Dissertation Summary: Thermal and Nonthermal Physics of Supernova Remnants
Supernova remnants (SNRs) are among the most powerful agents in the
galactic environment. Supernova explosions and remnant shock waves play a key role in galaxy evolution: they inject kinetic and thermal energy into the ISM and their ejecta contribute to the enrichment of their host galaxy.
Understanding and modeling emission from SNRs contributes to the knowledge
of physics in strong shocks, a study that includes such far ranging topics
as the earth's bow shock, extragalactic jets and star formation.
Since supernova shocks are one of the few mechanism known to be capable of
providing adequate energy to supply the pool of galactic cosmic rays,
supernova remnants have long been the suspected source of cosmic rays, at
least up to the slight steepening in the cosmic ray spectrum at 1015 eV
known as the ``knee''.
However, until recently the nonthermal proceses could only be studied
through radio synchrotron emission, involving electron energies 0.1-10 GeV, a
relatively uninteresting range.
Occasionally an entire field is driven forward by observations of a
single object. Early Einstein and OSO 8
observations of SN 1006 were well fit by a powerlaw spectrum so Becker et al. (1980) proposed
SN 1006 as a Crab-like object. Reynolds & Chevalier (1981) responded
with a different source of synchrotron emission - the high energy counterparts to
the electrons producing radio synchrotron. Sounding rocket observations
made by Vartanian, Lum & Ku (1985) revealed oxygen lines, signaling
the presence of thermal emission. Hamilton, Sarazin & Szymkowiak (1986)
replied with a lineless thermal model to describe the spectrum. Finally, 1995 ASCA
observations by Koyama et al. revealed that SN 1006 had both thermal and
nonthermal emission. At this point it became clear that understanding
SN 1006 would involve separating thermal and nonthermal components -
requiring a new level of sophistication in models.
Predictions were made by Reynolds (1998) that synchrotron emission would
extend up to X-ray energies and then roll off due to remnant age,
particle escape or radiative losses. These early models seemed to agree
with pre-ASCA X-ray total flux measurements of SN 1006 (Reynolds 1996).
The goal of my thesis was to cross the gap between theory and observations
by testing a synchrotron model, with full single particle emissivity, on
actual X-ray spectra, a much more stringent test that fitting pre-ASCA
integrated X-ray fluxes, and to make the physics in synchrotron models
available to the X-ray community to describe synchrotron
X-rays from SNR shocks.
The escape-limited synchrotron model, publicly released as SRESC in XSPEC
11, uses the radio spectral index and flux as inputs and includes the
full single-particle emissivity (Reynolds 1998). The model contains a single free
parameter, a frequency which describes the rolloff in the synchrotron
spectrum. This parameter measures the product of the maximum MHD
wavelength scattering the electrons and the magnetic field strength.
I have demonstrated that a detailed model of synchrotron emission is capable of
describing the spectrum of SN 1006, a dominantly nonthermal supernova. This marks the first time synchrotron X-ray spectra have been described by models which describe the physical mechanism behind the radiation. I have have shown that the shock wave
of a young supernova remnant such as SN 1006 AD can accelerate electrons to TeV energies where they produce synchrotron radiation in the X-ray band, with implications for the acceleration of ions.
From the X-ray spectrum and TeV observations (Tanimori et al. 1998) it is possible to uniquely determine the magnetic field (suggesting SN 1006 is far from equipartition) and deduce the relativistic electron acceleration efficiency in the shock, larg
er by a factor of two than previous estimates.
The realization of X-ray synchrotron models also has important
consequences for thermal emission in SNRs.
Fits to the integrated spectrum of SN 1006 reveal both nonthermal
emission and a thermal component with high abundances (including indications of half a solar mass of iron), predicted by nucleosynthesis models of Type Ia
supernova (Iwamoto et al. 1999), but never previously detected.
My results indicate that joint thermal and nonthermal fitting, using
sophisticated thermal and nonthermal models, will be required for
analysis of remnants suspected of nonthermal emission, possibly for the
majority of SNRs. Accurate accounting for a possible nonthermal
component will be a necessary precondition to using nucleosynthesis
models to interpret abundances in SNRs.
Having demonstrated that the integrated spectrum of SN1006 is well described by synchrotron models
I have now turned to spatially resolved of the synchrotron models. Preliminary results indicate
that all regions, even the center, which shows prominent silicon lines,
have a significant amount of nonthermal emission. This analysis of SN
1006 with these models will provide a stringent test of
abundances measured in paper I.
