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Abstract We propose a scenario that can describe a broad range of fast radio burst (FRB) phenomenology, from nonrepeating bursts to highly prolific repeaters. Coherent radio waves in these bursts are produced in the polar cap region of a magnetar, where magnetic field lines are open. The angle between the rotation and magnetic axes, relative to the angular size of the polar cap region, partially determines the repetition rate and polarization properties of FRBs. We discuss how many of the properties of repeating FRBs—such as their lack of periodicity, energetics, small polarization angle (PA) swing, spectro–temporal correlation, and inferred low source density— are explained by this scenario. The systematic PA swing and the periodic modulation of long-duration bursts from nonrepeaters are also natural outcomes. We derive a lower limit of about 400 on the Lorentz factor of FRB sources applying this scenario to bursts with a linear polarization degree greater than 95%. 

Abstract Gamma-ray bursts (GRBs) are among the most energetic events in the Universe, driven by relativistic jets launched from black holes (BHs) formed during the collapse of massive stars or after the merger of two neutron stars. The jet power depends on the BH spin and the magnetic flux accreted onto it. In the standard thin disk model, jet power is limited by insufficient magnetic flux, even when the spin approaches maximum possible value. In contrast, the magnetically arrested disk (MAD) state limits jet energy by extracting significant angular momentum, braking BH rotation. We propose a unified model incorporating both standard thin disk and MAD states, identifying a universal curve for jet power per accretion rate as a function of the magnetic flux ratio, ${\Delta }_{\mathrm{eq}}={({\Phi }_{\mathrm{BH}}/{\Phi }_{\mathrm{MAD}})}_{\mathrm{eq}}$, at spin equilibrium. For long GRBs (lGRBs), the model predicts a maximum jet energy of ∼1.5% of the accretion energy, occurring at Δ_eq ∼ 0.4, where the BH equilibrium spin isa ∼ 0.5. Both long and short GRBs are unlikely to be produced by a MAD: for short GRBs, this requires an accreted mass orders of magnitude smaller than that available, while for lGRBs, the narrow progenitor mass distribution challenges the ability to produce the observed broad distribution of jet energies. This framework provides a consistent explanation for both standard and luminous GRBs, emphasizing the critical role of magnetic flux. Both long and short GRBs require magnetic flux distributions that peak around 10²⁷G cm². 

ABSTRACT Kilonovae are optical transients following the merger of neutron star binaries, which are powered by the r-process heating of merger ejecta. However, if a merger remnant is a long-lived supramassive neutron star supported by its uniform rotation, it will inject energy into the ejecta through spin-down power. The energy injection can boost the peak luminosity of a kilonova by many orders of magnitudes, thus significantly increasing the detectable volume. Therefore, even if such events are only a small fraction of the kilonova population, they could dominate the detection rates. However, after many years of optical sky surveys, no such event has been confirmed. In this work, we build a boosted kilonova model with rich physical details, including the description of the evolution and stability of a proto neutron star, and the energy absorption through X-ray photoionization. We simulate the observation prospects and find the only way to match the absence of detection is to limit the energy injection by the newly born magnetar to only a small fraction of the neutron star rotational energy, thus they should collapse soon after the merger. Our result indicates that most supramassive neutron stars resulting from binary neutron star mergers are short lived and they are likely to be rare in the Universe. 

ABSTRACT Most fast radio burst (FRB) models can be divided into two groups based on the distance of the radio emission region from the central engine. The first group of models, the so-called ‘nearby’ or magnetospheric models, invoke FRB emission at distances of 109 cm or less from the central engine, while the second ‘far-away’ models involve emission from distances of 1011 cm or greater. The lateral size for the emission region for the former class of models (≲107 cm) is much smaller than the second class of models (≳109 cm). We propose that an interstellar scattering screen in the host galaxy is well-suited to differentiate between the two classes of models, particularly based on the level of modulations in the observed intensity with frequency, in the regime of strong diffractive scintillation. This is because the diffractive length scale for the host galaxy’s interstellar medium scattering screen is expected to lie between the transverse emission-region sizes for the ‘nearby’ and the ‘far-away’ class of models. Determining the strength of flux modulation caused by scintillation (scintillation modulation index) across the scintillation bandwidth (∼1/2πδts) would provide a strong constraint on the FRB radiation mechanism when the scatter broadening (δts) is shown to be from the FRB host galaxy. The scaling of the scintillation bandwidth as ∼ν4.4 may make it easier to determine the modulation index at ≳ 1 GHz. 

Abstract We present the first multiepoch broadband radio and millimeter monitoring of an off-nuclear tidal disruption event (TDE) using the Very Large Array, the Atacama Large Millimeter/submillimeter Array, the Allen Telescope Array, the Arcminute Microkelvin Imager Large Array, and the Submillimeter Array. The off-nuclear TDE AT 2024tvd exhibits double-peaked radio light curves and the fastest-evolving radio emission observed from a TDE to date. With respect to the optical discovery date, the first radio flare rises faster thanF_ν ∼ t⁹at Δt = 88–131 days and then decays as fast asF_ν ∼ t⁻⁶. The emergence of a second radio flare is observed at Δt ≈ 194 days with an initial fast rise ofF_ν ∼ t¹⁸and an optically thin decline ofF_ν ∼ t⁻¹². We interpret these observations in the context of a self-absorbed and free–free absorbed synchrotron spectrum, while accounting for both synchrotron and inverse Compton cooling. We find that a single prompt outflow cannot easily explain these observations and that it is likely that either there is only one outflow that was launched at Δt ∼ 80 days or there are two distinct outflows, with the second launched at Δt ∼ 170–190 days. The nature of these outflows, whether sub-, mildly, or ultrarelativistic, is still unclear, and we explore these different scenarios. Finally, we find a temporal coincidence between the launch time of the first radio-emitting outflow and the onset of a power-law component in the X-ray spectrum, attributed to inverse Compton scattering of thermal photons. 

