Two snapshots of the same lighthouse. One is taken from a boat sailing straight into the beam, the other from the shore, where the beam only grazes the edge of the sky. From these two frames, you can reconstruct how the lighthouse light is distributed across angles without ever getting close to it. That is roughly what astronomers did with two gamma-ray bursts they consider twins.
The first burst, GRB 090510, was caught in 2009. It was as bright as a headlight in your face: we were looking almost straight down its jet. The second, GW170817, was born from the merger of two neutron stars in 2017 and turned out to be a million times fainter. For a long time, astronomers could not figure out why: either we see it from the side, or the jet itself is structured inhomogeneously.
The authors of the new study assumed these are the same type of system, just seen at different angles: the first along its axis, the second at an angle of about 20 degrees. Then they took an unexpected step: they abandoned the usual Doppler shift that works for a point source. Instead, they considered a cone of emission filled with matter. For such a cone, the brightness we see depends only on how much energy is emitted in our direction, while the color of the burst depends on the speed of matter at that point. This allowed them to compare the two bursts without a single free parameter.
From the ratio of energies, (7.45±1.41)×$10^5$, and peak energies, 36.4±9.8, they obtained an index n = 3.76±0.29. This means that the energy emitted per unit solid angle grows as the matter speed raised to the power 3.76. This value rules out four popular models of jet structure at significance levels from 4.7σ to 13σ: a uniform flow without energy structure, structure only in baryon loading, a minimally structured jet, and a uniform cone seen from its edge.
Using two anchors from the literature, the Lorentz factor of the core of GRB 090510 (at least 1200) and the viewing angle of GW170817 (19–25 degrees), they converted the slope into an angular profile. It turned out that outside the core with a half-opening angle of 2–5 degrees, the energy falls off as the angle to the power −7.4, and along the line of sight, the matter speed corresponds to a Lorentz factor of 33. The same profile gave the radius where the gamma-ray emission is produced: 2.7×$10^{13}$ centimeters. This accounts for 0.41 seconds of the 1.74-second delay between the gravitational waves and the gamma-ray burst; the rest is the time it took the jet to break through the shell of matter ejected during the merger.
Previously, to learn the shape of a jet, astronomers had to monitor its afterglow for months and make assumptions about the density of the surrounding gas. Now, two instantaneous spectra sufficed. The faintness of GW170817 turned out not to be a consequence of viewing the bright core from the side, but an intrinsic property of the jet: along our line of sight, the matter simply emits less energy. This changes the view of short gamma-ray bursts and their connection to neutron star mergers. The lighthouse that seemed dim actually shines differently, and now we know how.
The delay between gravitational waves and gamma rays in GW170817 was 1.74 seconds. Of that, 0.41 seconds is explained by the angular spread of the emission, while the rest is the time it took the jet to break through the shell of merger-ejected matter.
In Carl Sagan's novel Contact, a signal from space arrives as a sequence of pulses that the protagonists decode as a structure. Here, the jet's structure is also reconstructed from two pulses, but instead of an alien message, they are two natural gamma-ray bursts seen from different angles.