Two Views of the Same Lighthouse: How We Learned the Shape of a Cosmic Jet ✛ ✎

Original: "The angular structure of the GW170817 jet from prompt emission alone"

arXiv:2609.04776v1 · 2026-09-04 · arXiv non-exclusive · independent analysis · 2 min

By comparing two identical explosions seen from different angles, astronomers reconstructed how light is distributed in an invisible jet.

A lighthouse looks different depending on where you view it from. If a boat sails straight into the beam, the light seems dazzling. If from the shore you see only the edge of the beam, the lighthouse looks dim. Two snapshots of the same lighthouse from different angles help you understand how its light is arranged. That is roughly how scientists figured out two cosmic explosions.

The first explosion, GRB 090510, 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 and turned out to be a million times fainter. For a long time, astronomers could not tell whether we were seeing it from the side or whether the jet itself was patchy.

A jet is a narrow stream of matter ejected in an explosion at nearly the speed of light. How bright it appears depends on the angle at which we view it.

The scientists proposed that both explosions are identical systems, just seen from different angles: the first along its axis, the second at about 20 degrees off-axis. They compared the brightness and color of the two bursts and obtained a simple number: the energy in the jet grows as the velocity of the material to the power of 3.76. This means the jet is not uniform: it is fast and bright at the center, but slows down and dims toward the edges. This explanation ruled out several earlier models. It turned out that the faintness of GW170817 is not a trick of viewing angle but a property of the jet itself. Along our line of sight, the material simply emits less energy. This finding helps us better understand how explosions work in the universe and why we see only a small fraction of them.

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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.

n = \frac{\ln(E_{\mathrm{iso}}^{(A)}/E_{\mathrm{iso}}^{(B)})}{\ln(E_{\mathrm{p,i}}^{(A)}/E_{\mathrm{p,i}}^{(B)})}

The ratio of the logarithms of isotropic energies and peak energies of two bursts gives the slope of the emitted energy versus Lorentz factor.

E_{\mathrm{p,i}} \propto \Gamma(\theta_{\mathrm{v}})

The peak energy of the spectrum is proportional to the Lorentz factor of the matter along the line of sight.

R_{\gamma} = 2\Gamma^2(\theta_{\mathrm{v}}) c T_{\mathrm{ang}}

The radius at which the gamma-ray burst emits is determined by the Lorentz factor and the angular spread time.

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Two Views of the Same Lighthouse: How We Learned the Shape of a Cosmic Jet ✛ ✎