arXiv subjects

B. M. Gaensler

Publications and source records attributed to B. M. Gaensler.

At least 163 records · Page 9Linked to original sources

arXiv2018

Ghost of a Shell: Magnetic Fields of Galactic Supershell GSH 006$-$15$+$7

We identify a counterpart to a Galactic supershell in diffuse radio polarisation, and use this to determine the magnetic fields associated with this object. GSH 006$-$15$+$7 has perturbed the polarised emission at 2.3$\,$GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS), acting as a Faraday screen. We model the Faraday rotation over the shell, and produce a map of Faraday depth over the area across it. Such models require information about the polarised emission behind the screen, which we obtain from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23$\,$GHz to 2.3$\,$GHz, to estimate the synchrotron background behind GSH 006$-$15$+$7. Using the modelled Faraday thickness we determine the magnitude and the plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We find a peak line-of-sight field strength of $|B_\parallel|_\text{peak} = 2.0\substack{+0.01 \\ -0.7}\,μ$G. Our measurement probes weak magnetic fields in a low-density regime (number densities of $\sim0.4\,$cm$^{-3}$) of the ISM, thus providing crucial information about the magnetic fields in the partially-ionised phase.

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arXiv2018

Dispersion and Rotation Measure of Supernova Remnants and Magnetized Stellar Winds: Application to Fast Radio Bursts

Recent studies of fast radio bursts (FRBs) have led to many theories associating them with young neutron stars. If this is the case, then the presence of supernova ejecta and stellar winds provide a changing dispersion measure (DM) and rotation measure (RM) that can potentially be probes of the environments of FRB progenitors. Here we summarize the scalings for the DM and RM in the cases of a constant density ambient medium and of a progenitor stellar wind. Since the amount of ionized material is controlled by the dynamics of the reverse shock, we find the DM changes more slowly than in previous simpler work, which simply assumed a constant ionization fraction. Furthermore, the DM can be constant or even increasing as the supernova remnant sweeps up material, arguing that a young neutron star hypothesis for FRBs is not ruled out if the DM is not decreasing over repeated bursts. The combined DM and RM measurements for the repeating FRB 121102 are consistent with supernova ejecta with an age of $\sim10^2-10^3\,{\rm yrs}$ expanding into a high density ($\sim100\,{\rm cm^{-3}}$) interstellar medium. This naturally explains its relatively constant DM over many years as well. Other FRBs with much lower RMs may indicate that they are especially young supernovae in wind environments or that their DMs are largely from the intergalactic medium. We therefore caution about inferring magnetic fields from simply by dividing an RM by DM, because these quantities could originate from distinct regions along the path an FRB propagates.

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arXiv2018

Galactic synchrotron distribution derived from 152 HII region absorption features in the full GLEAM survey

We derive the synchrotron distribution in the Milky Way disk from HII region absorption observations over -40° < l < 40° at six frequencies of 76.2, 83.8, 91.5, 99.2, 106.9, and 114.6 MHz with the GaLactic and Extragalactic All-sky Murchison widefield array survey (GLEAM). We develop a new method of emissivity calculation by taking advantage of the Haslam et al., (1981) map and known spectral indices, which enable us to simultaneously derive the emissivity and the optical depth of HII regions at each frequency. We show our derived synchrotron emissivities based on 152 absorption features of HII regions using both the method previously adopted in the literature and our improved method. We derive the synchrotron emissivity from HII regions to the Galactic edge along the line of sight and, for the first time, derive the emissivity from HII regions to the Sun. These results provide direct information on the distribution of the Galactic magnetic field and cosmic-ray electrons for future modelling.

