The Dawes Review 13: A New Look at The Dynamic Radio Sky

TL;DR

This review synthesizes the state of the dynamic radio sky, focusing on image-domain transients evolving on timescales from seconds to years and looking ahead to SKA-era facilities. It connects fundamental emission physics—synchrotron, free-free, stellar coherent and incoherent processes—with the diverse classes of radio transients, from GRB afterglows and SNe to AGN variability, TDEs, and long-period transients. It provides a framework for evaluating surveys via a modern figure of merit, discusses detection and classification pipelines (including ML and citizen science), and highlights the critical need for real-time, commensal, multi-wavelength follow-up. The paper also surveys the progress of large-scale radio transient surveys to date and outlines the anticipated gains from future facilities (SKA, DSA-2000, ngVLA, LOFAR2.0), emphasizing the ability to characterize populations, constrain rates, and potentially discover new classes of radio transients. Overall, it charts a path from early targeted studies to comprehensive, population-level understanding of the dynamic radio sky in the SKAO era.

Abstract

Astronomical objects that change rapidly give us insight into extreme environments, allowing us to identify new phenomena, test fundamental physics, and probe the Universe on all scales. Transient and variable radio sources range from the cosmological, such as gamma-ray bursts, to much more local events, such as massive flares from stars in our Galactic neighbourhood. The capability to observe the sky repeatedly, over many frequencies and timescales, has allowed us to explore and understand dynamic phenomena in a way that has not been previously possible. In the past decade, there have been great strides forward as we prepared for the revolution in time domain radio astronomy that is being enabled by the SKA Observatory telescopes, the SKAO pathfinders and precursors, and other `next generation' radio telescopes. Hence it is timely to review the current status of the field, and summarise the developments that have happened to get to our current point. This review focuses on image domain (or `slow') transients, on timescales of seconds to years. We discuss the physical mechanisms that cause radio variability, and the classes of radio transients that result. We then outline what an ideal image domain radio transients survey would look like, and summarise the history of the field, from targeted observations to surveys with existing radio telescopes. We discuss methods and approaches for transient discovery and classification, and identify some of the challenges in scaling up current methods for future telescopes. Finally, we present our current understanding of the dynamic radio sky, in terms of source populations and transient rates, and look at what we can expect from surveys on future radio telescopes.

Figures (11)