X-ray and Radio Morphology
High resolution observations of the X-ray and radio morphology of young SNRs can make significant contributions to the understanding of shock physics in SNRs. The unresolved edges visible in the radio can place limits on scattering ahead of the shock. Pol
arization provides crucial information about the orientation of the magnetic field as well as providing information about the degree of order and turbulence. Radio and X-ray correlations may suggest the presence of nonthermal X-ray emission.
3C 397 (Dyer & Reynolds, 1999)
is one of the highest surface brightness Galactic SNRs in both X-ray and radio and has an unusual, rectangular morphology, in both bands.
High resolution observations of 3C 397 provided an opportunity to test methods of determining spectral index variations, since a thorough study of 3C 397 was carried out by Anderson & Rudnick in 1993. While not drastically different, our dataset was
an incremental improvement, with additional arrays at 6 and 20 cm. With either the standard spectral index technique or a linear regression method we were unable to replicate their results, although we found that the magnitude of our variations was simila
r to the the variations they had found.
One of the most striking X-ray features of 3C 397 is a hotspot, not correlated with any feature in the radio. While a logical explanation is a radio-quiet pulsar, timing searches reveal nothing within the resolution of ASCA, and there are no differences i
n spectra between the point source and the rest of the remnant. We carried out a thorough X-ray analysis (Safi-Harb et al. accepted for ApJ) of ROSAT, ASCA and RXTE observations and found that the X-ray spectrum demanded a multiple component fit which cou
ld support one of two scenarios: a young ejecta-dominated remnant of a core collapse supernova, or a medium-aged SNR in a dense interstellar medium.
3C 396
is a roughly spherical SNR, with a complex structure of arcs and filaments in the radio. Filamentary structure is important for the study of the diffusion of relativistic particles behind the shock wave, and can also contribute to our understanding of pos
t-shock turbulence. This SNR demonstrates multiwavelength polarization (Dyer & Reynolds, in preparation), allowing us to determine the direction of the magnetic field. The polarization is strongest along the filaments, with the magnetic field parallel
to the filamentary structure.
Due to high absorption, X-ray observations of 3C 396 have lagged the radio. 3C 396 was observed by ASCA but not ROSAT. We hope to see that remedied by our successful A02 Chandra proposal. I am observational PI on the observation of 3C 396 scheduled for la
te spring.
Maser Kinematics of Roberts 22
The enormous signal to noise of maser emission provides a rare opportunity to investigate the kinematics of the emission region. Roberts 22 is a southern hemisphere Type II OH-IR star. In 1969 strong 1612 MHz maser emission was discovered by Manchester, G
oss, & Robinson. Roberts 22 is unusual among OH-IR stars: reflection nebula identify the hidden central star as a A2 Ie, rather than an M star and it exhibits a relatively high OH outflow velocity. HST observations (Sahai et al 1999) reveal an intrica
te bipolar planetary nebula with a high degree of point symmetry. Observations with the Australian Telescope Compact Array provided full tracks at 1612 MHz, 1665 MHz and 1667 MHz, the most complete observation of the maser emission to date. The results o
f this study are intriguing: At 1612 and 1665 MHz the peak of the maser emission at each velocity traces out an arc segment, suggestive of a ellipse. Plots of position angle vs. velocity trace out a parabola. Sophisticated models fitting programs develope
d by Athol Kemball based on parameterization by Bowers (1991) were run on the velocity, position and flux information, without conclusive identification. We can eliminate most standard geometries, including an expanding spherical shell and simple rotating
disk. Painstaking work to bring the HST image into the radio reference frame resulted in a position better than 0.6 in accuracy. This places the maser emission not at the center of the optical nebula but along the ``northern spur'', a region of supp
ressed emission.
REFERENCES
Low Frequency Observations of the Galactic Plane
Low frequency interferometric observations can help disentangle three issues in SNR studies: intrinsic properties of the explosion, the character of the SNR environment, and observational constraints. Nonlinear models of shock acceleration predict that th
e spectra from young SNRs should be slightly concave rather than power laws - flattening toward higher energies. This is one of the few methods by which limits can be set on the magnetic field independent of the electron energy. Most SNR spectral index st
udies have been restricted to the small range from 6 to 20 cm (a factor of 1.7 in electron energies). Observations as low as 74 MHz increase the electron energy baseline.
In many cases low frequency measurements may be affected by free-free absorption - allowing us to use SNRs to probe the ISM on very small spatial scale. Our recent work on W49B (Lacey et al., in progress) has found small scale absorption features that cor
relate with HI measurements. This is only one of several remnants for which we have low frequency observations so we will be able to test the inhomogeneous of the ISM throughout that region of the galactic plane.
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Kristy Kathleen Dyer
2000-12-13