arXiv subjects

Athol J. Kemball

Publications and source records attributed to Athol J. Kemball.

12 recordsLinked to original sources

arXiv2026

MeerKAT discovery of a high-redshift strongly-lensed hydroxyl gigamaser

At low redshifts, hydroxyl megamasers (OHMs) have been shown to trace galaxy mergers, obscured starbursts, high molecular gas densities, and candidate dual supermassive black hole systems. Given this astrophysical utility, exploring these sources at larger cosmological look-back times is therefore of key interest. While previous OHM surveys have been limited to redshifts of $z \lesssim 0.25$, the ability to expand the OHM frontier is significantly enhanced with new high-sensitivity radio facilities such as MeerKAT. In this Letter, we report the discovery of an OHM in the gravitational lens system HATLAS J142935.3-002836 at $z = 1.027$, the most distant OHM source yet detected. The spectrum has blended 1667 and 1665 MHz emission and exhibits a highly complex profile, with spectral components ranging in widths of $<8$ km s$^{-1}$ to $\sim300$ km s$^{-1}$. The integrated (magnification uncorrected) luminosity of log($L_{\rm OH} / L_{\odot}$) = 5.51 $\pm$ 0.67 makes this the most apparently luminous OHM known to date. In the same wide-band dataset, we have also detected a previously unknown ${\rm H I}$ absorption line. The signal-to-noise ratio of over 150 with just a 4.7 h observation highlights the potential that MeerKAT and the future Square Kilometre Array mid-frequency array offer to explore the high-redshift OHM universe.

astro-ph.GA↗

arXiv2025

A Search for Exoplanet Candidates in TESS 2-min Light Curves using Joint Bayesian Detection

In this work, we apply an exploratory joint Bayesian transit detector (Taaki et al. 2020), previously evaluated using Kepler data, to the 2 min simple aperture photometry light curve data in the continuous viewing zone for the Transiting Exoplanet Survey Satellite (TESS) over three years of observation. The detector uses Bayesian priors, adaptively estimated, to model unknown systematic noise and stellar variability incorporated in a Neyman-Pearson likelihood ratio test for a candidate transit signal; a primary goal of the algorithm is to reduce overfitting. The detector was adapted to the TESS data and refined to improve outlier rejection and suppress false alarm detections in post-processing. The statistical performance of the detector was evaluated using transit injection tests, where the joint Bayesian detector achieves an 80.0 % detection rate and a 19.1% quasi-false alarm rate at a detection threshold τ = 10; this is a marginal, although not statistically significant, improvement of 0.2% over a reference sequential detrending and detection algorithm. In addition, a full search of the input TESS data was performed to evaluate the recovery rate of known TESS objects of interest (TOI) and to perform an independent search for new exoplanet candidates. The joint detector has a 73% recall rate and a 63% detection rate for known TOI; the former considers a match against all detection statistics above threshold while the latter considers only the maximum detection statistic.

astro-ph.EP↗

arXiv2022

Polarized Maser Emission with In-Source Faraday Rotation

We discuss studies of polarization in astrophysical masers with particular emphasis on the case where the Zeeman splitting is small compared to the Doppler profile, resulting in a blend of the transitions between magnetic substates. A semi-classical theory of the molecular response is derived, and coupled to radiative transfer solutions for 1 and 2-beam linear masers, resulting in a set of non-linear, algebraic equations for elements of the molecular density matrix. The new code, PRISM, implements numerical methods to compute these solutions. Using PRISM, we demonstrate a smooth transfer between this case and that of wider splitting. For a J=1-0 system, with parameters based on the $v=1, J=1-0$ transition of SiO, we investigate the behaviour of linear and circular polarization as a function of the angle between the propagation axis and the magnetic field, and with the optical depth, or saturation state, of the model. We demonstrate how solutions are modified by the presence of Faraday rotation, generated by various abundances of free electrons, and that strong Faraday rotation leads to additional angles where Stokes-Q changes sign. We compare our results to a number of previous models, from the analytical limits derived by Goldreich, Keeley and Kwan in 1973, through computational results by W. Watson and co-authors, to the recent work by Lankhaar and Vlemmings in 2019. We find that our results are generally consistent with those of other authors given the differences of approach and the approximations made.

astro-ph.IM↗

arXiv2022

Testing Primordial Black Hole Dark Matter with ALMA Observations of the Gravitational Lens B1422+231

We examine the flux density ratio anomaly in the quadruply-imaged strong gravitational lens, B1422+231, and consider the contribution of $10-10^3M_{\odot}$ primordial black holes (PBHs) as a potential dark matter constituent. We describe the first flux density ratio measurement of B1422+231 in the millimeter-wave band using the Atacama Large Millimeter Array (ALMA). This fills an important multi-wavelength gap in our knowledge of this key lensed system. The flux density of the quasar at 233 GHz is dominated by synchrotron emission and the source size is estimated to be 66.9 pc. The observed flux density ratios at 233 GHz are similar to those measured in radio, mid-infrared and optical bands, which cannot be explained by a simple smooth mass model of the lens galaxy. We examine the probability of the flux density ratio anomaly arising from PBH microlensing using ray tracing simulations. The simulations consider the cases where 10% and 50% of dark matter are $10-10^3M_{\odot}$ PBHs with a power law mass function. Our analysis shows that the anomalous flux density ratio for B1422+231 can be explained by a lens model with a significant fraction of dark matter being PBHs. This study demonstrates the potential for new constraints on PBH dark matter using ALMA observations of multiply imaged strong gravitational lenses.

