Fast Radio Bursts: Origins, Discoveries, and Mysteries of the Cosmic Radio Flashes

1. Discovery of the First FRB (Lorimer Burst, 2007)

The first FRB was discovered accidentally in 2007 by Duncan Lorimer and his team while analyzing archival data, meaning it was not detected in real-time but was discovered during a later analysis of previously recorded observations, from the Parkes radio telescope in Australia.

Key Details About the Discovery:

• It was a bright and short-lived radio pulse lasting only a few milliseconds.
• This FRB came from an extragalactic source, meaning it originated far beyond our Milky Way galaxy.
• It is now known as the Lorimer Burst and is considered the first officially recognized FRB.

2. Real-Time Detection of FRBs by CHIME (Since 2019)

Starting from 2019, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) began detecting FRBs in real-time in the 400–800 MHz frequency range.

3. Key Properties of FRBs

Sporadic Nature

• FRBs occur unpredictably from different parts of the sky, making them difficult to detect and study.
• This randomness means we cannot predict when or where an FRB will appear.

High Luminosity

• FRBs are among the most luminous radio signals detected.
• Despite lasting for only milliseconds, they release an enormous amount of energy, sometimes comparable to what the Sun emits in a whole day.

Occurrence Rate

• It is estimated that there are about 1,000 FRBs occurring across the entire sky per day.
• However, we detect only a small fraction of these because our telescopes cover only a limited portion of the sky at any given time.

Duration Range

• FRBs can last anywhere between 0.1 milliseconds to 100 milliseconds.
• This duration depends on their emission mechanism and propagation effects through space.

2. Extragalactic Origin of FRBs

How Do We Know FRBs Come from Other Galaxies?

• The key evidence comes from dispersion measure (DM).
• Dispersion Measure (DM): This is the total number of free electrons that radio waves encounter as they travel through space.

• Most FRBs have such a high DM value that it cannot be explained by sources within our own galaxy.

• This strongly suggests that FRBs originate from distant galaxies beyond the Milky Way.

3. Repeating vs. One-Time FRBs (Non-Repeating FRBs)

One-Off FRBs

• Most FRBs occur only once and are never detected again.
• The cause of these one-time bursts is still uncertain, but possible explanations include:
  • Cataclysmic events (e.g., neutron star mergers, black hole formation).
  • Explosive magnetic activity in exotic astrophysical objects.

Repeating FRBs

• Some FRBs have been observed to repeat over time, meaning they come from a source that does not get destroyed after emitting a burst.
• These repeaters might be caused by highly magnetized neutron stars (magnetars) or other long-lived astrophysical objects.
• First Discovered Repeater: FRB 20121102A. It is located in a star-forming dwarf galaxy at a redshift $z=0.19$. 
• Redshift $z$ is a measure of how far away an object is; $z=0.19$ means this galaxy is about 2.5 billion light-years away.
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Periodic FRBs: FRB 20121102A & FRB 20180916B

Some Repeaters Show Periodicity

• While most FRBs appear randomly, some repeaters exhibit quasi-periodic behavior, meaning their bursts occur at regular intervals.

Two Key Periodic Repeaters:

1. FRB 20121102A

   • This FRB bursts in closely grouped clusters separated by a longer gap of ~157 days.
   • This suggests that it might be linked to a rotating neutron star or a binary system where the emission becomes visible periodically.

2. FRB 20180916B

   • This FRB follows a shorter periodic cycle of ~16 days between bursts.
   • This periodicity hints at a possible binary system where the FRB source moves in and out of alignment with Earth.

What Causes These Periodic FRBs?

• Possible explanations include:
• Neutron stars in binary systems: The periodicity could be caused by a neutron star orbiting another massive object.
• Precession of a magnetar: A strongly magnetized neutron star might wobble as it spins, leading to periodic bursts.
• Interaction with a stellar wind: If the FRB source is near a companion star, bursts may be influenced by the stellar wind.

4. The Two Main Theoretical Models of FRBs

Since FRBs last only for a few milliseconds, they must originate from extremely compact and energetic astrophysical sources. The two primary models proposed are:

A. Magnetar/Soft Gamma Repeater (SGR) Model

• Magnetars are a special type of neutron star with extremely strong magnetic fields.
• Occasionally, they undergo violent magnetic reconnections or starquakes, releasing bursts of X-rays, gamma rays, and radio waves.
• Soft Gamma Repeaters (SGRs) are a type of magnetar that emits repeated gamma-ray bursts.
• This model suggests that FRBs may be radio bursts associated with magnetar activity.
• Many studies have favored this model based on observed properties of FRBs.

