Tag Archives: #fastradiobursts

CHIME Telescope Detects More Than 500 Mysterious Fast Radio Bursts in its First Year of Operation (Astronomy)

Observations quadruple the number of known radio bursts and reveal two types: one-offs and repeaters.

To catch sight of a fast radio burst is to be extremely lucky in where and when you point your radio dish. Fast radio bursts, or FRBs, are oddly bright flashes of light, registering in the radio band of the electromagnetic spectrum, that blaze for a few milliseconds before vanishing without a trace.

These brief and mysterious beacons have been spotted in various and distant parts of the universe, as well as in our own galaxy. Their origins are unknown, and their appearance is unpredictable. Since the first was discovered in 2007, radio astronomers have only caught sight of around 140 bursts in their scopes.

Now, a large stationary radio telescope in British Columbia has nearly quadrupled the number of fast radio bursts discovered to date. The telescope, known as CHIME, for the Canadian Hydrogen Intensity Mapping Experiment, has detected 535 new fast radio bursts during its first year of operation, between 2018 and 2019.

Scientists with the CHIME Collaboration, including researchers at MIT, have assembled the new signals in the telescope’s first FRB catalog, which they will present this week at the American Astronomical Society Meeting.

The new catalog significantly expands the current library of known FRBs, and is already yielding clues as to their properties. For instance, the newly discovered bursts appear to fall in two distinct classes: those that repeat, and those that don’t. Scientists identified 18 FRB sources that burst repeatedly, while the rest appear to be one-offs. The repeaters also look different, with each burst lasting slightly longer and emitting more focused radio frequencies than bursts from single, nonrepeating FRBs.

These observations strongly suggest that repeaters and one-offs arise from separate mechanisms and astrophysical sources. With more observations, astronomers hope soon to pin down the extreme origins of these curiously bright signals.

“Before CHIME, there were less than 100 total discovered FRBs; now, after one year of observation, we’ve discovered hundreds more,” says CHIME member Kaitlyn Shin, a graduate student in MIT’s Department of Physics. “With all these sources, we can really start getting a picture of what FRBs look like as a whole, what astrophysics might be driving these events, and how they can be used to study the universe going forward.”

Seeing flashes

CHIME comprises four massive cylindrical radio antennas, roughly the size and shape of snowboarding half-pipes, located at the Dominion Radio Astrophysical Observatory, operated by the National Research Council of Canada in British Columbia, Canada. CHIME is a stationary array, with no moving parts. The telescope receives radio signals each day from half of the sky as the Earth rotates.

A sky map of FRBs based on CHIME detections reveals bursts distributed evenly across the night sky. Credits:Image: Courtesy of CHIME

While most radio astronomy is done by swiveling a large dish to focus light from different parts of the sky, CHIME stares, motionless, at the sky, and focuses incoming signals using a correlator — a powerful digital signaling processor that can work through huge amounts of data, at a rate of about 7 terabits per second, equivalent to a few percent of the world’s internet traffic.

“Digital signal processing is what makes CHIME able to reconstruct and ‘look’ in thousands of directions simultaneously,” says Kiyoshi Masui, assistant professor of physics at MIT, who will lead the group’s conference presentation. “That’s what helps us detect FRBs a thousand times more often than a traditional telescope.”

Over the first year of operation, CHIME detected 535 new fast radio bursts. When the scientists mapped their locations, they found the bursts were evenly distributed in space, seeming to arise from any and all parts of the sky. From the FRBs that CHIME was able to detect, the scientists calculated that bright fast radio bursts occur at a rate of about 800 per day across the entire sky — the most precise estimate of FRBs overall rate to date.

“That’s kind of the beautiful thing about this field — FRBs are really hard to see, but they’re not uncommon,” says Masui, who is a member of MIT’s Kavli Institute for Astrophysics and Space Research. “If your eyes could see radio flashes the way you can see camera flashes, you would see them all the time if you just looked up.”

Mapping the universe

As radio waves travel across space, any interstellar gas, or plasma, along the way can distort or disperse the wave’s properties and trajectory. The degree to which a radio wave is dispersed can give clues to how much gas it passed through, and possibly how much distance it has traveled from its source.

