The simplest, yet unbelievable, explanation
is that the source is extragalactic and the
excess DM is contributed by the electrons in
the intergalactic medium (IGM) — placing
the source of the Lorimer burst at a redshift
of z ~ 0.3, a distance of ~ 1 billion parsecs.
The emitted power at the source would have
been 10 42 erg/s, about a billion times more
luminous than the brightest radio pulsars
ever observed in the Milky Way.
The Population of FRBs
Over the next decade, such radio bursts
were detected at multiple radio telescopes
— Parkes, Green Bank (West Virginia), Are-
cibo (Puerto Rico), and Molonglo (near Can-
berra, Australia) — and came to be known
as Fast Radio Bursts (FRBs). To date, only 26
bursts have been reported in the literature,
but considering the narrow fields-of-view of
radio telescopes and the survey durations,
the expected sky rate of FRBs is large — 10 3
per s ky per day above a peak flux density of
1 Jy at an observing frequency of 1.4 GHz
(Lawrence et al., 2016).
Despite this prodigious rate, we have little
knowledge about the sources that emit
FRBs and the emission mechanisms that
allow such luminous coherent bursts. Until
this work, even the distance to any FRB was
only estimated from the excess DM. Due to
the paucity of observational constraints,
there are more theoretical models of FRBs
than the total number of observations (see
box at right). In the future, FRBs are pro-
jected to serve as excellent cosmological
probes of the electron and baryon distribu-
tion in the Universe.
The Repeater
FRB 121102 was discovered by the 300-me-
ter Arecibo Observatory during a survey of
the Galactic plane with a DM of 557 pc/cm 3
(Spitler et al., 2014). In follow up Arecibo ob-
January 2018 / 2017 Year in Review
Cold Plasma Dispersion
When electromagnetic waves pass through interstellar plasma, the inertia
of electrons moving in response to the electric fields causes the lower fre-
quency waves to propagate slower than the higher frequency waves. For
non-relativistic, diffuse plasma, the pulse arrival time difference between
two frequencies is given by
where the dispersion measure
is the integral of the elec-
tron density from the source to the observer, ν is the radio frequency and
m e , e and c are the mass and charge of an electron and the speed of light,
respectively. The Milky Way interstellar medium (ISM) contribution to the DM
along different lines of sight has been characterized using pulsar DM mea-
surements, H i maps and Galactic models. Any excess in DM would have to
be attributed to either excess electrons near the source or the intergalactic
medium (IGM).
servations conducted in 2015, eleven more
bursts were found at the same location with
the same DM (Spitler et al., 2016), earning
FRB 121102 the moniker “Repeater.” None of
the other FRBs, even after several follow up
observations of various durations, have yet
been observed to repeat.
It is not clear at this time whether the Re-
peater belongs to a separate population from
the rest of the FRBs or whether all FRBs are a
homogeneous population — but the much
higher sensitivity of Arecibo compared to
other radio telescopes allowed Arecibo to
detect fainter bursts; ones that are likely to
be more frequent than bright bursts and may
Theoretical Models for FRBs
Due to the very short timescale (few milliseconds) and the bright, often po-
larized emission, it is almost necessary to invoke a compact magnetic field to
produce an FRB, making some varieties of neutron stars an obvious choice
for FRB sources. However, the observed energy scales of FRBs are far higher
than those of galactic radio pulsars. A plethora of models have been pro-
posed including magnetar giant flares, Crab-like giant pulses from young
extragalactic pulsars, planets in pulsar magnetospheres, asteroids impacting
neutron stars, neutron star mergers, neutron stars collapsing into blackholes,
black hole-neutron star mergers, magnetar pulse-wind interactions, flares
from nearby stars, quark novae, and axion stars. For a more complete review,
please see Katz, 2016.
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