arXiv subjects

Joseph R. Callingham

Publications and source records attributed to Joseph R. Callingham.

At least 37 records · Page 2Linked to original sources

arXiv2025

The Serpent Eating Its Own Tail: Dust Destruction in the Apep Colliding-Wind Nebula

Much of the carbonaceous dust observed in the early universe may originate from colliding wind binaries (CWBs) hosting hot, luminous Wolf-Rayet (WR) stars. Downstream of the shock between the stellar winds there exists a suitable environment for dust grain formation, and the orbital motion of the stars wraps this dust into richly structured spiral geometries. The Apep system is the most extreme WR-CWB in our Milky Way: two WR stars produce a complex spiral dust nebula, whose slow expansion has been linked to a gamma-ray burst progenitor. It has been unclear whether the O-type supergiant 0.7" distant from the WR+WR binary is physically associated with the system, and whether it affects the dusty nebula. Multi-epoch VLT/VISIR and JWST/MIRI observations show that this northern companion star routinely carves a cavity in the dust nebula - the first time such an effect has been observed in a CWB - which unambiguously associates the O star as a bound component to the Apep system. These observations are used together with a new geometric model to infer the cavity geometry and the orbit of the WR+WR binary, yielding the first strong constraints on wind and orbital parameters. We confirm an orbital period of over 190 years for the inner binary - nearly an order of magnitude longer than the next longest period dust-producing WR-CWB. This, together with the confirmed classification as a hierarchical triple, cements Apep as a singular astrophysical laboratory for studying colliding winds and the terminal life stages of the most massive star systems.

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arXiv2025

Strongly polarised radio pulses from a new white-dwarf-hosting long-period transient

Long-period transients (LPTs) are a new and enigmatic class of objects that produce bright pulsations in the radio, with periods far exceeding those seen in rotationally powered pulsars. The proposed progenitors for LPTs are contested, with white dwarfs or magnetars being likely candidates. Here, we present the discovery of ILT\,J163430+445010, a new LPT detected in a blind search for Stokes\,V transients in the LOFAR Two-Metre Sky Survey. Unusual for LPTs, J1634+44 shows pulses that are 100\% circularly polarised, as well as pulses that are 100\% linearly polarised, with the polarisation state changing from pulse to pulse. We detect 19 pulses in total, each with a total polarisation fraction of $\sim100\%$ and a pulse duration of at most 10\,s. The pulses show a periodicity at $841.24808\pm0.00015$\,s, implying a low duty cycle of $0.012$. J1634+44 has a marginally detected counterpart in the ultraviolet GALEX MIS survey and the ultraviolet/optical UNIONS survey, suggesting that it contains a white dwarf with an effective temperature between 15000\,K and 33000\,K. We do not detect J1634+44 with a deep $J$-band exposure with UKIRT at a $3σ$ AB magnitude limit of 24.7, ruling out a main-sequence star or ultracool dwarf with a spectral type earlier than M7. The pulses from J1634+44 follow a particular pattern, with two pulses being produced every five periods after a waiting time of two or three periods. This pattern could be a result of spin-orbit coupling in a binary system with a 5:2 or 5:3 resonance, where a companion induces beamed radio emission on the white dwarf. The companion is most likely an ultracool dwarf or another white dwarf, making J1634+44 unique among the currently known sample of LPTs.

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arXiv2025

Searching for planet-induced radio signal from the young close-in planet host star HIP 67522

