Cooling off with a kilonova - lower limit on the expansion velocity of GW170817

Itai Linial*, Re'em Sari

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


GW170817 was the first detection of a binary neutron star merger via gravitational waves. The event was observed over a wide range of the electromagnetic spectrum, revealing a thermal kilonova dominating the optical signal during the first ∼15 days, and a non-thermal synchrotron emission that has continued to rise ∼200 days post-merger, dominating the radio and X-ray emission. At early times, when the kilonova is still dominant, the synchrotron emitting electrons can efficiently cool by up-scattering the kilonova photos through inverse-Compton. Yet, the cooling frequency is not observed up to the X-ray band. This can only be explained if the source is moving at least at a mildly relativistic velocity. We find a lower limit on the source's bulk Lorentz factor of Γ > 2.1 at 9 days. This lower limit is model independent and relies directly on the observed quantities, providing additional robust evidence to the relativistic motion in this event at early times.

Original languageAmerican English
Pages (from-to)624-627
Number of pages4
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - 11 Feb 2019

Bibliographical note

Funding Information:
IL thanks support from the Adams fellowship. RS is supported by an ISF and an iCore grant. We thank Tsvi Piran, Kenta Hotokezaka and Sivan Ginzburg for comments and discussion.

Publisher Copyright:
© 2018 The Author(s).


  • Gravitational waves
  • Radiation mechanisms: general


Dive into the research topics of 'Cooling off with a kilonova - lower limit on the expansion velocity of GW170817'. Together they form a unique fingerprint.

Cite this