Radio Emission Research Articles

The TRAPUM Large Magellanic Cloud pulsar survey with MeerKAT – I. Survey set-up and first seven pulsar discoveries

ABSTRACT The Large Magellanic Cloud (LMC) presents a unique environment for pulsar population studies due to its distinct star formation characteristics and proximity to the Milky Way. As part of the TRAPUM (TRAnsients and PUlsars with MeerKAT) Large Survey Project, we are using the core array of the MeerKAT radio telescope (MeerKAT) to conduct a targeted search of the LMC for radio pulsars at L-band frequencies, 856–1712 MHz. The excellent sensitivity of MeerKAT, coupled with a 2-h integration time, makes the survey 3 times more sensitive than previous LMC radio pulsar surveys. We report the results from the initial four survey pointings which have resulted in the discovery of seven new radio pulsars, increasing the LMC radio pulsar population by 30 per cent. The pulse periods of these new pulsars range from 278 to 1690 ms, and the highest dispersion measure is 254.20 $\mathrm{ pc} \, \mathrm{ cm}^{-3}$. We searched for, but did not find any significant pulsed radio emission in a beam centred on the SN 1987A remnant, establishing an upper limit of 6.3 $\mu \mathrm{ Jy}$ on its minimum flux density at 1400 MHz.

Beyond the Rotational Deathline: Radio Emission from Ultra-long Period Magnetars

ABSTRACT Motivated by the recent detection of ultralong-period radio transients, we investigate new models of coherent radio emission via low-altitude electron–positron pair production in neutron stars (NSs) beyond rotationally powered curvature radiation deathlines. We find that plastic motion (akin to ‘continental drift’) and qualitatively similar thermoelectric action by temperature gradients in the crusts of slowly rotating, highly magnetized NSs could impart mild local magnetospheric twists. Regardless of which mechanism drives twists, we find that particle acceleration initiates pair cascades across charge-starved gaps above a mild critical twist. Cascades are initiated via resonant inverse-Compton scattered photons or curvature radiation, and may produce broad-band coherent radio emission. We compute the pair luminosity (maximum allowed radio luminosity) for these two channels, and derive deathlines and ‘active zones’ in $P-\dot{P}$ space from a variety of considerations. We find these twist-initiated pair cascades only occur for magnetar-like field strengths $B \gtrsim 10^{14}$ G and long periods: $P_{\rm RICS} \gtrsim 120 \,\, (T/10^{6.5} {\rm K})^{-5} \, {\rm s}$ and $P_{\rm curv} \gtrsim 150 \,\, ({\rm v_{\rm pl}}/10^{3} {\, \rm cm \, yr^{-1}})^{-7/6} \, {\rm s}$. Using a simplified geometric model, we find that plastic motion or thermoelectrically driven twists might naturally reproduce the observed luminosities, time-scales, and timing signatures. We further derive ‘active zones’ in which rotationally powered pair creation occurs via resonantly scattered photons, beyond standard curvature deathlines for pulsars. All cascades are generically accompanied by simultaneous (non-)thermal X-ray/UV counterparts which might be detectable with current instrumentation.

A nebular origin for the persistent radio emission of fast radio bursts.

Fast radio bursts (FRBs) are millisecond-duration, bright (approximately Jy) extragalactic bursts, whose production mechanism is still unclear1. Recently, two repeating FRBs were found to have a physically associated persistent radio source of non-thermal origin2,3. These two FRBs have unusually large Faraday rotation measure values2,3, probably tracing a dense magneto-ionic medium, consistent with synchrotron radiation originating from a nebula surrounding the FRB source4-8. Recent theoretical arguments predict that, if the observed Faraday rotation measure mostly arises from the persistent radio source region, there should be a simple relation between the persistent radio source luminosity and the rotation measureitself7,9. Here we report the detection of a third, less luminous persistent radio source associated with the repeating FRB source FRB 20201124A at a distance of 413 Mpc, substantially expanding the predicted relation into the low luminosity-low Faraday rotation measure regime (<1,000 rad m-2). At lower values of the Faraday rotation measure, the expected radio luminosity falls below the limit-of-detection threshold for present-day radio telescopes. These findings support the idea that the persistent radio sources observed so far are generated by a nebula in the FRB environment and that FRBs with low Faraday rotation measure may not show a persistent radio source because of a weaker magneto-ionic medium. This is generally consistent with models invoking a young magnetar as the central engine of the FRB, in which the surrounding ionized nebula-or the interacting shock in a binary system-powers the persistent radio source.

