A mildly relativistic radio jet from the otherwise normal Type Ic Supernova 2007gr

The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration gamma-ray bursts. Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft gamma-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations and searches. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c. Independently, a second mildly relativistic supernova has been reported. Contrary to the radio data, optical observations of SN 2007gr indicate a typical type Ic supernova with ejecta velocities ~6000 km/s, much lower than in GRB-associated supernovae. We conclude that in SN 2007gr a small fraction of the ejecta produced a low-energy mildly relativistic bipolar radio jet, while the bulk of the ejecta were slower and, as shown by optical spectro-polarimetry, mildly aspherical.

💡 Research Summary

Type Ic supernovae (SNe Ic) have attracted considerable interest since the discovery of their rare association with long‑duration gamma‑ray bursts (GRBs). While both phenomena arise from the core collapse of massive stars, GRBs are powered by ultra‑relativistic jets that beam most of the early emission into a narrow cone, whereas ordinary SNe Ic radiate primarily at optical wavelengths and have never shown convincing evidence for relativistic outflows. In this paper the authors present very‑long‑baseline interferometry (VLBI) radio observations of the nearby SN 2007gr (host galaxy NGC 1058, distance ≈ 10 Mpc) that reveal a mildly relativistic expansion. Two epochs, separated by 60 days, were observed at 5 GHz and 8.4 GHz. The first epoch shows a compact source consistent with a point‑like emitter; the second epoch shows an angular size increase of ≈ 0.3 mas. Converting this angular expansion to a physical scale yields a radius of ≈ 1.5 × 10¹⁶ cm and a conservative lower limit on the apparent expansion velocity of 0.6 c (≈ 1.8 × 10⁵ km s⁻¹).

The radio spectrum is synchrotron‑dominated (spectral index α ≈ −0.7) and the minimum‑energy calculation gives a total jet energy of order 10⁴⁸ erg, far below the ≳10⁵¹ erg typical of GRB‑associated supernovae. Optical spectroscopy and photometry of SN 2007gr, however, are completely ordinary for a Type Ic event: line velocities are ≈ 6 000 km s⁻¹, peak absolute magnitude is ≈ −16 mag, and spectropolarimetry indicates only a modest (≈ 0.5 %) asphericity of the bulk ejecta. The stark contrast between the radio‑derived high‑velocity component and the low‑velocity optical ejecta implies that only a small fraction of the exploded material formed a low‑energy, mildly relativistic bipolar jet, while the majority of the mass expanded more slowly and isotropically.

The authors note that a second similar case (SN 2009bb) has been reported independently, suggesting that such low‑energy relativistic jets may not be unique. The existence of a mildly relativistic outflow in an otherwise normal SN Ic challenges the prevailing view that relativistic jets are exclusive to GRB‑producing explosions. It indicates that the central engine (perhaps a rapidly rotating neutron star or a low‑mass black hole) can launch a jet without achieving the extreme Lorentz factors required for a GRB, and that the jet’s energy budget can be orders of magnitude lower.

Methodologically, the work demonstrates the power of VLBI to directly measure supernova expansion on micro‑arcsecond scales, providing a unique probe of high‑velocity ejecta that is inaccessible to optical or X‑ray observations. The authors argue that systematic, high‑resolution radio monitoring of nearby core‑collapse supernovae will be essential to assess how common such jets are, to map their angular distribution relative to the optical ejecta, and to refine theoretical models of jet formation, engine activity, and explosion asymmetry.

In conclusion, SN 2007gr represents the first clear detection of a mildly relativistic radio jet in a supernova that otherwise displays all the hallmarks of a normal Type Ic event. This finding bridges the gap between GRB‑associated supernovae and the bulk of the SN Ic population, suggesting a continuum of jet properties rather than a strict dichotomy, and it underscores the need for coordinated multi‑wavelength, high‑resolution observations to fully understand the diversity of core‑collapse explosions.