[Paper Review] Radio Imaging of SN 1993J: The Story Continues

M. F. Bietenholz, N. Bartel+8 more|arXiv (Cornell University)|Mar 9, 2011

Gamma-ray bursts and supernovae13 references4 citations

TL;DR

This paper presents the latest Very Long Baseline Interferometry (VLBI) radio images of supernova SN 1993J at 1.7 GHz, observed in 2010, using an in-beam calibrator technique to achieve a background rms of 3.7 μJy bm⁻¹. The study confirms the supernova's shell remains nearly circular, with expansion slowing to ~30% of its initial velocity and evolving approximately as r ∝ t⁰.⁸, while revealing a significant thickening of the shell over time, particularly after ~8 years, indicating deviations from self-similar evolution.

ABSTRACT

We present the most recent VLBI images of SN 1993J, taken at 1.7 GHz on 2010 March 5-6, along with a discussion of its evolution with time. The new image is the latest in a sequence covering almost the entire lifetime of the supernova. For these latest observations we used an "in beam calibrator" technique, and obtained a background rms brightness of 3.7 micro-Jy/beam. The supernova shell remains quite circular in outline. Modulations in brightness are seen around the rim which evolve relatively slowly, having remained generally similar over the last several years of observation. We determine the outer radius of the supernova using visibility-plane model-fitting. The supernova has slowed down to around 30% of its original expansion velocity, and continues to expand with radius approximately proportional to t^0.8, however, deviations from a strict power-law evolution are seen. We do not find any clear-cut evidence for systematically frequency-dependent evolution, suggesting that the radii as determined from visibility-plane model-fitting continue to provide reasonable estimates of the physical outer shock-front radius.

Motivation & Objective

• To image SN 1993J at 1.7 GHz using high-resolution VLBI to study its long-term evolution.
• To determine the physical expansion rate and shock front radius using visibility-plane model-fitting.
• To investigate deviations from self-similar power-law expansion and assess frequency-dependent evolution.
• To compare the reliability of u-v plane model-fitting versus image-plane CPM methods for radius measurements.
• To examine the evolution of shell thickness (outer vs. inner radius ratio) over time.

Proposed method

• VLBI observations were conducted using a global array of 18 telescopes from the European VLBI Network and NRAO at 1.7 GHz on 2010 March 5–6.
• An in-beam calibrator technique was employed, pointing between SN 1993J and M81*, its phase reference source, to correct for atmospheric phase delays.
• Visibility-plane model-fitting was applied to the u-v data to simultaneously fit the outer and inner angular radii (θ_out and θ_in) of the supernova shell.
• The Chevalier mini-shell model was used to interpret the expansion as r ∝ t^m, with m determined from the fit to the visibility data.
• The results were compared with those from the Common Point Method (CPM) applied by Marti-Vidal et al. to assess methodological differences.
• Frequency-dependent evolution was tested by comparing radii derived from multi-frequency data, with no clear frequency dependence found.

Experimental results

Research questions

• RQ1 Does the expansion of SN 1993J continue to follow a power-law evolution, and if not, what deviations are observed?
• RQ2 What is the true physical radius of the outer shock front, as estimated from u-v plane model-fitting?
• RQ3 How does the thickness of the supernova shell (θ_out / θ_in) evolve over time, and what does this imply for shock dynamics?
• RQ4 Is the u-v plane model-fitting method more reliable than the image-plane CPM for measuring angular radii in extended sources like SN 1993J?
• RQ5 Is there evidence for frequency-dependent evolution in the supernova's expansion or brightness distribution?

Key findings

• The outer radius of SN 1993J evolves approximately as r ∝ t⁰.⁸, but with measurable deviations from a strict power law, especially after t ≈ 1 yr.
• The supernova's expansion velocity has slowed to about 30% of its initial value, indicating significant deceleration over time.
• The shell thickness, quantified by the ratio θ_out / θ_in, increases systematically after t ≈ 8 yr, reaching ~1.7 by t ≈ 16.9 yr, indicating non-uniform shock evolution.
• No clear frequency dependence was found in the expansion or brightness distribution, supporting the use of u-v plane model-fitting for estimating the true outer shock radius.
• The u-v plane model-fitting method produced radii with significantly less scatter than the CPM method, contradicting claims of CPM's superior accuracy.
• The observed thickening of the shell is larger than predicted by simulations, suggesting additional physical processes such as instabilities in the unshocked ejecta may be at play.