Letters of Intent received in 2019

LoI 2021-2073
GA Symposium: The dawn of cosmology & multi-messenger studies with fast radio bursts

Topics

1) Recent results from FRB surveys
2) Emission mechanism theories
3) Observational constraints on emission mechanisms
4) Progenitor Models
5) Observational constraints on progenitors
6) Propagation effects on the observed FRB bursts
7) Host galaxies and FRB environments
8) Cosmological uses of FRBs
9) Is there an FRB zoo?
10) Multi-messenger and Multi-wavelength observations of FRBs

Rationale

Introduction and Motivation:

Brilliant bursts of electromagnetic radiation from exploding stars in the distant Universe have transformed our understanding of the Cosmos over the last two decades. One such burst, the Lorimer burst, was discovered in 2007, and it is the exemplar of the class we now know as fast radio bursts (FRBs). FRBs are characterised by millisecond-duration radio pulses appearing at random locations in the sky at cosmological distances, and are one of the most tantalising astrophysical phenomena of the last decade. Over a few thousand FRBs are estimated to occur over the entire sky each day. The intriguing combination of cosmological origin, along with their estimated high radio luminosities and correspondingly large brightness temperatures is perhaps what makes FRBs most compelling. Almost every radio telescope in the world is currently undertaking large-area surveys at radio frequencies ranging from 100 MHz up to tens of GHz to discover, study and understand these bursts.

With the development of new instrumentation and software, we have now reached a point where radical changes in the field occur on timescales of a few months or so. As a result, the quest to answer the fundamental questions of their enigmatic nature, progenitors, environments, spatial distribution and their potential for use as cosmological probes is gaining enormous momentum. The aim of the symposium is to facilitate the essential confluence of data and theory at a time when many of the latest experiments will have been running for sufficient time to make significant progress in answering these questions.

FRBs have three measurable quantities, which provide us with clues regarding their origin – pulse duration, dispersion measure and flux density. A constraint on the nature of the progenitor can made from light travel-time arguments using the observed time duration of the bursts and the inferred distance scale, which suggests that FRB progenitors likely involve compact objects. Their large dispersion measures (DMs), the integrated electron column densities, are believed to be effective proxies for distance. The measured DM values significantly exceed the maximum Galactic contribution from the interstellar medium (ISM) implying distances of several gigaparsecs and relatively high redshifts. The radio data alone encode information about the progenitor source and its environment, the host galaxy, the intergalactic medium (IGM), intracluster medium (ICM), the circumgalactic medium (CGM) and our own Galaxy. As a result, FRBs are sensitive to a whole collection of propagation effects like no other cosmological population. FRB science aims to combine this information and synergise it with multi-wavelength observations in order to exploit them as unique probes of cosmology.

Prospects for using FRBs as cosmological probes:

Several theories predict FRBs to be extremely potent tools to study the nature of the magneto-ionic intergalactic medium, establish a chromatic scattering-redshift relation, possibly test Einstein’s Equivalence Principle, measure the dark energy equation of state, examine baryonic feedback processes in galaxies, constrain the characteristic radial density profiles of CGM halos, and even detect and examine the elusive baryonic matter in the tenuous IGM. FRBs are an exquisitely high-precision probe of baryonic matter because the effect of dispersion accounts for every single ionised baryon that lies between the burst and the telescope. More recently, it has been suggested that gravitationally lensed FRBs could independently provide constraints on two of the most important cosmological parameters: the Hubble constant and cosmic curvature. FRBs detectable with next generation telescopes like the SKA and FAST at redshifts of 3 and above, are expected to carry the imprint of the epoch of helium reionization. The detection and characterisation of this epoch may hold crucial information to aid in searches for the signature of the much sought after epoch of hydrogen reionization, at yet higher redshifts. It has also been suggested that FRBs can be used to improve galaxy-cluster kinetic Sunyaev–Zeldovich (kSZ) measurements of the growth rate and amplitude of cosmic density fluctuations, which is an important goal of the Dark Energy Spectroscopic Instrument (DESI) surveys. It is obvious that several FRB science goals complement existing multi-wavelength probes of cosmology, quite neatly.