• Figure 1: Transient phase space showing radio luminosity versus the product of timescale and observing frequency for different transient source classes, following cordes_dynamic_2004. Note that the luminosity assumes sources are beamed into only $1$ sr and no relativistic beaming, which may or may not be appropriate for individual objects, while the timescales are just the observed variability timescales and ignore more constraining limits such as the finite sizes of e.g. stellar sources. For sources with relativistic beaming the true brightness temperature could be significantly lower (e.g. readhead_equipartition_1994), while for some stellar sources the true brightness temperature could be significantly higher, as suggested by the arrows. The diagonal lines show contours of brightness temperature, with coherent emitters having $T_B>10^{12}\,$K. Adapted from pietka_variability_2015 and nimmo_burst_2022, with select sources added: the binary neutron star merger GW170817 mooley_strong_2018; LFBOTs ho_at2018cow_2019coppejans_mildly_2020ho_koala_2020; relativistic TDEs mimica_radio_2015andreoni_very_2022; flare stars/brown dwarfs hallinan_periodic_2007rose_periodic_2023route_second_2016zic_askap_2019; long-period radio pulsars caleb_discovery_2022wang_discovery_2025; Galactic Centre radio transients hyman_powerful_2005wang_discovery_2021; white dwarf pulsars pelisoli_53-min-period_2023de_ruiter_sporadic_2025 and long-period transients wang_detection_2025lee_interpulse_2025wang_discovery_2021caleb_emission-state-switching_2024hurley-walker_29_2024hurley-walker_long-period_2023hurley-walker_radio_2022dong_chime_2025. In particular we highlight the range of sources from Table \ref{['t_lpts']} that are filling out the centre of this space, straddling the coherent/incoherent divide: long-period radio pulsars are upward-pointing triangles, GCRTs are diamonds, pulsing white dwarf binaries are right-pointing triangles, and long period transients (LPTs) are squares.
• Figure 2: Model GRB lightcurves, computed using ryan_gamma-ray_2020 and inspired by piran_physics_2004. These models use the simplest possible model for a relativistic jet. The top dash-dotted curves are for infinite frequency (ignoring any effects of self-absorption), while the lower solid curves are for a frequency of 1 GHz. All curves are normalised at 10 d. For both frequencies, we show jet opening angles of $\theta_{\rm jet}=5\deg$ (blue), $15\deg$ (orange), and $30\deg$ (green) with an on-axis observer ($\theta_{\rm obs}=0$). The infinite frequency models show jet breaks when the jet has decelerated to a bulk Lorentz factor of $1/\theta_{\rm jet}$, after which they have a similar power-law behaviour with $F_\nu \propto \nu^{-p}$ (black dotted line), where $p$ is the power-law index of the electron distribution. We also show a jet seen by an off-axis observer, potentially an 'orphan afterglow', since the observer would miss any high-energy emission (red dashed curve). This has similar late-time behaviour but is much fainter at early times.
• Figure 3: Plot showing the relevant timescales of different classes of radio transients. Approximate limits of variability timescales are shown for different sources and different mechanisms. We also separate the highly-polarised largely coherent transients in the top red box from the synchrotron afterglow (Section \ref{['s_afterglow']}) in the bottom blue box. We roughly delineate the timescales for traditional transient searches that find sources individually in each epoch and associate them across epochs ($\mathrel{ \vcenter{ \ialign{ \cr$>$\cr {}$∼$\cr {} } } }\,$hours, e.g. swinbank_lofar_2015rowlinson_identifying_2019pintaldi_scalable_2022) and those that use image-subtraction or related techniques to find shorter-timescale variability at a reduced computational cost ($\mathrel{ \vcenter{ \ialign{ \cr$<$\cr {}$∼$\cr {} } } }\,$hours, e.g. wang_radio_2023fijma_new_2024smirnov_tron1_2025).
• Figure 4: Radio lightcurves of a diverse set of synchrotron transients, following examples like ho_koala_2020 and coppejans_mildly_2020. The sources include: long GRBs, sub-energetic GRBs, and short GRBs (circles; Section \ref{['s_grbs']}), supernovae (hexagons; Section \ref{['s_sne']}), TDE (squares; Section \ref{['s_tdes']}), FBOTs (pluses; Section \ref{['s_fbots']}), changing-look AGN (diamonds; Section \ref{['s_agn']}), and the potential orphan afterglow FIRST J141918.9+394036. Data are primarily at 7--10 GHz, except FIRST J141918.9+394036 (1.4 GHz, but which was scaled following mooley_late-time_2022), a few GRBs at 5 GHz, and a few TDEs at 15 GHz; in those cases no spectral correction or $K$-correction has been applied. Data were compiled by A. Gulati and are from alexander_discovery_2016alexander_radio_2017anderson_early_2024andreoni_very_2022berger_grb_2001berger_host_2001berger_jet_2000berger_radio_2003berger_afterglow_2005bietenholz_radio_2021bright_radio_2022brown_late-time_2017cendes_mildly_2022cendes_radio_2021cendes_ubiquitous_2024cenko_afterglow_2011cenko_multiwavelength_2006cenko_swift_2012chandra_comprehensive_2008chandra_discovery_2010chrimes_multi-wavelength_2024coppejans_mildly_2020djorgovski_afterglow_2001eftekhari_associating_2018fong_short_2014fong_decade_2015fong_broadband_2021frail_450_2000frail_complete_2003frail_accurate_2005frail_energetic_2006frail_enigmatic_2000frail_radio_1999galama_bright_2000galama_continued_2003goodwin_radio_2023goodwin_systematic_2025greiner_unusual_2013hajela_eight_2025hancock_grb111209a_2012harrison_broadband_2001harrison_optical_1999ho_at2018cow_2019ho_koala_2020horesh_are_2021horesh_delayed_2021horesh_unusual_2015kulkarni_radio_1998lamb_short_2019laskar_radio_2023laskar_reverse_2016laskar_vla_2018laskar_first_2022law_discovery_2018leung_search_2021levan_heavy-element_2024margutti_embedded_2019margutti_signature_2013mattila_dust-enshrouded_2018meyer_late-time_2025moin_radio_2013mooley_late-time_2022oconnor_structured_2023pasham_multiwavelength_2015perley_afterglow_2014perley_grb_2008price_grb_2002-1rhodes_rocking_2024rol_grb_2007rose_late-time_2024schroeder_long-lived_2025schroeder_radio_2024sfaradi_off-axis_2024soderberg_afterglow_2006soderberg_constraints_2004soderberg_redshift_2004soderberg_relativistic_2006stein_tidal_2021taylor_discovery_1998van_der_horst_detailed_2008zauderer_radio_2013. See https://github.com/ashnagulati/Transient_Comparison_Plots
• Figure 5: Gaia DR3 colour-magnitude diagram showing the stars in an updated version of the Sydney Radio Stars Catalogue. The colour scale shows the radio luminosity based on the maximum flux density of each star in the SRSC and the Gaia rgeo distance. The grey background points show the Gaia DR2 CMD for reference pedersen_diverse_2019, and we annotate some of the major features. The Sun is indicated by the red $\odot$ symbol. Individual stars are indicated by the coloured circles and are labeled with their types. We show a rough translation between Gaia $G_{BP}-G_{RP}$ colour and effective temperature determined from synthetic photometry (based on stsci_development_team_pysynphot_2013) and an extinction vector zhang_empirical_2023. Even cooler sources such as brown dwarfs are located off the right edge of the figure, and are not included as they do not have Gaia measurements. Figure is adapted from driessen_sydney_2024.