Non-thermal electrons in an eruptive solar event: Magnetic structure, confinement, and escape into the heliosphere

Filament eruptions and coronal mass ejections (CMEs) reveal large-scale instabilities of magnetic structures in the solar corona. Some of them are accompanied by radio emission, which at decimetric and longer wavelengths is a signature of electron acceleration that may be different from the acceleration in impulsive flares. The radio emission is part of the broadband continua at decimetre and metre wavelengths called type IV bursts. In this article we investigate a particularly well-observed combination of a filament eruption seen in Halpha and at extreme ultraviolet (EUV) wavelengths and a moving type IV burst on 2021 August 24. The aim is to shed light on the relationship between the large-scale erupting magnetic structure and the acceleration and transport of non-thermal electrons. We used imaging observations of a moving radio source and associated burst groups with the refurbished Nan c ay Radioheliograph and whole-Sun radio spectrography from different ground-based and space-borne instruments, in combination with X-ray, radio, and in situ electron observations at tens of keV from Solar Orbiter and EUV imaging by SDO/AIA. The radio sources are located with respect to the erupting magnetic structure traced by the filament (EUV 30.4 nm), and the timing of the electrons detected in situ is compared with the timing of the different radio emissions. We find that the moving radio source is located at the top of the erupting magnetic structure outlined by the filament, which we interpret as a magnetic flux rope. The flux rope erupts in a strongly non-radial direction, guided by the overlying magnetic field of a coronal hole. The electrons detected at Solar Orbiter are found to be released mainly in two episodes, 10 to 40 minutes after the impulsive phase. The releases coincide with two groups of radio bursts, which originate respectively on the flank and near the top of the erupting flux rope. The observation allows an unusually clear association between a moving type IV radio burst, an erupting magnetic flux rope as core structure of a CME, and particle releases into the heliosphere. Non-thermal electrons are confined in the flux rope. Electrons escape to the heliosphere mainly in two distinct episodes, which we relate to magnetic reconnection between the flux rope and ambient open field lines.

A comparative study of radio signatures from winds and jets: modelling synchrotron emission and polarization

ABSTRACT Outflows driven by active galactic nuclei (AGNs) are seen in numerous compact sources; however, it has remained unclear how to distinguish between the driving mechanisms, such as winds and jets. Therefore, our study aims to offer observational insights from simulations to aid in this distinction. Specifically, in this paper, we investigate the evolution of wide-angled moderately relativistic magnetized winds and analyse their non-thermal radio emission and polarization properties. We find that the evolution of winds varies depending on factors such as power, density, and opening angle, which in turn influence their observable characteristics. Additionally, different viewing angles can lead to varying observations. Furthermore, we note distinctions in the evolution of winds compared to jets, resulting in disparities in their observable features. Jets typically exhibit a thin spine and hotspot(s). Winds manifest broader spines or an ‘hourglass-shaped’ bright emission in the cocoon, which are capped by bright arcs. Both display high polarization coinciding with the bright spine and hotspots/arcs, although these regions are relatively compact and localized in jets when compared to winds. We emphasize the importance of high resolution, as we demonstrate that emission features from both jets and winds can become indistinguishable at lower resolutions. The distribution of polarization is largely unaffected by resolution, though lower polarization becomes more noticeable when the resolution is decreased.

