SN 1993J VLBI (IV): A Geometric Determination of the Distance to M81 with the Expanding Shock Front Method

TL;DR

This paper presents a geometric distance measurement to galaxy M81 using VLBI observations of supernova 1993J's expanding shock front, achieving a precise estimate that aligns with previous measurements.

Contribution

It introduces the Expanding Shock Front Method (ESM) applied to VLBI and optical data for supernovae, providing a direct geometric distance to M81.

Findings

Distance to M81 is estimated at 3.96 Mpc with ~7% total error.

The method confirms the supernova's shock front expansion aligns with optical velocities.

Results are consistent with previous HST-based distance measurements.

Abstract

We compare the angular expansion velocities, determined with VLBI, with the linear expansion velocities measured from optical spectra for supernova 1993J in the galaxy M81, over the period from 7 d to ~9 yr after shock breakout. We estimate the distance to SN 1993J using the Expanding Shock Front Method (ESM). We find the best distance estimate is obtained by fitting the angular velocity of a point halfway between the contact surface and outer shock front to the maximum observed hydrogen gas velocity. We obtain a direct, geometric, distance estimate for M81 of D=3.96+-0.05+-0.29 Mpc with statistical and systematic error contributions, respectively, corresponding to a total standard error of $+-0.29 Mpc. The upper limit of 4.25 Mpc corresponds to the hydrogen gas with the highest observed velocity reaching no farther out than the contact surface a few days after shock breakout. The lower limit of 3.67 Mpc corresponds to this hydrogen gas reaching as far out as the forward shock for the whole period, which would mean that Rayleigh-Taylor fingers have grown to the forward shock already a few days after shock breakout. Our distance estimate is 9+-13 % larger than that of 3.63+-0.34 Mpc from the HST Key Project, which is near our lower limit but within the errors.