S&A Colloquium: Fernando Camilo (SARAO)
2023-11-01 15:00 - 16:00 (Australia/Sydney) | WA
MeerKAT at 5

Abstract

The MeerKAT radio telescope in South Africa was inaugurated 5 years ago. In this talk I’ll provide a broad overview of science results to date. Among other examples, I’ll show: how an exquisitely sensitive continuum image is constraining the star formation history of the universe, putting it into tension with UV+IR-derived evolution; the discovery of OH megamasers at redshift > 0.5; how the study of neutral hydrogen down to column densities of ~5e17/cm^2 is enabling Local Group science outside the Local Group, and revealing how gas is shaped and removed from galaxies in cluster environments. Come see how MeerKAT combined with a pulsar timing backend developed in Australia is enabling unprecedented tomographic mapping of the Local Bubble and constraints on a stochastic background of gravitational waves. I’ll also cover the discovery and study of numerous extragalactic and Galactic transients – including still mysterious sources with rotation periods up to tens of minutes – done in part jointly with the MWA and ASKAP.
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S&A Colloquium: Yuzhe (Robert) Song (Swinburne)
2023-11-08 15:00 - 16:00
Gamma-ray Emission from Nearby Stellar Flares and Undetected Pulsars

Abstract

The continuing survey of the gamma-ray sky by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope enables us to carry out population studies in gamma-rays. We developed novel stacking techniques to analyse Fermi-LAT data beyond the limit of point source sensitivity, which subsequently have been applied to various types of astrophysical sources.

So far, less than 10% of known pulsars are detected in gamma-rays. Stacking analysis of pulsars off the Galactic plane shows a significant detection, with a pulsar-like spectral index and a characteristic flux well below the Fermi-LAT point source sensitivity. Using COMPAS, a rapid binary population synthesis code, we provide a best fit model of distributions of pulsar properties at birth that can describe our observations.

Young ultra-cool dwarfs are extremely magnetically active and produce highly energetic flares, yet the Sun is the only isolated main sequence star detected in gamma-rays. We present a temporal analysis of red dwarf TVLM 513-46546 and reported periodicity in gamma-rays. Stacking Fermi-LAT data during flare events identified from optical and X-ray flare surveys reports an upper limit of flare gamma-ray emission. It likely originates from decay of neutral pions generated from the relativistic protons interacting within the stellar atmosphere. If confirmed, this could be catastrophic on exoplanet atmospheres and significantly reduces the habitability on these worlds.
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S&A Colloquium: Joshua Pritchard (University of Sydney)
2023-11-15 15:00 - 16:00
Searches for Stellar Radio Activity in Circular Polarisation

Abstract

Transient radio bursts produced in the coronae of stellar atmospheres and within planetary magnetospheres often feature a high degree of circular polarisation. These events reveal the presence of strong magnetic fields, and trace particle acceleration driven by magnetic reconnection, space weather, and auroral current systems. The detection of stellar radio emission has historically been challenged by the overwhelmingly higher surface density of extra-Galactic sources that dominate the radio sky, which produce a large number of false positive associations to optically identified stars. Our knowledge of the radio star population has therefore been primarily driven by targeted studies of the most active, nearby systems, impacting the inference of population statistics and discovery of new regimes of stellar radio activity.

In this talk I will present a body of work that exploits the circular polarisation of magnetically driven stellar radio emission to identify large samples of radio stars in widefield surveys. I will describe the application of this technique to two widefield surveys with the Australian Square Kilometre Array Pathfinder (ASKAP—the Rapid ASKAP Continuum Survey (RACS) and the Variables and Slow Transients (VAST) Pilot Survey—which resulted in 229 detections of a sample of 76 radio stars. Through repeat sampling of this group our observations place constraints on the statistical fraction of the M-dwarf population that produce radio bursts and provide a forecast of expected detection rates in future surveys with ASKAP and the Square Kilometre Array. Finally, I will discuss the utility of circular polarisation searches to uncover novel regimes of stellar radio activity. I will highlight the discovery of rotationally modulated auroral radio pulses from an old, slowly rotating M9.5 ultracool dwarf in a two year monitoring campaign with ASKAP and the Australia Telescope Compact Array. This object is the oldest and slowest rotator among known aurorally active ultracool dwarfs and displays a rich variety of radio burst features, forming an important probe of the dynamo mechanism responsible for strong magnetism in ageing cool stars.
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S&A Colloquium: Prajval Shastri (ICRAR / RRI)
2023-11-22 15:00 - 16:00
Different Angles on Accreting Supermassive Black Holes

Abstract

Supermassive black holes appear to inhabit the centres of most galaxies and co-evolve with them, thus playing a significant role in regulating galaxy assembly. We are able to spot these black holes to very high redshifts when they accrete matter, which is also what causes them to impact their environments out to spatial scales that are well beyond their gravitational sphere of influence. In this talk I will discuss the understanding that has emerged from multiple-frequencies studies of the systematics of accreting supermassive black holes.

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S&A Colloquium: Amit Seta (ANU)
2023-11-29 15:00 - 16:00
Magnetic fields in the Milky Way from pulsar observations

Abstract

Pulsars can act as an excellent probe of the Milky Way magnetic field. The average strength of the Galactic magnetic field component parallel to the line of sight can be estimated as 1.232 RM/DM, where RM and DM are the rotation and dispersion measure. However, this assumes that the thermal electron density and magnetic field of the interstellar medium are uncorrelated. Using numerical simulations and observations, we test the validity of this assumption. Based on magnetohydrodynamical simulations of driven turbulence, we show that the correlation between the thermal electron density and the small-scale magnetic field increases with increasing Mach number of the turbulence. We find that the assumption of uncorrelated thermal electron density and magnetic fields is valid only for subsonic and transsonic flows, but for supersonic turbulence, the field strength can be severely overestimated. We then correlate existing pulsar observations from the Australia Telescope National Facility with regions of enhanced thermal electron density probed by CO, Zeeman, neutral hydrogen and Halpha observations. Using these observational data, we show that the thermal electron density and magnetic fields are largely uncorrelated over kpc scales. Thus, we conclude that the relation provides a good estimate of the magnetic field on Galactic scales, but might break down on sub-kpc scales.
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