Searching for pulsars, magnetars, and fast radio bursts in the sculptor galaxy using MeerKAT

Abstract The Sculptor Galaxy (NGC 253), located in the Southern Hemisphere, far off the Galactic Plane, has a relatively high star-formation rate of about 7 M⊙ yr−1 and hosts a young and bright stellar population, including several super star clusters and supernova remnants. It is also the first galaxy, apart from the Milky Way Galaxy to be associated with two giant magnetar flares. As such, it is a potential host of pulsars and/or fast radio bursts in the nearby Universe. The instantaneous sensitivity and multibeam sky coverage offered by MeerKAT therefore make it a favourable target. We searched for pulsars, radio-emitting magnetars, and fast radio bursts in NGC 253 as part of the TRAPUM large survey project with MeerKAT. We did not find any pulsars during a four-hour observation, and derive a flux density limit of 4.4 μJy at 1400 MHz, limiting the pseudo-luminosity of the brightest putative pulsar in this galaxy to 54 Jy kpc2. Assuming universality of pulsar populations between galaxies, we estimate that detecting a pulsar as bright as this limit requires NGC 253 to contain a pulsar population of ⪞20 000. We also did not detect any single pulses and our single pulse search flux density limit is 62 mJy at 1284 MHz. Our search is sensitive enough to have detected any fast radio bursts and radio emission similar to the brighter pulses seen from the magnetar SGR J1935+2154 if they had occurred during our observation.

Signatures of Active Galactic Nucleus Feedback Modes: A Green Bean Galaxy with 150 kpc Jet-induced Radio Emission

Jetted active galactic nuclei (AGN) hosting extended photoionized nebulae provide us with a unique view of the timescales associated with AGN activity. Here, we present a new green bean galaxy at z = 0.304458 ± 0.000007 with large-scale jet-induced radio emission. The spectral energy distributions of the radio components show steep spectral indices (α = −0.85 to −0.92 for the extended regions, and α = −1.02 for the faint radio core), and spectral age modeling of the extended radio emission indicates that the lobes are >6 Myr old. It is unclear whether the jet is active, or is a remnant with an off-time of 2–3 Myr. Several detached clouds lie around the host galaxy up to 37.8 kpc away from the nucleus, and their ionization profile indicates a decline (∼2 dex) in the AGN ionizing photon production over the past ∼0.15 Myr. Furthermore, we measure a blueshift for one of the clouds that is spatially coincident with the path of the radio jet. The cloud is likely illuminated by the photoionizing AGN, and potentially underwent an interaction with the relativistic jet. Our multiwavelength analysis suggests that RGB1 was in a phase of jet production prior to the radiatively efficient accretion phase traced by the detached cloud emission. It is unclear whether RGB1 transitioned into a low-excitation radio galaxy or an inactive galaxy over the past ∼0.15 Myr, or whether the extended radio and optical emission trace distinct accretion phases that occurred in succession.

Ubiquitous Late Radio Emission from Tidal Disruption Events

We present radio observations of 23 optically discovered tidal disruption events (TDEs) on timescales of ∼500–3200 days postdiscovery. We detect nine new TDEs that did not have detectable radio emission at earlier times, indicating a late-time brightening after several hundred (and up to 2300) days; an additional seven TDEs exhibit radio emission whose origin is ambiguous or may be attributed to the host galaxy or an active galactic nucleus. We also report a new rising component in one TDE previously detected in the radio at ∼103 days. While the radio emission in some of the detected TDEs peaked on a timescale ≈2–4 yr, over half of the sample still show rising emission. The range of luminosities for the sample is ∼1037–1039 erg s−1, about 2 orders of magnitude below the radio luminosity of the relativistic TDE Sw J1644+57. Our data set indicates ∼40% of all optical TDEs are detected in radio hundreds to thousands of days after discovery, and that this is probably more common than early radio emission peaking at ∼102 days. Using an equipartition analysis, we find evidence for a delayed launch of the radio-emitting outflows, with delay timescales of ∼500–2000 days, inferred velocities of ≈0.02–0.15c, and kinetic energies of ∼1047–1049 erg. We rule out off-axis relativistic jets as a viable explanation for this population, and conclude delayed outflows are a more likely explanation, possibly from delayed disk formation. We conclude late radio emission marks a fairly ubiquitous but heretofore overlooked phase of TDE evolution.

