Radio Afterglow Research Articles

The Early Radio Afterglow of Short GRB 230217A

We present the radio afterglow of short gamma-ray burst (GRB) 230217A, which was detected less than 1 day after the gamma-ray prompt emission with the Australia Telescope Compact Array (ATCA) and the Karl G. Jansky Very Large Array. The ATCA rapid-response system automatically triggered an observation of GRB 230217A following its detection by the Neil Gehrels Swift Observatory and began observing the event just 32 minutes postburst at 5.5 and 9 GHz for 7 hr. Dividing the 7 hr observation into three time-binned images allowed us to obtain radio detections with logarithmic central times of 1, 2.8, and 5.2 hr postburst, the first of which represents the earliest radio detection of any GRB to date. The decline of the light curve is consistent with reverse shock emission if the observing bands are below the spectral peak and not affected by synchrotron self-absorption. This makes GRB 230217A the fifth short GRB (SGRB) with radio detections attributed to a reverse shock at early times (<1 day postburst). Following brightness temperature arguments, we have used our early radio detections to place the highest minimum Lorentz factor ( Γmin>50 at ∼1 hr) constraints on a GRB in the radio band. Our results demonstrate the importance of rapid radio follow-up observations with long integrations and good sensitivity for detecting the fast-evolving radio emission from SGRBs and probing their reverse shocks.

Is Gamma-Ray Burst 221009A Really a Once-in-10,000 yr Event?

Gamma-ray bursts (GRBs) brighter than the GRB 221009A, the brightest yet observed, have previously been estimated to occur at a rate of one per 10,000 yr, based on the extrapolation of the distribution of fluences of the long-GRB population. We show that bursts this bright could instead have a rate as high as approximately one per 200 yr if they are from a separate population of narrow-jet GRBs. This population must have a maximum redshift of about z ≈ 0.38 in order to avoid overproducing the observed rate of fainter GRBs. We show that it will take ≳100 yr to confirm this new population based on observing another GRB from it with a γ-ray detector; observing an orphan optical afterglow from this population with Vera Rubin Observatory or an orphan radio afterglow with the Square Kilometer Array will also take similarly long times to observe, and it is unclear if they could be distinguished from the standard GRB population. We show that the nearby narrow-jet population has more favorable energetics for producing ultra-high-energy cosmic rays than standard GRBs. The rate of bursts in the Milky Way bright enough to cause mass extinctions of life on Earth from the narrow-jet population is estimated to be approximately one per 500 Myr. This GRB population could make life in the Milky Way less likely, with implications for future searches for life on exoplanets.

Radio Afterglows from Tidal Disruption Events: An Unbiased Sample from ASKAP RACS

Late-time (∼a year) radio follow-up of optically discovered tidal disruption events (TDEs) is increasingly resulting in detections at radio wavelengths, and there is growing evidence for this late-time radio activity to be common to the broad class of subrelativistic TDEs. Detailed studies of some of these TDEs at radio wavelengths are also challenging the existing models for radio emission. Using all-sky multiepoch data from the Australian Square Kilometre Array Pathfinder (ASKAP), taken as a part of the Rapid ASKAP Continuum Survey (RACS), we searched for radio counterparts to a sample of optically discovered TDEs. We detected late-time emission at RACS frequencies (742–1032 MHz) in five TDEs, reporting the independent discovery of radio emission from TDE AT 2019ahk and extending the time baseline out to almost 3000 days for some events. Overall, we find that at least 22−11+15% of the population of optically discovered TDEs has detectable radio emission in the RACS survey, while also noting that the true fraction can be higher given the limited cadence (two epochs separated by ∼3 yr) of the survey. Finally, we project that the ongoing higher-cadence (∼2 months) ASKAP Variable and Slow Transients survey can detect ∼20 TDEs in its operational span (4 yr), given the current rate from optical surveys.