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arXiv2018

Detection of Linear Polarization in the Radio Remnant of Supernova 1987A

Supernova 1987A in the Large Magellanic Cloud has proven a unique laboratory to investigate particle acceleration in young supernova remnants. Here we report the first detection of linear polarization of the supernova's synchrotron emission from imaging observations at frequencies spanning from 20 to 50 GHz, carried out with the Australia Telescope Compact Array between October 2015 and May 2016. The direction of the radio polarization, corrected for Faraday rotation, points to a primarily radial magnetic field across the inner ring, encompassing both the reverse and forward shocks. The magnetic field strength peaks over the high-emissivity eastern sites, where efficient cosmic ray acceleration likely takes place under quasi-parallel shocks at high Mach numbers. The mean fraction of polarized emission in the brightest sites is $2.7\pm0.2\%$ at 22 GHz and $3.5\pm0.7\%$ at 44 GHz. In the inner remnant, non-radial components of the polarized emission appear to be more prevalent. However, the low significance detection in the central regions limits interpretation.

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arXiv2018

Limits on radio emission from meteors using the MWA

Recently, low frequency, broadband radio emission has been observed accompanying bright meteors by the Long Wavelength Array (LWA). The broadband spectra between 20 and 60 MHz were captured for several events, while the spectral index (dependence of flux density on frequency, with $S_ν\propto ν^α$) was estimated to be $-4\pm1$ during the peak of meteor afterglows. Here we present a survey of meteor emission and other transient events using the Murchison Widefield Array (MWA) at 72-103 MHz. In our 322-hour survey, down to a $5σ$ detection threshold of 3.5 Jy/beam, no transient candidates were identified as intrinsic emission from meteors. We derived an upper limit of -3.7 (95% confidence limit) on the spectral index in our frequency range. We also report detections of other transient events, like reflected FM broadcast signals from small satellites, conclusively demonstrating the ability of the MWA to detect and track space debris on scales as small as 0.1 m in low Earth orbits.

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arXiv2018

The CHIME Fast Radio Burst Project: System Overview

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a novel transit radio telescope operating across the 400-800-MHz band. CHIME is comprised of four 20-m x 100-m semi-cylindrical paraboloid reflectors, each of which has 256 dual-polarization feeds suspended along its axis, giving it a >200 square degree field-of-view. This, combined with wide bandwidth, high sensitivity, and a powerful correlator makes CHIME an excellent instrument for the detection of Fast Radio Bursts (FRBs). The CHIME Fast Radio Burst Project (CHIME/FRB) will search beam-formed, high time-and frequency-resolution data in real time for FRBs in the CHIME field-of-view. Here we describe the CHIME/FRB backend, including the real-time FRB search and detection software pipeline as well as the planned offline analyses. We estimate a CHIME/FRB detection rate of 2-42 FRBs/sky/day normalizing to the rate estimated at 1.4-GHz by Vander Wiel et al. (2016). Likely science outcomes of CHIME/FRB are also discussed. CHIME/FRB is currently operational in a commissioning phase, with science operations expected to commence in the latter half of 2018.

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arXiv2018

SMSS J130522.47-293113.0: a high-latitude stellar X-ray source with pc-scale outflow relics?

We report the discovery of an unusual stellar system, SMSS J130522.47-293113.0. The optical spectrum is dominated by a blue continuum together with emission lines of hydrogen, neutral and ionized helium, and the N III, C III blend at around 4640-4650 Angstrom. The emission line profiles vary in strength and position on timescales as short as 1 day, while optical photometry reveals fluctuations of as much as ~0.2 mag in g on timescales as short as 10-15 min. The system is a weak X-ray source (f_{0.3-10} = (1.2 +/- 0.1) x 10^{-13} ergs cm^{-2} s^{-1} in the 0.3-10 keV band) but is not detected at radio wavelengths (3-sigma upper limit of 50 microJy at 5.5 GHz). The most intriguing property of the system, however, is the existence of two "blobs", a few arcsec in size, that are symmetrically located 3.8 arcmin (2.2 pc for our preferred system distance of ~2 kpc) each side of the central object. The blobs are detected in optical and near-IR broadband images but do not show any excess emission in Halpha images. We discuss the interpretation of the system, suggesting that the central object is most likely a nova-like CV, and that the blobs are relics of a pc-scale accretion-powered collimated outflow.