Follow-up LOFAR observations of the τ Boötis exoplanetary system

Context. Observing the radio emission from exoplanets is among the most promising methods to detect their magnetic fields and a measurement of an exoplanetary magnetic field will help constrain the planet’s interior structure, star-planet interactions, atmospheric escape and dynamics, and habitability. Recently, circularly polarized bursty and slow emission from the τ Boötis (τ Boo) exoplanetary system was tentatively detected using LOFAR (LOW-Frequency ARray) beamformed observations. If confirmed, this detection will be a major contribution to exoplanet science. However, follow-up observations are required to confirm this detection. Aims. Here, we present such follow-up observations of the τ Boo system using LOFAR. These observations cover 70% of the orbital period of τ Boo b including the orbital phases of the previous tentative detections. Methods. We used the BOREALIS pipeline to mitigate radio frequency interference and to search for bursty and slowly varying radio signals. BOREALIS was previously used to find the tentative radio signals from τ Boo. Results. Our new observations do not show any signs of bursty or slow emission from the τ Boötis exoplanetary system. Conclusions. The cause for our non-detection is currently degenerate. It is possible that the tentative radio signals were an unknown instrumental systematic or that we are observing variability in the planetary radio emission due to changes in its host star. More radio data (preferably multi-site) and ancillary observations (e.g. magnetic maps) are required to further investigate the potential radio emission from the τ Boötis exoplanetary system.

Non-thermal radio emission in Sakurai's Object

The very late thermal pulse (VLTP) affects the evolution of sim 20&lt;!PCT!&gt; of 1--8\,$ M_ stars, repeating the last red giant phases within a few years and leading to the formation of a new, but hydrogen-poor, nebula within the old planetary nebula. The strong dust formation in the latter obscures the optical and near-infrared radiation of the star. We aimed to determine the reheating timescale of the central star in Sakurai's Object, which is an important constraint for the poorly understood VLTP evolution. We observed the radio continuum emission of Sakurai's Object for almost 20 years, from 2004 to 2023. Continuous, multi-frequency observations proved to be essential for distinguishing between phases dominated by photoionization and shock ionization. The flux density fluctuates by more than a factor of 40 within months to years. The spectral index remained negative between 2006 and 2017 and has been close to zero since 2019. The emission region has been only barely resolved since 2021. Non-thermal radio emission observed from 2004 to 2017 traces shocks induced by wind interactions due to discrete mass-loss events. Thermal emission dominates from 2019 to 2023 and may indicate photoionization of the nebula by the central star.

Prevalence of Compact Nuclear Radio Emission in Post-merger Galaxies and Its Origin

Post-merger galaxies are unique laboratories to study the triggering and interplay of star formation and active galactic nucleus (AGN) activity. Combining new, high-resolution Jansky Very Large Array (VLA) observations with archival radio surveys, we have examined the radio properties of 28 spheroidal post-merger galaxies. We detect 18 radio sources in our post-merger sample and find a general lack of extended emission at (sub)kiloparsec scales, indicating the prevalence of compact, nuclear radio emission in these post-merger galaxies, with the majority (16/18; 89%) characterized as low luminosity. Using multiwavelength data, we determine the origin of the radio emission, discovering 15 new radio AGNs and three radio sources likely associated with star-forming (SF) processes. Among the radio AGNs, almost all are low luminosity (13/15; 87%), inconsistent with a relativistic jet origin. We discover a new dual AGN (DAGN) candidate, J1511+0417, and investigate the radio properties of the DAGN candidate J0843+3549. Five of these radio AGNs are hosted by a SF or SF-AGN composite emission-line galaxy, suggesting that radio AGN activity may be present during periods of SF activity in post-mergers. The low-power jets and compact morphologies of these radio AGNs also point to a scenario in which AGN feedback may be efficient in this sample of post-mergers. Lastly, we present simulated, multifrequency observations of the 15 radio AGNs with the Very Long Baseline Array and the very-long-baseline interferometry capabilities of the Next-Generation VLA to assess the feasibility of these instruments in searches for supermassive black hole binaries.

