Long wavelength observations of supernova remnants

Low frequency or long wavelength (LW ≡ λ ≥ ~ 1 m) observations offer unique insights into supernova remnants (SNRs) and their interaction with the interstellar medium (ISM). The relatively high surface brightness sensitivity and large field of view are well matched to the nonthermal radiation and extended morphology of SNRs. They address the incompleteness in SNR catalogs by uncovering new remnants, with discovery of the barrel-shaped SNR G7.06-0.12 presented here as an example. Continuum spectra constrain particle acceleration and cosmic ray theories, since the radio spectrum reflects the energy distribution of the synchrotron emitting relativistic electrons. Furthermore, LW observations compliment higher frequency images because spectral measurement errors are reduced due to the wide frequency coverage. They also can detect the unshocked ejecta inside young shell-type SNRs through absorption, and can also delineate other forms of ionized material inside remnants such as filaments. LW SNR observations can also constrain the distribution of ionized gas in the ISM by measuring the line of sight free-free absorption. Measurements of co-distant samples of SNRs in nearby galaxies are especially useful for studying the energetics, star formation history, ISM, and cosmic ray gas properties of external systems. These studies are enhanced by extending them to longer wavelengths if good angular resolution can be maintained, and new and planned ground-based systems such as the 74 MHz VLA, GMRT, and the Low Frequency Array (LOFAR) are beginning to achieve this. Extending such studies to space-based observations at even longer wavelengths will be even more powerful.