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Article
Multiwavelength Observations of Fast Radio Bursts
Luciano Nicastro1,*
, Cristiano Guidorzi1,2,3
, Eliana Palazzi1
, Luca Zampieri4
, Massimo
T uratto4
and Angela Gardini5
1INAF—Osservatorio di Astroﬁsica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, I-40129 Bologna,
Italy; guidorzi@fe.infn.it (C.G.); eliana.palazzi@inaf.it (E.P .)
2Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, I-44122, Ferrara, Italy
3INFN—Sezione di Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
4INAF—Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy;
luca.zampieri@inaf.it (L.Z.); massimo.turatto@inaf.it (M.T.)
5Observatorio Astronómico Calar Alto s/n, Sierra de los Filabres, E-04550 Gergal, Almería, Spain; gardini@caha.es
*Correspondence: luciano.nicastro@inaf.it; Tel.: +39-051-6398778
Received: 24 February 2021; Accepted: 11 March 2021; Published: 23 March 2021
Abstract: The origin and phenomenology of the Fast Radio Burst (FRB) remains unknown despite more
than a decade of efforts. Though several models have been proposed to explain the observed data, none
is able to explain alone the variety of events so far recorded. The leading models consider magnetars as
potential FRB sources. The recent detection of FRBs from the galactic magnetar SGR J1935+2154 seems to
support them. Still, emission duration and energetic budget challenge all these models. Like for other
classes of objects initially detected in a single band, it appeared clear that any solution to the FRB enigma
could only come from a coordinated observational and theoretical effort in an as wide as possible energy
band. In particular, the detection and localisation of optical/NIR or/and high-energy counterparts
seemed an unavoidable starting point that could shed light on the FRB physics. Multiwavelength (MWL)
search campaigns were conducted for several FRBs, in particular for repeaters. Here we summarize the
observational and theoretical results and the perspectives in view of the several new sources accurately
localised that will likely be identiﬁed by various radio facilities worldwide. We conclude that more
dedicated MWL campaigns sensitive to the millisecond–minute timescale transients are needed to address
the various aspects involved in the identiﬁcation of FRB counterparts. Dedicated instrumentation could
be one of the key points in this respect. In the optical/NIR band, fast photometry looks to be the only
viable strategy. Additionally, small/medium size radiotelescopes co-pointing higher energies telescopes
look a very interesting and cheap complementary observational strategy.
Keywords: FRB; radio transient sources; fast transient; multiwavelength observations
1. Introduction
The multiwavelength (MWL) approach to study transient astronomical events has demonstrated its
effectiveness in solving many puzzles in astronomy, both related to “local” and extragalactic sources. The
Gamma Ray Burst (GRB) phenomenon represents a perfect example. Wide area detectors (e.g., high-energy
monitors) or speciﬁc surveys monitoring large areas of the sky can detect events that only last for a short
period of time. Being able to reduce positional uncertainties and perform timely MWL observations
using sensitive and high-enough resolution instruments could be the only way to discriminate among
the possible progenitors. This is a key point when the transient astronomical event is only detected in a
Universe 2021 ,7, 76; doi:10.3390/universe7030076 www.mdpi.com/journal/universearXiv:2103.07786v2  [astro-ph.HE]  25 Mar 2021
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Universe 2021 ,7, 76 2 of 48
single band of the electromagnetic spectrum or when multiple sources could produce that event. Studying
the source emission in a as wide as possible spectral band justiﬁes huge efforts in terms of observational
time and manpower. It, typically, turns out to be the only way to solve the most challenging questions in
astrophysics.