B. Gravitational Collapse of a Supra-Massive Spinning Neutron Star

• Some neutron stars are formed with very high masses, making them supra-massive (heavier than the normal neutron star limit).
• If such a neutron star is spinning rapidly, centrifugal forces can temporarily prevent it from collapsing into a black hole.
• Over time, it loses rotational energy and eventually collapses, producing an intense radio burst in the final moments before forming a black hole.
• This model suggests that some one-time FRBs could be due to the catastrophic collapse of such massive neutron stars.

Evidence Supporting the Magnetar Model

A. Discovery of FRB 20200428A from a Galactic Magnetar (SGR 1935+2154)

• In April 2020, astronomers detected a low-luminosity FRB (FRB 20200428A) from within our own Milky Way.
• This FRB was accompanied by a simultaneous X-ray burst, confirming that at least some FRBs originate from magnetars.
• The source of this FRB was identified as the magnetar SGR 1935+2154, already known as a SGR (soft gamma repeater).
• This discovery strongly supports the idea that some FRBs are caused by magnetars experiencing sudden magnetic activity.

B. Why Magnetars Are a Favorable Explanation

• They are highly energetic and can produce the observed FRB brightness.
• They undergo frequent magnetic flares, which can explain the repeating nature of some FRBs.
• The FRB 20200428A event proved that magnetars can produce FRB-like signals.

Counterarguments and Limitations of the Magnetar Model

A. No FRB-SGR Connection Observed by FAST

• The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in China, the world’s largest single-dish radio telescope, has not detected any FRBs from known magnetars.
• If magnetars were the sole cause of FRBs, we would expect to see more FRBs from known magnetars, but this has not been the case.
• This suggests that magnetar-driven FRBs might be extremely rare or that another mechanism is also involved in producing FRBs.

B. Lack of High-Energy Counterparts for Most FRBs

• If FRBs were linked to magnetar activity, we would expect them to be accompanied by high-energy emissions such as X-rays or gamma rays.
• However, most observed FRBs do not show any associated X-ray or gamma-ray bursts.
• Searches for high-energy radiation from FRBs have only set upper limits, meaning any such emissions are either very weak or completely absent.

C. FRBs with Periodic Behavior Suggest Another Mechanism

• Some repeating FRBs, such as FRB 20180916B, show periodic activity, suggesting a binary system or a neutron star in a precessing orbit.
• This periodicity is not easily explained by magnetar bursts, implying that some FRBs may have a different origin.

1. Key Observational Trends in FRBs

• Repeaters vs. Non-Repeaters:
  • Repeating FRBs tend to last longer (i.e., have a longer duration) than non-repeating FRBs.
  • Repeaters have narrower spectral widths, leading to a wider variation in spectral index values.
  • The spectral index of non-repeaters is usually around 1.5, while for repeaters, it varies broadly from -2 to 1.5.

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2. Statistical Evidence for Two FRB Categories

• The study focuses on peak radio luminosity densities of non-CHIME FRBs.
• The statistical analysis suggests that FRBs fall into two distinct categories:
1. High-luminosity density FRBs (more energetic bursts).
2. Lower-luminosity density FRBs (weaker bursts).

Estimating Redshifts of Repeating and Non-Repeating FRBs

The redshift of an object is a measure of how much its light has been stretched due to the expansion of the universe. It helps determine the distance of the source and provides insights into the cosmological properties of FRBs. However, estimating redshifts for FRBs is challenging because they are short-lived and do not emit light across multiple wavelengths, making direct spectroscopic measurements difficult.

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1. How Redshifts Are Estimated for Different FRB Types

A. Repeating FRBs – Redshifts from Host Galaxies

• Some repeating FRBs have been traced back to their host galaxies, whose redshifts can be measured accurately using optical spectroscopy.
• This method provides a direct and reliable way to determine the redshift of the FRB source.
• Example: FRB 20121102A is a well-known repeater found in a dwarf galaxy at redshift $z=0.19$.

B. Non-Repeating FRBs – Redshifts from Dispersion Measures (DMs)

• For most non-repeating FRBs, their host galaxies remain unknown, so redshifts cannot be directly measured.
• Instead, their redshifts are estimated using their observed dispersion measures (DMs).
• Dispersion Measure (DM):
  • The DM of an FRB represents the total column density of free electrons along the line of sight.
  • Since intergalactic plasma contributes to the DM, astronomers use models of the electron distribution in the universe to estimate the redshift.
  • This method is less accurate than direct host galaxy redshifts but provides an approximate cosmological distance for non-repeating FRBs.

