Tracy E. Clarke

A subset of active galactic nuclei (AGN) manifest as compact radio sources with peaked radio spectra, commonly referred to as peaked-spectrum (PS) sources. Generally, PS sources are interpreted as young radio jets, but the early evolution of the jet is rarely observed and not fully understood. Here, we present VT J182357-234511 (VT J1823-2345), a radio transient in the Very Large Array Sky Survey (VLASS) that resembles a PS source. Notable properties of VT J1823-2345 include (i) brightening by a factor of 3 between 1996 and 2021 at 1.4 GHz, (ii) peaked spectra with high-frequency peaks at ν _p,obs ≳ 6 GHz, (iii) a peaked 3 GHz light curve that has been fading since 2023, (iv) a resolved Gaussian (major-axis) size of 0.4 mas in 2024, and (v) a linear polarization fraction p _L ≃ 0.73% at 8.8 GHz with an extreme Faraday rotation measure RM _obs ∼ +12,000 rad m ⁻² . On the sky, VT J1823-2345 coincides with a supernova remnant and an F-type star, but no background galaxy is seen. Based on the multiwavelength properties, we disfavor Galactic transients, explosive extragalactic transients, and blazar variability as plausible explanations. We argue that VT J1823-2345 is consistent with a “short-lived” parsec-scale PS source with a decades-old jet that does not grow into a large-scale radio galaxy. From comparing the full VLASS light curves, we speculate that VT J1823-2345 is similar to many VLASS nuclear transients reported in the literature, which may represent an emerging class of transient radio jets, perhaps related to episodic AGN activity or jetted tidal disruption events.

We present results from a deep MeerKAT 1.28 GHz observation of A520, a galaxy cluster in the middle of a merger and a treasure trove of merger-generated phenomena. MeerKAT’s combination of high angular resolution and sensitivity to extended radio emission lets us uncover unprecedented detail of the cluster’s giant halo and puzzling features beyond the halo. We find that the main radio halo is sharply bounded along the direction of the NE–SW merger axis by the two X-ray shock fronts. In the perpendicular direction, the halo wings smoothly span over 2 Mpc, with the spectrum steepening in the outskirts. We detect faint radio emission upstream of the main (SW) bow shock; the emissivity jump across this shock can be used to constrain the physical mechanisms at work there. We discover a tail of radio emission extending past the NE shock to the gas-poor, high-entropy “dark subcluster” in the NE outskirts. The tail and subcluster have a higher radio/X-ray brightness ratio than the main halo’s, suggesting efficient reacceleration of cosmic rays. We also discover two peripheral radio relics beyond the cluster virial radius, and a faint, ∼2 Mpc long, broad radio bridge connecting the dark subcluster to one of the relics along the direction perpendicular to the merger axis. The spectral slopes of the dark subcluster and the bridge are consistent with that of the main halo. This bridge and both relics may be caused by another giant merger-generated shock front at the virial radius.

We present late-time ( δt > 3000 days) optical (Keck), X-ray (Chandra and the Nuclear Spectroscopic Telescope Array), and radio (the Very Large Area, the Atacama Large Millimeter/submillimeter Array, and the Upgraded Giant Metrewave Radio Telescope) observations of the Type Ic-BL (SN Ic-BL) SN 2012ap. Previous studies of this supernova (SN) have suggested that it stands out as a key example of a weak engine-driven explosion due to the lack of gamma-ray burst (GRB) detection and mildly relativistic ejecta. Recently, radio sky surveys revealed the rebrightening of the radio emission from this SN, highlighting the possibilities of a density enhancement at large radii or the existence of an off-axis relativistic jet. While the late-time optical spectra do not exhibit the broad emission lines seen in other interacting supernovae (SNe), our analysis of the broadband radio and X-ray emission implies that both scenarios are plausible. If a density enhancement is responsible for the radio rebrightening, it has to result from a change in the mass-loss rate and/or wind velocity, possibly due to the transition of the progenitor from a red supergiant to a Wolf–Rayet star. If the late-time radio component is a result of an off-axis relativistic jet, we find that an energetic narrow jet viewed at θ _obs ≥ 80° is needed. In this scenario, SN 2012ap is not a result of a weak engine-driven explosion, and instead, it is similar to other GRBs. However, radio rebrightenings of SN Ic-BL are not enough on their own to determine the existence of off-axis jets, and our planned Very Long Baseline Array observation will help reveal the true nature of this SN.