Fast Radio Bursts (FRBs) are among the most studied astrophysical transients, still their origin and
whether there are multiple types of progenitors and emission mechanisms are still open questions (see [ 1–5]
for a review). Are ( apparently ) one-off and repeating events representative of distinct samples or are they
the realization of a very wide timescales distribution of the same objects class? Can the periodicity seen in
a few FRBs be reconciled with the proposed models? Is the behavior distinctive of a class or sub-class of
FRBs? Of the about 140 distinct sources known [https://wis-tns.weizmann.ac.il/] (accessed on 1 March 2021)
(but we are aware that many more have been discovered but remain unpublished at the time of writing this
review), only for a bunch of them a MWL search campaign was possible, in particular for the two repeating
(and periodic) sources FRB 20121102A (commonly referred as FRB 121102) and FRB 20180916B (also referred
as FRB 180916 and FRB 180916.J0158+65, see below for the details). What makes FRB searches even more
challenging than for other transients is the duration of the event (before its flux falls below our detection
limit) at “all” wavelengths. For example while a short GRB detected in the g-rays can also last a few tens of
milliseconds [ 6,7], it can remain detectable at other wavelengths for days or longer. In the FRB case, even though
some of the many , still viable, emission mechanisms predict a sort of afterglow emission similar to that of GRBs,
they also predict a very weak signal on time scales of (at most) minutes after the radio burst. Therefore it seems
much more promising searching for an almost simultaneous, ms-duration burst also at wavelengths outside the
radio band. The recently proposed unified magnetar models by Lu et al. (2020) [ 8] and Margalit et al. (2020a) [9]
support this scenario. This would then require MWL simultaneous observational campaigns and the use of
detectors capable of acquiring data at a high cadence. Fine time resolution is normal for high-energy detectors
on-board satellites, much less for on-ground optical/NIR cameras.
In spite of the lack of MWL detections, possibly due to the limited capabilities of existing instruments,
there is no doubt that also non-detections in FRB follow-up campaigns remain of great importance. Collecting
observational data and flux upper limits at all wavelengths are helpful to constrain rate and spectral properties,
as well as to identify periods of active emission phases and then estimate the probability of events detection.
This is the case for repeating FRBs (rFRBs). Upper limits on fluxes are also relevant to constrain the fluence
ratios between the high energy bands (optical and X-/ g-ray) and the radio band, which in turn put constraints
on the proposed FRB emission models (e.g., [ 10], http://frbtheorycat.org (accessed on 1 March 2021)). At the
current stage of the FRB research, observational data that can rule out theories represent a highly valuable
work. Vacuum synchrotron maser, plasma synchrotron maser and synchrotron maser from magnetized
shocks, coherent curvature emission, are among the most invoked mechanisms (see e.g., [ 4,5,11] for a review)
but, as it was the case for GRBs, the controversy on which radiation mechanism fits best the data may last
awhile before reaching a final conclusion. With the additional complication of the (apparent/real) dicothomy
of one-off and repeating bursts. Meanwhile, coordinated MWL observing campaigns, in particular of rFRBs,
represent a key point to verify/challenge their predictions.
As new MWL observational data are being published on the transient emission from the Galactic
magnetars SGR J1935+2154 [ 12–18], 1E 1547.0–5408 [ 19], XTE J1810–197 [ 20], Swift J1818.0–1607 [ 21],
similarities with the FRB phenomena become more and more striking, and then the possible common
physical processes involved [ 8]. On the other hand MWL campaigns on FRB 20180916B can rule out the
occurrence of magnetar giant ﬂares (MGF) ( E<104547erg) either simultaneous to a few radio bursts,
or in general during some of the radio-burst active phases [ 22–26] and constrain the possible associated
persistent X-ray luminosity to < 21040erg s1[22], which is still decades above the observed persistent
luminosity of magnetars. In addition, the possible existence of a population of extragalactic magnetars
that are equally or even more active than their Galactic siblings and that can emit even more energetic
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Universe 2021 ,7, 76 3 of 48
ﬂares [ 27,28], as was also the recent case of a MGF from NGC 253 (Sculptor Galaxy) at 3.5 Mpc [ 29–31],
adds to the case of monitoring the high-energy activity of nearby rFRBs. In parallel, the expected growing
sample of rFRBs in the coming years will enable a systematic search for past activity hidden in the optical
and high-energy surveys, as was done for the few known cases (e.g., [32–36]).
Regarding the FRBs host galaxies, as of today 13 have been ﬁrmly identiﬁed. Such limited sample
does not yet allow us to draw solid conclusions about potential progenitors as observational selection
biases could play an important role. However statistical studies of stellar masses and star formation rates
(SFRs) suggest that, at least some of them, are consistent with the host galaxies of core-collapse supernovae
(CCSNe), but not with the hosts of long GRBs (LGRBs) and superluminous supernovae (SLSNe-I) [ 37–39].
This strengthens the possibility that FRBs are produced by magnetars. As a larger sample of FRB hosts
becomes available, possibly with offset distribution and local environment studies, it may turn up evidence
for alternate magnetar formation channels or call for a second progenitor scenario for FRBs.
