arXiv subjects

Hayley Bignall

Publications and source records attributed to Hayley Bignall.

16 recordsLinked to original sources

arXiv2026

First monitoring campaign of a Main-sequence Radio Pulse emitter: the case of CU Vir

CU Vir, a magnetic hot star, is the first discovered Main-sequence Radio Pulse emitter (MRP) characterized by its ability to produce periodic radio pulses via electron cyclotron maser emission. Although significant advancements have been made in understanding MRPs, their temporal properties remain mostly unexplored. To overcome this limitation, we conducted a pilot study with the Australia Telescope Compact Array, in which we observed pulses from CU Vir at 36 epochs over $1-3$ GHz. In this frequency range, CU Vir produces two $\approx 100\%$ circularly polarized pulses, called `leading' and `trailing' pulses per rotation period. We find significant differences in the variability indices exhibited by the two pulses as a function of frequencies, with the leading pulse showing higher variability throughout our observing band. This result could be explained in the scenario of centrifugal breakout events in the magnetosphere of an oblique rotator causing correlated fluctuations across frequencies, along with intrinsic instabilities associated with coherent emission. In addition, we discover jittering in the arrival phases of pulses that must be considered in future monitoring campaigns. The pulses also exhibit a systematic shift to later arrival times during the course of our observing campaign, allowing us to refine the rotation period to $0.5206882$ days. Finally, we estimate that $\sim 30$ pulses will be needed to extract global pulse properties for the leading or trailing pulses. This relatively small number strongly motivates more extensive monitoring campaigns of MRPs, both to validate our results, and also to pinpoint the origin of the observed temporal variations.

astro-ph.SR↗

arXiv2026

Time Domain Studies of Active Galactic Nuclei with the SKA telescopes

Time domain studies of active galactic nuclei (AGN) at radio wavelengths probe physical processes near the central engine via intrinsic variability, in particular within the relativistic jets, as well as small-scale structures in the local Galactic interstellar medium (ISM) via scintillation and scattering effects. Recent discoveries reinforce the expectation that the high sensitivity, large field-of-view, and broadband frequency coverage of the SKA telescopes will help to revolutionise our understanding of AGN populations and the evolution of jets, and allow detailed modelling of the structure and dynamics of scattering plasma in the local ISM over a large fraction of the sky.

astro-ph.GA↗

arXiv2024

Demographics of black holes at $<$100 R$_{\rm g}$ scales: accretion flows, jets, and shadows

Using the Event Horizon Telescope (EHT), the gravitationally lensed rings around the supermassive black holes (SMBHs) in Messier 87 (M87) and Sagittarius A* (Sgr A*) have now been successfully imaged at a resolution under 10 gravitational radii (R$_{\rm g}$ $ = \rm{GM/c^2}$). To expand studies beyond M87 and Sgr A*, we have constructed the Event Horizon and Environs (ETHER) sample, a comprehensive database encompassing approximately 3.15 million SMBH mass estimates, $\sim$ 20,000 Very-Long Baseline Interferometry (VLBI) radio flux densities, and $\sim$ 36,000 hard X-ray flux densities. This database is designed to identify and optimize target selection for the EHT and its upgrades on the ground and in space. We have identified a Gold Sample (GS) of nearby low-luminosity Active Galactic Nuclei (AGNs) within it that are ideal for studying jet bases and potentially imaging black hole shadows. We observed 27 of these AGNs using the EHT from 2022 to 2024, providing an opportunity to resolve and image accretion flows and jets at resolutions of $\leq$ 100 R$_{\rm g}$. Only a few SMBHs have sufficiently high enough flux density to be imaged at scales of $\leq$ 50 R$_{\rm g}$ with the present EHT. Among these are M87, Sgr A*, NGC4594 (Sombrero/M104), NGC4261, and NGC4374 (Messier 84/M84). Of these, NGC4261, Sombrero, and M84 have been observed and/or are scheduled for deep imaging with EHT+ALMA from 2023 to 2025. Sombrero, NGC4261, M84, NGC4278, and NGC5232 are clearly detected in our EHT+ALMA observations in 2022, indicating that the 230 GHz flux density from the accretion flows is significantly high. Ongoing imaging of the ETHER GS will enable measurements of black hole mass and spin, help constrain General Relativity, and enrich our understanding of jet launching and accretion inflows across a broad multi-parameter space, including black hole mass, spin, accretion rate, and orientation.

