Swift and NuSTAR observations of GW170817: Detection of a blue kilonova

Abstract

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...

Localization of binary neutron star mergers with second and third generation gravitational-wave detectors

“…This multimessenger data provided convincing answers to many outstanding questions. For instance, the detection of a short gamma ray burst (GRB) 1.7 seconds after GW170817 [24][25][26], and subsequent kilonova [27][28][29][30][31][36][37][38][39][40][41][42][43][44], confirmed that BNS mergers are a progenitor of these events. Lanthanide signatures in the kilonova light curves also showed BNS mergers to be a major site for nucleosynthesis of elements heavier than iron [40,44,47,48].…”

ALMA and GMRT Constraints on the Off-axis Gamma-Ray Burst 170817A from the Binary Neutron Star Merger GW170817

“…The individual light curves of the SGRB sample are shown in light blue. Already the Swift non-detections presented in Evans et al (2017b; downward pointing triangles in Figure 2) revealed that the afterglow is >1.5 dex fainter than the faintest SGRB with detected afterglow (downward pointing triangles in Figure 2). The deep Chandra observation by Troja et al (2017b) at 2.3days after GW170817 excluded any afterglow brighter than > -10 erg s 39 1 , i.e., a factor of 10 below the Swift upper limits.…”

Afterglows and Kilonovae Associated with Nearby Low-luminosity Short-duration Gamma-Ray Bursts: Application to GW170817/GRB 170817A

“…Fitting parameters for different lines are shown in Table 1 Table 2. Panel (a): The fitting of X-ray data (including black upper limits and red data-points) (Evans et al 2017;Troja et al 2017) with the two-component jet model. …”

“…Table 1. Panel (a): The fitting of X-ray data (including black upper limits and red data-points) (Evans et al 2017;Troja et al 2017) with the structured jet model. Fitting parameters for different lines are shown in Table 1 Table 2.…”

“…The relevant fitting parameters are given in Table 1 and Table 2. The X-ray upper limits are given by Swift-XRT and NuSTAR (Evans et al 2017), while the two detections of Chandra are indicated by the red datapoints (Troja et al 2017). For the optical band, we choose r-band to fit and the data are collected in the literature (Andreoni et al 2017;Arcavi et al 2017;Coulter et al 2017;Drout et al 2017;Kilpatrick et al 2017;Pian et al 2017;Shappee et al 2017;Smartt et al 2017).…”