On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

Merging neutron stars offer an excellent laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart (EM170817) with gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic data set, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultrarelativistic jets. Instead, we suggest that breakout of a wide-angle, mildly relativistic cocoon engulfing the jet explains the low-luminosity gamma rays, the high-luminosity ultraviolet-optical-infrared, and the delayed radio and x-ray emission. We posit that all neutron star mergers may lead to a wide-angle cocoon breakout, sometimes accompanied by a successful jet and sometimes by a choked jet.

We present the photometric calibration of the Swift Ultraviolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux and the photometric zero-points (which are accurate to better than 4 per cent) for each of the seven UVOT broad-band filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position-dependent uniformity, and instrument response over the 1600-8000 Å operational range. Because the UVOT is a photon-counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results.

With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and X-ray observations by Swift and the Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the binary neutron star merger GW 170817. The bright, rapidly fading ultraviolet emission indicates a high mass (≈ 0.03 solar masses) wind-driven outflow with moderate electron fraction (Y e ≈ 0.27). Combined with the X-ray limits, we favor an observer viewing angle of ≈ 30• away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultra-relativistic, highly collimated ejecta (a gamma-ray burst afterglow). One-sentence summaryWe report X-ray and UV observations of the first binary neutron star merger detected via gravitational waves. Main TextAt 12:41:04.45 on 2017 August 17 (UT times are used throughout this work), the Laser Interferometric GravitationalWave Observatory (LIGO) and Virgo Consortium (LVC) registered a strong gravitational wave (GW) signal (LVC trigger G298048; (1)), later named GW 170817 (2). Unlike previous GW sources reported by LIGO, which involved only black holes (3), the gravitational strain waveforms indicated a merger of two neutron stars. Binary neutron star mergers have long been considered a promising candidate for the detection of an electromagnetic counterpart associated with a gravitational wave source. Two seconds later, the Gamma-Ray Burst Monitor (GBM) on the Fermi spacecraft triggered on a short (duration ≈ 2 s) gamma-ray signal consistent with the GW localization, GRB 170817A (4, 5). 330°00'00" 300°00'00" 270°00'00" 240°00'00" 210°00'00" 180°00'00" 150°00'00" 120°00'00" 90°00'00" 30°0°Figure 1: Skymap of Swift XRT observations, in equatorial (J2000) coordinates. The grey probability area is the GW localization (13), the blue region shows the Fermi-GBM localization, and the red circles are Swift-XRT fields of view. UVOT fields are colocated with a field of view 60% of the XRT. The location of the counterpart, EM 170817, is marked with a large yellow cross. The early 37-point mosaic can be seen, centred on the GBM probability. The widely scattered points are from the first uploaded observing plan, which was based on the singledetector GW skymap. The final observed plan was based on the first 3-detector map (11), however we show here the higher-quality map (13) so that our coverage can be compared to the final probability map (which was not available at the time of our planning; (7)).Swift satellite (6) in its low-Earth orbit meant that the GW and gamma-ray burst (GRB) localizations were occulted by the Earth (7) and so not visible to its Burst Alert Telescope (BAT). These discoveries triggered a world-wide effort to find, localize and characterize the EM counterpart (8). We present UV and X-ray observations conducted as part of t...

Abstract. We present an updated calibration of the Swift/UVOT broadband ultraviolet (uvw1, uvm2, and uvw2) filters. The new calibration accounts for the ~1% per year decline in the UVOT sensitivity observed in all filters, and makes use of additional calibration sources with a wider range of colours and with HST spectrophotometry. In this paper we present the new effective area curves and instrumental photometric zeropoints and compare with the previous calibration.