The short answer: nobody knows. It may well be that this program will help
providing a better answers!
More concretely, fast radio bursts (FRBs) are short, non-repeating pulses
observed at radio frequencies (Lorimer et al. 2007,
Sci, 318, 777; Thornton et al. 2013,
Sci, 341, 53). They have been serendipitously identified in surveys carried
out at the Parkes telescope. Despite similarities with the so-called "perytons",
due to radio interference of terrestrial origin (Burke-Spolaor et al.
2011, ApJ 727, 18), they are now widely accepted as genuine astronomical
phenomena. Located at high Galactic latitudes, a few such events per year are
currently discovered, which remarkably corresponds to a rate over the entire sky
as high as ~104 day-1 (Thornton et al. 2013). Most
interestingly, FRB pulses unambiguously show electromagnetic dispersion (Figure,
left): pulse arrival times are increasingly delayed with decreasing frequency,
consistent with wave propagation through a cold plasma. The delay is dependent
on the dispersion measure DM, i.e., the column density of free electrons
encountered by the wave. FRB pulse lags indicate much higher values of the DM
than expected for the Milky Way, indicating an extragalactic origin (figure,
right). Typical assumptions for the ISM and IGM densities lead to estimates
z ~ 0.5-1. However, a significant contribution to the DM could arise in
the immediate environment of the progenitor. This would invalidate this distance
estimation, even allowing a Galactic origin. With this caveat in mind, the
energies corresponding to extragalactic distances are
~1038-1040 erg and the rate is ~10-3
yr-1 per galaxy.
Figure, left. Dynamic FRB spectrum showing the delay of pulse arrival
times at lower frequencies. The inset shows the broadening of the pulse shape.
Both effects are due to electromagnetic dispersion.
Right. The dispersion
measures (DMs) of FRBs (open triangles and squares) as a function of Galactic
latitude, compared to pulsars (crosses); FRBs clearly stand out suggesting an
extragalactic origin. From Thornton et al. (2013).
Theoretical proposals. The lack of counterparts at short wavelengths and
the uncertain distance and energetics scales has led to a plethora of progenitor
models. The non-repetitiveness (Lorimer et al. 2007) suggests a catastrophic
event, and the short timescales unavoidably point to a compact object progenitor
due to causality arguments. Stellar-mass black holes and neutron stars (NSs) are
prime suspects.
Hypergiant flares of soft gamma repeaters (SGRs) have a rate comparable to FRBs
and can accommodate the inferred energetics (Popov & Postnov
2007, arXiv:0710.2006). However, known SGR giant flares develop over much
longer timescales (>100 ms). The properties and spectra of FRB pulses are not so
different from those of pulsar spikes, suggesting NSs (both strongly and
normally magnetized) as natural FRB progenitor candidates. FRBs may be produced
in the merging of a binary NS system (Totani 2013, PASJ, 65,
L12) due to the forced synchronization of the magnetic fields. Other models
involve the collapse of a supra-massive NS (that is, a rotationally-supported NS
with a mass larger then the maximum allowed mass for a non-rotating NS), either
immediately (102-103 s; Zhang 2014, ApJ, 780,
L21) or long (103-106 yr; Falcke & Rezzolla 2014, A&A, 562, 137)
after the NS formation. A highly-magnetized NS (a "magnetar") could be the
engine of late-time activity in a fraction of both short and long gamma-ray
bursts (GRBs) — in this case, we could even expect that FRBs may be
associated with a fraction of GRBs. Due to different collimation geometries, not
each FRB would be paired with an observed GRB. Interestingly, some of the above
events (such as NS mergers) are among the most copious sources of gravitational
waves (GWs). FRBs can thus offer an extra avenue to localize and characterize GW
events in the forthcoming advanced LIGO/VIRGO era.
At the other extreme, more mundane explanations have been proposed, including
flaring variable stars in the Galaxy (Loeb et al. 2014,
MNRAS, 439, L46), based on spatial coincidence of bright stars inside the
(large) FRB error boxes, though the limited statistics again limit any
conclusion. In fact, the lengthy list reported above reflects both our ignorance
and excitement when faced with a completely new, unknown astrophysical
phenomenon.
FRBs as comoslogical probes. Should FRBs be confirmed to occur at
extragalactic distances, they would offer a new avenue to probe environments in
the distant Universe, after QSOs, GRBs, and SNe. As the DM reveals the total
line-of-sight electron content, FRBs are excellent probes of the ionised
component of the IGM and host ISM. In addition, the frequency-dependent pulse
broadening due to scattering and multi-path propagation (observed in two of the
known FRBs to date) provides a diagnostic of the density inhomogeneities and
turbulent properties of the gas. However, disentangling the host contribution to
the total DM and pulse broadening requires more knowledge of the host
environment, which the shorter wavelength observations can provide. Direct
redshift measurements are critical if the DMs of a population of FRBs at a range
of redshifts and sight-lines are to be used for tomography of the ionised IGM.
The potential of FRBs will likely rise once their production mechanism is better
constrained.
Our proposal
We aim at detecting and characterizing the optical and near-infrared
counterparts of FRBs. The first key step is simply to localize such objects,
assuming that they have detectable optical emission. No emission at short
wavelengths has ever been detected before! A plain detection would be a major
breakthrough, enabling a number of tests and inferences on FRBs. First, optical
emission would enable spectroscopy, which in turn is the direct avenue to
measuring or constraining the FRB redshift. This would thus settle once and for
all the distance scale, as well as fix their energetics and volumetric rate. A
precise FRB localization would also enable searchs of a quiescent counterpart,
be it a host galaxy, a Galactic star, or a gas cloud. The identification of such
an object and its properties would provide tremendous clues on the FRB
progenitor.
Once detected, measuring the spectral and timing properties of the FRB
emission would allow to feed models with precious information. Broad-band
photometry would constrain the spectral shape of the emission (thermal?
power-law?), and potentially detect the effect of dust extinction (if any).
Light curve monitoring would reveal the presence of multiple emission
components, such as kilonova emission (expected from binary NS mergers) or even
a regular SN contribution.
