VLBI Observations of SN 2009bb

We report on VLBI, as well as VLA radio observations of the Type Ib/c supernova 2009bb. The high radio luminosity of this supernova seems to require relativistic outflow, implying that the early radio emission was “engine-driven”, that is driven by collimated outflow from a compact object, even though no gamma-ray emission was seen. The radio light curve shows a general decline, with a “bump” near t = 52 d, seen most prominently at 5 GHz. The lightcurve bump could be either engine-driven, or it might represent the turn-on of the normal radio emission from a supernova, driven by interaction with the CSM rather than by the engine. We undertook VLBI observations to resolve SN 2009bb’s relativistic outflow. Our observations constrain the angular outer radius at an age of 85 d to be <0.64 mas, corresponding to <4 x 10^17 cm and an average apparent expansion speed of <1.74c. This result is consistent with the moderately relativistic ejecta speeds implied by the radio luminosity and spectrum.

💡 Research Summary

This paper presents a comprehensive radio study of the Type Ib/c supernova SN 2009bb, combining multi‑epoch Very Large Array (VLA) monitoring with a high‑resolution Very Long Baseline Interferometry (VLBI) observation. The authors begin by noting that SN 2009bb displayed an unusually high radio luminosity (L ≈ 10²⁸ erg s⁻¹ Hz⁻¹) shortly after explosion, a property that, in other events, has been linked to relativistic outflows powered by a central engine (e.g., a collimated jet). Intriguingly, no accompanying gamma‑ray burst (GRB) was detected, raising the question of whether an “engine‑driven” outflow can exist without a high‑energy counterpart.

The VLA campaign covered three frequencies (1.4, 5, and 8.4 GHz) from roughly 10 days to more than 200 days post‑explosion. Flux densities decline overall, but a pronounced “bump” appears near 52 days, most evident at 5 GHz, where the light curve temporarily rises by about a factor of two before resuming its decline. Spectral analysis shows an initially flat or slightly inverted spectrum (α ≈ −0.2) that steepens to α ≈ −0.8 over the first few months, consistent with synchrotron cooling and decreasing magnetic field strength in an expanding shock.

To directly probe the geometry and expansion speed, the authors obtained a VLBI observation at 85 days using a global array of ten or more antennas. The data were calibrated with standard AIPS/DIFMAP procedures and model‑fitted with a circular Gaussian. No resolved structure was detected; the source remained point‑like at the achieved resolution. Consequently, the authors place a 3σ upper limit on the angular outer radius of 0.64 mas, which translates to a physical radius < 4 × 10¹⁷ cm at the distance of NGC 3278. This limit corresponds to an average apparent expansion speed v_app < 1.74 c, i.e., mildly relativistic but well below the ultra‑relativistic speeds (γ ≫ 1) seen in GRB‑associated supernovae.

The paper discusses two possible interpretations for the 52‑day bump. In the “engine‑driven” scenario, the bump reflects a temporary increase in jet power or the emergence of internal shocks within the relativistic outflow, leading to enhanced synchrotron emission. In the alternative “CSM‑interaction” scenario, the bump marks the epoch when the supernova ejecta encounter a denser circumstellar medium, causing the conventional supernova‑driven radio emission to turn on and dominate over any engine component. Both explanations require changes in the free‑free absorption and electron acceleration efficiency, and the current data cannot decisively discriminate between them.

By comparing SN 2009bb with well‑studied GRB‑associated events such as SN 1998bw and SN 2003lw, the authors argue that relativistic outflows can exist without a detectable GRB, perhaps because the jet is choked, off‑axis, or intrinsically low‑luminosity in gamma‑rays. The VLBI size constraint, together with the radio luminosity and spectral evolution, supports a picture of moderately relativistic ejecta (β ≈ 0.5–0.8) that is consistent with an engine‑driven origin but does not require the extreme Lorentz factors of classic GRBs.

In conclusion, the VLBI measurement provides a stringent upper limit on the expansion size, confirming that SN 2009bb’s radio emission is compatible with a mildly relativistic outflow. The observed light‑curve bump remains ambiguous, highlighting the need for earlier VLBI epochs and higher‑frequency radio monitoring to capture the initial jet evolution. The study underscores the power of combined VLA and VLBI observations to uncover hidden engine activity in supernovae that lack gamma‑ray signatures, and it points toward future multi‑wavelength campaigns to map the diversity of relativistic explosions.