Falling merger debris could fuel—and frustrate—a second cosmic jet

A lingering signal after a brief burst

Short gamma-ray bursts typically produce their initial emission in less than two seconds. Yet a fraction also show softer X-ray or gamma-ray emission lasting tens to hundreds of seconds. Some extended signals are dominated by nonthermal radiation, while others include a substantial thermal component resembling emission from hot material.

What sustains this activity remains uncertain. In a preprint posted to arXiv on November 6, Zhiwei Chen, Marie Cassing, Paramvir Singh and Luciano Rezzolla investigate one possibility: a secondary jet powered by debris falling back toward a black hole formed after a neutron-star merger. Their study establishes possible dynamical and energy conditions, rather than reporting a newly observed jet.

The same debris supplies fuel and resistance

Earlier simulations incorporating general relativity, magnetized fluid dynamics and neutrino transport found that some merger debris remains gravitationally bound. Although initially thrown outward, it eventually returns. At late times, this fallback can surround the remnant in an approximately spherical distribution rather than remaining concentrated around its equator.

That geometry creates competing effects. Returning material with sufficient angular momentum may settle into an orbiting accretion flow and supply a secondary jet. Meanwhile, debris returning near the rotation axis can partially refill the channel cleared by the initial jet, obstructing the later outflow.

The authors also reassess an earlier suggestion that radioactive decay alone could power thermal extended emission. Under their adopted heating and photon-diffusion assumptions, they estimate luminosities of roughly 10^43–10^44 ergs per second, below those typically inferred for extended emission. They argue that treating the local gas temperature as the effective radiation temperature overestimates the available luminosity. This motivates using fallback as fuel for renewed accretion, not just as a radioactively heated emitter.

Estimating how much material could feed the engine

The team reanalyzed material from an existing simulation of two equal-mass neutron stars, each with 1.36 times the Sun’s mass. In that simulation, the remnant collapsed into a black hole about 3.7 milliseconds after merger.

Using the debris energy and angular momentum, the researchers estimated return times and classified trajectories according to whether material would plunge directly into the black hole or avoid immediate capture. Approximately 52% of the bound fallback avoided prompt capture.

That is an upper limit on the potential accretion-flow supply, not a measurement of how much actually forms a disk. Returning streams must still dissipate orbital energy and redistribute angular momentum. The model therefore explores cases in which about 5.2%, 26% or 52% of all bound fallback joins the accretion flow.

Three possible outcomes for the secondary jet

The researchers then use a semi-analytic toy model to follow the advancing jet head through inward-moving debris. Their reference setup assumes a secondary engine starting 10 seconds after merger and operating for 100 seconds. Jet power follows the declining fallback rate, with an assumed 10% conversion efficiency from accreted rest-mass energy into one jet.

Three outcomes emerge. A sufficiently strong jet can penetrate the envelope with much of its energy remaining in the relativistic outflow. Stronger interaction with the debris instead transfers substantial energy into the surrounding cocoon, producing a cocoon-dominated breakout. If the jet’s trailing material catches its head before escape, the jet is classified as choked.

More fuel, a narrower jet or a less densely refilled polar channel favors jet-dominated escape. The strength of polar refilling remains unknown, so the authors test channels containing 1%, 10% and 100% of the undisturbed fallback density.

Compatible energies, but not a confirmed explanation

For the case with 10% polar refilling, estimated nonthermal X-ray energies are approximately 10^49–10^51 ergs, while thermal estimates mainly fall around 10^48–10^49 ergs. These concern the 0.3–10 kiloelectronvolt band and are isotropic-equivalent energies: the energies inferred if the observed radiation were emitted equally in every direction.

The estimates overlap the observed extended-emission energy range, with the thermal branch reaching its lower-energy portion. However, this comparison depends on simplified radiation prescriptions, including an assumed 1% conversion of surviving jet energy into nonthermal X-rays. Some comparison events also have assumed rather than measured redshifts.

The authors do not fit individual brightness histories or calculate detailed time-dependent radiation transport. Their fallback calibration comes from one merger simulation, and they do not self-consistently determine disk formation, secondary-jet launch timing or polar refilling. Thermal and nonthermal contributions could also coexist rather than follow a strict division.

The work is a preprint, not a peer-reviewed confirmation. Its result is a feasibility argument: fallback-fed secondary jets could provide enough energy and sometimes escape their surroundings, but whether they explain observed extended emission remains open.