New fast radio burst found, showing that host galaxies may muddy their signals

A new repeating fast radio burst (FRB) has been identified in a dwarf galaxy. The newly discovered FRB has similar features to the first classified, but some differences suggest a distinct cosmic environment.

The first FRB was discovered in 2007 and dubbed the “Lorimer burst”, but the exact origin of many of them is still unknown.

Fast radio bursts are pulses of radio-frequency electromagnetic radiation. The radio emissions have durations in the order of milliseconds or faster, and display the delay between bursts (dispersion measure) characteristic of radio pulsars. They can be used as tools to study the stuff between galaxies – the intergalactic medium – but some may be better suited to probing the vast gulfs between galaxies depending on the FRB’s locale.

Publishing their results in Nature, researchers from the Chinese Academy of Sciences have detected a new FRB, called FRB 20190520B and identified its host galaxy – the dwarf galaxy known as J160204.31−111718.5. The dwarf galaxy has a relatively small redshift of z = 0.241, corresponding to a distance from Earth of about 3 billion light years.

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The new FRB was detected with China’s Five hundred-meter Aperture Spherical Telescope (FAST) – the world’s largest single dish radio telescope – in drift-scan mode as part of the Commensal Radio Astronomy FAST Survey (CRAFTS) in 2019.

FRB 20190520B has many similarities to the Lorimer burst, but its unique properties suggest it inhabits a cosmic environment different to other FRB sources.

Corresponding author of the Nature paper, Dr Di Li, an astrophysicist at the Chinese Academy of Sciences, spoke with Cosmos about the team’s findings.

The Lorimer burst “source was never ‘heard’ from again, despite the initial ‘loudness’ of the signal and significant efforts to follow it up. Around 500 distinctive FRB sources have been identified, but only about 5% give out bursts repeatedly,” says Li.

To identify the new FRB’s host galaxy, the team overlayed its radio-frequency observations on an optical imageobtained using the Canada-France-Hawaii Telescope/MegaCam. And, hey presto, they were able to pinpoint the FRB’s home galaxy.

In (b), The infrared J-band image by Subaru/MOIRCS shows emission only at the location of the peak of the optical light profile of the host galaxy. The inset is a region matching the inset in a. In (c), the radio VLA image shows a compact persistent source at the FRB location. Credit: D. Li et al.

Li says that FRBs can tell us about the intergalactic medium by interacting with the particles between galaxies. “The radio signal will be dispersed by intergalactic electrons. Such dispersion will produce a chirp in the burst signal, i.e., the delay of arrival of the lower frequencies of the same burst. By measuring the level of delay, one can estimate the intervening electron density, thus providing a handle of this important component of cosmic matter.”

The authors write that “although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs,” there is at least one case in which the host galaxy’s extreme properties have altered the dispersion measure of the FRB.

FRB 20190520B appears to be another case of the host galaxy warping the dispersion measurement. Its host galaxy has its own competing radio source, and its dispersion measure is nearly an order of magnitude higher than other galaxies which host FRBs.

“FRB 20190520B has the highest confirmed electron density of any FRB’s environment. Along with the first known repeating FRB, FRB 20190520B is only the second FRB source with a persistent radio source counterpart. They could represent the youth of all FRBs in an evolutionary picture. It also provides a potential correction when relating signal dispersion to the intergalactic medium. For certain sources, the local contribution can overwhelm that of the intergalactic medium,” explains Li.

The authors say this is reason to believe that detailing the properties of other fast radio bursts is going to have to involve looking at the environment in which they live.

 “Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications,” they write.

“Active repeaters are especially useful for FRB research as they allow for more efficient follow-up observations. The number of active repeaters is still in the single digits. FRB 20190520B is the only one so far to be persistently active, i.e., it never ‘sleeps’. The famous first repeater, FRB 20121102A, for example, can be extremely diligent when active, but then is seen to ‘turn off’ for months. Not FRB 20190520B,” says Li.

Li says he hopes that astronomers will be able to determine the origin of FRBs in the near future. “The holy grail in this field is systematic multiple wavelength capture of bursting events. Given the pace of progress in this field, we may converge onto a consistent picture of FRBs’ astronomical origin in a few years.”

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