HIP 67522 is a 17 Myr old 1.2$M_\odot$ dwarf, and the only such young star known to host two close-in gas giant planets. The inner planet likely orbits close enough to its host to power magnetic star-planet interactions. In the radio domain, magnetic star-planet interaction is expected to produce a unique signature: electron cyclotron maser emission (ECME), beamed in phase with the orbit of the close-in planet. We conducted the longest radio monitoring campaign of a G dwarf host star to date to search for ECME, totaling $135\,$h on HIP 67522 over a period of five months with the Australia Telescope Compact Array (ATCA) between $1.1-3.1$GHz. We find that HIP 67522 is active in the radio, with emission that varies stochastically, with a duty cycle of $69\%$ above $0.24$mJy, and frequent bursts. Both the bursts and the quiescent emission are consistent with the canonical Güdel-Benz relation, and show a positive spectral index and brightness temperatures $\geq 10^{10}$K, indicating likely a flaring origin. Our observations cover $61\%$ of the innermost planet's orbit, including multiple visits of the quadrature where planet-induced ECME detection is most likely for this system. However, no orbital modulation or persistent polarization of the radio emission was detected. Our upper limit on circularly polarized emission from HIP 67522 suggests a $<0.7\%$ conversion efficiency of the magnetic power generated in the star-planet interaction into radio waves, unless the emission was missed by our phase or frequency coverage, or was absorbed in the circumstellar plasma. HIP 67522 is a system with one of the highest expected powers of star-planet interaction among known systems and shows strong indication of planet-induced flaring, motivating observations at other wavelengths to probe for further dissipation pathways.

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arXiv2025

The First Large Absorption Survey in HI (FLASH): II. Pilot Survey data release and first results

The First Large Absorption Survey in HI (FLASH) is a large-area radio survey for neutral hydrogen in the redshift range 0.4 1$, and appear to be a mixture of intervening and associated systems. Interestingly, around two-thirds of the lines found in this untargeted sample are detected against sources with a peaked-spectrum radio continuum, which are only a minor (5-20%) fraction of the overall radio-source population. The overall detection rate for HI absorption lines in the Pilot Surveys (0.3 to 0.5 lines per ASKAP field) is a factor of two below the expected value. One possible reason for this is the presence of a range of spectral-line artefacts in the Pilot Survey data that have now been mitigated and are not expected to recur in the full FLASH survey. A future paper will discuss the host galaxies of the HI absorption systems identified here.

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arXiv2025

Catching the wisps: Stellar mass-loss limits from low-frequency radio observations

The winds of low-mass stars carry away angular momentum and impact the atmospheres of surrounding planets. Determining the properties of these winds is necessary to understand the mass-loss history of the star and the evolution of exoplanetary atmospheres. Due to their tenuous nature, the winds of low-mass main-sequence stars are difficult to detect. The few existing techniques for measuring these winds are indirect, with the most common inference method for winds of low-mass stars being astrospheric Lyman-$α$ absorption combined with complex hydrodynamical modelling of the interaction between the stellar wind and the interstellar medium. Here, we employ a more direct method to place upper limits on the mass-loss rates of low-mass stars by combining observations of low-frequency coherent radio emission, the lack of free-free absorption, and a simple stellar wind model. We determine upper limits on the mass-loss rate for a sample of 19 M dwarf stars detected with the LOFAR telescope at 120--168 MHz, reaching a sensitivity within an order of magnitude of the solar mass-loss rate for cold stars with a surface magnetic field strength of $\sim$100 G. The sensitivity of our method does not depend on distance or spectral type, allowing us to find mass-loss rate constraints for stars up to spectral type M6 and out to a distance of 50 pc, later and farther than previous measurements. With upcoming low-frequency surveys with both LOFAR and the Square Kilometre Array, the number of stars with mass-loss rate upper limits determined with this method could reach $\sim$1000.

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arXiv2024

Core prominence as a signature of restarted jet activity in the LOFAR radio-galaxy population

(abridged) Characterizing duty cycles of recurrent phases of dormancy and activity in supermassive black holes in active galactic nuclei is crucial in understanding impact of energy released on host galaxies and their evolution. However, identifying sources in quiescent and restarted phases is challenging. Our goal is to identify and characterize a substantial sample of radio galaxies in restarted phase and explore core prominence as a signature of this activity. We expand our prior study from a $30\,\mathrm{deg^2}$ area in Lockman Hole to a larger $424\,\mathrm{deg^2}$ region in Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) field using visually selected core-dominated radio galaxies. We used 144 MHz LOFAR survey images to identify galaxies with restarting jets. By applying selection criteria, including radio core dominance, low surface brightness extended emission, spectral index properties, and morphology, we found 69 candidate restarted radio galaxies. These candidates show diverse intrinsic morphology, spanning FRI, FRII, core-with-halo, and asymmetric forms, suggesting different progenitors. Among these, nine galaxies exhibit ultra-steep spectrum extended emission combined with high radio core prominence, representing previous and current epochs of jet activity. This subset supports a model where the switch-on and switch-off mechanism occurs with fast duty cycle. The restarted candidates span radio luminosities from log$_{10}$(L$_\mathrm{144 MHz}$/$\mathrm{WHz^{-1}}$) = 23.24 to 26.80, with sizes between 88 and 1659 kpc, including 16 giant radio galaxies. Their total stellar content aligns with massive elliptical galaxies. Our findings at $z<0.4$ suggest that many restarting galaxies are not found in rich cluster environments, consistent with broader radio-galaxy population. This study confirms core prominence as an effective parameter for selecting restarted radio sources.

