Follow-up of the neutron star bearing gravitational-wave candidate events S190425z and S190426c with MMT and SOAR

Research output: Contribution to journalArticle


  • G. Hosseinzadeh
  • P. S. Cowperthwaite
  • S. Gomez
  • V. A. Villar
  • R. Margutti
  • E. Berger
  • R. Chornock
  • K. Paterson
  • W. Fong
  • V. Savchenko
  • P. Short
  • K. D. Alexander
  • P. K. Blanchard
  • J. Braga
  • M. L. Calkins
  • R. Cartier
  • D. L. Coppejans
  • T. Eftekhari
  • T. Laskar
  • C. Ly
  • L. Patton
  • I. Pelisoli
  • D. E. Reichart
  • G. Terreran
  • P. K. G. Williams

Colleges, School and Institutes


On 2019 April 25.346 and 26.640 UT the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave (GW) observatory announced the detection of the first candidate events in Observing Run 3 that contained at least one neutron star (NS). S190425z is a likely binary neutron star (BNS) merger at d L = 156 ± 41 Mpc, while S190426c is possibly the first NS-black hole (BH) merger ever detected, at d L = 377 ± 100 Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5 m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1 m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/near-infrared (NIR) kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short gamma-ray burst (SGRB) afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40% for S190425z and 60% for S190426c. For an on-axis jet typical of SGRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field γ-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the γ-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg2 are likely essential....


Original languageEnglish
JournalThe Astrophysical Journal
Publication statusPublished - 18 Jul 2019


  • binaries: close, gravitational waves, methods, observational, stars: black holes, stars: neutron