The Enigma of Fast Radio Bursts

Patrick Das Gupta

Department of Physics and Astrophysics,

University of Delhi, Delhi-110007

I. Introduction

Most cosmic bodies emit electromagnetic radiation at various wavelengths spanning from long wavelength radio-waves to very high frequency gamma-rays. Why is that so? Imagine listening to a radio set on a stormy evening with incessant lightning and thunder going on outside while time to time some corner of the dark sky brightens up due to a lightning flash followed by a loud thunder. In case the radio station you have tuned in is transmitting amplitude modulated (AM) signals then lightning outside, in general, is accompanied by hissing and bursting noise coming out of the radio set.

This is due to the fact that lightning is largely caused by the flow of huge number of energetic electrons from the turbulent clouds to the earth because of the high electrical potential difference between the charged clouds and the earth. Since electric current is nothing but flow of charges, these downward moving electrons constitute enormous currents that lead to emission of light. Electric currents lasting for a short duration, each time there is a flash of lightning, generate not only  radio waves (since accelerated charge particles radiate electromagnetic waves) but also visible light (since collisions of energetic electrons with atoms in the atmosphere excite the atoms to higher electronic energy levels so that when these atoms get de-excited they emit visible light).

After all, most of us are familiar with household phenomena of similar nature occurring in our day to day lives. For example, when we see tiny sparks at night as soon as   we take off our sweater quickly or comb our hair in the winter season (since cold air is extremely dry with humidity being low). One may also recollect the sparks we see near a electric switch when we turn it off. All these happen because of flow of electrons from negative potential to the positive one. In other words, it is rather natural that electromagnetic radiation from cosmic sources, majority of which involve charge particles in motion, would be of broad-band spectrum with wavelengths ranging from tens of metres (radio-waves) to tiny fractions of angstrom (gamma-rays).

In fact, one of the prime motivation of the Indian space-based astronomy mission ASTROSAT is to study astrophysical objects like gamma-ray burst sources, pulsars, hot accretion discs around black holes and compact objects, binary stars, etc. at multiple wavelengths. Which is why the ASTROSAT satellite has optical, UV and X-ray telescopes on board pointing in the same direction. One of the ways its instruments were calibrated was by observing the well studied Crab pulsar (which was formed after a supernova explosion that was seen and recorded by Chinese monks in 1054 AD).

II. Pulsars: Cosmic Lighthouses

Pulsars are very rapidly spinning compact objects made essentially of neutrons, formed after the compact iron core of a massive star collapses gravitationally in just few milli-seconds time interval. During the core collapse, electrons fall into the iron nuclei and undergo inverse beta decay whereby electrons combine with protons to become neutrons releasing neutrinos. Conservation of angular momentum turns an initially slow rotating iron core into a fast spinning neutron star as the core collapses from a radius of few thousand kilometres to about 10 kilometres. This is analogous to what happens when we sit, with our arms stretched out horizontally, in a chair that can spin about the vertical direction, with someone setting the chair spinning. When we bring our palms close to the chest, the chair spins faster because of angular momentum conservation and the fact that moment of inertia decreases by our action.

These neutron stars carry tremendously large surface magnetic field strengths ranging from about 100 billion Gauss to few million billion Gauss (as in the case of magnetars, a variety of neutron stars with enormously high magnetic field strengths, discovered in 1999). These city size compact objects are found to have masses ranging from one solar mass to just over two solar masses. But how do we know about the existence of such stars that are composed mainly of neutrons with just a fraction of protons and electrons?

Rapid rotation of neutron stars endowed with large magnetic fields lead to incredibly high electric fields around them, since changing magnetic field gives rise to electric field (after all, generators and dynamos use this principle to create electrical energy out of mechanical energy). Such high electric fields cause charge particles to get accelerated along magnetic field lines that radiate electromagnetic waves predominantly in the forward direction within a narrow cone because of Doppler effect. Hence, if the magnetic axis is inclined with respect to the spin axis of a neutron star, radiation emerging effectively out of a cone with its axis along the magnetic axis are seen as narrow pulses of radio-waves arriving periodically at the radio-telescope provided the precessing cone sweeps across the line of sight that is directed from the telescope to the rotating neutron star.

