Abstract
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 language | English (US) |
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Article number | L25 |
Journal | Astrophysical Journal Letters |
Volume | 848 |
Issue number | 2 |
DOIs | |
State | Published - Oct 20 2017 |
Keywords
- X-rays: binaries
- galaxies: individual (NGC 4993)
- gamma-ray burst: individual (GRB 170817A)
- gravitational waves
- stars: neutron
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science