Late Time Afterglow Observations Reveal a Collimated Relativistic Jet in the Ejecta of the Binary Neutron Star Merger GW170817

Abstract

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.

Fermi GBM Observations of GRB 150101B: A Second Nearby Event with a Short Hard Spike and a Soft Tail

“…When jets plow through dense environments they deposit a fraction of their energy in a hot cocoon (Ramirez-Ruiz et al 2002;Pe'er et al 2006), which may occur in binary neutron star mergers as the ultrarelativistic jet that powers the SGRB pushes through material ejected just before merger (Lazzati et al 2017). GRB 170817A has odd behavior in both prompt and afterglow emission, the origin of which is a matter of some debate (Abbott et al 2017a;Alexander et al 2017Alexander et al , 2018Bromberg et al 2017;Haggard et al 2017;Kasliwal et al 2017;Margutti et al 2017;Mooley et al 2018aMooley et al , 2018bTroja et al 2017Troja et al , 2018aGottlieb et al 2018;Lazzati et al 2018;Lyman et al 2018;Nynka et al 2018;Ruan et al 2018;Veres et al 2018). Possible interpretations include a structured ultrarelativistic jet (e.g., Alexander et al 2018), a jet and cocoon together (e.g., Abbott et al 2017a), or a cocoon model (e.g., Kasliwal et al 2017) where the shock breakout produces the harder peak.…”

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

“…While short GRBs can be observed to cosmological distances, they are believed to be rather tightly beamed, so that only a small fraction of BNS mergers would be accompanied by a gamma ray burst (GRB) counterpart [126]. However, observations of GW170817 imply that the gamma ray burst (GRB) emission is structured [25] with a broader, weak emission, possibly powered by a cocoon [47,127,128]. Since the gamma ray burst (GRB) emission is likely to be essentially concurrent with the merger, it will be difficult to use GW observations to provide advanced warning to gamma ray burst (GRB) satellites.…”

The Merger of Two Compact Stars: A Tool for Dense Matter Nuclear Physics

“…The first one assumes that the emission was intrinsically sub-luminous and quasi-isotropic [58,59]. The second one assumes instead a standard short GRB emission, that was observed off-axis [60]. While at the moment, about a hundred days after the event, both possibilities can explain the data, the analysis of the future time-evolution of the synchrotron emission will ultimately be able to distinguish between these two scenarios, telling therefore if GRB170817A was a standard short GRB seen off-axis or if it belongs to a new class of phenomena [61].…”