Geometry of the precessing, magnetically dominated, structured-jet model proposed for GRB 250702B. (Image by SHAO)

Prof. AN Tao from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences has proposed a novel "precessing magnetic jet engine" model to explain the peculiar gamma-ray burst (GRB) 250702B, a rare cosmic explosion discovered on July 2, 2025. 

This GRB exhibited periodic flares approximately every 47 minutes over more than three hours. The new model elucidates the physical origin of this "heartbeat" and resolves the mysteries surrounding its extremely hard spectrum and apparent excess energy.  

Results were published in the Astrophysical Journal Letters on December 2.

GRB 250702B was detected by high-energy observatories, including the Fermi satellite and Konus-Wind. Its uniqueness lies in its temporal structure. The entire burst lasted approximately 3.2 hours and included three distinct, intense gamma-ray pulses with intervals that were integer multiples of a base period of about 2,825 seconds. Interestingly, approximately one day prior to this event, China's "Einstein Probe" satellite detected a softer X-ray burst at the same location, acting as a precursor to the main event. This combination of "early warm-up plus hour-scale heartbeat" is extremely rare in GRB observations.

GRBs are among the most violent explosions in the universe. Existing theories either could not explain the precise timing or resulted in an unnatural "energy monster." Thus, a new model was urgently needed to offer a more reasonable explanation.

To resolve these puzzles, Prof. AN Tao proposed a new comprehensive picture in which a rapidly spinning black hole produces a slowly rotating "cosmic lighthouse."

Specifically, the model envisions a black hole (with a mass several times that of the Sun) surrounded by a slightly tilted, thick accretion disk feeding it matter. Due to general relativity effects, this thick disk does not rotate simply but precesses like a wobbling gyroscope. As the black hole accretes matter, it launches a highly magnetized, relativistic jet. This jet is not uniform, it resembles a layered flame with a "spine-sheath" structure: a narrow, fast, and bright core (spine) surrounded by a wider, slower, and fainter layer (sheath).

If the jet were viewed directly along its axis, it would appear as a continuously bright gamma-ray source. However, for GRB 250702B, the Earth's line of sight is slightly offset from the jet axis. As the jet slowly precesses, the narrow, bright "spine" only sweeps across our line of sight briefly, once per precession cycle. This continuous engine appears as a cosmic heartbeat flashing every 47 minutes. Throughout the 3.2-hour activity, the truly bright gamma pulses last only about 100 seconds each, resulting in a "duty cycle" of about 2.6%, which is a natural consequence of this geometric effect.

The model also explains why GRB 250702B appeared so energetic. The study suggests that the jet is dominated by strong magnetic fields, acting as an efficient "cosmic generator" capable of naturally producing the observed hard-spectrum and bright gamma rays without requiring extreme modifications to conventional models. By adopting a structured jet in which the jet core is brighter and more energetic than the outer layers, and by considering a mildly off-axis viewing geometry, the model reproduces the observed afterglow while keeping the intrinsic energy within the normal range of long GRBs. This adjustment brings the calculated intrinsic energy back within the normal range for long GRBs.

Essentially, GRB 250702B is no longer an inexplicable monster, but an extreme yet understandable case of a bright long GRB.

One significant implication of this work is that it does not strictly specify the progenitor of GRB 250702B (e.g., micro-tidal disruption events where a stellar-mass black hole disrupts a star, a black hole transpiring into the envelope of a helium star, the tidal disruption of a white dwarf by an intermediate-mass black hole, and ultra-long GRBs, among others). Instead, it provides a generalized "engine landscape." As long as a tilted thick disk forms around a black hole and launches a highly magnetized jet, the "precessing jet + spine-sheath jet structure + geometric viewing effect" mechanism can operate. This indicates that the new model has broad applicability and the potential to explain a wider range of gamma-ray burst phenomena.

Prof. AN outlined several predictions for verification through future observations: High-time-resolution gamma-ray polarization observations should reveal a regular rotation of the polarization angle coinciding with the "heartbeat" cycle. Frequency domain analysis of light curves should reveal fingerprints of this period, even during relatively "quiet" phases. In later stages, Very Long Baseline Interferometry (VLBI) observations could directly resolve the angular size and shape of the jet, providing direct visual evidence for the model.

He noted that they will continue to use the "Einstein Probe" X-ray telescope, the Five-hundred-meter Aperture Spherical radio Telescope (FAST), and international large radio telescope arrays to conduct long-term, multi-wavelength follow-up observations of this source and similar extreme explosive phenomena, aiming to further unveil the secrets behind the universe's most violent engines.