For each of the 535 FRBs that CHIME detected, Masui and his colleagues measured its dispersion, and found that most bursts likely originated from far-off sources within distant galaxies. The fact that the bursts were bright enough to be detected by CHIME suggests that they must have been produced by extremely energetic sources. As the telescope detects more FRBs, scientists hope to pin down exactly what kind of exotic phenomena could generate such ultrabright, ultrafast signals.

Scientists also plan to use the bursts, and their dispersion estimates, to map the distribution of gas throughout the universe.

“Each FRB gives us some information of how far they’ve propagated and how much gas they’ve propagated through,” Shin says. “With large numbers of FRBs, we can hopefully figure out how gas and matter are distributed on very large scales in the universe. So, alongside the mystery of what FRBs are themselves, there’s also the exciting potential for FRBs as powerful cosmological probes in the future.”

This research was supported by various institutions including the Canada Foundation for Innovation, the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto, the Canadian Institute for Advanced Research, McGill University and the McGill Space Institute via the Trottier Family Foundation, and the University of British Columbia.

Featured image: The large radio telescope CHIME, pictured here, has detected more than 500 mysterious fast radio bursts in its first year of operation, MIT researchers report. Credits:Image: Courtesy of CHIME


Provided by MIT

Famous Fast Radio Burst FRB20180916B Just Barely Lets Itself be Captured (Planetary Science)

Two international teams of astronomers (with significant Dutch involvement) have published two scientific papers with new information about the famous fast radio burst FRB20180916B. In a study published in the Astrophysical Journal Letters, they measured the radiation from the bursts at the lowest possible frequencies. In a study published in Nature Astronomy, they examined the bursts in the greatest possible detail. While the articles provide new information, they also raise new questions.

In 2007, the first fast radio burst (FRB) was discovered. But what exactly causes the bursts is not yet clear. Since 2020, scientists have suspected a connection to strongly magnetic neutron stars called magnetars. One of the best-known fast radio bursts is FRB20180916B. This FRB was discovered in 2018 and is only 500 million light-years away from us in another galaxy. The FRB is the closest so far and has a burst pattern that repeats every 16 days: four days of bursts, 12 days of relative quiet. That predictability makes it an ideal object for researchers to study.

Lowest radio signals ever

An international team of researchers led by Ziggy Pleunis (a graduate of the University of Amsterdam, now McGill University, Montreal, Canada) studied the FRB with the European network of LOFAR radio telescopes. They had tuned the LOFAR antennas to between 110 and 188 MHz. Those are almost the lowest possible frequencies the telescope can receive. They caught 18 bursts. This was unexpected because FRBs usually transmit in high frequencies. FRB20180916B thus breaks the low-frequency record. Incidentally, the researchers suspect that the burst emits radiation in even lower frequencies and will be looking for that in the near future.

Besides records, the observations also provide new insights. The low-level radio emission was quite clean and arrived later than bursts with higher radio emission. Co-author Jason Hessels (Netherlands Institute for Radio Astronomy ASTRON and University of Amsterdam) says, “At different times, we see radio bursts with different radio frequencies. Possibly the FRB is part of a binary star. If so, we would have a different view at different times of where these enormously powerful bursts are generated.”

Artistic view of the Effelsberg telescope pointing its dish at the galaxy 500 million light-years from Earth where the famous fast radio burst FRB20180916B sends out bursts of flashes with regularity. Credit: Daniëlle Futselaar/ASTRON/HST

A team of researchers led by Kenzie Nimmo (ASTRON and University of Amsterdam, the Netherlands) used the European VLBI network of radio telescopes, which includes one of ASTRON’s 12 Westerbork telescopes in Drenthe and the 100-meter Effelsberg telescope in Germany. They looked in the greatest detail ever at the so-called polarized microstructure of the eruptions. The astronomers saw that the burst pattern of FRB20180916B varied from microsecond to microsecond. The most logical explanation for the variation seems to be a “dancing” magnetosphere enveloping a neutron star.

