arXiv subjects

Kelvin Wandia

Publications and source records attributed to Kelvin Wandia.

6 recordsLinked to original sources

arXiv2026

The PARADIGM Project II: Characterising Nuclear and Diffuse Radio Components in Local U/LIRGs

Disentangling SF and AGN emission is essential for understanding galaxy evolution, yet remains challenging in merging systems where both processes are enhanced and spatially intertwined. Galaxy mergers drive gas inflows that simultaneously fuel nuclear SBs and BH accretion, shaping morphology from nuclear ($\lesssim 250$~pc) to large-scale ($\gtrsim 500$~pc) regions. Radio interferometry provides an unobscured view, but separating compact nuclear SBs, AGN, and diffuse SF requires multiscale, multi-frequency observations. We present a systematic method to characterise multiscale radio properties in 15 local ($z\lesssim 0.1$) U/LIRGs ($L_{\mathrm{IR}} > 10^{11}\mathrm{L}_{\odot}$). Using \emph{e}-MERLIN and VLA at 1.4, 6.0 and 33.0~GHz, we probe physical scales from $\sim 10$--$250$~pc to $\sim 0.5$--$3.0$~kpc. We decompose radio emission into nuclear (compact cores and nuclear extended) and large-scale (total and diffuse) components, comparing morphological properties (emission fractions, sizes, luminosities, surface densities) and investigating correlations with source classes, merger stages, and $L_{\mathrm{IR}}$. We find: i) nuclear emission contributes $\sim$50\% of total radio emission on average; ii) total multiscale diffuse emission (SF-related) contributes $\sim$80\% to total power; iii) nuclear emission components act together to correlate with total radio and infrared luminosities, which increase with merger stage, whilst diffuse emission at larger scales shows no clear dependence on nuclear processes; iv) sources with radio excess (lower $q_{\mathrm{IR}}$) show lower nuclear luminosity ratios $L_{\mathrm{R,33}}^{\mathrm{N}}/L_{\mathrm{R,6}}^{\mathrm{N}}$, indicating a deficit of high-$ν$ radio emission; since 33.0~GHz traces recent SF, this suggests the radio excess is dominated by non-thermal emission at lower $ν$, likely AGN-related, rather than enhanced SF.

astro-ph.GA↗

arXiv2026

Upper Limits on Radio Emission from the K2-18 System

Stellar and planetary magnetic fields play a crucial role in the habitability of a planet and the integrity of its atmosphere. The recently claimed detection of biosignatures, methane, carbon dioxide and dimethyl sulfide/disulfide, in the atmosphere of K2-18 b, a sub-Neptune orbiting an M dwarf star present an intriguing question regarding the stellar magnetic environment and the resistance of the planet's magnetosphere (if it exists) to erosion by magnetic activity from the host. To probe for radio emission from the system, we have conducted observations using the Karl G. Jansky Very Large Array (VLA) at S, C and X-bands (2-4, 4.5-7.5 and 8-10 GHz respectively) to search for coherent and incoherent radio emission. We detect no radio emission associated with incoherent emission mechanisms. We report $3σ$ Stokes I upper limits of $49.8\ μ\rm{Jybeam}^{-1}$ at S-band, $17.7\ μ\rm{Jybeam}^{-1}$at C-band and $18.0\ μ\rm{Jybeam}^{-1}$ at X-band and an upper limit of the ratio of the radio to the total bolometric luminosity of $\log L_\text{R}/\log L_\text{bol}<-8.8$. We have also searched for short duration bursts associated with coherent emission mechanisms at C and X-bands . No signals above a $3σ$ significance threshold are detected. Although no signals are detected our radio observations offer constraints, albeit limited, on the stellar magnetic environment supporting recent X-ray observations indicating K2-18 is a very faint emitter. Our results also contextualise any planetary transmission spectra by providing constraints on the activity level of the host.

astro-ph.SR↗

arXiv2026

Radio Activity from the Rapidly Rotating T dwarf 2MASS 2228-4310

We present the detection of 2MASS J22282889-4310262 (2M2228), a T6/T6.5 brown dwarf, using the Karl G. Jansky Very Large Array (VLA) archival data observed at C band (4-8 GHz) over two observing epochs ($2\times96$ minutes). 2M2228 is detected at time and frequency averaged Stokes I and V peak flux densities of $67.3\pm4.9\ μ \rm{Jy beam}^{-1}$ and $14.4\pm3.0\ μ\text{Jy beam}^{-1}$ in the first epoch and $107.2\pm5.2\ μ\rm{Jy\ beam}^{-1}$ and $-20.7\pm1.2\ μ\text{Jy beam}^{-1}$ in the second epoch. This discovery constitutes the eighth and, notably, the most rapidly rotating T dwarf detected to date at radio wavelengths. Our observations reveal highly polarised bursts at fractional polarisation ratios $f_\text{c}>50$%. Using Stokes I light curves, we measure occurrence intervals of $\sim47$ and $\sim58$ minutes in the two observing epochs respectively with the first burst aligning within a half period timescale of the the previously measured mid infrared photometric period of $85.8\pm0.32$ minutes. We attribute the emission to the electron cyclotron maser emission (ECME) and constrain the magnetic field strength to $B\gtrsim1.4$ kG. We emphasise that the periods inferred are provisional considering the short observing durations. The combination of previously demonstrated atmospheric stability and newly detected radio emission in 2M2228 makes it a promising laboratory for testing magnetospheric currents-driven auroral models and for guiding future coordinated James Webb Space Telescope (JWST) and radio observations to probe the link between auroral activity and atmospheric dynamics in T-type brown dwarfs.

