[Paper Review] e-VLBI observations of SN2001em - an off-axis GRB candidate

Z. Paragi, M. A. Garrett+6 more|arXiv (Cornell University)|May 23, 2005

Gamma-ray bursts and supernovae3 citations

TL;DR

This study presents the first e-VLBI observations of the faint Type Ib/c supernova SN2001em, demonstrating real-time, high-resolution radio interferometry for transient sources. Despite marginal detection at 1.6 GHz with a 571 μJy/beam flux density and unresolved size estimates (2.45 mas Gaussian, 1.09 mas uniform disk), the data suggest either fading emission or an inverted radio spectrum, indicating free-free or synchrotron self-absorption—unusual but not unprecedented in supernovae.

ABSTRACT

Studying transient phenomena with the Very Long Baseline Interferometry (VLBI) technique faces severe difficulties because the turnaround time of the experiments from the observations to the scientific result is rather long. The e-VLBI technique has made it possible to transfer the data from a number of European VLBI Network (EVN) telescopes to the central data processor at JIVE through optical fibres, and correlate them in real time. The main goal of this paper is to introduce this rapidly developing new technique, by presenting observational results from a recent experiment. We observed SN2001em, a Type Ib/c supernova with an e-VLBI array and the Multi-Element Radio Linked Interferometer Network (MERLIN) in the UK. The source is marginally detected in our observations. We cannot make definite conclusions whether it is resolved at 1.6 GHz or not. Our data show that SN2001em either started fading in the last couple of months, or its radio spectrum is inverted at low frequencies,indicating free-free or synchrotron self-absorption. This is quite unusual, but not unprecedented in radio SNe.

Motivation & Objective

• To demonstrate the feasibility of e-VLBI for rapid, real-time imaging of faint, transient radio sources like supernovae.
• To investigate the nature of SN2001em’s late-time radio emission, which appeared two years post-explosion and was previously unresolved at higher frequencies.
• To determine whether SN2001em is resolved or fading by comparing e-VLBI data with prior high-frequency observations.
• To assess the scientific turnaround time and data quality of e-VLBI for time-domain astronomy.

Proposed method

• e-VLBI data were collected in real time from six European VLBI Network (EVN) telescopes and Arecibo, with data streamed via optical fiber over the GÉANT network to the JIVE correlator.
• Phase-referencing was applied using the calibrator J2145+1115, observed in an 11-minute cycle to correct for atmospheric and instrumental delays.
• Self-calibration was performed over 30 minutes with a 10-minute solution interval, using Westerbork-Arecibo baselines for phase calibration.
• Data were correlated in real time using the EVN Data Processor, with real-time fringe plots monitored via the Internet for quality control.
• Model fitting was applied to the uv data using circular Gaussian and uniformly bright disk models to estimate source size and flux.
• Sensitivity estimates were computed for each baseline, with Arecibo and Westerbork providing the most sensitive baselines (136–379 μJy sensitivity).

Experimental results

Research questions

• RQ1 Can e-VLBI enable real-time, high-resolution imaging of faint transient radio sources like SN2001em?
• RQ2 Is SN2001em resolved at 1.6 GHz, or is its marginal detection due to low signal-to-noise or source fading?
• RQ3 Does the observed flux density at 1.6 GHz indicate a spectral turnover or spectral index reversal compared to earlier high-frequency measurements?
• RQ4 What physical mechanism—such as free-free absorption or synchrotron self-absorption—could explain an inverted spectrum in a radio supernova?
• RQ5 How does the e-VLBI pipeline enable faster scientific turnaround compared to traditional VLBI?

Key findings

• SN2001em was marginally detected with a correlated flux density of 571 μJy/beam at 1.6 GHz, significantly fainter than prior 8.4 GHz measurements of ~1.5–1.8 mJy.
• The source is consistent with a 900 μJy compact component with a size of 2.45 mas (Gaussian model), or a uniformly bright disk of 1.09 mas diameter, though uncertainties are large due to low signal-to-noise.
• The measured flux density suggests a spectral index of approximately +0.3 between 1.6 GHz and 8.4 GHz, indicating an inverted spectrum, which is unusual but not unprecedented in supernovae.
• The lack of strong fringes on the Westerbork-Arecibo baseline and weak phase solutions suggest the source may be slightly resolved, though not definitively.
• The source may have faded significantly in the months prior to the observation, or the emission region may be optically thick at low frequencies due to free-free or synchrotron self-absorption.
• The entire data processing and initial results were available within days, demonstrating the potential of e-VLBI for rapid response to transient phenomena.