ABSTRACT We describe how gravitational lensing of fast radio bursts (FRBs) is affected by a plasma screen in the vicinity of the lens or somewhere between the source and the observer. Wave passage through a turbulent medium affects gravitational image magnification, lensing probability (particularly for strong magnification events), and the time delay between images. The magnification is suppressed because of the broadening of the angular size of the source due to scattering by the plasma. The time delay between images is modified as the result of different dispersion measures (DM) along photon trajectories for different images. Each of the image light curves is also broadened due to wave scattering so that the images could have distinct temporal profiles. The first two effects are most severe for stellar and sub-stellar mass lens, and the last one (scatter broadening) for lenses and plasma screens at cosmological distances from the source/observer. This could limit the use of FRBs to measure their cosmic abundance. On the other hand, when the time delay between images is large, such that the light curve of a transient source has two or more well-separated peaks, the different DMs along the wave paths of different images can probe density fluctuations in the IGM on scales ≲10−6 rad and explore the patchy reionization history of the universe using lensed FRBs at high redshifts. Different rotation measures (RM) along two-image paths can convert linearly polarized radiation from a source to partial circular polarization. 

ABSTRACT We show that the 216.8 ± 0.1 ms periodicity reported for the fast radio burst (FRB) 20191221A is very constraining for burst models. The high accuracy of burst periodicity (better than one part in 103), and the 2 per cent duty cycle (ratio of burst duration and interburst interval), suggest a pulsar-like rotating beam model for the observed activity; the radio waves are produced along open field lines within ∼107 cm of the neutron star surface, and the beam periodically sweeps across the observer as the star spins. According to this picture, FRB 20191221A is a factor ∼1012 scaled up version of galactic pulsars with one major difference, whereas pulsars convert rotational kinetic energy to EM waves and the outbursts of 20191221A require conversion of magnetic energy to radiation. 

ABSTRACT We describe how the observed polarization properties of an astronomical object are related to its intrinsic polarization properties and the finite temporal and spectral resolutions of the observing device. Moreover, we discuss the effect that a scattering screen, with non-zero magnetic field, between the source and observer has on the observed polarization properties. We show that the polarization properties are determined by the ratio of observing bandwidth and coherence bandwidth of the scattering screen and the ratio of temporal resolution of the instrument and the variability time of screen, as long as the length over which the Faraday rotation induced by the screen changes by ∼π is smaller than the size of the screen visible to the observer. We describe the conditions under which a source that is 100 per cent linearly polarized intrinsically might be observed as partially depolarized, and how the source’s temporal variability can be distinguished from the temporal variability induced by the scattering screen. In general, linearly polarized waves passing through a magnetized scattering screen can develop a significant circular polarization. We apply the work to the observed polarization properties of a few fast radio bursts (FRBs), and outline potential applications to pulsars. 

ABSTRACT A repeating source of fast radio bursts (FRBs) is recently discovered from a globular cluster of M81. Association with a globular cluster (or other old stellar systems) suggests that strongly magnetized neutron stars, which are the most likely objects responsible for FRBs, are born not only when young massive stars undergo core-collapse, but also by mergers of old white dwarfs. We find that the fractional contribution to the total FRB rate by old stellar populations is at least a few per cent, and the precise fraction can be constrained by FRB searches in the directions of nearby galaxies, both star-forming and elliptical ones. Using very general arguments, we show that the activity time of the M81-FRB source is between 104 and 106 yr, and more likely of the order of 105 yr. The energetics of radio outbursts put a lower limit on the magnetic field strength of 10$$^{13}\,$$G, and the spin period $$\gtrsim 0.2\,$$s, thereby ruling out the source being a milli-second pulsar. The upper limit on the persistent X-ray luminosity (provided by Chandra), together with the high FRB luminosity and frequent repetitions, severely constrains (or rules out) the possibility that the M81-FRB is a scaled-up version of giant pulses from Galactic pulsars. Finally, the 50-ns variability time of the FRB light curve suggests that the emission is produced in a compact region inside the neutron star magnetosphere, as it cannot be accounted for when the emission is at distances $$\gtrsim 10^{10}\rm \, cm$$. 

ABSTRACT We describe three different methods for exploring the hydrogen reionization epoch using fast radio bursts (FRBs) and provide arguments for the existence of FRBs at high redshift (z). The simplest way, observationally, is to determine the maximum dispersion measure (DMmax) of FRBs for an ensemble that includes bursts during the reionization. The DMmax provides information regarding reionization much like the optical depth of the cosmic microwave background to Thomson scattering does, and it has the potential to be more accurate than constraints from Planck, if DMmax can be measured to a precision better than 500 pccm−3. Another method is to measure redshifts of about 40 FRBs between z of 6 and 10 with $${\sim}10{{\ \rm per\ cent}}$$ accuracy to obtain the average electron density in four different z-bins with $${\sim}4{{\ \rm per\ cent}}$$ accuracy. These two methods do not require knowledge of the FRB luminosity function and its possible redshift evolution. Finally, we show that the reionization history is reflected in the number of FRBs per unit DM, given a fluence limited survey of FRBs that includes bursts during the reionization epoch; we show using FIRE simulations that the contribution to DM from the FRB host galaxy and circumgalactic medium during the reionization era is a small fraction of the observed DM. This third method requires no redshift information but does require knowledge of the FRB luminosity function.