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) |
|---|---|
| Article number | L25 |
| Journal | Astrophysical Journal Letters |
| Volume | 848 |
| Issue number | 2 |
| DOIs | |
| State | Published - Oct 20 2017 |
Funding
We dedicate this work to the memory of Neil Gehrels, one of the original PIs for our Chandra proposal and an active participant in the early months of the program. Neil’s stewardship of Swift has influenced our entire community—this short γ-ray burst and its coincidence with a LIGO-Virgo GW source would have thrilled him. The authors also owe a debt of gratitude to Belinda Wilkes and the Chandra scheduling, data processing, and archive teams. Their incredibly fast work was essential to making these time-sensitive observations possible. We also thank our anonymous referee for their timely review and useful comments. We thank Sean McWilliams for his useful input. This work was supported by Chandra Award Number GO7-18033X, issued by the Chandra X-Ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration (NASA) under contract NAS8-03060. D.H. acknowledges support from the Canadian Institute for Advanced Research (CIFAR). D.H., M.N., and J.J.R. also acknowledge support from a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant and a Fonds de recherche du Québec-Nature et Technologies (FRQNT) Nouveaux Chercheurs Grant. P.A.E. acknowledges UKSA support. J.A.K. acknowledges the support of NASA grant NAS5-00136.
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