astro-ph.CO↗

arXiv2020

Bayesian Methods for Joint Exoplanet Transit Detection and Systematic Noise Characterization

The treatment of systematic noise is a significant aspect of transit exoplanet data processing due to the signal strength of systematic noise relative to a transit signal. Typically the standard approach to transit detection is to estimate and remove systematic noise independently of and prior to a transit detection test. If a transit signal is present in a light curve, the process of systematic noise removal may distort the transit signal by overfitting and thereby reduce detection efficiency. We present a Bayesian framework for joint detection of transit signals and systematic noise characterization and describe the implementation of these detectors as optimal Neyman-Pearson likelihood ratio tests. The joint detectors reduce to closed form as matched filters under the assumption of a Gaussian Bayesian prior for the systematic noise. The performance of the exploratory detectors was evaluated in injection tests and show $\sim 2\%$ improvement in overall detection efficiency relative to the standard approach. We find that joint detection efficiency is specifically improved for short-period, low transit-depth exoplanet transits, providing evidence in support of the hypothesis that joint detection may indeed help to mitigate overfitting. In addition, an initial feasibility test to detect known exoplanets in Kepler data using the joint detectors produced encouraging preliminary results.

astro-ph.EP↗

arXiv2020

Halo Counts-in-cells for Cosmological Models with Different Dark Energy

We examine the counts-in-cells probability distribution functions that describe dark matter halos in the Dark Energy Universe Simulations (DEUS). We describe the measurements between redshifts $z=0$ to $z=4$ on both linear and non-linear scales. The best-fits of the gravitational quasi-equilibrium distribution (GQED), the negative binomial distribution (NBD), the Poisson-Lognormal distribution (PLN), and the Poisson-Lognormal distribution with a bias parameter (PLNB) are compared to simulations. The fits agree reasonably consistently over a range of redshifts and scales. To distinguish quintessence (RPCDM) and phantom ($w$CDM) dark energy from $Λ$ dark energy, we present a new method that compares the model parameters of the counts-in-cells probability distribution functions. We find that the mean and variance of the halo counts-in-cells on $2-25h^{-1}$Mpc scales between redshifts $0.65<z<4$ show significant percentage differences for different dark energy cosmologies. On $15-25h^{-1}$Mpc scales, the $g$ parameter in NBD, $ω$ parameter in PLN, $b$ and $C_b$ parameters in PLNB show larger percentage differences for different dark energy cosmologies than on smaller scales. On $2-6h^{-1}$Mpc scales, kurtosis and the $b$ parameter in the GQED show larger percentage differences for different dark energy cosmologies than on larger scales. For cosmologies explored in the DEUS simulations, the percentage differences between these statistics for the RPCDM and $w$CDM dark energy cosmologies relative to $Λ$CDM generally increases with redshift from a few percent to significantly larger percentages at $z=4$. Applying our method to simulations and galaxy surveys can provide a useful way to distinguish among dark energy models and cosmologies in general.

astro-ph.CO↗

arXiv2019

Review of Zeeman Effect Observations of Regions of Star Formation

(Edited for length) The Zeeman effect is the only observational technique available to measure directly the strength of magnetic fields in regions of star formation. We review the physics of the Zeeman effect and its practical use in both extended gas and in masers. We discuss observational results for the five species for which the Zeeman effect has been detected in the ISM -- H~I, OH, and CN in extended gas and OH, CH$_3$OH, and H$_2$O in masers. These species span densities from $\sim10$ cm$^{-3}$ to $\sim10^{10}$ cm $^{-3}$, which allows magnetic fields to be measured over the full range of cloud densities. However, there are significant limitations, including that only the line-of-sight component of the magnetic field strength can usually be measured and that there are often significant uncertainties about the physical conditions being sampled, particularly for masers. We discuss statistical methods to partially overcome these limitations. The results of Zeeman observations are that the mass to magnetic flux ratio is subcritical (gravity dominates magnetic support) at lower densities but supercritical for $N_H \gtrsim 10^{22}$ cm$^{-2}$. Above $n_H\sim 300$ cm$^{-3}$, which is roughly the density at which clouds typically become self-gravitating, the strength of magnetic fields increases approximately as $B \propto n^{2/3}$, which suggest that magnetic fields do not provide significant support at high densities. This is consistent with high-density clouds being supercritical. However, magnetic fields have a large range in strengths at any given density, so the role of magnetic fields should differ significantly from one cloud to another. And for maser regions the dependence of field strength on density may have a slightly lower slope. Turbulent reconnection theory seems to best match the Zeeman observational results.