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arXiv2023

Phenomenology and periodicity of radio emission from the stellar system AU Microscopii

Stellar radio emission can measure a star's magnetic field strength and structure, plasma density and dynamics, and the stellar wind pressure impinging on exoplanet atmospheres. However, properly interpreting the radio data often requires temporal baselines that cover the rotation of the stars, orbits of their planets and any longer-term stellar activity cycles. Here we present our monitoring campaign on the young, active M dwarf AU Microscopii with the Australia Telescope Compact Array between 1.1 and 3.1 GHz. With over 250 hours of observations, these data represent the longest radio monitoring campaign on a single main-sequence star to date. We find that AU Mic produces a wide variety of radio emission, for which we introduce a phenomenological classification scheme predicated on the polarisation fraction and time-frequency structure of the emission. The radio emission detected on AU Mic can be broadly categorised into five distinct types of bursts, and broadband quiescent emission. The radio bursts are highly circularly polarised and periodic with the rotation period of the star, implying that the emission is beamed. It is therefore most likely produced by the electron cyclotron maser instability. We present a model to show that the observed emission can be explained with auroral rings on the magnetic poles. The total intensity of the broadband emission is stochastic, but we show that its circular polarisation fraction is also periodic with the rotation of the star. We present a qualitative model to describe the periodicity in the polarisation fraction of the broadband emission using gyromagnetic emission, and infer a magnetic obliquity of at least 20 degrees from the observed variation in polarisation fraction. Finally, we show that the radio emission might be evolving on long timescales, hinting at a potential stellar magnetic activity cycle.

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arXiv2023

Radio emission as a stellar activity indicator

Radio observations of stars trace the plasma conditions and magnetic field properties of stellar magnetospheres and coronae. Depending on the plasma conditions at the emitter site, radio emission in the metre- and decimetre-wave bands is generated via different mechanisms such as gyrosynchrotron, electron cyclotron maser instability, and plasma radiation processes. The ongoing LOFAR Two-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most sensitive wide-field radio sky surveys ever conducted. Because these surveys are untargeted, they provide an opportunity to study the statistical properties of the radio-emitting stellar population in an unbiased manner. Here, we perform an untargeted search for stellar radio sources down to sub-mJy level using these radio surveys. We find that the population of radio-emitting stellar systems is mainly composed of two distinct categories: chromospherically active stellar (CAS) systems and M dwarfs. We also seek to identify signatures of a gradual transition within the M-dwarf population from chromospheric/coronal acceleration close to the stellar surface similar to that observed on the Sun, to magnetospheric acceleration occurring far from the stellar surface similar to that observed on Jupiter. We determine that radio detectability evolves with spectral type, and we identify a transition in radio detectability around spectral type M4, where stars become fully convective. Furthermore, we compare the radio detectability vs spectra type with X-ray and optical flare (observed by TESS) incidence statistics. We find that the radio efficiency of X-ray/optical flares, which is the fraction of flare energy channelled into radio-emitting charges, increases with spectral type. These results motivate us to conjecture that the emergence of large-scale magnetic fields in CAS systems and later M dwarfs leads to an increase in radio efficiency.

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arXiv2022

Radio masers on WX UMa: hints of a Neptune-sized planet, or magnetospheric reconnection?

