Department of Physics, Oregon State University, 301 Weniger Hall, Corvallis, Oregon 97331, USA.
Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA.
Phys Rev Lett. 2018 Jun 15;120(24):241103. doi: 10.1103/PhysRevLett.120.241103.
The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multiwavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubt on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multiwavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30° away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.
双中子星（BNS）合并 GW170817 是首个在引力波和多波长电磁辐射中探测到的天体物理源。伽马射线脉冲的几乎同时观测证明了 BNS 合并至少与一些短伽马射线暴（GRB）有关。然而，伽马射线脉冲很微弱，这使得 BNS 合并与典型的亮而高度相对论性的短 GRB 外流的关联受到质疑。在这里，我们表明，具有相对论性、高能核心的结构喷流，周围是较慢且能量较低的翼片，会产生余晖发射，其特征是随着时间的推移而变亮，最近在 GW170817 的余晖中就看到了这种情况。最初，我们只能看到相对较慢的物质向我们移动。随着时间的推移，喷流的越来越大的部分变得可见，增加了余晖的亮度。GW170817 的多波长余晖的后期出现和亮度增加，使我们能够约束其喷出物的几何形状，从而揭示出存在一个指向地球外约 30°的偏轴喷流。我们的结果证实了 BNS 合并和短 GRB 的单一起源：GW170817 产生了具有高度相对论性核心和典型短 GRB 的结构喷流。我们没有看到明亮的爆发，因为它是从地球方向射来的。然而，在引力波探测到的合并中，大约每 20 个中就有一个会伴随着一个明亮的、典型的短 GRB。
