[Paper Review] VLBI observations of SN 2008iz: I. Expansion velocity and limits on anisotropic expansion

A. Brunthaler, I. Martí‐Vidal+10 more|arXiv (Cornell University)|Mar 24, 2010

Gamma-ray bursts and supernovae44 references22 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, indicating a higher-than-expected expansion velocity inconsistent with synchrotron self-absorption dominance. The results suggest SN 2008iz is a type II supernova, likely a core-collapse event obscured by a dense molecular cloud, with no evidence for asymmetric explosion and upper limits on X-ray emission consistent with absorption.

ABSTRACT

We present observations of the recently discovered supernova 2008iz in M82 with the VLBI High Sensitivity Array at 22 GHz, the Very Large Array at frequencies of 1.4, 4.8, 8.4, 22 and 43 GHz, and the Chandra X-ray observatory. The supernova was clearly detected on two VLBI images, separated by 11 months. The source shows a ring-like morphology and expands with a velocity of ~23000 km/s. The most likely explosion date is in mid February 2008. The measured expansion speed is a factor of ~2 higher than expected under the assumption that synchrotron self-absorption dominates the light curve at the peak, indicating that this absorption mechanism may not be important for the radio emission. We find no evidence for an asymmetric explosion. The VLA spectrum shows a broken power law, indicating that the source was still optically thick at 1.4 GHz in April 2009. Finally, we report upper limits on the X-ray emission from SN 2008iz and a second radio transient recently discovered by MERLIN observations.

Motivation & Objective

• To measure the expansion velocity of SN 2008iz using VLBI imaging to constrain its physical properties and explosion mechanism.
• To investigate whether anisotropic expansion or asymmetric explosion is present in the supernova's radio emission.
• To determine the radio spectral index and turnover frequency to assess optical depth and emission mechanisms.
• To set upper limits on X-ray emission from SN 2008iz and a second transient, assessing absorption and luminosity.

Proposed method

• VLBI observations were conducted using the High Sensitivity Array at 22 GHz on two epochs separated by ~11 months to resolve the expanding shell.
• The Very Large Array (VLA) observed SN 2008iz at 1.4–43 GHz to construct a broadband radio spectrum and determine spectral index and turnover frequency.
• Chandra X-ray observations were used to derive upper limits on X-ray luminosity, accounting for absorption by a dense molecular cloud.
• Phase-referencing techniques with M81* and quasars were applied to correct for ionospheric and atmospheric delays in VLBI data.
• The expansion velocity was derived from the angular size change between the two VLBI epochs, assuming a distance of 3.6 Mpc.
• A broken power-law model was fitted to the VLA spectrum to determine the turnover frequency and spectral indices in optically thick and thin regimes.

Experimental results

Research questions

• RQ1 What is the true expansion velocity of SN 2008iz, and how does it compare to theoretical expectations based on synchrotron self-absorption?
• RQ2 Is the supernova explosion asymmetric, as indicated by deviations in the shell morphology or velocity structure?
• RQ3 What is the radio spectral index and turnover frequency, and what do they imply about optical depth and emission mechanisms?
• RQ4 What is the X-ray luminosity of SN 2008iz, and how does absorption by the interstellar medium affect detectability?
• RQ5 How does SN 2008iz compare to other type II radio supernovae like SN 1993J in terms of luminosity, expansion, and spectral evolution?

Key findings

• The VLBI images show a shell-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 synchrotron self-absorption (SSA) dominance, indicating SSA is not the dominant absorption mechanism.
• The most likely explosion date is mid-February 2008, constrained between January 22 and March 24.
• No evidence for asymmetric expansion is found; the two VLBI images show high self-similarity, suggesting a spherically symmetric outflow.
• The VLA spectrum is best fitted by a broken power law with a turnover frequency of 1.5 ± 0.1 GHz and a spectral index of -1.08 ± 0.08 in the optically thin regime.
• The X-ray luminosity upper limits are ~1.5 × 10⁴¹ erg s⁻¹ at ~75 days and ~1.5 × 10³⁹ erg s⁻¹ at ~200 days, consistent with absorption by a dense molecular cloud (N(H₂) = 5.4 × 10²² cm⁻², A_V = 24.4 mag).