astro-ph.GA↗

arXiv2019

Interstellar Scintillation Monitoring of the RadioAstron Blazars

The RadioAstron space radio telescope provides a unique opportunity to study the extreme brightness temperatures ($\mathrm{T_B }$) in AGNs with unprecedented long baselines of up to 28 Earth diameters. Since interstellar scintillation (ISS) may affect the visibilities observed with space VLBI (sVLBI), a complementary ground based flux density monitoring of the RadioAstron targets, which is performed near in time to the VLBI observation, could be beneficial. The combination/comparison with the sVLBI data can help to unravel the relative influence of source intrinsic and ISS induced effects, which in the end may alter the conclusions on the $\mathrm{T_B }$ measurements from sVLBI. Since 2013, a dedicated monitoring program has been ongoing to observe the ISS of RadioAstron AGN targets with a number of radio telescopes. Here we briefly introduce the program and present results from the statistical analysis of the Effelsberg monitoring data. We discuss the possible effects of ISS on $\mathrm{T_B }$ measurements for the RadioAstron target B0529+483 as a case study.

astro-ph.GA↗

arXiv2019

Spica and the annual cycle of PKS B1322-110 scintillations

PKS B1322-110 is a radio quasar that is located only 8.5' in angular separation from the bright B star Spica. It exhibits intra-day variability in its flux density at GHz frequencies attributed to scintillations from plasma inhomogeneities. We have tracked the rate of scintillation of this source for over a year with the Australia Telescope Compact Array, recording a strong annual cycle that includes a near-standstill in August and another in December. The cycle is consistent with scattering by highly anisotropic plasma microstructure, and we fit our data to that model in order to determine the kinematic parameters of the plasma. Because of the low ecliptic latitude of PKS B1322-110, the orientation of the plasma microstructure is poorly constrained. Nonetheless at each possible orientation our data single out a narrow range of the corresponding velocity component, leading to a one-dimensional constraint in a two-dimensional parameter space. The constrained region is consistent with a published model in which the scattering material is associated with Spica and consists of filaments that are radially oriented around the star. This result has a 1% probability of arising by chance.

astro-ph.GA↗

arXiv2018

Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of Intra-Day Variability

We present the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The~IDV observations were performed with the Effelsberg 100-m radio telescope at 4.8\,GHz, focusing on the statistical properties of IDV in a relatively large sample of compact active galactic nuclei (AGN). We investigated the dependence of rapid ($<$3 day) variability on various source properties through a likelihood approach. We found that the IDV amplitude depends on flux density and that fainter sources vary by about a factor of 3 more than their brighter counterparts. We also found a significant difference in the variability amplitude between inverted- and flat-spectrum radio sources, with the former exhibiting stronger variations. $γ$-ray loud sources were found to vary by up to a factor 4 more than $γ$-ray quiet ones, with 4$σ$ significance. However a galactic latitude dependence was barely observed, which suggests that it is predominantly the intrinsic properties (e.g., angular size, core-dominance) of the blazars that determine how they scintillate, rather than the directional dependence in the interstellar medium (ISM). We showed that the uncertainty in the VLBI brightness temperatures obtained from the space VLBI data of the RadioAstron satellite can be as high as $\sim$70\% due to the presence of the rapid flux density variations. Our statistical results support the view that IDV at centimeter wavelengths is predominantly caused by interstellar scintillation (ISS) of the emission from the most compact, core-dominant region in an AGN.

astro-ph.GA↗

arXiv2017

Scintillation kinks, bumps and wiggles in the radio spectrum of the quasar PMN J1106-3647

