arXiv subjects

Malcolm Gray

Publications and source records attributed to Malcolm Gray.

7 recordsLinked to original sources

arXiv2026

Measuring Magnetic Fields Near and Far with the SKA via the Zeeman Effect

Zeeman splitting in spectral lines -- both in emission and absorption -- provides direct estimates of magnetic field strength and direction in magnetized gas in our own Milky Way and in external galaxies. We discuss the potential for using the Square Kilometre Array (SKA) to measure the Zeeman effect in targets spanning an enormous range of distance: from cometary comas in the solar system, through Galactic molecular clouds, HI filaments in the cold neutral medium, high-velocity clouds, the Fermi Bubbles, and photodissociation regions (PDRs) traced by radio recombination lines, to OH masers and megamasers in nearby and distant starburst galaxies, and to cold neutral gas in damped Ly-alpha absorbing systems at cosmological redshifts. We update the sensitivity calculations of Robishaw et al. (2015) and indicate, for each science goal, whether it will be achievable with Array Assembly 4 (AA4) of SKA-Mid, with the staged delivery of AA*, or only with the full SKA buildout. Zeeman measurements will probe the magnetic field in situ in the warm and cold neutral interstellar medium, complementing SKA Faraday rotation programs; radio recombination lines, stackable across hundreds of transitions, extend this reach to HII regions and PDRs. In external galaxies, SKA-Mid will enable Zeeman studies of OH kilomasers in nearby starburst systems, substantially expand the census of megamaser Zeeman detections beyond the Arecibo sky, and probe magnetic fields in damped Lyman-alpha systems to field limits well below those currently achievable, opening a new window on the role of magnetic fields in galaxy formation and cosmic evolution.

astro-ph.GA↗

arXiv2025

First VLBI Imaging of SiO $v=0$, $J=1 \rightarrow 0$ Masers in VY Canis Majoris

We achieved the first VLBI detections of the ground vibrational state ($v=0$) $^{28}$SiO (hereafter, SiO) and $^{29}$SiO masers of the $J=1\rightarrow 0$ rotational transitions, towards the 25 \Msun ~red supergiant (RSG) star, VY Canis Majoris (VY CMa), taking advantage of the high sensitivity of the VLBI Exploration of Radio Astrometry (VERA) telescopes that coordinate with the Nobeyama 45 m telescope. In addition, we successfully detected the SiO $J=1\rightarrow 0$ transition in the $v=3$ state towards VY CMa for the first time with VLBI. The SiO $J=1\rightarrow 0$ maser spot in $v=0$ state was detected in the cross-power spectra taken with the baselines involving the Nobeyama 45-m telescope. The combination of previously reported absolute astrometry and the relative astrometry technique allowed us to derive the location of the SiO $v=0$ maser spot, {(RA, DEC) = ( 7${\rm ^h}$ 22${\rm ^m}$ 58.$^{\rm s}$32, $-$25$^{\circ}$ 46$^{\prime}$ 3.$^{\prime\prime}$4) in J2000 at an absolute positional accuracy of $\sim$100 milliarcseconds (mas). The SiO $v=0$ maser spot is offset by the amount of ($\Delta$RA, $\Delta$DEC)=($-$150, $-$300) (mas) to the southwest of the stellar position, suggesting that the $v = 0$ maser spot is associated with its outflow activity.} This observational study demonstrates that the brightest SiO $v=0$ maser spot is compact (3 mas), producing an extremely high brightness of $\sim$ 10$^7$ K. This indicates that the SiO $v=0$ maser action may originate from strong shocks in the stellar wind emanating from this extreme RSG that leads to its intense mass ejection.

astro-ph.SR↗

arXiv2020

VLBI20-30: a scientific roadmap for the next decade -- The future of the European VLBI Network