The nearby M dwarf WX UMa has recently been detected at radio wavelengths with LOFAR. The combination of its observed brightness temperature and circular polarisation fraction suggests that the emission is generated via the electron-cyclotron maser instability. Two distinct mechanisms have been proposed to power such emission from low-mass stars: either a sub-Alfvénic interaction between the stellar magnetic field and an orbiting planet, or reconnection at the edge of the stellar magnetosphere. In this paper, we investigate the feasibility of both mechanisms, utilising the information about the star's surrounding plasma environment obtained from modelling its stellar wind. Using this information, we show that a Neptune-sized exoplanet with a magnetic field strength of 10-100 G orbiting at ~0.034 au can accurately reproduce the observed radio emission from the star, with corresponding orbital periods of 7.4 days. Due to the stellar inclination, a planet in an equatorial orbit is unlikely to transit the star. While such a planet could induce radial velocity semi-amplitudes from 7 to 396 m s$^{-1}$, it is unlikely that this signal could be detected with current techniques due to the activity of the host star. The application of our planet-induced radio emission model here illustrates its exciting potential as a new tool for identifying planet-hosting candidates from long-term radio monitoring. We also develop a model to investigate the reconnection-powered emission scenario. While this approach produces less favourable results than the planet-induced scenario, it nevertheless serves as a potential alternative emission mechanism which is worth exploring further.

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arXiv2021

The GLEAMing of the First Supermassive Black Holes

We present the results of a new selection technique to identify powerful ($L_{\rm 500\,MHz}>10^{27}\,$WHz$^{-1}$) radio galaxies towards the end of the Epoch of Reionisation. Our method is based on the selection of bright radio sources showing radio spectral curvature at the lowest frequency ($\sim 100\,$MHz) combined with the traditional faintness in $K-$band for high redshift galaxies. This technique is only possible thanks to the Galactic and Extra-galactic All-sky Murchison wide-field Array (GLEAM) survey which provides us with 20 flux measurements across the $70-230\,$MHz range. For this pilot project, we focus on the GAMA 09 field to demonstrate our technique. We present the results of our follow-up campaign with the Very Large Telescope, Australian Telescope Compact Array and the Atacama Large Millimetre Array (ALMA) to locate the host galaxy and to determine its redshift. Of our four candidate high redshift sources, we find two powerful radio galaxies in the $1 5.5$ has a high $25-50\%$ success rate.

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arXiv2021

The TESS View of LOFAR Radio-Emitting Stars

The recent detection of the M dwarf GJ 1151 at 144 MHz low radio frequencies using LOFAR has been interpreted as evidence of an exoplanet magnetically interacting with its host star. This would be the first exoplanet detected around a main sequence star by a radio telescope. Radial velocity confirmation of such a planet has proven inconclusive, and it remains possible that the radio emission could be generated by a stellar coronal process. Using data from TESS, we shed light on this question by probing the stellar activity and flares of GJ 1151 as well as fourteen other M dwarfs detected by LOFAR. GJ 1151 and three other star-planet interaction candidates are found to be inactive, with no rotational modulation and few, if any, flares. The remainder of the LOFAR detected M dwarfs flare frequently. We consider it unlikely that stellar activity is responsible for the bright, circularly-polarized emission from GJ 1151 and its three analogs, and support the star-planet magnetic interaction interpretation.

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arXiv2021

High-resolution international LOFAR observations of 4C~43.15 -- Spectral ages and injection indices in a high-z radio galaxy

Radio sources with steep spectra are preferentially associated with the most distant galaxies, the $α-z$ relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low-frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a $z = 2.4$ radio galaxy at frequencies between $121$ MHz and $166$ MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the $α-z$ relation. We targeted 4C 43.15 with the High Band Antennas (HBAs) of the \textit{International LOFAR Telescope} (ILT) with a range of baselines up to $1300\ \mathrm{km}$. At the central frequency of $143$ MHz we achieve an angular resolution of $\sim 0.3''$. By complementing our data with archival \textit{Very Large Array} (VLA) data we study the spectral index distribution across 4C 43.15 between $55\ \mathrm{MHz}$ and $8.4\ \mathrm{GHz}$ at resolutions of $0.4''$ and $0.9''$. With a magnetic field strength of $B = 5.2$ nT and fitted injection indices of $α^\mathrm{north}_\mathrm{inj} = -0.8$ and $α^\mathrm{south}_\mathrm{inj} = -0.6$, fitting a Tribble spectral ageing model results in a spectral age of $τ_\mathrm{spec} = 1.1 \pm 0.1$ Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the $α-z$ relation.