We report radio observations of the quasar PMN J1106-3647. Our data, taken with the Australia Telescope Compact Array, show large variations in the amplitude and shape of its spectrum, on a short time-scale. A great variety of spectral features is evident, including: sharp kinks; broad spectral peaks; and wiggles. No two spectra are alike. We interpret the variations as interstellar scintillation of a radio source that is compact, but not point-like. Under this interpretation, complex spectral structure can arise purely refractively, under high magnification conditions, or from interference between waves that have been scattered by small-scale density fluctuations (diffractive scintillation). Both effects may be playing a role in J1106-3647, and we tentatively identify kinks with the former, and wiggles with the latter. Diffractive scintillation of AGN is uncommon, as the fringe visibility is low for all but the most compact radio sources. Refractive interpretation of the kink implies that the source has a sharp, concave boundary. Our data are consistent with a mildly boosted synchrotron source, provided the scattering material is at a distance ~50 pc from us.

astro-ph.HE↗

arXiv2017

Extreme radio-wave scattering associated with hot stars

We use data on extreme radio scintillation to demonstrate that this phenomenon is associated with hot stars in the solar neighbourhood. The ionized gas responsible for the scattering is found at distances up to 1.75pc from the host star, and on average must comprise 1.E5 distinct structures per star. We detect azimuthal velocities of the plasma, relative to the host star, up to 9.7 km/s, consistent with warm gas expanding at the sound speed. The circumstellar plasma structures that we infer are similar in several respects to the cometary knots seen in the Helix, and in other planetary nebulae. There the ionized gas appears as a skin around tiny molecular clumps. Our analysis suggests that molecular clumps are ubiquitous circumstellar features, unrelated to the evolutionary state of the star. The total mass in such clumps is comparable to the stellar mass.

astro-ph.GA↗

arXiv2016

Real-time detection of an extreme scattering event: constraints on Galactic plasma lenses

Extreme scattering events (ESEs) are distinctive fluctuations in the brightness of astronomical radio sources caused by occulting plasma lenses in the interstellar medium. The inferred plasma pressures of the lenses are $\sim 10^3$ times the ambient pressure, challenging our understanding of gas conditions in the Milky Way. Using a new survey technique, we have discovered an ESE while it was in progress. We report radio and optical follow-up observations. Modelling of the radio data demonstrates that the lensing structure is a density enhancement and that the lens is diverging, ruling out one of two competing physical models. Our technique will uncover many more ESEs, addressing a long-standing mystery of the small-scale gas structure of the Galaxy.

astro-ph.GA↗

arXiv2015

Time domain studies of Active Galactic Nuclei with the Square Kilometre Array

Variability of radio-emitting active galactic nuclei can be used to probe both intrinsic variations arising from shocks, flares, and other changes in emission from regions surrounding the central supermassive black hole, as well as extrinsic variations due to scattering by structures in our own Galaxy. Such interstellar scattering also probes the structure of the emitting regions, with microarcsecond resolution. Current studies have necessarily been limited to either small numbers of objects monitored over long periods of time, or large numbers of objects but with poor time sampling. The dramatic increase in survey speed engendered by the Square Kilometre Array will enable precision synoptic monitoring studies of hundreds of thousands of sources with a cadence of days or less. Statistics of variability, in particular concurrent observations at multiple radio frequencies and in other bands of the electromagnetic spectrum, will probe accretion physics over a wide range of AGN classes, luminosities, and orientations, as well as enabling a detailed understanding of the structures responsible for radio wave scattering in the Galactic interstellar medium.

astro-ph.HE↗

arXiv2015

Microarcsecond structure of an AGN Jet via Interstellar Scintillation

We have used the broadband backend available at the ATCA to study the fast interstellar scintillation of quasar PKS 1257-326, resolving the core shift as a function of frequency on scales less than 10 microarcseconds. In this short paper we discuss the jet direction implied from the microarcsecond-scale core shift in PKS 1257-326.

