Swift sGRB Analysis of 39 Events Finds No Evidence for Exploding Primordial Black Holes’ Backwards Gamma-Ray Bursts

The search for exploding primordial black holes represents a compelling frontier in astrophysics, potentially revealing insights into the very early universe and the nature of gravity. Stefano Profumo from the Santa Cruz Institute for Particle Physics and University of California, Santa Cruz, along with Kally Wen from Lynbrook High School, recently conducted a systematic investigation of short gamma-ray burst catalogues, seeking the distinctive signature of a black hole undergoing final-stage Hawking radiation. This research focuses on identifying “backwards bursts”, brief and intense flashes of energy predicted to accompany terminal black hole evaporation, characterised by rapidly increasing brightness and minimal lingering afterglow. By developing a detailed modelling framework and comparing theoretical predictions with observations from 39 well-characterised bursts detected by the Swift satellite, the team establishes new constraints on the prevalence of exploding primordial black holes, offering valuable insights into the conditions of the early universe and complementing existing searches for these exotic objects.

Primordial Black Hole Evaporation in GRB Catalogs

This research explores the possibility of detecting primordial black holes (PBHs) through the gamma rays emitted as they evaporate, a process predicted by Hawking radiation. PBHs, formed in the early universe, are considered potential candidates for dark matter, and their evaporation could produce detectable signals. Scientists leveraged existing catalogs of gamma-ray bursts (GRBs) as a potential source of these evaporation signatures. The core of the analysis involves creating theoretical models of what a PBH evaporation event would look like in terms of gamma-ray emission, considering the energy spectrum and duration.

These models were then compared to data from gamma-ray telescopes, including Swift, Fermi, and the Interplanetary Network, to search for matching signals. Researchers employed statistical tests to determine whether any observed GRBs could be consistent with being PBH evaporation events, or if they were more likely to originate from standard astrophysical processes. The study did not find any statistically significant evidence for PBH evaporation events within the analyzed GRB catalogs. Despite this null result, the research demonstrates a robust methodology for searching for these signatures in gamma-ray data, providing a framework for future investigations with more sensitive instruments.

The non-detection allows scientists to place limits on the possible abundance and mass range of PBHs, ruling out certain combinations of properties that would have produced detectable signals. These results contribute to the ongoing effort to constrain the properties of dark matter candidates, including PBHs. The research emphasizes the need for more sensitive gamma-ray telescopes to improve the chances of detecting PBH evaporation events, with instruments like HAWC, H. E. S. S., and future missions being crucial. Refining the theoretical models of PBH evaporation, and combining gamma-ray observations with other types of data, such as gravitational waves and cosmic rays, could also enhance future searches.

No Black Hole Evaporation Signals Found

Scientists conducted a systematic search for the distinctive signal of terminal black hole evaporation within catalogs of short gamma-ray bursts, seeking a “backwards burst”, a short, spectrally hard transient expected to exhibit monotonically increasing flux and minimal afterglow. The research team developed a forward-modeling framework to directly compare theoretical light curves from evaporating black holes with observed burst data, accounting for detector response and particle content. Analyzing 35 well-characterized short gamma-ray bursts detected by the Swift satellite, lacking detected afterglows, the study revealed that all events exhibited fast-rise, slow-decay temporal profiles inconsistent with the predicted black hole evaporation signature. Model comparison using established statistical criteria decisively favored conventional GRB models, either FRED or ERCA, over the primordial black hole template for every burst analyzed, leading to an upper bound on the local rate of terminal primordial black hole explosions.

Detailed analysis of temporal characteristics revealed that the observed bursts consistently displayed a rapid rise in brightness followed by a slower decay, a pattern opposite to the slow-rise, fast-decay profile predicted for terminal black hole evaporation. The research team also examined four additional GRB events detected by both Swift and Fermi, and four more detected with the Interplanetary Network, finding consistent results. This study establishes a robust template-matching approach that can be scaled to larger multi-instrument catalogs, providing valuable insight into the potential contribution of primordial black holes to the observed gamma-ray background.

No Evaporating Black Holes Found in Bursts

This research presents a systematic investigation into the possibility of identifying terminal evaporation events from primordial black holes within catalogs of short gamma-ray bursts. The team developed a detailed forward-modeling framework to compare predicted light curves from evaporating black holes with observed burst data, accounting for detector response and particle content. Analyzing a sample of well-characterized short gamma-ray bursts, the study found that none exhibited the distinctive temporal profile expected from a black hole undergoing final-stage evaporation, specifically, a fast rise and slow decay without significant afterglow. Model comparison using established statistical criteria consistently favored conventional burst models over the predicted black hole evaporation template for every event examined, leading to an upper bound on the local rate of terminal primordial black hole explosions.

The methodology established provides a robust template-matching approach that can be applied to larger datasets from multiple instruments, offering a foundation for future investigations into this unique signature of gravity and early-universe physics. The authors acknowledge that the estimated horizon distance for detecting these events, and therefore the derived upper bound on the explosion rate, relies on assumptions about the energy emitted during evaporation and the sensitivity of the detectors used. Future research could refine these estimates through more detailed modeling of the evaporation process and analysis of data from additional instruments, potentially extending the search to a wider range of black hole masses and distances.