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arXiv2021

Evidence for cold plasma in planetary nebulae from radio observations with the LOw Frequency ARray (LOFAR)

We present observations of planetary nebulae with the LOw Frequency ARray (LOFAR) between 120 and 168 MHz. The images show thermal free-free emission from the nebular shells. We have determined the electron temperatures for spatially resolved, optically thick nebulae. These temperatures are 20 to 60% lower than those estimated from collisionally excited optical emission lines. This strongly supports the existence of a cold plasma component, which co-exists with hot plasma in planetary nebulae. This cold plasma does not contribute to the collisionally excited lines, but does contribute to recombination lines and radio flux. Neither of the plasma components are spatially resolved in our images, although we infer that the cold plasma extends to the outer radii of planetary nebulae. However, more cold plasma appears to exist at smaller radii. The presence of cold plasma should be taken into account in modeling of radio emission of planetary nebulae. Modelling of radio emission usually uses electron temperatures calculated from collisionally excited optical and/or infrared lines. This may lead to an underestimate of the ionized mass and an overestimate of the extinction correction from planetary nebulae when derived from the radio flux alone. The correction improves the consistency of extinction derived from the radio fluxes when compared to estimates from the Balmer decrement flux ratios.

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arXiv2020

The GLEAM 4-Jy (G4Jy) Sample: II. Host-galaxy identification for individual sources

The entire southern sky (Declination, $δ<$ 30 deg) has been observed using the Murchison Widefield Array (MWA), which provides radio imaging of $\sim$2-arcmin resolution at low frequencies (72-231 MHz). This is the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, and we have previously used a combination of visual inspection, cross-checks against the literature, and internal matching to identify the 'brightest' radio-sources ($S_{\mathrm{151MHz}} >$ 4 Jy) in the extragalactic catalogue (Galactic latitude, $|b| >$ 10 deg). We refer to these 1,863 sources as the GLEAM 4-Jy (G4Jy) Sample, and use radio images (of $\leq$ 45-arcsec resolution), and multi-wavelength information, to assess their morphology and identify the galaxy that is hosting the radio emission (where appropriate). Details of how to access all of the overlays used for this work are available at https://github.com/svw26/G4Jy. Alongside this we conduct further checks against the literature, which we document in this paper for individual sources. Whilst the vast majority of the G4Jy Sample are active galactic nuclei with powerful radio-jets, we highlight that it also contains a nebula, two nearby, star-forming galaxies, a cluster relic, and a cluster halo. There are also three extended sources for which we are unable to infer the mechanism that gives rise to the low-frequency emission. In the G4Jy catalogue we provide mid-infrared identifications for 86% of the sources, and flag the remainder as: having an uncertain identification (129 sources), having a faint/uncharacterised mid-infrared host (126 sources), or it being inappropriate to specify a host (2 sources). For the subset of 129 sources, there is ambiguity concerning candidate host-galaxies, and this includes four sources (B0424$-$728, B0703$-$451, 3C 198, and 3C 403.1) where we question the existing identification.