Periods and Frequency Drifts of Groups of the Decimetric Spikes in Two Solar Flares

We studied the radio emission occurring as narrowband decimetric spikes observed during the 10 May 2022 and 26 August 2022 flares. In the radio spectra, these spikes were distributed in groups that occurred quasi-periodically with the periods 5.1 s in the 10 May 2022 flare and 9.1 s in the 26 August 2022 flare. In some parts of these groups, even subgroups of spikes distributed with the quasi-periods of 0.19 s (10 May 2022 flare), and 0.17 s and 0.21 s (26 August 2022 flare) were found. Some of these subgroups even drifted to higher or lower frequencies, which was observed for the first time. At the time of the dm-spikes observation, a pair of reconnecting loops are identified in the SDO/AIA EUV observations of the 10 May 2022 flare, one of which is interpreted as belonging to a small erupting filament. We propose that these loops reconnect in the dynamic quasi-periodic regime (the period 0.19 s) and this reconnection is modulated by an oscillation of one of the interacting loops (the period 5.1 s). Accelerated electrons from this process are trapped in reconnecting plasma outflows, and thus the drifting groups of spikes are generated. The 26 August 2022 flare is a complex event with several systems of bright loops; nevertheless, it also shows a disintegrating erupting filament similar to the 10 May 2022 flare, meaning that the dm-spikes are likely generated by similar reconnection processes.

Late-time Radio Flares in Tidal Disruption Events

Radio monitoring unveiled late (hundreds to a thousand days) radio flares in a significant fraction of tidal disruption events. We propose that these late-time radio flares are a natural outcome if the surrounding density profile flattens outside the Bondi radius. At the Bondi radius, the outflow is optically thin (above a few GHz) to synchrotron self-absorption. As more and more material is swept up, the radio emission rises asymptotically as ∝ t 3 until the outflow begins to decelerate. A detection of such a rise and a late-time maximum constrains the black hole mass and the mass and energy of the radio-emitting outflow. We show that this model can give reasonable fits to some observed light curves, leading to reasonable estimates of the black hole and outflow masses. We also find that the slope of the density profile within the Bondi radius determines whether an early-time (∼102 days) radio peak exists.

Probing particle acceleration in Abell 2256: From 16 MHz to gamma rays

Merging galaxy clusters often host spectacular diffuse radio synchrotron sources. These sources can be explained by a non-thermal pool of relativistic electrons that are accelerated by shocks and turbulence in the intracluster medium. The origin of the pool and details of the cosmic ray transport and acceleration mechanisms in clusters are still open questions. Due to the often extremely steep spectral indices of diffuse radio emission, it is best studied at low frequencies. However, the lowest frequency window available to ground-based telescopes (10−30 MHz) has remained largely unexplored as radio frequency interference and calibration problems related to the ionosphere become severe. Here, we present LOFAR observations from 16 to 168 MHz targeting the famous cluster Abell 2256. In the deepest-ever images at decametre wavelengths, we detected and resolved the radio halo, radio shock, and various steep spectrum sources. We measured standard single power-law behaviour for the radio halo and radio shock spectra, with spectral indices of α = −1.56 ± 0.02 from 24 to 1500 MHz and α = −1.00 ± 0.02 from 24 to 3000 MHz, respectively. Additionally, we found significant spectral index and curvature fluctuations across the radio halo, indicating an inhomogeneous emitting volume. In contrast to the straight power-law spectra of the large-scale diffuse sources, the various AGN-related sources showed extreme steepening towards higher frequencies and flattening towards low frequencies. We also discovered a new fossil plasma source with a steep spectrum between 23 and 144 MHz, with α = −1.9 ± 0.1. Finally, by comparing radio and gamma-ray observations, we ruled out purely hadronic models for the radio halo origin in Abell 2256, unless the magnetic field strength in the cluster is exceptionally high, which is unsupportable by energetic arguments and inconsistent with the knowledge of other cluster magnetic fields.

Interstellar Scintillations of Sources B0821+394 and B1812+412 as Observed by the LPA LPI Radio Telescope

The search for long-term variability of compact components of radio sources B0821+394 and B1812+412 over an interval of ten years was carried out. The LPA LPI radio telescope with an operating frequency of 111 MHz was used for observations. According to our estimates, the characteristic time of variability for both sources is 1.5–2.5 years. It is shown that the observed variability is not related to intrinsic variations in the radiation flux, but is due to refractive scintillation on inhomogeneities of the interstellar medium. From the obtained upper estimates of the apparent angular dimensions of the sources, it follows that the main contribution to the scattering of radio emission is made by turbulent plasma concentrated in sufficiently thin screens, the distance to which does not exceed 300–400 pc.