Magnetically driven phenomena such as flaring events and aurorae lead ultracool dwarfs (UCDs) to emit at radio frequencies. Despite decades of scrutiny, a comprehensive physical understanding of their radio emission at different frequencies remains elusive, spurring on additional study of these complex objects. The Very Large Array (VLA) Low-band Ionosphere and Transient Experiment (VLITE) is a commensal instrument operating at 340 MHz on the VLA. A key advantage of 340 MHz observations is their sensitivity to circumstellar disks and planets at understudied distances from the stellar disk, intermediate between GHz and low MHz sensitivities. Hard-to-find coronal mass ejections are also predicted to be detectable at 340 MHz. However, this frequency regime is relatively unprobed in UCD studies, with few searches and no published detections to date. Here we highlight our investigation of the nearby M7–M7 binary, EI Cancri. EI Cancri AB is magnetically active, yet it has an uncharacteristically long 83-day candidate rotation period within the system. With the VLITE detection of the EI Cancri system, we present the first ever detection of UCD emission at 340 MHz.

We present new Very Long Baseline Array (VLBA) imaging of a megahertz peaked-spectrum source that was found using commensal low-frequency data taken with the Karl G. Jansky Very Large Array (VLA). The source, J0330–2730, was identified in multiepoch data from the VLA Low-band Ionosphere and Transient Experiment (VLITE). VLITE continuously collects low-frequency data at 340 MHz during regular VLA observations. Our analysis of the VLITE light curve demonstrates that J0330–2730 has significant 340 MHz flux variability at the ∼20% level over a timescale of approximately 1 yr. Our VLBA images reveal a resolved, double-lobed morphology with a projected linear size of 64 pc. We consider plausible mechanisms that could explain the observed 340 MHz variability and the source properties on milliarcsecond scales. We rule out variable Doppler boosting and conclude that refractive interstellar scintillation or variable free–free absorption are the most likely explanations. We argue that the properties of J0330–2730 are consistent with the class of compact symmetric objects and consider the evolutionary stage of the source. The extent of the resolved lobes revealed by the VLBA is significantly smaller than predictions based on the turnover–size relation for a standard synchrotron self-absorbed jet model. We discuss possible explanations for the departure from the turnover–size relation, including jet formation by a transient phenomenon such as a tidal disruption event or a “frustrated jet” impeded by the presence of dense gas or a high-pressure environment. This study highlights the potential of VLITE for the identification of compact and young radio sources.

To examine how active galactic nucleus (AGN) feedback shapes the intracluster medium (ICM) and fuels black hole accretion in the cool-core galaxy cluster A2597, we present deep (∼600 ks) Chandra X-ray observations complemented by archival GMRT radio and SINFONI near-infrared data. Radio-mode AGN activity has inflated seven X-ray cavities and driven one to three potential weak shocks ( M ∼ 1.05 − 1.14 ) extending to ∼150 kpc, suggesting recurrent outbursts occurring on ∼10 ⁷ yr timescales. We also detect a narrow, ∼57 kpc X-ray surface brightness deficit—a potential plasma depletion layer—likely shaped by residual sloshing motions that amplified magnetic fields and/or displaced gas within the cluster core. Although the AGN injects ∼10 ⁴⁴ erg s ⁻¹ of energy, comparable to the cluster’s cooling luminosity, radiative cooling persists at ∼15 M _⊙ yr ⁻¹ , replenishing the billion-solar-mass cold gas reservoir at the heart of the brightest cluster galaxy. Sustaining this level of activity requires a continuous fuel supply, yet the estimated Bondi accretion power (∼2 × 10 ⁴³ erg s ⁻¹ ) falls an order of magnitude short of the observed cavity power, suggesting that “hot” gas fueling is insufficient. Instead, archival Atacama Large Millimeter/submillimeter Array observations continue to support a chaotic cold accretion scenario, where turbulence-driven condensation fuels the AGN at rates exceeding Bondi accretion, sustaining a self-regulated feedback cycle that repeatedly shapes the core of A2597.

