Astronomers Record a Gamma-Ray Burst Whose Engine Kept Firing for 27 Days, a New Record
GRB 220706A kept flaring in X-rays for about 51 days after detection, and its central engine stayed active roughly 20 days longer than any burst on record. No single explanation fits every detail.
Astronomers have observed the longest-lasting central engine activity ever recorded from a gamma-ray burst. The burst, designated GRB 220706A, occurred at a redshift of 0.8577, and its engine stayed active for about 27 days in the burst's own rest frame. That is around 20 days longer than the previous record. The paper was posted to the arXiv preprint server on Sept. 18 and explores several mechanisms that could be powering the event.
Gamma-ray bursts are among the most energetic explosions in the universe. They are typically linked either to the collapse of massive stars into black holes, which produces long-duration bursts that often come with a supernova, or to mergers of neutron stars, which produce short-duration ones. Ultra-long bursts are a rare subclass in which the object driving the explosion stays active far longer than in typical events. Only a handful of confirmed examples exist, and what causes them is still debated.
GRB 220706A was first detected on July 6, 2022, and kept flaring in X-rays for nearly a month. A team led by Benjamin P. Gompertz of the University of Birmingham studied it across the spectrum. They used X-ray data from the Swift X-ray Telescope, the NICER instrument and the Chandra X-ray Observatory, optical and infrared imaging from four ground-based telescopes, and radio and millimeter observations from NOEMA and the Karl G. Jansky Very Large Array. The follow-up covered the period from about 100 seconds after the initial burst to Oct. 5, 2023.
The team found that GRB 220706A ranks eighth-longest among 549 gamma-ray bursts analyzed. Its initial burst lasted about 15.6 hours, longer than even the previously known archetypal ultra-long bursts. X-ray observations showed repeated flaring that continued for about 51 days after the burst was first seen. Because the burst sits at a redshift of 0.8577, the expansion of the universe stretches that clock, and in the burst's own frame the engine activity lasted at least 27 days.
The researchers then weighed what could keep an engine running that long. A newborn magnetar, a rapidly spinning neutron star with an extremely strong magnetic field, is one candidate, but the team argued against it. As they note, "the typical timescales of such flares are of the order of seconds to minutes, and nascent magnetars are not expected to have fully formed crusts at timescales relevant to GRB 220706A."
A tidal disruption event, in which a black hole tears apart a star, is geometrically possible given that the burst sits at the center of its host galaxy. But its X-ray and optical behavior looked more like a standard gamma-ray burst. The team ultimately favored the death of a star through collapse paired with a supernova, based on similarities to a previously confirmed case.
They are open about the limits of that conclusion. No single proposed mechanism clearly explains every observed detail. The extra optical light that they interpret as a supernova could instead be an unusual optical counterpart to the ongoing X-ray flaring, mimicking a supernova's brightness and timing by coincidence.
The result is a new benchmark for how long a stellar explosion's engine can keep running. Because the work is a preprint, it has not yet gone through peer review.