astro-ph.SR↗

arXiv2025

Radio Emission from a Nearby M dwarf Binary

We present the detection of the binary system 2MASS J02132062+3648506 AB using the Karl G. Jansky Very Large Array (VLA) archive data observed at 4-8 GHz. The system is a triple consisting of a tight binary ($\sim0.2"$) of two M dwarfs of spectral class M4.5 and M6.5 and a wide T3 brown dwarf companion ($\sim$16.4"). The binary displays coronal and chromospheric activity as traced by previously measured X-ray flux and H$α$ emission. We detect the unresolved binary at a peak flux density of $\sim356\ μ\mathrm{Jybeam}^{-1}$ at a signal-to-noise ratio (SNR) of $\sim36$ and determine a radio luminosity of $\mathrm{log}L_R/\mathrm{log}L_\mathrm{bol}\approx-7.76$. The radio emission is quiescent, polarised at a mean circular polarisation fraction $f_\mathrm{c}=45.20 \pm 1.58$ % and exhibits a spectral index $α=-0.44\pm0.07$ . We probe the binary using the Enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN) with an angular resolution of $\sim40$ mas at 5 GHz and detect a component at a peak flux density of $\sim90\ μ$Jy $\mathrm{beam}^{-1}$ at a SNR $\sim5$ . We propose a gyrosynchrotron origin for the radio emission and estimate a magnetic field strength $B<174.86$ G, an emitting region of size $L<1.54$ times the radius of the M4.5 primary and a plasma number density $n_\mathrm{e}<2.91\times10^5\ \mathrm{cm}^{-3}$. The brown dwarf companion is not detected. Additionally, we have analysed observations of 2MASS J04183483+213127, a chromospherically active L5 brown dwarf which is also not detected. Accordingly, we place $3σ$ flux density upper limits at $36.9\ μ$Jy $\mathrm{beam}^{-1}$ and $42.3\ μ$Jy $\mathrm{beam}^{-1}$ for Stokes I and V respectively.

astro-ph.SR↗

arXiv2024

Conducting High Frequency Radio SETI using ALMA

The Atacama Millimeter/Submillimeter Array (ALMA) remains unparalleled in sensitivity at radio frequencies above 35 GHz. In this paper, we explore ALMA's potential for narrowband technosignature detection, considering factors such as the interferometer's undistorted field of view, signal dilution due to significant drift rates at high frequencies and the possibility of spectral confusion. We present the first technosignature survey using archival ALMA data in Band 3, focusing on two spectral windows centred on 90.642 GHz and 93.151 GHz. Our survey places new limits at these frequencies on the prevalence of extraterrestrial transmitters for 28 galactic stars, selected from the Gaia DR3 catalogue. We employ a stellar 'bycatch' method to sample these objects within the undistorted field of view of four ALMA calibrators. For the closest star in our sample, we find no evidence of transmitters with EIRP_min > 7 x 10^17 W. To the best of our knowledge, this represents the first technosignature search conducted using ALMA data.

astro-ph.IM↗

arXiv2023

An Interferometric SETI Observation of Kepler-111 b

The application of Very Long Baseline Interferometry (VLBI) to the Search for Extraterrestrial Intelligence (SETI) has been limited to date, despite the technique offering many advantages over traditional single-dish SETI observations. In order to further develop interferometry for SETI, we used the European VLBI Network (EVN) at $21$~cm to observe potential secondary phase calibrators in the Kepler field. Unfortunately, no secondary calibrators were detected. However, a VLBA primary calibrator in the field, J1926+4441, offset only $\sim1.88'$ from a nearby exoplanet Kepler-111~b, was correlated with high temporal $\left(0.25 \ \rm{s}\right)$ and spectral $\left(16384 \times 488\ \rm{Hz \ channels}\right)$ resolution. During the analysis of the high-resolution data, we identified a spectral feature that was present in both the auto and cross-correlation data with a central frequency of $1420.424\pm0.0002$ MHz and a width of 0.25 MHz. We demonstrate that the feature in the cross-correlations is an artefact in the data, associated with a significant increase in each telescope's noise figure due to the presence of \ion{H}{i} in the beam. This would typically go unnoticed in data correlated with standard spectral resolution. We flag (excluded from the subsequent analysis) these channels and phase rotate the data to the location of Kepler-111~b aided by the GAIA catalogue and search for signals with $\rm{SNR}>7$. At the time of our observations, we detect no transmitters with an Equivalent Isotropically Radiated Power (EIRP) > $\sim4\times10^{15}$ W.

astro-ph.GA↗