We present results from very deep (485 ks) Chandra X-ray observations of the relaxed, cool core cluster A2029 ( z = 0.0767). A2029 hosts one of the longest, most continuous sloshing spirals ever observed, which we find extends nearly 600 kpc from the cluster core. In addition to providing detailed views of the sloshing spiral, imaging and spectroscopic analysis reveals intracluster medium (ICM) substructure related to the merger history, including a broad “splash” of cooler gas and a potential merger shock. The radio lobes of the central WAT source show evidence of alignment with the sloshing motions, consistent with ICM bulk flow, rather than host-galaxy motion, as the primary driver of lobe bending. Comparison to a 1:10 mass-ratio off-axis merger simulation indicates that the observed ICM structures are relics of a second core passage of a subcluster ∼4 Gyr after the start of the merger, where the “splash” feature is revealed to be a wake of cool gas trailing behind the subcluster. Overall, our results suggest that A2029 is still settling from past interactions—showing that even the initially most relaxed-looking clusters can be hiding a rich history of dynamical activity.

Galaxy mergers are predicted to be a critical stage of merger-driven SMBH growth and galaxy evolution. Systems exhibiting multiple active galactic nuclei (AGN) are an important observational tool for constraining the prevalence and properties of merging SMBHs. Most multi-AGN selection methods focus on the identification of AGN pairs, while few focus on the more elusive triple AGN, which are a prediction of hierarchical growth and natural laboratories for studying SMBH dynamics. These systems are rare; only two confirmed cases exist locally ( z < 0.1). Here, we present a new, confirmed triple AGN system, WISEJ121857.42+103551.2/WISEJ121901.77+103515.0, with nuclear separations of 22.6 and 97 kpc, identified within a sample of spatially resolved mid-IR dual AGN. The system exhibits multiwavelength evidence for three distinct AGN hosted in an ongoing galaxy merger. To confirm the nature of this system, we present new radio observations with the Karl G. Jansky Very Large Array in A-configuration at 3, 10, and 15 GHz, providing subarcsecond resolutions, and the Very Long Baseline Array at 4.9 GHz, providing milliarcsecond resolutions. These observations confirm this system as a triple AGN, and the first triple AGN in which all three nuclei host radio AGN.

We present the discovery of a 421 s long period transient using the CHIME telescope, CHIME J0630+25. The source is localized to R.A. = 06:30:38.4 ± 1 ′ decl. = 25:26:23 ± 1 ′ using voltage data acquired with the CHIME baseband system. A timing analysis shows that a model including a glitch is preferred over a nonglitch model with dF / F = 1.3 × 10 ⁻⁶ , consistent with other glitching neutron stars. The timing model suggests a surface magnetic field of ∼1.5 × 10 ¹⁵ G and a characteristic age of ∼1.28 × 10 ⁶ yr. A separate line of evidence to support a strong local magnetic field is an abnormally high rotation measure of RM = −347.8(6) rad m ⁻² relative to CHIME J0630+25’s modest dispersion measure of 22(1) pc cm ⁻² , implying a dense local magneto-ionic structure. As a result, we believe that CHIME J0630+25 is a magnetized, slowly spinning, isolated neutron star. This marks CHIME J0630+25 as the longest period neutron star and the second-longest period neutron star with an inferred magnetar-like field. Based on dispersion measure models and comparison with pulsars with distance measurements, CHIME J0630+25 is located at a nearby distance of 170 − 100 + 310 pc (95.4%), making it an ideal candidate for follow-up studies.