Long-Lived Radio Echo Signals Extreme Cosmic Explosion, Potential "Hidden" Gamma-Ray Burst
New research describes the discovery of a long-lived radio echo from an extreme cosmic explosion. This observation, accepted for publication in The Astrophysical Journal, represents a potential example of a "hidden explosion."
Specifically, it could be the radio afterglow of a powerful gamma-ray burst that was initially unobserved.
An alternative explanation for the phenomenon involves a star being disrupted by an intermediate-mass black hole—a class of black holes that has been challenging to detect. Both scenarios indicate the observation of a rare, extreme cosmic event's aftermath.
Understanding Gamma-Ray Bursts
Gamma-ray bursts (GRBs) are short, intense jets of high-energy radiation, releasing energy equivalent to the Sun's entire lifespan in seconds. They originate from the collapse of massive stars forming black holes. While these jets are emitted multidirectionally, only those pointed towards Earth are initially observed as flashes.
When the emission is directed away, only a slowly fading afterglow is detectable, known as an "orphan afterglow." These orphan afterglows have been theorized for decades but have been difficult to locate due to the absence of an initial high-energy signal, requiring astronomers to survey vast sky regions.
Discovery by ASKAP Telescope
The Australian SKA Pathfinder (ASKAP), a 36-antenna radio telescope, was used to scan wide areas of the sky for long-lived radio transients. During one survey, a previously unobserved radio source, designated ASKAP J005512-255834, was identified.
The source rapidly brightened, emitting 10³² Watts of radio energy, comparable to billions of Suns, before gradually fading. This behavior is distinct from typical radio transients, which usually evolve quickly or flare repeatedly. Instead, ASKAP J005512-255834's activity is consistent with the prolonged echo of a single, powerful explosion.
Observations indicated that ASKAP J005512-255834 was bright in radio wavelengths but showed minimal signal in visible light or X-rays. This characteristic aligns with expectations for an orphan afterglow: the expanding, fading emission of a cosmic jet initially not aimed at Earth, which becomes detectable as it decelerates and spreads.
Location and Potential Causes
ASKAP J005512-255834 is located in an irregularly structured, star-forming galaxy approximately 1.7 billion light-years from Earth. Its position within the galaxy is offset from the central nucleus, appearing to be within a compact star-forming region, possibly a nuclear star cluster. This location prompts further investigation into the environments that host such powerful cosmic events.
Researchers considered and ruled out several alternative explanations, including stars, pulsars, and supernovae, due to the source's unique characteristics. The only other scenario that accounts for the observed radio behavior is the tidal disruption of a star by an intermediate-mass black hole (IMBH). IMBHs are a rare class of black holes positioned between stellar-mass and supermassive black holes. While such events are considered rare at radio wavelengths, this possibility cannot be entirely excluded. A confirmation of this scenario would represent the first discovery of its kind, on par with identifying an orphan gamma-ray burst.
Future Implications
ASKAP J005512-255834 has been identified as a strong candidate for an orphan gamma-ray burst afterglow. This discovery was made by specifically searching for the long-lived radio echoes of previously unobserved explosions. Researchers intend to apply this method to find more orphan afterglows, aiming to develop a more comprehensive understanding of the gamma-ray burst population, including those not initially detected by their high-energy flashes.