This white paper describes the science case for Very Long Baseline Interferometry (VLBI) and provides suggestions towards upgrade paths for the European VLBI Network (EVN). The EVN is a distributed long-baseline radio interferometric array, that operates at the very forefront of astronomical research. Recent results, together with the new science possibilities outlined in this vision document, demonstrate the EVN's potential to generate new and exciting results that will transform our view of the cosmos. Together with e-MERLIN, the EVN provides a range of baseline lengths that permit unique studies of faint radio sources to be made over a wide range of spatial scales. The science cases are reviewed in six chapters that cover the following broad areas: cosmology, galaxy formation and evolution, innermost regions of active galactic nuclei, explosive phenomena and transients, stars and stellar masers in the Milky Way, celestial reference frames and space applications. The document concludes with identifying the synergies with other radio, as well as multi-band/multi-messenger instruments, and provide the recommendations for future improvements. The appendices briefly describe other radio VLBI arrays, the technological framework for EVN developments, and a selection of spectral lines of astrophysical interest below 100 GHz. The document includes a glossary for non-specialists, and a list of acronyms at the end.

astro-ph.IM↗

arXiv2018

Constraining Theories of SiO Maser Polarization: Analysis of a $pi$/2 EVPA Change

The full theory of polarized SiO maser emission from the near-circumstellar environment of Asymptotic Giant Branch stars has been the subject of debate, with theories ranging from classical Zeeman origins to predominantly non-Zeeman anisotropic excitation or propagation effects. Features with an internal electric vector position angle (EVPA) rotation of $\sim \pi /2$ offer unique constraints on theoretical models. In this work, results are presented for one such feature that persisted across five epochs of SiO $\nu=1, J=1-0$ VLBA observations of TX Cam. We examine the fit to the predicted dependence of linear polarization and EVPA on angle ($\theta$) between the line of sight and the magnetic field against theoretical models. We also present results on the dependence of $m_c$ on $\theta$ and their theoretical implications. Finally, we discuss potential causes of the observed differences, and continuing work.

astro-ph.SR↗

arXiv2015

Measuring Magnetic Fields Near and Far with the SKA via the Zeeman Effect

The measurement of Zeeman splitting in spectral lines---both in emission and absorption---can provide direct estimates of the magnetic field strength and direction in atomic and molecular clouds, both in our own Milky Way and in external galaxies. This method will probe the magnetic field in the warm and cold neutral components of the interstellar medium, providing a complement to the extensive SKA Faraday studies planning to probe the field in the ionized components.

astro-ph.GA↗

arXiv2014

Amplification of OAM Radiation by Astrophysical Masers

We extend the theory of astrophysical maser propagation through a medium with a Zeeman-split molecular response to the case of a non-uniform magnetic field, and allow a component of the electric field of the radiation in the direction of propagation: a characteristic of radiation with orbital angular momentum. A classical reduction of the governing equations leads to a set of nine differential equations for the evolution of intensity-like parameters for each Fourier component of the radiation. Four of these parameters correspond to the standard Stokes parameters, whilst the other five represent the $z$-component of the electric field, and its coupling to the conventional components in the $x-y$-plane. A restricted analytical solution of the governing equations demonstrates a non-trivial coupling of the Stokes parameters to those representing orbital angular momentum: the $z$-component of the electric field can grow from a background in which only Stokes-$I$ is non-zero. A numerical solution of the governing equations reveals radiation patterns with a radial and angular structure for the case of an ideal quadrupole magnetic field perpendicular to the propagation direction. In this ideal case generation of radiation orbital angular momentum, like polarization, can approach 100 per cent.

astro-ph.GA↗

arXiv2011

Flares and Proper Motions of Ground-State OH Masers in W75N

The star-forming region W75N hosts bright OH masers that are observed to be variable. We present observations taken in 2008 of the ground-state OH maser transitions with the Very Long Baseline Array (VLBA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN) and with the Nancay Radio Telescope in 2011. Several of the masers in W75N were observed to be flaring, with the brightest 1720-MHz maser in excess of 400 Jy. The 1720-MHz masers appear to be associated with the continuum source VLA 1, unlike the bright flaring 1665- and 1667-MHz masers, which are associated with VLA 2. The 1720-MHz masers are located in an outflow traced by water masers and are indicative of very dense molecular material near the H II region. The magnetic field strengths are larger in the 1720-MHz maser region than in most regions hosting only main-line OH masers. The density falls off along the outflow, and the order of appearance of different transitions of OH masers is consistent with theoretical models. The 1665- and 1667-MHz VLBA data are compared against previous epochs over a time baseline of over 7 years. The median maser motion is 3.5 km/s, with a scatter that is comparable to thermal turbulence. The general pattern of maser proper motions observed in the 1665- and 1667-MHz transitions is consistent with previous observations.

astro-ph.GA↗