One of the universe's deep mysteries just got a lot stranger. Astrophysicists have discovered a clue that could help explain why, every once in a while, superfast bursts of radio waves flash across Earth from deep space. But the clue — a repeating 16-day pattern in one of the bursts, undermines one of the most popular theories for where the bursts are coming from.
Fast radio bursts (FRBs) have likely happened for billions of years. But humans only discovered them in 2007, and have detected only a few dozen of them since. And in June 2019, astronomers finally tracked an FRB to its home galaxy.
But no one knows what causes them. Because these bursts are so rare, unusual and bright — considering that they're visible from billions of light-years across space — physicists have tended to assume they come from a cataclysmic event, such as the collision of stars.
This repeating pattern, however, suggests that something else is going on, that there's some sort of natural machine in the universe for pumping regular shrieks of radio energy across space.
Researchers looking at data from the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB) first spotted this FRB, known as FRB 180916.J0158+65, in 2019. In January 2020, they published a paper in the journal Nature that reanalyzed old data and found more than one burst from FRB 180916.J0158+65. They traced this FRB back to a relatively nearby spiral galaxy. What's new in this latest paper, published Feb. 3 to the arXiv database, is the regular pattern in the bursts. The FRB, they found, goes through four-day cycles of regular activity, bleating out radio waves into space on an almost hourly basis. Then it goes into a 12-day period of silence. Sometimes the source seems to skip its usual four-day awake periods, or lets out only a single burst. CHIME/FRB is able to watch the FRB only some of the time, they noted, so it's likely the detector misses many FRBs during the awake period.
No one knows what this pattern means, the researchers noted in a statement, but this pattern doesn't fit neatly into any existing explanations for FRBs.
In general, patterns like this in astrophysics are often related to a spinning object or orbiting celestial bodies. Neutron stars often seem to strobe regularly from the perspective of X-ray detectors on Earth, because hot spots on their surface spin in and out of view like a lighthouse beacon. And tiny planets may dim the light of the stars they orbit everytime they pass between that star and Earth.
In other words, for astrophysics, patterns tend to indicate rotation. But no one knows if this pattern governs all FRBs or just some of them.
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Currently, it appears there are two types of engines driving the FRB's. One could be the result of the particle acceleration by the magnetic fields of a neutron star, the remnant of a super-massive star after a previous supernova explosion has blown the surface off the star leaving only a ball of packed neutrons. The other could be the radio signals emitted from theoretical cosmic strings.Reply
To explain more fully, FRBs may be created in the closed field line regions of magnetar (a particular type of neutron star with an extremely strong magnetic field) magnetospheres. Crustal neutron slippage on the surface of the magnetar causes magnetic reconnection and thus particle acceleration, producing coherent emission. To allow emission to escape, the magnetars must have a low-density plasma in the closed field line regions, and hence must have low magnetospheric twist. Signals above a few MeV (million electron volts) are expected to be contained by photon splitting and magnetic pair production in the magnetosphere.
It is also suspected that another signature of FRB's may arise from cosmic strings coming from non-negligible electromagnetic radiation emitted by cusps, portions of the string which double back on themselves due to its considerable size, l c ∼ r 1/3 R 2/3 , where r is the thickness of the string and R is the radius of the string loop. Given that cosmic strings have yet to be observed, and assuming cusp decay produces radiation across all frequencies, FRBs could, in principle, provide a possible observational testing ground for cosmic strings' signatures.
The causation of FRB's is taking us to the very boundaries of astrophysics and string theory. It is truly an exciting time in the world of physics, my favorite subject outside of history.
possibly aliasing effect in the instrument during sampling.Reply
Aliasing is an effect which causes different signals to become indistinguishable from each other during sampling. Aliasing is characterized by the altering of the output compared to the original signal because resampling or interpolation resulted in a lower wave resolution in the audio signal. Anti-aliasing filters are routinely used to correct this problem in both audio and video.Reply
In audio, aliasing is the result of a lower resolution sampling, which translates to poor sound quality and static. This occurs when the audio signal is sampled at a lower resolution than the original recording. When the sinusoidal audio wave is converted to a digital wave using a lower resolution sample, only a few specific points of the wave are taken as data. This results in a wave with a lower frequency than the original, translating to a loss of data and audio quality.
The following is from "A Repeating Fast Radio Burst" by L. G. Spitler, P. Scholz, J. W. T. Hessels, et al:
"FRB 121102 was discovered in the PALFA survey, a deep search of the Galactic plane
at 1.4 GHz for radio pulsars and fast radio bursts (FRBs) using the 305-m William E. Gordon Telescope at the Arecibo Observatory and the 7-beam Arecibo L-band Feed Array (ALFA).
...the (Arecibo) data were processed using standard radio-frequency interference (RFI) excision, dispersion removal, and single-pulse-search algorithms implemented in the PRESTO software suite and associated data reduction pipelines."
(PRESTO is a large suite of pulsar search and analysis software developed by Scott Ransom (Harvard-Smithsonian Center for Astrophysics) mostly from scratch. It was primarily designed to efficiently search for binary millisecond pulsars from long observations of globular clusters (although it has since been used in several surveys with short integrations and to process a lot of X-ray data as well). It is written primarily in ANSI C, with many of the recent routines in Python. According to Steve Eikenberry, PRESTO stands for: PulsaR Exploration and Search TOolkit!)
The data processing suite used at Arecibo, of the ones located, makes no mention of RF (radio frequency) aliasing, where aliasing seems to apply to audio and video only and not to radio signals. If these radio waves are later converted to audio, their output would be synthetic in nature and not prone to aliasing.