an age of between 500 and 2500 yr. The point source has a substantially harder spectrum, with a power-law photon index less than or approximately equal to 1. A search for periodic modulation from the point source yields upper limits ranging from 10%-35% for periods between 0.025 and 1000 s, depending on the assumed pulse shape. No aperiodic variability on timescales of from 10(exp 3) to 10(exp 8) s is required, although a factor of approximately 2 change between the Einstein and ROSAT eras is possible. We show that the point source cannot represent thermal emission from the surface of a young neutron star and is unlikely to be explained as nonthermal, Crab-like X-ray pulses or a small synchrotron nebula. The most likely models involve accretion-powered systems -- either a wind-fed neutron star with a massive companion or a low-mass X-ray binary. In all probability, this is the youngest X-ray binary in the Galaxy.

Helfand, David J.↗