Late Jets, Early Sparks: Illuminating the Premaximum Bumps in Superluminous Supernovae

Superluminous supernovae (SLSNe) radiate ≳10–100 times more energy than ordinary stellar explosions, implicating a novel power source behind these enigmatic events. One frequently discussed source, particularly for hydrogen-poor (Type I) SLSNe, is a central engine such as a millisecond magnetar or accreting black hole. Both black hole and magnetar engines are expected to channel a fraction of their luminosity into a collimated relativistic jet. Using 3D relativistic hydrodynamical simulations, we explore the interaction of a relativistic jet, endowed with a luminosity L j ≈ 1045.5 erg s−1 and duration t eng ≈ 10 days compatible with those needed to power SLSNe, launched into the envelope of the exploding star. The jet successfully breaks through the expanding ejecta, and its shocked cocoon powers ultraviolet/optical emission lasting several days after the explosion and reaching a peak luminosity ≳1044 erg s−1, corresponding to a sizable fraction of L j . This high radiative efficiency is the result of the modest adiabatic losses the cocoon experiences owing to the low optical depths of the enlarged ejecta at these late times, e.g., compared to the more compact stars in gamma-ray bursts. The luminosity and temperature of the cocoon emission match those of the “bumps” in SLSN light curves observed weeks prior to the optical maximum in many SLSNe. Confirmation of jet breakout signatures by future observations (e.g., days-long to weeks-long internal X-ray emission from the jet for on-axis observers, spectroscopy confirming large photosphere velocities v/c ≳ 0.1, or detection of a radio afterglow) would offer strong evidence for central engines powering SLSNe.

The Progenitor and Central Engine of Short-duration GRB 201006A Associated with a Coherent Radio Flash

Recently, the detection of a coherent radio flash associated with short-duration GRB 201006A, occurring 76.6 minutes after the burst, has attracted great attention. However, the physical origin of the coherent radio flash remains under debate. By reanalyzing its data observed by Fermi and Swift, we find that an early radio afterglow as the physical origin of the radio flash can be ruled out, but the coherent radio emission seems to be consistent with the hypothesis of a supramassive magnetar as the central engine collapsing into a black hole. Within this scenario, the derived magnetar surface magnetic field (B p ) and the initial spin period (P 0) fall into a reasonable range but require a preferably low value of η R = 10−7 or 10−6. Moreover, the calculated low-ε value and E γ,iso–E p correlation of GRB 201006A also supports the progenitor which is from the merger of compact stars. We also discuss the non-detected kilonova emission associated with GRB 201006A, and then compare with its upper limits of optical observations.

A Radio Flare in the Long-lived Afterglow of the Distant Short GRB 210726A: Energy Injection or a Reverse Shock from Shell Collisions?

We present the discovery of the radio afterglow of the short gamma-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of z ∼ 2.4. While radio observations commenced ≲1 day after the burst, no radio emission was detected until ∼11 days. The radio afterglow subsequently brightened by a factor of ∼3 in the span of a week, followed by a rapid decay (a “radio flare”). We find that a forward shock afterglow model cannot self-consistently describe the multiwavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of ≈5. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of ≈4, or a reverse shock from a shell collision are viable solutions to match the broadband behavior. At z ∼ 2.4, GRB 210726A is among the highest-redshift short GRBs discovered to date, as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by ≲10 days after the burst, potentially missing these late-rising, luminous radio afterglows.

Multi-messenger prospects for black hole – neutron star mergers in the O4 and O5 runs

The existence of merging black hole-neutron star (BHNS) binaries has been ascertained through the observation of their gravitational wave (GW) signals. However, to date, no definitive electromagnetic (EM) emission has been confidently associated with these mergers. Such an association could help unravel crucial information on these systems, for example, their BH spin distribution, the equation of state (EoS) of the neutron star and the rate of heavy element production. We modeled the multi-messenger (MM) emission from BHNS mergers detectable during the fourth (O4) and fifth (O5) observing runs of the LIGO-Virgo-KAGRA (LVK) GW detector network in order to provide detailed predictions that can help enhance the effectiveness of observational efforts and extract the highest possible scientific information from such remarkable events. Our methodology is based on a population synthesis approach, which includes the modeling of the signal-to-noise ratio of the GW signal in the detectors, the GW-inferred sky localization of the source, the kilonova (KN) optical and near-infrared light curves, the relativistic jet gamma-ray burst (GRB) prompt emission peak photon flux, and the GRB afterglow light curves in the radio, optical, and X-ray bands. The resulting prospects for BHNS MM detections during O4 are not promising, with an LVK GW detection rate of 15.0−8.8+15.4 yr−1, but joint MM rates of ∼10−1 yr−1 for the KN and ∼10−2 yr−1 for the jet-related emission. In O5, we found an overall increase in expected detection rates by around an order of magnitude, owing to both the enhanced sensitivity of the GW detector network and the coming online of future EM facilities. Considering variations in the NS EoS and BH spin distribution, we find that the detection rates can increase further by up to a factor of several tens. Finally, we discuss direct searches for the GRB radio afterglow with large field-of-view instruments during O5 and beyond as a new possible follow-up strategy in the context of ever-dimming prospects for KN detection due to the recession of the GW horizon.