astro-ph.GA↗

arXiv2018

Constraining Theories of Polarized SiO Maser Transport: Multi-Epoch Analysis of a $π/2$ Electric Vector Rotation Feature

The detailed polarization mechanisms of SiO masers originating from the near circum\-stellar environment of Asymptotic Giant Branch stars are not yet definitively known. Prevailing theories are broadly classified as either Zeeman or non-Zeeman in origin, the latter including effects such as anisotropic pumping or anisotropic resonant scattering. The predicted behavior of the linear and circular polarization fractions and electric vector position angle vary by theory. In particular, individual maser features that exhibit a rotation in linear polarization of $\sim π/2$ as a function of frequency over their extent can be utilized as a test of several maser polarization transport theories. In this paper, we analyze one SiO ($ν=1$, $J=1-0$) maser feature toward the Mira variable, TX Cam that exhibits this internal polarization rotation and persists across five epochs (spanning $\sim3$ months). We compare our results to the predictions by several maser polarization theories and find that the linear polarization across the feature is consistent with a geometric effect for a saturated maser originating when the angle between the projected magnetic field and the line of sight ($θ$) crosses the Van Vleck angle $θ_F \sim 55^{\circ}$. However, the electric vector position angle (EVPA) exhibits a smooth rotation across the spatial extent of the feature rather than the expected abrupt $π/2$ flip. We discuss possible explanations for this discrepancy and alternative theories. Circular polarization across the feature is also analyzed and it is the most accurately described by Zeeman effects giving rise to a circular polarization fraction of the form $m_c$ is approximately proportional to $\cos θ$.

astro-ph.SR↗

arXiv2009

Multi-epoch imaging polarimetry of the SiO masers in the extended atmosphere of the Mira variable TX Cam

We present a time series of synoptic images of the linearly-polarized v=1, J=1-0 SiO maser emission toward the Mira variable, TX Cam. These data comprise 43 individual epochs at an approximate biweekly sampling over an optical pulsation phase range of 0.68 to 1.82. The images have an angular resolution of ~500 microarcsec and were obtained using the Very Long Baseline Array (VLBA), operating in the 43 GHz band in spectral-line, polarization mode. We have previously published the total intensity time series for this pulsation phase range; this paper serves to present the linearly-polarized image sequence and an associated animation representing the evolution of the linear polarization morphology over time. We find a predominantly tangential polarization morphology, a high degree of persistence in linear polarization properties over individual component lifetimes, and stronger linear polarization in the inner projected shell than at larger projected shell radii. We present an initial polarization proper motion analysis examining the possible dynamical influence of magnetic fields in component motions in the extended atmospheres of late-type, evolved stars.

astro-ph.SR↗

arXiv2009

Grain Growth and Density Distribution of the Youngest Protostellar Systems

We present dust opacity spectral indexes (beta) of the youngest protostellar systems (so-called Class 0 sources), L1448 IRS 2, L1448 IRS 3, and L1157, obtained between 1.3 mm and 2.7 mm continua, using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). The unprecedented compact configuration and image fidelity of CARMA allow a better detection of the dust continuum emission from Class 0 sources, with a less serious missing flux problem normally associated with interferometry. Through visibility-modeling at both 1.3 mm and 2.7 mm simultaneously, as well as image- and visibility-comparison, we show that beta of the three Class 0 sources are around or smaller than 1, indicating that dust grains have already significantly grown at the Class 0 stage. In addition, we find a radial dependence of beta, which implies faster grain growth in the denser central regions and/or dust segregation. Density distributions of the Class 0 sources are also addressed by visibility-modeling.

astro-ph.SR↗

arXiv2007

Stellar masers, circumstellar envelopes, and supernova remnants

This paper reviews recent advances in the study or circumstellar masers and masers found toward supernova remnants. The review is organized by science focus area, including the astrophysics of extended stellar atmospheres, stellar mass-loss processes and outflows, late-type evolved stellar evolution, stellar maser excitation and chemistry, and the use of stellar masers as independent distance estimators. Masers toward supernova remnants are covered separately. Recent advances and open future questions in this field are explored.

astro-ph↗

arXiv2001

Sub-milliarcsec-scale structure of the gravitational lens B1600+434

In the gravitational lens system B1600+434 the brighter image, A, is known to show rapid variability which is not detected in the weaker image, B (Koopmans & de Bruyn 2000). Since correlated variability is one of the fundamental properties of gravitational lensing, it has been proposed that image A is microlensed by stars in the halo of the lensing galaxy (Koopmans & de Bruyn 2000). We present VLBA observations of B1600+434 at 15 GHz with a resolution of 0.5 milliarcsec to determine the source structure at high spatial resolution. The surface brightness of the images are significantly different, with image A being more compact. This is in apparent contradiction with the required property of gravitational lensing that surface brightness be preserved. Our results suggest that both the lensed images may show two-sided elongation at this resolution, a morphology which does not necessarily favour superluminal motion. Instead these data may suggest that image B is scatter-broadened at the lens so that its size is larger than that of A, and hence scintillates less than image A.

astro-ph↗