Featured image: Artistic view of LOFAR’s so-called Superterp in Drenthe, the Netherlands, where low frequency radio waves from the fast radio burst FRB20180916B were captured. The FRB is located in a spiral galaxy 500 million light years from Earth. Credit: Daniëlle Futselaar/ASTRON/HST


Reference: (1) LOFAR Detection of 110–188 MHz Emission and Frequency-Dependent Activity from FRB 20180916B. By: Z. Pleunis et al. In: The Astrophysical Journal Letters, 9 April 2021. iopscience.iop.org/article/10. … 847/2041-8213/abec72
Preprint: arxiv.org/abs/2012.08372 (2) Highly polarized microstructure from the repeating FRB 20180916B. By: K. Nimmo et al. In: Nature Astronomy, 22 March 2021. dx.doi.org/10.1038/s41550-021-01321-3
Preprint: arxiv.org/abs/2010.05800


Provided by Netherlands Research School for Astronomy

Could FRB’s are Produced By Axion Stars? (Astronomy)

Summary:

◉ According to Iwazaki and colleagues, FRBs arise from the collisions between axion stars and neutron stars.

◉ Axions are one of most promising candidates of dark matter. A prominent feature of axions is that they are converted to radiations under strong magnetic fields. The axions form axion stars known as oscillaton made of axions bounded gravitationally.

◉ According to them, when the axion stars collide with neutron stars having strong magnetic fields, the electric fields are generated, which make electrons in atmospheres of neutron stars coherently oscillate. Thus, the electrons emit coherent radiations with the frequency given by the axion mass. Since the electrons are much dense in the atmospheres, the large amount of radiations with the frequency ma/2π ≃ 2.4 GHz can be produced in the collisions. The total amount of the energy of the radiations is given by 10-¹²M (10km/10²km)² ∼10⁴³GeV, where the radii of the neutron stars and the axion stars are supposed to be 10km and 10²km, respectively. This is their production mechanism of FRBs.

◉ They also proposed that such radiations are also produced by the collision of white dwarfs and axion stars. But, have wider bandwidths to that of neutron stars. These features can be observable only if the white dwarfs have very strong magnetic fields ≥ 10^9G.


Fast Radio Bursts have recently been discovered at around 1.4 GHz frequency. The durations of the bursts are typically a few milliseconds. The origin of the bursts has been suggested to be extra-galactic owing to their large dispersion measures. This suggests that the large amount of the energies ∼ 10⁴³GeV/s is produced at the radio frequencies. The event rate of the burst is estimated to be ∼ 10-³per year in a galaxy. Furthermore, no gamma or X ray bursts associated with the bursts have been detected. Follow up observations of FRBs do not find any signals from the direction of the FRB. To find progenitors of the bursts, several models have been proposed. They ascribe FRBs to traditional sources such as neutron star-neutron star mergers, magnetors, black holes, etc..

Artist impression of FRB’s © gettyimages

Now, Iwazaki and colleagues model ascribes FRBs to axions, which are one of most promising candidates of dark matter. A prominent feature of axions is that they are converted to radiations under strong magnetic fields. The axions form axion stars known as oscillaton made of axions bounded gravitationally. The axion stars are condensed objects of axion miniclusters, which have been shown to be produced after the QCD phase transition and to form the dominant component of dark matter in the Universe. Furthermore, the axion miniclusters have been shown to form the axion stars by gravitationally losing their kinetic energies. Thus, the axion stars are the dominant component of dark matter. Iwazaki and colleagues in their recent work, proposed a progenitor of the FRBs that the FRBs arise from the collisions between the axion stars and neutron stars.