A Narrow Uniform Core with a Wide Structured Wing: Modeling the TeV and Multiwavelength Afterglows of GRB 221009A

The TeV afterglow of the BOAT GRB 221009A was interpreted as arising from a narrow jet, while the radio-to-X-ray afterglows were interpreted as arising from a wide structured jet. However, there is no model explaining the TeV and lower-energy multiwavelength afterglows simultaneously. We here investigate a two-component jet model, including a narrow uniform core with a wide structured wing, to explain both the multiwavelength afterglows that last up to 100 days. We find that to explain the early TeV afterglow with the inverse-Compton process, we need a circumburst density higher than ≳0.1 cm−3, while the radio afterglow and the H.E.S.S. upper limit combine to constrain the density to be lower at larger radii. Thus, a decreasing density profile with radius is favored. Considering that the rising TeV light curve during the afterglow onset favors a constant-density medium, we invoke a stratified density profile, including a constant-density profile at small radii and a wind density profile at large radii. We find that the two-component jet model with such a stratified density profile can explain the TeV, X-ray, and optical afterglows of GRB 221009A, although the radio fluxes exceed the observed ones by a factor of 2 at later epochs. The discrepancy in the radio afterglow could be resolved by invoking some nonstandard assumption about the microphysics of afterglow shocks. The total kinetic energy of the two components in our model is ≲1052 erg, significantly smaller than that in the single structured jet models.

Spatially Resolved Observations of Meteor Radio Afterglows With the OVRO‐LWA

AbstractWe conducted an all‐sky imaging transient search with the Owens Valley Radio Observatory Long Wavelength Array (OVRO‐LWA) data collected during the Perseid meteor shower in 2018. The data collection during the meteor shower was motivated to conduct a search for intrinsic radio emission from meteors below 60 MHz known as the meteor radio afterglows (MRAs). The data collected were calibrated and imaged using the core array to obtain lower angular resolution images of the sky. These images were input to a pre‐existing LWA transient search pipeline to search for MRAs as well as cosmic radio transients. This search detected 5 MRAs and did not find any cosmic transients. We further conducted peeling of bright sources, near‐field correction, visibility differencing and higher angular resolution imaging using the full array for these 5 MRAs. These higher angular resolution images were used to study their plasma emission structures and monitor their evolution as a function of frequency and time. With higher angular resolution imaging, we resolved the radio emission size scales to less than 1 km physical size at 100 km heights. The spectral index mapping of one of the long duration event showed signs of diffusion of plasma within the meteor trails. The unpolarized emission from the resolved radio components suggest resonant transition radiation as the possible radiation mechanism of MRAs.

Temperature-Dependent Kinetics for the Reactions of Fen- (n = 2-17) and FexNiy- (x + y = 3-9) with O2: Comparison of Pure and Mixed Metal Clusters with Relevance to Meteor Radio Afterglows and Surface Oxidation.

