ing into account how electrons are distrib-
uted in the Milky Way and the Universe, the
DM can provide a rough distance estimate
to the source.
All FRBs show dispersion measures that sig-
nificantly exceed the expected values from
the electrons in our Milky Way. This indi-
cates that FRBs originate at cosmological
distances. Those detected lie billions of light
years distant, and are around a trillion times
more luminous than the brightest pulsars
in our Galaxy. There is no clear solution to
scale pulsar emission mechanisms to match
the luminosity and recurrence rate of FRBs.
A large number of possible scenarios have
been proposed: from giant magnetar flares
and colliding neutron stars, to exotic models
invoking axions and cosmic strings (see e.g.,
Platts et al., 2018; Petroff et al., 2019).
An important step forward in the field oc-
curred in 2012 with the discovery of multiple
bursts from the same source — FRB 121102
(Spitler et al., 2014 and 2016; Scholz et al.,
2016). This discovery rules out cataclysmic
models, at least for this particular source. A
handful of similar repeating FRBs have been
discovered since. It remains unclear if all
FRBs have the capability of repeating, or if
there are two distinct classes of FRBs: repeat-
ing and non-repeating. To date, only a small
fraction of the observed population of FRBs
repeat; perhaps more observing time for
longer durations and more constant moni-
toring with a more sensitive instrument is re-
quired to detect bursts, we just do not know.
While single-dish radio telescopes are pow-
erful FRB detectors, they do not have the
resolution to localize their host galaxy. Since
FRB 121102 exhibits repeating bursts, this
allowed for follow-up observations with the
Karl G. Jansky Very Large Array (VLA), the Eu-
ropean VLBI Network (EVN), Gemini North,
and the Hubble Space Telescope. In 2017
January 2020
they uncovered the precise location of FRB
121102, confirming its extragalactic nature;
the source was found within a low-metallic-
ity star-forming region of an irregular dwarf
galaxy some 3 billion light years distant
(Chatterjee et al., 2017; Marcote et al., 2017;
Tendulkar et al., 2017; and Bassa et al., 2017).
Interestingly, the radio bursts from FRB
121102 have an extremely high rotation
measure — a rotation of the plane of polar-
ization that occurs during the propagation
of electromagnetic waves in a magnetized
plasma (Michilli et al., 2018). They were also
found spatially coincident with a luminous
persistent radio counterpart (Chatterjee et
al., 2017; Marcote et al., 2017). This extreme
environment suggests a possible connec-
tion between FRBs and other energetic
transients, such as long gamma-ray bursts
(Metzger et al., 2017). However, the observa-
tions are also consistent with models invok-
ing extreme objects such as neutron stars or
massive black holes (Chatterjee et al., 2017;
Michilli et al., 2018).
Within the last year, three new localizations
have been reported; so far, all are non-re-
peaters. In all three cases, the observed host
galaxies are radically different from the first
repeating FRB: they are all located in massive
galaxies: two reside in the outskirts of ellip-
ticals, and one in a spiral galaxy (Bannister et
al., 2019; Ravi et al., 2019; and Prochaska et
al., 2019).
The large discrepancies between the local
environment and host of the first repeater,
FRB 121102, when compared with those of
the apparently non-repeating sources, deep-
ened the idea of two distinct classes of FRBs:
repeating and non-repeating. Cleary, we re-
quired more localizations of both repeating
and non-repeating FRBs to clarify the nature
of these events.
GeminiFocus
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