A Deep Chandra X-Ray Study of Neutron Star Coalescence GW170817
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 ({{Δ }}t∼ 2 days) and Swift ({{Δ }}t∼ 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 ({{Δ }}t∼ 15{--}16 days). We detect X-ray emission from SSS17a with {L}0.3{--10{keV}}={2.6}-0.4+0.5× {10}38 erg 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 ({L}0.3{--10{keV}}∼ 9× {10}38 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2017
- DOI:
- 10.3847/2041-8213/aa8ede
- arXiv:
- arXiv:1710.05852
- Bibcode:
- 2017ApJ...848L..25H
- Keywords:
-
- galaxies: individual: NGC 4993;
- gamma-ray burst: individual: GRB 170817A;
- gravitational waves;
- stars: neutron;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- ApJL, published 2017 Oct 16