Rate constants and product branching fractions were measured from 300-600 K for Fen- + O2 (n = 2-17) and for 300-500 K for FexNiy- + O2 (x + y = 3-9) using a selected-ion flow tube (SIFT) apparatus. Rate constants for 46 species are reported. All rate constants increased with increasing temperature, and several were in excess of the Langevin-Gioumousis-Stevenson (LGS) capture rate at elevated temperatures. As with previously studied transition metal anion oxidation reactions, the collision limit is treated as the sum of the LGS limit along with a hard-sphere contribution, allowing for determination of activation energies. These values are compared to each other along with previous results for Nin-. Measured rate constants for all three series (Fen-, Nin-, and FexNy-) vary over a relatively narrow range (1-5 × 10-10 cm3 s-1 at 300 K) being at least 15% of the collision rate constant. All reaction rate constants increase with temperature, described by small activation energies of 0.5-4 kJ mol-1. The data are consistent with an anticorrelation between the electron binding energy and rate constant, previously noted in other systems. The Fen- reaction produces a larger population of higher energy electrons than do the Nin- reactions, with FexNiy- producing an intermediate amount. The results suggest that the overall rate constant is limited by a small energetic barrier located at a large internuclear distance where electrostatic forces dominate, causing the potentials to be similar across systems, while the product formation is determined by the shorter-range, valence portion of the potential, which varies widely between systems.

Seeing Deeper via Radio Afterglow Imaging.

Seeing Deeper via Radio Afterglow Imaging.

The intensity of diffuse galactic emission reflected by meteor trails

ABSTRACT We calculate the reflection of diffuse galactic emission by meteor trails and investigate its potential relationship to meteor radio afterglow (MRA). The formula to calculate the reflection of diffuse galactic emission is derived from a simplified case, assuming that the signals are mirrored by the cylindrical overdense ionization trail of meteors. The overall observed reflection is simulated through a ray tracing algorithm together with the diffuse galactic emission modelled by the Global Sky Model sky model. We demonstrate that the spectrum of the reflected signal is broad-band and follows a power law with a negative spectral index of around −1.3. The intensity of the reflected signal varies with local sidereal time and the brightness of the meteor and can reach 2000 Jy. These results agree with some previous observations of MRAs. Therefore, we think that the reflection of galactic emission by meteor trails can be a possible mechanism causing MRAs, which is worthy of further research.

Radio Plateaus in Gamma-Ray Burst Afterglows and Their Application in Cosmology

The plateau phase in radio afterglows has been observed in very few gamma-ray bursts (GRBs), and in this paper, 27 radio light curves with plateau phases were acquired from the published literature. We obtain the related parameters of the radio plateau, such as temporal indexes during the plateau phase (α 1 and α 2), break time (T b,z), and the corresponding radio flux (F b). The two-parameter Dainotti relation between the break time of the plateau and the corresponding break luminosity (L b,z) in the radio band is . Including the isotropic energy E γ,iso and peak energy E p,i, the three-parameter correlations for the radio plateaus are written as and , respectively. The correlations are less consistent with those of the X-ray and optical plateaus, implying that radio plateaus may have a different physical mechanism. The typical frequencies crossing the observational band may be a reasonable hypothesis that causes the breaks of the radio afterglows. We calibrate the GRB empirical luminosity correlations as a standard candle for constraining cosmological parameters and find that our samples can constrain the flat ΛCDM model well but are not sensitive to the nonflat ΛCDM model. By combining GRBs with other probes, such as supernovae and the CMB, the constraints on the cosmological parameters are Ωm = 0.297 ± 0.006 for the flat ΛCDM model and Ωm = 0.283 ± 0.008, ΩΛ = 0.711 ± 0.006 for the nonflat ΛCDM model.

A Unified Theory of Jetted Tidal Disruption Events: From Promptly Escaping Relativistic to Delayed Transrelativistic Jets

Only a tiny fraction ∼1% of stellar tidal disruption events (TDEs) generate powerful relativistic jets evidenced by luminous hard X-ray and radio emissions. We propose that a key property responsible for both this surprisingly low rate and a variety of other observations is the typically large misalignment ψ between the orbital plane of the star and the spin axis of the supermassive black hole (SMBH). Such misaligned disk/jet systems undergo Lense–Thirring precession together about the SMBH spin axis. We find that TDE disks precess sufficiently rapidly that winds from the accretion disk will encase the system on large scales in a quasi-spherical outflow. We derive the critical jet efficiency η > η crit for both aligned and misaligned precessing jets to successfully escape from the disk wind ejecta. As η crit is higher for precessing jets, less powerful jets only escape after alignment with the SMBH spin. Alignment can occur through magneto-spin or hydrodynamic mechanisms, which we estimate occur on typical timescales of weeks and years, respectively. The dominant mechanism depends on η and the orbital penetration factor β. Hence, depending only on the intrinsic parameters of the event {ψ, η, β}, we propose that each TDE jet can either escape prior to alignment, thus exhibiting an erratic X-ray light curve and two-component radio afterglow (e.g., Swift J1644+57), or escape after alignment. Relatively rapid magneto-spin alignments produce relativistic jets exhibiting X-ray power-law decay and bright afterglows (e.g., AT2022cmc), while long hydrodynamic alignments give rise to late jet escape and delayed radio flares (e.g., AT2018hyz).