astro-ph.HE↗

arXiv2014

Very Long Baseline Interferometry with the SKA

Adding VLBI capability to the SKA arrays will greatly broaden the science of the SKA, and is feasible within the current specifications. SKA-VLBI can be initially implemented by providing phased-array outputs for SKA1-MID and SKA1-SUR and using these extremely sensitive stations with other radio telescopes, and in SKA2 by realising a distributed configuration providing baselines up to thousands of km, merging it with existing VLBI networks. The motivation for and the possible realization of SKA-VLBI is described in this paper.

astro-ph.IM↗

arXiv2011

First geodetic observations using new VLBI stations ASKAP-29 and WARK12M

We report the results of a successful 7 hour 1.4 GHz VLBI experiment using two new stations, ASKAP-29 located in Western Australia and WARK12M located on the North Island of New Zealand. This was the first geodetic VLBI observing session with the participation of these new stations. We have determined the positions of ASKAP-29 and WARK12M. Random errors on position estimates are 150-200 mm for the vertical component and 40-50 mm for the horizontal component. Systematic errors caused by the unmodeled ionosphere path delay may reach 1.3 m for the vertical component.

astro-ph.IM↗

arXiv2008

Extremely Anisotropic Scintillations

A small number of quasars exhibit interstellar scintillation on time-scales less than an hour; their scintillation patterns are all known to be anisotropic. Here we consider a totally anisotropic model in which the scintillation pattern is effectively one-dimensional. For the persistent rapid scintillators J1819+3845 and PKS1257-326 we show that this model offers a good description of the two-station time-delay measurements and the annual cycle in the scintillation time-scale. Generalising the model to finite anisotropy yields a better match to the data but the improvement is not significant and the two additional parameters which are required to describe this model are not justified by the existing data. The extreme anisotropy we infer for the scintillation patterns must be attributed to the scattering medium rather than a highly elongated source. For J1819+3845 the totally anisotropic model predicts that the particular radio flux variations seen between mid July and late August should repeat between late August and mid November, and then again between mid November and late December as the Earth twice changes its direction of motion across the scintillation pattern. If this effect can be observed then the minor-axis velocity component of the screen and the orientation of that axis can both be precisely determined. In reality the axis ratio is finite, albeit large, and spatial decorrelation of the flux pattern along the major axis may be observable via differences in the pairwise fluxes within this overlap region; in this case we can also constrain both the major-axis velocity component of the screen and the magnitude of the anisotropy.

astro-ph↗

arXiv2008

The nature of gamma ray blazar candidate PMN J1326-5256

A comparison of AGN detected at gamma ray energies by EGRET with flat-spectrum radio sources observed in surveys for intraday variability reveals that a remarkably high fraction of EGRET blazars show significant interstellar scintillation at centimetre wavelengths. Scintillating AGN will therefore be targets of interest for GLAST, scheduled for launch in early 2008. We suggest that the variable, scintillating flat-spectrum radio source PMN J1326-5256 is associated with the unidentified EGRET source 3EG J1316-5244. We describe the properties of PMN J1326-5256 and present recent results of monitoring with the ATCA and Ceduna radio telescopes.

astro-ph↗

arXiv2004

The Radio Variability of the Gravitational Lens PMN J1838-3427

We present the results of a radio variability study of the gravitational lens PMN J1838-3427. Our motivation was to determine the Hubble constant by measuring the time delay between variations of the two quasar images. We monitored the system for 4 months (approximately 5 times longer than the expected delay) using the Australia Telescope Compact Array at 9 GHz. Although both images were variable on a time scale of a few days, no correlated intrinsic variability could be identified, and therefore no time delay could be measured. Notably, the fractional variation of the fainter image (8%) was greater than that of the brighter image (4%), whereas lensed images of a point source would have the same fractional variation. This effect can be explained, at least in part, as the refractive scintillation of both images due to the turbulent interstellar medium of the Galaxy.

astro-ph↗