Artistic conception of a neutron star with an ultra strong magnetic field, called a magnetic field, which emits radio waves (red). Magnetars are one of the main contenders for Fast Radio Bursts. Credit: Bill Saxton, NRAO / AUI / NSF
In radio astronomy, a fast radio (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond to a few milliseconds, caused by some mysterious high-energy astrophysical process that is not yet known. has discovered. Astronomers estimate that the average FRB releases as much energy in a millisecond (one thousandth of a second) as the Sun emits in 3 days (which is more than 250,000 seconds).
Duncan Lorimer and his student David Narkevic discovered the first FRB in 2007, and it is commonly known as Lorimer Burst. Since then, many other FRBs have been detected. One of them, FRB 180916, is extremely mysterious because it pulses regularly every 16.35 days.
Now, astronomers have only found the second example of a very active and repeated fast radio burst with a weaker but persistent compact source of radio emission between bursts. The discovery raises new questions about the nature of these mysterious objects and also about their usefulness as tools for studying the nature of intergalactic space. Scientists used the Karl G. Jansky Very Large Array (VLA) from the National Science Foundation and other telescopes to study the object, first discovered in 2019.
The object, named FRB 190520, was found by the five-hundred-meter spherical aperture radio telescope (FAST) in China. An explosion of the object occurred on May 20, 2019 and was found in the data of this telescope in November of that year. Follow-up observations with FAST showed that, unlike many other FRBs, it emits frequent and repeated bursts of radio waves.
Fast Radio Burst FRLA 190520 VLA image (red), combined with an optical image, when the FRB is exploding. Credit: Niu, et al .; Bill Saxton, NRAO / AUI / NSF; CFHT
Observations with the VLA in 2020 identified the location of the object, and this allowed visible light observations with the Subaru telescope in Hawaii to show that it is on the outskirts of a dwarf galaxy nearly 3 billion years old. light of the Earth. VLA observations also found that the object constantly emits weaker radio waves between bursts.
“These characteristics make it very similar to the first FRB whose position was determined, also by the VLA, in 2016,” said Casey Law of Caltech. This development was a breakthrough as it provided the first information about the environment and distance of an FRB. However, its combination of repeated bursts and persistent radio emission between bursts, coming from a compact region, differentiated the 2016 object, called FRB 121102, apart from all other known FRBs, so far.
The FRB 190520 region, seen in visible light, with the VLA image of Radio Fast animating between the exploding and non-exploding object. Credit: Niu, et al .; Bill Saxton, NRAO / AUI / NSF; CFHT
“Now we have two like this, and that raises some important questions,” Law said. Law is part of an international team of astronomers who report their findings to the journal Nature.
The differences between FRB 190520 and FRB 121102 and all others reinforce the possibility suggested above that there may be two different types of FRB.
“Are those who repeat different from those who do not? What about persistent radio broadcasting, is it common? “Said Kshitij Aggarwal, a graduate student at the University of West Virginia (WVU).
Astronomers suggest that there may be two different mechanisms that produce FRB or that the objects that produce them may act differently at different stages in their evolution. The main candidates for FRB sources are super-dense neutron stars that remain after a massive star explodes as a supernova, or neutron stars with ultra-strong magnetic fields, called magnets.
Location of FRB 190520 in the sky. Credit: Bill Saxton, NRAO / AUI / NSF
A feature of FRB 190520 calls into question the usefulness of FRBs as tools for studying material between them and the Earth. Astronomers often analyze the effects of intervening material on radio waves emitted by distant objects to know this faint material. One of these effects occurs when radio waves pass through space that contains free electrons. In this case, the higher frequency waves travel faster than the lower frequency waves.
This effect, called scattering, can be measured to determine the density of electrons in the space between the object and the Earth, or, if the density of electrons is known or assumed, to provide an approximate estimate of the distance to the object. ‘object. The effect is often used to make distance estimates on the pulsars.
This did not work for FRB 190520. An independent measure of the distance based on the Doppler shift of light from the galaxy caused by the expansion of the Universe placed the galaxy almost 3 billion light-years from Earth. . However, the explosion signal shows an amount of scattering that would normally indicate a distance of approximately 8 to 9.5 billion light-years.
“This means that there is a lot of material near the FRB that would confuse any attempt to use it to measure interstellar gas,” Aggarwal said. “If this is the case for others, we cannot count on the use of FRB as cosmic criteria,” he added.
Astronomers speculated that FRB 190520 could be a “newborn”, still surrounded by dense material ejected by the supernova explosion that left the neutron star behind. As this material dissipates, the dispersion of the explosion signals would also decrease. Under the “newborn” scenario, they said, repeated outbreaks could also be a feature of younger FRBs and decrease with age.
“The FRB field is moving very fast right now and new discoveries are coming out every month. However, there are still big questions left, and this object gives us challenging clues about those questions,” said Sarah Burke-Spolaor of WVU.
Reference: “Repeated Rapid Radio Associated with a Persistent Radio Source” June 8, 2022, Nature.DOI: 10.1038 / s41586-022-04755-5
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