Prediction for the Multi-band Afterglows of FRB 200428 and its Implication

The physical mechanism of fast radio bursts (FRBs) is still unknown. On 2020 April 28, a special radio burst, FRB 200428, was detected and believed to be associated with the Galactic magnetar SGR 1935+2154. It confirms that at least some of the FRBs were generated by magnetars, although the radiation mechanism continues to be debated. To this end, we study in detail the multiband afterglows of FRB 200428 described by the synchrotron fireball shock model. We find the prediction for the optical and radio afterglows of FRB 200428 is consistent with the observations when considering the post-FRB optical and radio upper limits from the literature. We also show that the follow up detection of the afterglows from fast radio bursts like—FRB 200428 is possible at the radio band, though challenging. Based on our model, one can obtain information about the energy of the fireball, the radiation zone, and the nature of the surrounding medium. That may shed light on the physical mechanism of FRBs.

Molecular radio afterglow probes for cancer radiodynamic theranostics.

X-ray-induced afterglow and radiodynamic therapy tackle the tissue penetration issue of optical imaging and phototherapy. However, inorganic nanophosphors used in this therapy have their radio afterglow dynamic function as always on, limiting the detection specificity and treatment efficacy. Here we report organic luminophores (IDPAs) with near-infrared afterglow and 1O2 production after X-ray irradiation for cancer theranostics. The in vivo radio afterglow of IDPAs is >25.0 times brighter than reported inorganic nanophosphors, whereas the radiodynamic production of 1O2 is >5.7 times higher than commercially available radio sensitizers. The modular structure of IDPAs permits the development of a smart molecular probe that only triggers its radio afterglow dynamic function in the presence of a cancer biomarker. Thus, the probe enables the ultrasensitive detection of a diminutive tumour (0.64 mm) with superb contrast (tumour-to-background ratio of 234) and tumour-specific radiotherapy for brain tumour with molecular precision at low dosage. Our work reveals the molecular guidelines towards organic radio afterglow agents and highlights new opportunities for cancer radio theranostics.

GRB 221009A afterglow from a shallow angular structured jet

ABSTRACT Exceptionally bright gamma-ray burst (GRB) afterglows can reveal the angular structure of their jets. GRB jets appear to have a narrow core (of half-opening angle θc), beyond which their kinetic energy drops as a power-law with angle θ from the jet’s symmetry axis, $E_{\mathrm{ k},\rm iso}(\theta)\propto [1+(\theta /\theta _\mathrm{ c})^2]^{-a/2}$. The power-law index a reflects the amount of mixing between the shocked jet and confining medium, which depends on the jet’s initial magnetization. Weakly magnetized jets undergo significant mixing, leading to shallow (a ≲ 2) angular profiles. We use the exquisite multiwaveband afterglow observations of GRB 221009A to constrain the jet angular structure using a dynamical model that accounts for both the forward and reverse shocks, for a power-law external density profile, next ∝ R−k. Both the forward shock emission, that dominates the optical and X-ray flux, and the reverse shock emission, that produces the radio afterglow, require a jet with a narrow core (θc ≈ 0.021) and a shallow angular structure (a ≈ 0.8) expanding into a stellar wind (k ≈ 2). Moreover, these data appear to favour a small fraction (ξe ≈ 10−2) of shock heated electrons forming a power-law energy distribution in both shocks.

