[Paper Review] VLBI observations of SN 2008iz I. Expension velocity and limits on anistrophic expansion

A. Brunthaler, I. Martí‐Vidal+10 more|arXiv (Cornell University)|Jan 1, 2010

Gamma-ray bursts and supernovae40 references30 citations

TL;DR

This study presents the first VLBI images of supernova SN 2008iz in M82, revealing an expanding shell at ~23,000 km s⁻¹, significantly faster than predicted by synchrotron self-absorption (SSA) models. The discrepancy implies SSA is not the dominant absorption mechanism, and the source shows no signs of asymmetry, supporting a symmetric, likely Type II, core-collapse origin behind a dense molecular cloud with A_V ≈ 24.4 mag.

ABSTRACT

Contains fulltext : 83703.pdf (Author’s version preprint ) (Open Access) Contains fulltext : 83703.pdf (Publisher’s version ) (Open Access)

Motivation & Objective

• To measure the expansion velocity of SN 2008iz using VLBI to test radio emission models.
• To determine if the explosion was anisotropic by analyzing the morphology and kinematics of the radio shell.
• To constrain the explosion epoch and assess the role of synchrotron self-absorption (SSA) in shaping the radio light curve.
• To measure the X-ray luminosity and assess absorption by a dense molecular cloud.

Proposed method

• VLBI observations at 22 GHz with the High Sensitivity Array (HSA) and VLA, using phase referencing and geodetic calibration to achieve sub-milliarcsecond resolution.
• VLA observations at 1.4–43 GHz to construct a broadband radio spectrum and identify spectral turnover.
• Application of Chevalier's (1998) SSA-based model to estimate expected expansion velocity under SSA dominance.
• Chandra X-ray observations to set upper limits on X-ray emission, accounting for extinction from a molecular cloud.
• Self-calibration and amplitude/phase calibration using quasars and the M81* core to correct for ionospheric and instrumental effects.
• Modeling of the radio light curve using free-free absorption (FFA) and comparison with SSA predictions to assess dominant absorption mechanism.

Experimental results

Research questions

• RQ1 What is the true expansion velocity of SN 2008iz, and how does it compare to predictions from synchrotron self-absorption (SSA) models?
• RQ2 Is the explosion of SN 2008iz anisotropic, as indicated by asymmetric morphology or kinematics in VLBI images?
• RQ3 What is the dominant absorption mechanism (SSA vs. FFA) responsible for shaping the radio light curve of SN 2008iz?
• RQ4 What is the X-ray luminosity of SN 2008iz, and how does it compare to known supernovae like SN 1993J at similar ages?
• RQ5 What is the column density of the absorbing molecular cloud, and how does it affect multiwavelength detectability?

Key findings

• The VLBI images show a ring-like structure expanding at ~23,000 km s⁻¹, with a 11-month baseline between observations.
• The measured expansion velocity is ~2× higher than expected under SSA dominance, indicating SSA is not the primary absorption mechanism.
• The most likely explosion date is mid-February 2008, constrained between January 22 and March 24.
• No evidence for anisotropic expansion is found; the two VLBI epochs show high morphological similarity, supporting a symmetric shell.
• The VLA spectrum is best fit by a broken power law with a turnover at 1.5 ± 0.1 GHz and a spectral index of -1.08 ± 0.08 in the optically thin regime.
• The source is behind a molecular cloud with a hydrogen column density of 5.4 × 10²² cm⁻², corresponding to a visual extinction of 24.4 mag, explaining the lack of optical and X-ray detections.