There are however, limitations to the use of radio data, as most of the science goals require an accurate distance estimate to the FRB. Indeed, the scientific payoff from an astrophysical source is only truly realised upon localisation. The true distance to an FRB requires identification of a host galaxy through radio interferometry, followed by photometry and/or an optical spectrum of the host. Several radio interferometers like the VLA, ASKAP, DSA-10, MeerKAT, APERTIF and CHIME are coming online and transforming into ‘FRB detection machines’ to yield real-time detections and instant localisations in the radio. Once localised, characterisation of the host galaxy, intervening media along the line-of-sight, and the diversity in FRB engines itself demands multi-wavelength and multi-messenger observations of varying cadences. If FRBs are detectable in follow-up multi-wavelength/multi-messenger observations, it will be the most straightforward way to answer the most fundamental open question of what are/makes FRBs.

Multi-wavelength observations of pulsars, supernovae and gamma-ray bursts sequentially across the entirety of the electromagnetic spectrum over the decades, have enabled us to study their evolution in extraordinary detail. Recent multi-messenger transient sources have led to the detection of gravitational waves from binary black hole/neutron star mergers, and even high-energy neutrinos from the active galaxy TXS 0506+056. The entire electromagnetic spectrum along with gravitational waves and neutrinos are now open for FRB detections. With the potential of multi-wavelength detections and their uses as effective cosmological probes, it appears that FRBs have formed a bridge across all astronomy.

Current status of the field:

Presently, new FRBs both repeating and non-repeating continue to be steadily discovered with existing instrumentation. Over 50 progenitor model theories have been proposed for both types of FRBs, ranging from flaring magnetars and the destruction of highly magnetised white dwarfs, to more exotic ones involving interactions between axion stars and black holes. The only stringent limitation for a model progenitor is that the FRB cannot be embedded in a medium too dense that gigahertz-frequency emissions would be suppressed. However, no direct observational evidence has yet been gathered to confirm the source of either type of FRB. While the electromagnetic emission provides us information about the intervening plasma, detections of gravitational waves associated with an FRB would actually prove its cataclysmic nature and provide a direct link with neutron star mergers, gamma-ray bursts and kilonovae. Neutrino detections coincident with FRBs on the other hand, would give us insight into hadronic accelerations and atomic decay processes associated with the source.

On going and planned real-time identifications of FRBs will permit quasi-real-time triggering of multi-wavelength instruments to search for afterglows through automated mechanisms such as VOEvents. An extremely exciting possibility is finding the electromagnetic counterpart to a gravitational wave burst either by providing triggers to, or receiving triggers from, the Advanced LIGO and VIRGO detectors. The better we can characterise the radio bursts and their associated multi-wavelength emission, the better our chances are of identifying the underlying emission mechanism. The full potential of FRBs will only be realised in the era of routine FRB detections and corresponding host galaxy identifications, which we expect to reach by 2021.

Planning the next decade of FRB astronomy:

The last few years have revolutionised FRB astronomy, and as a community we are on the brink of answering some of the open questions regarding the nature and uses of FRBs. The MeerKAT telescope in South Africa has started scanning the skies for FRBs along with the robotic MeerLICHT optical telescope to identify possible contemporaneous optical flashes that FRBs might produce. The UTMOST, ASKAP and CHIME telescopes are making significant advances in studying the low DM (100–1000 pc/cc) population in the relatively local Universe. The ASKAP, DSA-10 and VLA telescopes are making huge strides towards real time detections and localisations. In, or near 2021, we will also have initial FRB survey results from the next generation telescopes like FAST and MeerKAT which will push our search horizons beyond redshifts of 2. We can expect to look forward to multiple new detections, host identifications and physical insights in the next couple of years.

At this stage in the field, it is vital that we take the opportunity to capitalise on the convergence of astronomers with expertise in cosmology, galactic dynamics and high-energy phenomena at the GA, to collaborate and strategise the next few years of FRB astronomy. This look forward to the future is essential, as the continuous improvement of current facilities and building of superb new facilities promise decades of exciting (astro)physics to follow.

SOC Members:

Keith Bannister, CSIRO, Australia
Victoria Kaspi, McGill University, Canada
Casey Law, Caltech, USA
Yuri Lyubarsky, Ben-Gurion University, Israel
Laura Spitler, Max Planck Institute for Radio astronomy, Germany
Nicolas Tejos, Pontificia Universidad Catolica de Valparaiso, Chile
Nina Wang, Chinese Academy of Sciences, China
Amanda Weltman, University of Cape Town, South Africa
Barak Zackay, Institute for Advanced Study, USA