Hence, rotating neutron stars whose cone of emission intersect the line of sight periodically due to the rotation of the compact star, can be observed as pulsars. In fact, Jocelyn Bell, a research student of radio-astronomer Anthony Hewish was the first to notice tiny periodic radio-pulses with a period of about 1.33 second from such a cosmic light-house in 1967. By now about 2300 radio-pulsars have been detected.

III. Fast Radio Bursts: A New Class of Cosmic Beasts

Around 2007, an astronomer from West Virginia university, Duncan Lorimer, was looking for radio-pulses in the old records of Parkes telescope data. To his surprise, he along with his other team members, discovered a bright but single radio-pulse lasting for about 5 milli-seconds in the archival data. Unlike pulses arriving from a standard pulsar at regular instants of time separated by a definite period, it was just a solitary pulse! This chance discovery has led to a new field in astrophysics - the study of Fast Radio Bursts (FRBs).

In the last 10 years or so, astronomers have detected so far about 30 extragalactic FRBs with the help of radio-telescopes operating at frequencies ranging from about 800 MHz to about 2 GHz. A typical FRB event is characterized by a single narrow radio-pulse, few milli-seconds wide, with a peak flux density ranging from about 0.5 Jy to about 2 Jy at 1.4 GHz (1 Jy =10^{-26} W/m^2/Hz). We must note that, in comparison, radio energy received from Sun at 1.4 GHz is more than about 600000 Jy. But this is an unfair comparison since these mysterious radio bursts are occurring way beyond the limits of Milky Way, at distances  larger than 10^{14} times Earth-Sun distance!

How does one know that these FRBs are extragalactic in nature? To understand this, one needs to consider propagation of electromagnetic waves in a cold plasma. It so happens that the gaseous matter that is spread all over our Milky Way, the so called interstellar medium (ISM), is mostly in ionized form. When radio waves propagate in such ionized ISM, high frequency components travel faster than the lower frequency ones. Radio astronomers define a quantity called the dispersion measure (DM) of a source, which is nothing but the integral of electron density over the path traversed by the waves from the source along the line of sight to the observer. Delay of arrival of low frequency waves compared to the high frequency ones is proportional to the DM.

Hence, larger the DM, more is the lagging behind of lower frequency radio waves. The distances of Galactic pulsars are routinely estimated from such time delays. It turns out that the FRBs detected in the directions sufficiently away from the Galactic plane have very high DM. Since these FRBs are sighted in directions where the Galactic electron densities are extremely low, observed large DM must be due to the enormous distance traversed by the radio waves through tenuous and ionized intergalactic medium (IGM) that exists between galaxies.

Therefore, by monitoring the arrival times of radio spikes seen at different wavelengths, one can estimate how far the FRBs are. It turns out that FRBs are located several billions of light years away from us. From the measured radio flux densities and estimated distances one infers that FRBs release 10^{38} erg to 10^{41} erg of energy in radio waves. 

Among all FRBs, FRB 121102 stands out. Radio bursts from this source recurs again and again. Not only that, it is found to be co-located in a dwarf galaxy which itself is a weak radio-source. As of now, only FRB 121102 is seen to repeat, rest of the FRBs being `one-shot’ catastrophic events. So far about 200 such transient bursts have been detected from FRB 121102.

Understanding the physical nature of FRBs is creating great deal of excitement among the astrophysicists, keeping them delightfully busy. Are they radiation from supramassive neutron stars or magnetars that collapse when their spins get slowed down? Could they be due to the interaction of electron winds from hot accretion discs around black holes with pulsar magnetospheres? Exotic models like white holes, collisions of asteroids and neutron stars, etc. have also been proposed. Only future investigations would reveal the true nature of FRBs.

Several astronomers of Indian origin are involved and active in the detection and study of FRBs like S. R. Kulkarni, Pawan Kumar, Shami Chatterjee, S. P. Tendulkar, T. Vachaspati, V. Ravi, D. Palaniswamy, J. Yadav, M. Lingam, M. Bagchi, etc. One hopes that ASTROSAT will soon lead to some crucial breakthrough in resolving the FRB conundrum. After all, using ASTROSAT, an exciting discovery has recently been made - A. R. Rao (TIFR), Dipankar Bhattacharya (IUCAA) and their team, have detected polarized X-rays emitted from several Gamma Ray Bursts!