Synthesis of praseodymium doped lutetium and gadolinium aluminum garnets modified by scandium and boron to improve luminescence properties

The structural and luminescence properties of the Lu3Al4Sc1O12:Pr and Gd3Al3Sc2O12:Pr garnet scintillators co-doped with boron (B3+) ions were investigated. The addition of B3+ and Sc3+ to the structure has a positive effect on the luminescence properties, especially on the photo, radio emission intensities, quantum efficiency, and results in a reduction of the decay time, which is necessary for high-quality scintillators. Structural properties of garnets have been determined after X-ray diffraction analysis coupled with Rietveld refinement. Surface morphology analysis was performed using a scanning electron microscope and particle size was also determined. Elemental composition was confirmed by the ICP-OES technique. Reflection, excitation (in the UV and VUV range), emission (upon excitation by UV or X-rays), decay time, and quantum efficiency at room temperature were measured. Furthermore, the influence of temperature on excitation, emission, and decay curves was also analyzed. The obtained luminescence properties indicated positive effect caused by doping.

A Surprising Excess of Radio Emission in Extremely Stable Quasars: A Unique Clue to Jet Launching?

Quasars are generally divided into jetted radio-loud and nonjetted radio-quiet ones, but why only 10% of quasars are radio-loud has been puzzling for decades. Other than jet-induced phenomena, black hole mass, or Eddington ratio, a prominent difference between jetted and nonjetted quasars has scarcely been detected. Here we show a unique distinction between them, and the mystery of jet launching could be disclosed by a prominent excess of radio emission in extremely stable quasars (ESQs; i.e., type 1 quasars with extremely weak variability in UV/optical over 10 yr). Specifically, we find that >25% of the ESQs are detected by the FIRST/VLASS radio survey, while only ∼6%–8% of the control sample, matched in redshift, luminosity, and Eddington ratio, are radio-detected. The excess of radio detection in ESQs has a significance of 4.4σ (99.9995%) and predominantly occurs at intermediate radio loudness with R ∼ 10–60. The radio detection fraction of ESQs also tends to increase in the ESQ samples selected with more stringent thresholds. Our results are in contrast to the common view that radio-loud quasars are likely more variable in UV/optical due to jet contribution. New clues/challenges posed by our findings highlight the importance of extensive follow-up observations to probe the nature of jets in ESQs and theoretical studies on the link between jet launching and ESQs. Moreover, our results make ESQs, an essential population that has never been explored, unique targets in the burgeoning era of time domain astronomy, like their opposite counterparts of quasars exhibiting extreme variability or changing-look features.

Absolute emission height determination of the radio emission components of PSR B2111+46 at multiple bands by relativistic phase shift method

Absolute emission height determination of the radio emission components of PSR B2111+46 at multiple bands by relativistic phase shift method

Flavor composition of ultrahigh-energy cosmic neutrinos: Measurement forecasts for in-ice radio-based EeV neutrino telescopes

In-ice radio detection is a promising technique to discover and characterize ultrahigh-energy (UHE) neutrinos, with energies above 1017 eV, adopted by present—ARA, ARIANNA, and RNO-G—and planned—IceCube-Gen2—experiments. So far, their ability to measure neutrino flavor had remained unexplored. We show and quantify how the neutrino flavor can be measured with in-ice radio detectors using two complementary detection channels. The first channel, sensitive to νe, identifies them via their charged-current interactions, whose radio emission is elongated in time due to the Landau-Pomeranchuk-Migdal effect. The second channel, sensitive to νμ and ντ, identifies events made up of multiple showers generated by the muons and taus they generate. We show this in state-of-the-art forecasts geared at IceCube-Gen2, for representative choices of the UHE neutrino flux. This newfound sensitivity could allow us to infer the UHE neutrino flavor composition at their sources—and thus the neutrino production mechanism—and to probe UHE neutrino physics. Published by the American Physical Society 2024

Constraints on fast radio burst emission in the aftermath of gamma-ray bursts

Constraints on fast radio burst emission in the aftermath of gamma-ray bursts