In their paper, they presented the details of their models. First of all, they derived the solutions of the axion stars by expanding the axion field and the gravitational fields in terms of eigen modes cos(nωt) with n integers, where the eigen value ω can be determined by solving axion field equation representing gravitationally bounded axions. It is given such that ω = ma – k²/2ma where k²/2ma( ≪ ma ) denotes a gravitational binding energy of an axion bounded by the axion star; k ≃ G×Ma×(m_a)², where G and Ma are the gravitational constant and the mass of the axion star, respectively. Then, they found that the mass Ma of the axion star is given in terms of the radius Ra = 1/k of the axion stars such that Ma = 1/(G × (ma)² × Ra). They obtained the value of the mass Ma by the comparison of the theoretical and observational even rate of FRBs in their production mechanism of the FRBs. They have found that the masses and radii of the axion stars are given by Ma ∼ 10-¹²M and Ra ∼ 10²km, respectively.

They also showed that the oscillating electric fields are generated on the axion stars under external magnetic fields. Thus, when the axion stars collide with neutron stars with strong magnetic fields, the electric fields are generated, which make electrons in atmospheres of neutron stars coherently oscillate. Thus, the electrons emit coherent radiations with the frequency given by the axion mass. Since the electrons are much dense in the atmospheres, the large amount of radiations with the frequency ma/2π ≃ 2.4 GHz can be produced in the collisions. The total amount of the energy of the radiations is given by 10-¹²M (10km/10²km)² ∼10⁴³GeV, where the radii of the neutron stars and the axion stars are supposed to be 10km and 10²km, respectively. This is their production mechanism of FRBs.


According to the mechanism, we can explain naturally the durations ( ∼ ms ) and amount of the energies ( 10^ 40erg ) of the bursts.

— said Iwazaki

They also discussed the optical properties of the atmospheres of neutron stars. The geometrical depth of the hydrogen atmospheres of old neutron stars is of the order of 0.1cm. The radiations emitted in the atmospheres can pass through them because they are shown to be optically thin for the radiations with the frequency (ma/2π) ≃ 2.4 GHz. They can also pass through magnetospheres of neutron stars. After they pass the magnetospheres, the radiations are circularly polarized owing to the absorption of the radiations with either right or left handed polarization. The circular polarization of a FRB has recently been observed.

Although the radiations produced by their mechanism are monochromatic having the frequency given by ω =ma/2π, the observed radiations have bandwidths at least wider than the range 1.2GHz∼ 1.6GHz used by actual observations. They showed that the bandwidths ωth ( ω ± ωth ) of FRBs arise from thermal fluctuations of electrons. Thus, the bandwidths are narrow. The presence of such narrow bandwidths owing to the thermal fluctuations is a distinctive feature of their model and can be tested observationally.


We have shown that the bandwidths are caused by the thermal fluctuations of electrons emitting the radiations

— said Iwazaki

While, in the actual collisions, the tidal forces of the neutron stars distort the formation of the axion stars. When the axion stars are close to the neutron stars, the gravitational forces of the neutron stars are stronger than those of axion stars binding themselves. Then, the axions freely fall to the neutron stars. But the coherence of the axions is kept because the number density of the axions in the volume (ma)-³ is quite large.

But guys, there are many studies which claimed that FRBs could be produced by the collision of axion stars and white dwarfs. For such cases, Iwazaki and colleagues found that the duration of the bursts is of the order of 0.1second and that the radiations have wider bandwidths than those of the radiations from neutron stars. These features can be observable only if the white dwarfs have very strong magnetic fields ≥ 10^9G. Although the number of such white dwarfs in a galaxy is still unknown, the production rate of the bursts is sufficiently large for them to be detectable if their number is larger than 10^6 in a galaxy.

The number of the white dwarfs with strong magnetic fields ≥ 109G in a galaxy is not known and the estimation of the number is difficult. Although we know the presence of such white dwarfs, the number of them could be very few. Thus, the event rate of the FRBs associated with the white dwarfs could be much small so that the FRBs are undetectable.

— said Iwazaki

If our production mechanism of FRBs is true, we can reach a significant conclusion that the axions are the dominant component of dark matter and their mass is about 10^-5 eV, which is in the window allowed by observational and cosmological constraints.

— said Prof. J. Arafune

Reference: Aiichi Iwazaki, J. Arafune, “Fast Radio Bursts from Axion Stars”, ArXiv, pp. 1-13, 2015. https://arxiv.org/abs/1412.7825


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