ROOM
82
Space Science
and a magnetic field vector perpendicular to one
another, and to the direction of propagation. As the
radio wave propagates from source to observer,
the plan of polarization of the electric field vector
rotates under the influence of a magnetic field.
This effect is called ‘Faraday Rotation’ and helps
astronomers quantify the strength of the magnetic
field of the medium, along the line-of-sight. The
magnetic field of the Milky Way has been mapped
extensively using this method with pulsars. FRBs
could help us understand and measure for the
very first time, the magnetic fields associated with
the intergalactic medium.
Last but not least, FRBs can be used as cosmic
rulers. A comparison of their observed dispersion
measure to their measured redshift, when coupled
with a model for the average electron density as a
function of redshift, enables one to measure the
time of flight of the photon, and hence the path
length. In this sense, FRBs represent cosmic rulers.
Mysteries surrounding FRBs
The FRB pulses themselves have so far only
been observed in the radio portion of the
electromagnetic spectrum and their high rate
is a major constraint on their theories of origin.
Despite the plethora of suggested progenitors, the
most plausible theories can be grouped into two
categories: FRBs are either caused by relatively
rare explosive collisions between old neutron
stars or white dwarfs, or they are more common,
periodic outbursts or flares from younger, rapidly
spinning pulsars.
The two categories have very different
implications. The collision-based models suggest
that FRBs are extremely energetic one-off events
that occur billions of light years away. The
outburst model on the other hand suggests that
FRBs are much less energetic, repeatable and
would only be a few million light years away, so
in astronomical terms - much closer. However,
despite all the theories, astronomers have no
conclusive proof yet as to what they might be.
Nonetheless, in 2016 an FRB discovered at
the Arecibo telescope in Puerto Rico, has been
found to repeat and has been localised to a dwarf
galaxy three billion light years away, opening
up possibilities for new theories and models.
Astronomers have spent several hundreds of
hours re-observing the known sky positions of
various other FRBs but this is the only FRB known
to repeat thus far. Each of the 26 FRBs detected is
unique in its own way but common characteristics
amongst these 26 pulses are that they last only a
few milliseconds and have dispersion measures well
Universe is composed of dark energy and dark
matter and only around five percent is ‘baryonic
matter’ or normal matter, since its constituents
are atoms.
Baryons make up objects of everyday life,
however only 50 percent are accounted for
and astrophysicists suspect that the remaining
50 percent is in the form of diffuse hot plasma
between galaxies. Studying these missing baryons
observationally has remained a challenge for many
years. But this is where FRBs could help out.
The FRB pulse carries a fingerprint of the
intervening medium it travels through and this
can be exploited as a cosmological probe of the
Universe. The dispersion measure of the FRB
along with a measurement of redshift of the galaxy
(an indication of how far away the galaxy is) can
provide a direct measurement of these ‘missing
baryons’ and consequently enable researchers to
‘weigh’ the Universe, as it were. Therefore, along
with information gleaned from the FRB data,
scientists should, in time, be able to solve the
missing matter problem.
Secondly, magnetic fields play very critical roles
in almost every aspect of astrophysics, ranging
from star formation to large-scale galactic and
intergalactic dynamics. They also have a significant
contribution to the hydrostatic balance in the
interstellar medium. Unfortunately, much remains
unknown about how these fields are generated
or how they are evolving. Polarization studies
are precisely what are required to answer these
questions of ‘Magnetogenesis’.
Radio emission from astronomical sources is
often linearly polarised with the electric field vector
The Molonglo radio
telescope near Canberra
in Australia is a Mills
cross design
interferometer and a
potential gold mine for
FRB discoveries.
The success of Molonglo radio telescope’s
observations has helped confirm the
extragalactic nature of one-off FRB events
Pablo A. Rosado