FRB 20121102A [ 40] was the ﬁrst FRB for which multiple bursts were detected, and is then known as
the “repeating FRB” [41]. Karl G. Jansky Very Large Array (VLA) sub-arcsec localisation allowed its host
galaxy at z'0.193 to be identiﬁed [ 42–44]. FRB 20180916B [ 45] was discovered by the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) and was immediately identiﬁed as a repeater. Follow-up very
long baseline interferometry (VLBI) campaigns led to its precise localisation and the identiﬁcation of the
host galaxy at a redshift z'0.0337 [46]. This identiﬁcation, second ever for a rFRB, immediately showed a
dichotomy with the case of the original repeater, with FRB 20180916B associated to a star-forming region
within a nearby massive spiral galaxy whereas FRB 20121102A host is a low-metallicity dwarf galaxy.
The subsequent continuous monitoring of FRB 20180916B by CHIME led to the ﬁrst identiﬁcation of a
periodicity in the active phases of a rFRB [ 47], recurring every 16.3 days and with an active window phase
of approx2.6days around the midpoint of the window. Thanks to the continuing monitoring and bursts
collection, a periodicity of 1615days in the FRB 20121102A bursts was later claimed by [ 48,49]. Models to
explain this recurring active phases are growing, with the most recent one invoking a potential connection
to ultra-luminous X-ray sources (ULXs), the closest known persistent super-Eddington sources [ 50]. More
about these two peculiar FRBs in the Section 5.
In this paper we review the outcome of most FRB MWL searches reported in the literature, discuss
the capabilities of present and being built instrumentation and what we believe are the most promising
strategies to adopt in future campaigns. In Section 2 we introduce magnetars and the FRB 20200428A
detected from SGR J1935+2154. We discuss the characteristics of the currently identified FRB host galaxies
in Section 3. A critical comparison of the various transient source hosts is also presented. In Section 4
we illustrate the various efforts and outcome from the observational campaigns and archival searches for
the high-energy counterpart of FRBs, from the optical band to the very high-energy (VHE) g-rays. We
focus in particular on coordinated observational campaigns, being the most promising approach in light
of the (quasi-)simultaneous MWL emission predicted by the magnetar-engine models. The most favoured
emission models are also briefly discussed. FRBs g-ray energetic is compared to the radio one and to that
of GRBs and galactic magnetars. In Section 5 optical and higher-energy observations of the two periodic
repeaters FRB 20121102A and FRB 20180916B are extensively discussed. The recent outcome from the MWL
observations performed during the April 2020 SGR J1935+2154 active phase are illustrated in Section 6. Our
conclusions are summarized in Section 7.
2. Magnetars
Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are thought to be magnetars, that
is, young neutron stars (NSs) with extremely high magnetic ﬁelds [ 51–53] and are among the candidates
for the sources of FRBs. About thirty magnetars [http://www.physics.mcgill.ca/~pulsar/magnetar/
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Universe 2021 ,7, 76 4 of 48
main.html] (accessed on 1 March 2021) are currently known in our Galaxy (and the Magellanic Clouds),
ﬁve of which exhibited transient radio pulsations. The recent detection of g-ray emission simultaneous
to a fast radio burst (FRB 20200428A) originated in the Galactic SGR J1935+2154 has demonstrated the
common origin of these phenomena. However the energetic for this event is of the order of 106times
that of a cosmological FRB at z1. We should point out that recently bursts just one decade more
energetic than FRB 20200428A were observed for FRB 20180916B [ 54], so it is not clear if it represents
just the tail of a population, as volumetric-rate estimates might suggest [ 8]. Assuming this is the case,
not only must emission models be able to explain the extremely wide range of radio ﬂuxes, but also the
radio-to- g-ray ﬂuence ratio of FRB 20200428A ( '2–4106in [12] and, more reliably, 3105in [13]),
which is more than ﬁve orders of magnitude greater than that of SGR 1806 20 as no FRB was observed
in the giant 27 December 2004 outburst of this SGR [ 55]. The Galactic FRB 20200428A is by far the most
radio-luminous such event detected from any Galactic magnetar. The brightest radio burst previously
seen from a magnetar was during the 2009 outburst of 1E 1547.0–408 and was three orders of magnitude
fainter. Thus, FRB 20200428A clearly suggests that magnetars can produce far brighter radio bursts than
has been previously known.
The prominent role of magnetars as promising candidates for extragalactic FRB sources has fostered a
number of complementary attempts to identify counterparts or associations with other classes of known
sources: since magnetars are believed to represent the endpoint of some core-collapsed progenitors of
long GRBs (e.g., [ 56–58]), as well as the result of a compact binary merger signalled by a short GRB
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