After the neutron star merger GW170817, a faint gamma-ray burst was detected, six orders of magnitude dimmer than typical short bursts. Interferometry showed the jet was relativistic and pointed about 20 degrees off our line of sight. However, geometry alone does not reveal the jet's angular structure: how energy and Lorentz factor vary with polar angle. It is this structure that converts observed brightness into true energy output, determines the fraction of bursts we actually see, and bears the imprint of the collimation mechanism. Usually, it is extracted from lengthy afterglow fitting, which requires knowledge of the ambient density and microphysical parameters that are not independently known. This work stands apart: nearby are studies of gamma-ray emission, cosmic rays, and jets, but direct neighbors solving the same problem from prompt emission alone are almost nonexistent.
The authors assumed that GRB 090510 and GW170817 represent the same type of astrophysical system, observed along the axis and at 20 degrees, respectively. This assumption is supported by independent gravitational-wave data: the masses of the binary systems and the emitted energies are compatible. The key step is abandoning point-source Doppler scaling. For an observer whose emission cone is filled with matter, the isotropic equivalent energy is simply 4π times the energy emitted per unit solid angle along the line of sight, and the peak energy is proportional to the local Lorentz factor. This allows expressing the energy ratio of the two bursts as the ratio of their peak energies raised to the power n, where n is the logarithmic slope of energy versus Lorentz factor. No free parameters or assumptions about the angle are needed. To translate the slope into an angular profile, two anchors from the literature were used: the Lorentz factor of the GRB 090510 core (≥1200) and the viewing angle of GW170817 (19–25 degrees).
From the energy ratio (7.45±1.41)×$10^5$ and peak energy ratio 36.4±9.8, the index n = 3.76±0.29 is obtained, meaning the emitted energy per unit solid angle scales as $Γ^3$.76. This rules out four common structures at significance levels from 4.7σ to 13σ: a uniform flow without energy structure (n=0), a structure solely in baryon loading (n=1), a minimal structured jet (n=2), and a uniform cone viewed from its edge (n=2.4). With the angular anchor, the profile emerges: outside a core with half-angle 2–5 degrees, energy falls as θ^−7.4, and the Lorentz factor along the line of sight is 33. The same profile fixes the emission radius Rγ = 2.7×$10^{13}$ cm, which contributes 0.41 s to the 1.74 s delay between gravitational waves and gamma rays; the rest is launch and jet breakout. Comparison with the kinetic energy from the afterglow yields a radiative efficiency proportional to the local Lorentz factor: η ∝ $Γ^0$.99±0.29.
This work shows that the angular structure of a short gamma-ray burst can be extracted from two prompt spectra alone, without afterglow, ambient density, or microphysical parameters. This differs from neighboring studies that rely on months-long afterglow fitting or numerical simulations. Here, for the first time, the structure is obtained solely from prompt emission, ruling out four popular models at high significance. The faintness of GW170817 is explained not by de-beaming of a bright core but by the jet's internal structure: along the line of sight, the matter simply emits less energy.
Two bursts provide only the slope, not the profile's shape. A third burst with an independently measured Lorentz factor would turn the slope into a shape and provide the first test capable of falsifying the initial assumption of identical systems. One could also search for other burst pairs viewed at different angles to refine the index n and test its universality. Of particular interest is measuring the angular dependence of the efficiency, which is currently determined only from central values.
The result directly affects the interpretation of short gamma-ray bursts and their connection to neutron star mergers. It also offers a new method for calibrating distances and energies for gravitational-wave events with electromagnetic counterparts.
Find a third burst with a known Lorentz factor and apply the same analysis to test the assumption. Systematically compare structures derived from prompt emission and afterglow for a large sample of short bursts.
In gamma-ray burst astrophysics, the collimation mechanism and the angular energy distribution remain open questions. This work links that problem to the observed diversity of short-burst brightnesses and to the delay between gravitational waves and gamma rays. It also touches on the long-standing debate over whether GW170817's faintness stems from geometry or internal structure, offering a way to distinguish these scenarios without model-dependent afterglow assumptions.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors compared two gamma-ray bursts, GRB 090510 and GW170817, assuming they are identical systems seen at different viewing angles. From the ratio of their energies and peak energies, they derived a power-law dependence of the emitted energy on the Lorentz factor with an index of 3.76, which rules out several popular models of jet structure. The resulting profile agrees with independent estimates from the afterglow and can explain the delay between the gravitational waves and the gamma-ray emission.
Where its strength is
The strength of this work lies in the fact that it extracts the angular structure of the jet from the prompt emission alone, without fitting the afterglow or knowing the microphysical parameters. This provides an independent, model-independent way to constrain the energy release profile.
Where it could go next
Worth trying
Why this may matter
This method makes it possible to extract the angular structure of jets from prompt emission, which matters for understanding the collimation mechanism and for estimating the true rates of neutron-star merger events. It can be applied to future observations of gravitational-wave events with electromagnetic counterparts.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.58, against a usual 0.64. It looks like few have drilled nearby.
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.