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2. Local Effects on Redshift Are Negligible

Apart from cosmological redshifts caused by the expansion of the universe, some local effects could, in theory, contribute to the overall observed redshift of an FRB. These effects include:

A. Doppler Shift Due to Motion of the Source

• If an FRB source is moving relative to the cosmic rest frame, its light experiences a Doppler shift (similar to how sound waves change pitch due to motion).
• However, even if the FRB source moves at a high speed of ~300 km/s, the corresponding Doppler shift is very small, meaning it does not significantly affect the redshift measurement.

B. Gravitational Redshift

• Light emitted from a strong gravitational field (such as near a neutron star or black hole) gets redshifted due to gravitational effects.
• However, in most cases, this contribution is small compared to the cosmological redshift and is usually ignored in FRB redshift estimations.
• Gravitational Redshift: Gravitational redshift is a phenomenon predicted by General Relativity, where light or electromagnetic radiation emitted from a strong gravitational field is shifted to longer (redder) wavelengths as it climbs out of the gravitational well. This effect occurs because gravity affects time, causing clocks in stronger gravitational fields to tick more slowly compared to those in weaker gravitational fields.
• Fast Radio Bursts (FRBs) are believed to originate from extreme astrophysical environments, such as:

1. Magnetars (Highly Magnetized Neutron Stars)
2. Neutron Star Mergers
3. Black Hole Accretion Disks

Since these objects have intense gravitational fields, the radiation emitted near their surfaces could experience gravitational redshift before escaping into space. However, in most cases, this redshift is small compared to the cosmological redshift caused by the expansion of the universe.

• Neutron Stars & Magnetars:

  • Neutron stars have very high densities, but their gravitational redshifts are typically small compared to the cosmological redshift of FRBs.
  • This means gravitational redshift does not significantly affect the observed FRB signals.

• Black Holes:

  • If an FRB originates from near a black hole, gravitational redshift could be much larger.
  • However, most FRBs do not show extreme redshifts, suggesting they are not emitted from very close to black holes.

1. What is Cosmological Redshift?

Cosmological redshift is the phenomenon where light from distant objects, such as Fast Radio Bursts (FRBs), galaxies, and quasars, is stretched to longer (redder) wavelengths due to the expansion of the universe.

2. How is Cosmological Redshift Relevant to FRBs?

Since FRBs originate from extragalactic sources, their signals travel through the expanding universe, causing their radio waves to be redshifted.

• For repeating FRBs, the redshift is determined directly from their host galaxies' spectra.
• For non-repeating FRBs, redshift is estimated using Dispersion Measure (DM), which provides a rough distance estimate based on the total number of free electrons along the line of sight.

3. Why is Cosmological Redshift Important for FRB Studies?

1. Confirms FRBs as Extragalactic Phenomena

   • The large observed redshifts indicate that most FRBs originate from distant galaxies, ruling out local sources like pulsars in the Milky Way.

2. Provides Distance Estimates

   • Using redshift, astronomers can estimate how far away an FRB occurred and compare it to other astrophysical objects.

3. Helps Understand the Early Universe

   • High-redshift FRBs can be used to probe the intergalactic medium (IGM) and cosmic evolution.

1. What is the Spectral Index (α)?

The spectral index (α) describes how the flux density Sν of an FRB changes with frequency ν according to the power law:

$$S_{\nu }\propto {\mathrm{\nu ^}}^{}$$α

• If α > 0, flux density increases with frequency.
• If α < 0, flux density decreases with frequency.
• The spectral index plays a crucial role in determining FRB luminosity and energy densities.

FRBs might not all be the same—they could be produced by two different astrophysical events, such as young magnetars or collapsing neutron stars. 

4. Difference Between Cosmological, Gravitational, and Doppler Redshift

Type of Redshift	Cause	Typical Effect on FRBs
Cosmological Redshift	Expansion of the universe	Dominates FRB observations (shifts them to longer wavelengths)
Gravitational Redshift	Strong gravity near source (e.g., neutron stars)	Usually small, negligible compared to cosmological redshift
Doppler Redshift	Motion of source relative to observer	Small, only noticeable if FRBs come from rapidly moving sources