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X-ray flares show GRB 220706A's engine still running 27 rest-frame days after the burst
Astronomers led by Benjamin Gompertz found GRB 220706A's central engine still flaring 27 rest-frame days after the burst, about 20 days past the old record. In their preprint, black hole accretion and magnetar models both have to be stretched to explain an engine that ran that long.
The Scientist · Science desk
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What happened
- GRB 220706A was first detected on July 6, 2022, and kept flaring in X-rays for about 51 days as seen from Earth.
- The team followed it with Swift, NICER, Chandra, four ground-based optical telescopes, NOEMA and the VLA, from about 100 seconds after trigger to Oct. 5, 2023.
- Its optical light was much fainter than its X-ray brightness predicted, making it a dark burst, most likely because of dust in the host galaxy.
- About 17 days in, extra optical light consistent with a supernova appeared, and depending on the dust correction it may be bright enough to count as superluminous.
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Why it matters
- constraint Any engine proposed for ultra-long bursts now has to keep a central source producing X-ray flares for at least 27 days in the burst's own frame.
- constraint The 27.25-day figure comes from the last flaring epoch observed, so it is a floor on the engine's lifetime, and the timing problem for each model can only grow.
- precedent If the superluminous reading survives the dust correction, this would be the first gamma-ray burst seen with a superluminous supernova, and models of ultra-long bursts would have to explain that pairing.
The 27-day figure is the observed one, corrected for distance. Light from redshift 0.8577 [2] reaches us with the intervals between signals stretched by the expansion of the universe [19]. Divide the roughly 51 days of observed X-ray flaring [5] by 1 + z, or 1.8577, and you get about 27.5 days [20]. The team's own figure is a little lower. "Corrected for redshift, this last epoch of flaring suggests that the central engine of GRB 220706A is still active at 27.25 rest-frame days after trigger," the authors wrote [6].
The record is for how late the flares came. GRB 210204A, the previous holder, had its latest flares about 20.6 rest-frame days earlier [7]. That puts the old mark near 6.65 days [21], with the new one about four times later [22]. The thing the 27-day figure doesn't tell you is how long the main burst lasted. On that measure GRB 220706A ran about 15.6 hours and ranks eighth among 549 bursts analyzed [8], roughly the top 1.5 percent [23].
Long bursts are usually blamed on a massive star collapsing into a black hole [18]. The ultra-long subclass has only a handful of confirmed members, and what causes them is still debated [17]. The team found black hole accretion from a massive, extended progenitor star plausible, but said it pushes standard models to their limits [12]. A magnetar engine, which has worked for some other ultra-long bursts, would need a lot of parameter adjustment here [13]. The objection is about timing: "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," the authors wrote [14].
A tidal disruption, where a black hole tears a star apart, is geometrically possible because the burst sits at the center of its galaxy [15]. In the team's assessment, none of the explanations fits perfectly [16]. I think the burst tightens the timing requirement on every candidate without yet ruling one out. The analysis so far is a preprint, posted to arXiv on Sept. 18 [3].
What to watch
- Whether follow-up analysis of the day-17 light confirms a superluminous supernova once the host galaxy's dust is better constrained.
- Whether the journal version of the Gompertz paper keeps the 27.25-day figure and its assessment that black hole accretion, magnetar and tidal disruption models all fit imperfectly.
- Another ultra-long burst with late X-ray flares and long follow-up, to test whether 27 rest-frame days is an outlier or common for the subclass.