Shock within a shock: Revisiting the radio flares of NS merger ejecta and gamma-ray burst-supernovae

Ben Margalit, Tsvi Piran

Research output: Contribution to journalArticlepeer-review

30 Scopus citations


Fast ejecta expelled in binary neutron star (NS) mergers or energetic supernovae (SNe) should produce late-time synchrotron radio emission as the ejecta shocks into the surrounding ambient medium. Models for such radio flares typically assume the ejecta expands into an unperturbed interstellar medium (ISM). However, it is also well known that binary NS mergers and broad-lined Ic SNe Ic can harbour relativistic jetted outflows. In this work, we show that such jets shock the ambient ISM ahead of the ejecta, thus evacuating the medium into which the ejecta subsequently collides. Using an idealized spherically symmetric model, we illustrate that this inhibits the ejecta radio flare at early times t < tcol ≈ 12 yr (Ej/1049 erg)1/3(n/1 cm−3)−1/3ej/0.1c)−5/3, where Ej is the jet energy, n the ISM density, and υej the ejecta velocity. We also show that this can produce a sharply peaked enhancement in the light curve at t = tcol. This has implications for radio observations of GW170817 and future binary NS mergers, gamma-ray burst (GRB) SNe, decade-long radio transients such as FIRST J1419, and possibly other events where a relativistic outflow precedes a slower moving ejecta. Future numerical work will extend these analytic estimates and treat the multidimensional nature of the problem.

Original languageEnglish (US)
Pages (from-to)4981-4993
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - 2020
Externally publishedYes

Bibliographical note

Funding Information:
BM thanks Eliot Quataert, Brian Metzger, Lorenzo Sironi, Aaron Tran, Stephen Ro, Adithan Kathirgamaraju, and Paz Beniamini for helpful conversations and comments. This work was conceived in interactions that were funded by the Gordon and Betty Moore Foundation through Grant GBMF5076. This research was supported in part by NASA through the NASA Hubble Fellowship grant no. HST-HF2-51412.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555 (BM), and by an Advanced ERC grant TReX (TP).

Publisher Copyright:
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society


  • Gamma-ray bursts
  • Neutron star mergers
  • Radiation mechanisms: non-thermal
  • Radio continuum: transients
  • Shock waves
  • Transients: supernovae


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