A Deep Chandra X-Ray Study of Neutron Star Coalescence GW170817

Daryl Haggard, Melania Nynka, John J. Ruan, Vicky Kalogera, S. Bradley Cenko, Phil Evans, Jamie A. Kennea

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199 Scopus citations


We report Chandra observations of GW170817, the first neutron star-neutron star merger discovered by the joint LIGO-Virgo Collaboration, and the first direct detection of gravitational radiation associated with an electromagnetic counterpart, Fermi short γ-ray burst GRB 170817A. The event occurred on 2017 August 17 and subsequent observations identified an optical counterpart, SSS17a, coincident with NGC 4993 (∼10″ separation). Early Chandra (Δt ∼ 2 days) and Swift (Δt ∼ 1-3 days) observations yielded non-detections at the optical position, but ∼9 days post-trigger Chandra monitoring revealed an X-ray point source coincident with SSS17a. We present two deep Chandra observations totaling ∼95 ks, collected on 2017 September 01-02 (Δt ∼ 15-16 days). We detect X-ray emission from SSS17a with L0.3-10 keV = 2.6+0.5-0.4 × 1038erg s-1, and a power law spectrum of Γ = 2.4 ± 0.8. We find that the X-ray light curve from a binary NS coalescence associated with this source is consistent with the afterglow from an off-axis short γ-ray burst, with a jet angled ≳23° from the line of sight. This event marks both the first electromagnetic counterpart to a LIGO-Virgo gravitational-wave source and the first identification of an off-axis short GRB. We also confirm extended X-ray emission from NGC 4993 (L0.3-10 keV ∼9 ×1038 erg s-1) consistent with its E/S0 galaxy classification, and report two new Chandra point sources in this field, CXOU J130948 and CXOU J130946.

Original languageEnglish (US)
Article numberL25
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Oct 20 2017

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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