A VLA Study of the Expansion of Tycho's Supernova Remnant

Abstract

We have measured the expansion of Tychos supernova remnant (SNR) over a 10 yr interval by comparing new VLA observations at 1375 MHz made in 1994 and 1995 with previous observations made in 1983 and 1984 using the same array congurations, bandwidths, calibrators, and integration times. To compute the expansion of the outer rim, we estimate the expansion rate for radial sectors of 4. The overall mean expansion obtained is but varies between 1A and 5A around the shell. The weighted 3A .05 average fractional expansion rate is 0.1126% yr~1, corresponding to a power-law index (expansion parameter) l 4 d ln R/d ln t \ 0.471, with an error of D6%. We also compute the expansion of interior features, and nd a comparable global expansion parameter. The value of l obtained is in excess of the pure Sedov value of 0.4, indicating that Tychos SNR is still in transition from an earlier phase to the Sedov adiabatic phase of evolution, and supporting expansion into a medium without a strong radial density gradient, as expected for a Type Ia supernova. In addition, the signicant local variations that we observe suggest that a unique average expansion rate is an oversimplied description for Tycho. Both radio ux and polarization were found to remain almost constant during the intervening 10 years.