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arXiv2020

The GLEAM 4-Jy (G4Jy) Sample: I. Definition and the catalogue

The Murchison Widefield Array (MWA) has observed the entire southern sky (Declination, $δ<$ 30 deg) at low radio-frequencies, over the range 72-231 MHz. These observations constitute the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, and we use the extragalactic catalogue (Galactic latitude, $|b| >$ 10 deg) to define the GLEAM 4-Jy (G4Jy) Sample. This is a complete sample of the 'brightest' radio-sources ($S_{\mathrm{151MHz}} >$ 4 Jy), the majority of which are active galactic nuclei with powerful radio-jets. Crucially, low-frequency observations allow the selection of such sources in an orientation-independent way (i.e. minimising the bias caused by Doppler boosting, inherent in high-frequency surveys). We then use higher-resolution radio images, and information at other wavelengths, to morphologically classify the brightest components in GLEAM. We also conduct cross-checks against the literature, and perform internal matching, in order to improve sample completeness (which is estimated to be $>$ 95.5%). This results in a catalogue of 1,863 sources, making the G4Jy Sample over 10 times larger than that of the revised Third Cambridge Catalogue of Radio Sources (3CRR; $S_{\mathrm{178MHz}} >$ 10.9 Jy). Of these G4Jy sources, 78 are resolved by the MWA (Phase-I) synthesised beam ($\sim$2 arcmin at 200 MHz), and we label 67% of the sample as 'single', 26% as 'double', 4% as 'triple', and 3% as having 'complex' morphology at $\sim$1 GHz (45-arcsec resolution). Alongside this, our value-added catalogue provides mid-infrared source associations (subject to 6-arcsec resolution at 3.4 micron) for the radio emission, as identified through visual inspection and thorough checks against the literature. As such, the G4Jy Sample can be used as a reliable training set for cross-identification via machine-learning algorithms. [Abstract abridged for arXiv submission.]

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arXiv2020

Searching for Dark Matter Signals from Local Dwarf Spheroidal Galaxies at Low Radio Frequencies in the GLEAM Survey

The search for emission from weakly interacting massive particle (WIMP) dark matter annihilation and decay has become a multi-pronged area of research not only targeting a diverse selection of astrophysical objects, but also taking advantage of the entire electromagnetic spectrum. The decay of WIMP particles into standard model particles has been suggested as a possible channel for synchrotron emission to be detected at low radio frequencies. Here, we present the stacking analysis of a sample of 33 dwarf spheroidal (dSph) galaxies with low-frequency (72 - 231 MHz) radio images from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. We produce radial surface brightness profiles of images centred upon each dSph galaxy with background radio sources masked. We remove ten fields from the stacking due to contamination from either poorly subtracted, bright radio sources or strong background gradients across the field. The remaining 23 dSph galaxies are stacked in an attempt to obtain a statistical detection of any WIMP-induced synchrotron emission in these systems. We find that the stacked radial brightness profile does not exhibit a statistically significant detection above the 95% confidence level of $\sim$1.5 mJy beam$^{-1}$. This novel technique shows the potential of using low-frequency radio images to constrain fundamental properties of particle dark matter.

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arXiv2020

No Massive Companion to the Coherent Radio-Emitting M Dwarf GJ 1151

The recent detection of circularly polarized, long-duration (>8 hr) low-frequency (~150 MHz) radio emission from the M4.5 dwarf GJ 1151 has been interpreted as arising from a star-planet interaction via the electron cyclotron maser instability. The existence or parameters of the proposed planets have not been determined. Using 20 new HARPS-N observations, we put 99th-percentile upper limits on the mass of any close companion to GJ 1151 at Msini < 5.6 M earth. With no stellar, brown dwarf, or giant planet companion likely in a close orbit, our data are consistent with detected radio emission emerging from a magnetic interaction between a short-period terrestrial-mass planet and GJ 1151.

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arXiv2019

Candidate radio supernova remnants observed by the GLEAM survey over $345^\circ < l < 60^\circ$ and $180^\circ < l < 240^\circ$

We examined the latest data release from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey covering $345^\circ < l < 60^\circ$, $180^\circ < l < 240^\circ$, using these data and that of the Widefield Infrared Survey Explorer to follow up proposed candidate Supernova Remnants from other sources. Of the 101 candidates proposed in the region, we are able to definitively confirm ten as SNRs, tentatively confirm two as SNRs, and reclassify five as Hii regions. A further two are detectable in our images but difficult to classify; the remaining 82 are undetectable in these data. We also investigated the 18 unclassified Multi-Array Galactic Plane Imaging Survey (MAGPIS) candidate SNRs, newly confirming three as SNRs, reclassifying two as Hii regions, and exploring the unusual spectra and morphology of two others.

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