Fossil gas and the electromagnetic precursor of supermassive binary black hole mergers

Philip Chang, Linda E. Strubbe, Kristen Menou, Eliot Quataert

Research output: Contribution to journalArticlepeer-review

56 Scopus citations


Using a 1D height integrated model, we calculate the evolution of an unequal mass binary black hole with a coplanar gas disc that contains a gap due to the presence of the secondary black hole. Viscous evolution of the outer circumbinary disc initially hardens the binary, while the inner disc drains on to the primary (central) black hole. As long as the inner disc remains cool and thin at low (rather than becoming hot and geometrically thick), the mass of the inner disc reaches an asymptotic mass typically ∼10-3-10-4 M. Once the semimajor axis shrinks below a critical value, angular momentum losses from gravitational waves dominate over viscous transport in hardening the binary. The inner disc then no longer responds viscously to the inspiraling black holes. Instead, tidal interactions with the secondary rapidly drive the inner disc into the primary. Tidal and viscous dissipation in the inner disc lead to a late time brightening in luminosity, L ∝t -5/4minus, where tminus is the time prior to the final merger. This late time brightening peaks ∼1 d prior to the final merger at ∼0.1LEdd. This behaviour is relatively robust because of self-regulation in the coupled viscous-gravitational evolution of such binary systems. It constitutes a unique electromagnetic signature of a binary supermassive black hole merger and may allow the host galaxy to be identified if used in conjunction with the Laser Interferometric Space Antenna localization.

Original languageEnglish (US)
Pages (from-to)2007-2016
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Sep 2010
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Accretion, accretion discs
  • Binaries: general
  • Black hole physics
  • Galaxies: active
  • Galaxies: nuclei
  • Gravitational waves
  • Quasars: general


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