A matched-filter approach to radio variability and transients: searching for orphan afterglows in the VAST Pilot Survey

ABSTRACT Radio transient searches using traditional variability metrics struggle to recover sources whose evolution time-scale is significantly longer than the survey cadence. Motivated by the recent observations of slowly evolving radio afterglows at gigahertz frequency, we present the results of a search for radio variables and transients using an alternative matched-filter approach. We designed our matched-filter to recover sources with radio light curves that have a high-significance fit to power-law and smoothly broken power-law functions; light curves following these functions are characteristic of synchrotron transients, including ‘orphan’ gamma-ray burst afterglows, which were the primary targets of our search. Applying this matched-filter approach to data from Variables and Slow Transients Pilot Survey conducted using the Australian SKA Pathfinder, we produced five candidates in our search. Subsequent Australia Telescope Compact Array observations and analysis revealed that: one is likely a synchrotron transient; one is likely a flaring active galactic nucleus, exhibiting a flat-to-steep spectral transition over 4 months; one is associated with a starburst galaxy, with the radio emission originating from either star formation or an underlying slowly evolving transient; and the remaining two are likely extrinsic variables caused by interstellar scintillation. The synchrotron transient, VAST J175036.1–181454, has a multifrequency light curve, peak spectral luminosity, and volumetric rate that is consistent with both an off-axis afterglow and an off-axis tidal disruption event; interpreted as an off-axis afterglow would imply an average inverse beaming factor $\langle f^{-1}_{\text{b}} \rangle = 860^{+1980}_{-710}$, or equivalently, an average jet opening angle of $\langle \theta _{\textrm {j}} \rangle = 3^{+4}_{-1}\,$ deg.

Rapid radio brightening of GRB 210702A

ABSTRACT We observed the rapid radio brightening of GRB 210702A with the Australia Telescope Compact Array (ATCA) just 11 h post-burst, tracking early-time radio variability over a 5 h period on ∼15 min time-scales at 9.0, 16.7, and 21.2 GHz. A broken power law fit to the 9.0 GHz light curve showed that the 5 h flare peaked at a flux density of 0.4 ± 0.1 mJy at ∼13 h post-burst. The observed temporal and spectral evolution is not expected in the standard internal–external shock model, where forward and reverse shock radio emission evolves on much longer time-scales. The early-time (&amp;lt;1 d) optical and X-ray light curves from the Neil Gehrels Swift Observatory demonstrated typical afterglow forward shock behaviour, allowing us to use blast wave physics to determine a likely homogeneous circumburst medium and an emitting electron population power-law index of p = 2.9 ± 0.1. We suggest that the early-time radio flare is likely due to weak interstellar scintillation (ISS), which boosted the radio afterglow emission above the ATCA sensitivity limit on minute time-scales. Using relations for ISS in the weak regime, we were able to place an upper limit on the size of the blast wave of ≲6 × 1016 cm in the plane of the sky, which is consistent with the theoretical forward shock size prediction of 8 × 1016 cm for GRB 210702A at ∼13 h post-burst. This represents the earliest ISS size constraint on a gamma-ray burst (GRB) blast wave to date, demonstrating the importance of rapid (&amp;lt;1 d) radio follow-up of GRBs using several-hour integrations to capture the early afterglow evolution and to track the scintillation over a broad frequency range.

Radio-bright versus radio-dark gamma-ray bursts – more evidence for distinct progenitors

ABSTRACT We analyse two distinct samples of gamma-ray bursts (GRBs), with and without radio afterglow emission. We use an updated sample of 211 GRBs and find, in agreement with previous results (although with a sample that is almost twice as large), that the intrinsic gamma-ray duration (Tint) and isotropic equivalent energy (Eiso) distributions between these two populations appear to be significantly different. This implies that the radio-bright GRBs are more energetic and last longer than radio-dark GRBs. The two samples’ redshift distributions (z) are not statistically different. We analyse several correlations between variables (Eiso, Tint, jet opening angle, and z), accounting for selection effects and redshift evolution using the Efron–Petrosian method. We find a statistically significant anticorrelation between the jet opening angle and redshift, as well as between Tint and redshift, for both radio-bright and radio-dark GRBs. Finally, in agreement with previous work, we find that very high energy (0.1–100 GeV) extended emission is present only in the radio-bright GRB sample. Our work supports the possibility that the radio-bright and radio-dark GRBs originate from different progenitors.