Researchers studying six very bright gamma-ray bursts discovered that the pulses composing these GRBs exhibited complex, time-reversible wavelike structures. In other words, each GRB pulse shows an event in which time appeared to repeat itself backwards.
Above – A model of temporally-reversed pulse structure. An impactor (red) produces variable radiation as it travels through axisymmetric, stretched clouds (blue).
They noticed this “mirroring” effect after realizing the “smoke” of limited instrumental sensitivity smeared out GRB light, giving moderately bright pulse light curves a three-peaked appearance and faint pulse light curves the shape of a simple bump. Only the brightest GRB pulse light curves exhibit the time-reversed wavelike structures.
Hakkila says that the time-reversible light curves do not necessarily violate natural laws of cause and effect. The research team believes that the most natural explanation is that a blast wave or a rapidly-ejected clump of particles radiates while being reflected within an expanding GRB jet or while moving through a symmetric distribution of clouds.
This discovery is intriguing, says Hakkila, in that it does not appear to have been predicted by theoretical models. Despite this, the discovery should provide astrophysicists with new tools in understanding the final death throes of massive stars and the physical processes that accompany black hole formation.
GRBs are the intrinsically brightest explosions known in the universe. They last from seconds to minutes, and originate during the formation of a black hole accompanying a beamed supernova or colliding neutron stars. The narrow beam of intense GRB radiation can only be seen when the jet points toward Earth, but such an event can be seen across the breadth of the universe.
Researchers demonstrate that the `smoke’ of limited instrumental sensitivity smears out structure in gamma-ray burst (GRB) pulse light curves, giving each a triple-peaked appearance at moderate signal-to-noise and a simple monotonic appearance at low signal-to-noise. They minimize this effect by studying six very bright GRB pulses (signal-to-noise generally over 100), discovering surprisingly that each exhibits complex time-reversible wavelike residual structures. These `mirrored’ wavelike structures can have large amplitudes, occur on short timescales, begin/end long before/after the onset of the monotonic pulse component, and have pulse spectra that generally evolve hard to soft, re-hardening at the time of each structural peak. Among other insights, these observations help explain the existence of negative pulse spectral lags, and allow us to conclude that GRB pulses are less common, more complex, and have longer durations than previously thought. Because structured emission mechanisms that can operate forwards and backwards in time seem unlikely, they look to kinematic behaviors to explain the time-reversed light curve structures. They conclude that each GRB pulse involves a single impactor interacting with an independent medium. Either the material is distributed in a bilaterally symmetric fashion, the impactor is structured in a bilaterally symmetric fashion, or the impactor’s motion is reversed such that it returns along its original path of motion. The wavelike structure of the time-reversible component suggests that radiation is being both produced and absorbed/deflected dramatically, repeatedly, and abruptly from the monotonic component.