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arXiv2018

A Serendipitous MWA Search for Narrow-band and Broad-band Low Frequency Radio Transmissions from 1I/2017 U1 'Oumuamua

We examine data from the Murchison Widefield Array (MWA) in the frequency range 72 -- 102 MHz for a field-of-view that serendipitously contained the interstellar object 'Oumuamua on 2017 November 28. Observations took place with time resolution of 0.5 s and frequency resolution of 10 kHz. %This observation was undertaken for another purpose but due to the MWA's extremely large field-of-view, 'Oumuamua was serendipitously observed simultaneously. Based on the interesting but highly unlikely suggestion that 'Oumuamua is an interstellar spacecraft, due to some unusual orbital and morphological characteristics, we examine our data for signals that might indicate the presence of intelligent life associated with 'Oumuamua. We searched our radio data for: 1) impulsive narrow-band signals; 2) persistent narrow-band signals; and 3) impulsive broadband signals. We found no such signals with non-terrestrial origins and make estimates of the upper limits on Equivalent Isotropic Radiated Power (EIRP) for these three cases of approximately 7 kW, 840 W, and 100 kW, respectively. These transmitter powers are well within the capabilities of human technologies, and are therefore plausible for alien civilizations. While the chances of positive detection in any given Search for Extraterrestrial Intelligence (SETI) experiment are vanishingly small, the characteristics of new generation telescopes such as the MWA (and in the future, the Square Kilometre Array) make certain classes of SETI experiment easy, or even a trivial by-product of astrophysical observations. This means that the future costs of SETI experiments are very low, allowing large target lists to partially balance the low probability of a positive detection.

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arXiv2018

Future Cosmological Constraints from Fast Radio Bursts

We consider the possible observation of Fast Radio Bursts (FRBs) with planned future radio telescopes, and investigate how well the dispersions and redshifts of these signals might constrain cosmological parameters. We construct mock catalogues of FRB dispersion measure (DM) data and employ Markov Chain Monte Carlo (MCMC) analysis, with which we forecast and compare with existing constraints in the flat $Λ$CDM model, as well as some popular extensions that include dark energy equation of state and curvature parameters. We find that the scatter in DM observations caused by inhomogeneities in the intergalactic medium (IGM) poses a big challenge to the utility of FRBs as a cosmic probe. Only in the most optimistic case, with a high number of events and low IGM variance, do FRBs aid in improving current constraints. In particular, when FRBs are combined with CMB+BAO+SNe+$H_0$ data, we find the biggest improvement comes in the $Ω_{\mathrm b}h^2$ constraint. Also, we find that the dark energy equation of state is poorly constrained, while the constraint on the curvature parameter $Ω_k$, shows some improvement when combined with current constraints. When FRBs are combined with future BAO data from 21cm Intensity Mapping (IM), we find little improvement over the constraints from BAOs alone. However, the inclusion of FRBs introduces an additional parameter constraint, $Ω_{\mathrm b}h^2$, which turns out to be comparable to existing constraints. This suggest that FRBs provide valuable information about the cosmological baryon density in the intermediate redshift Universe, independent of high redshift CMB data.

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arXiv2018

Advanced Diagnostics for the Study of Linearly Polarized Emission. II: Application to Diffuse Interstellar Radio Synchrotron Emission

Diagnostics of polarized emission provide us with valuable information on the Galactic magnetic field and the state of turbulence in the interstellar medium, which cannot be obtained from synchrotron intensity alone. In Paper I (Herron et al. 2017b), we derived polarization diagnostics that are rotationally and translationally invariant in the $Q$-$U$ plane, similar to the polarization gradient. In this paper, we apply these diagnostics to simulations of ideal magnetohydrodynamic turbulence that have a range of sonic and Alfvénic Mach numbers. We generate synthetic images of Stokes $Q$ and $U$ for these simulations, for the cases where the turbulence is illuminated from behind by uniform polarized emission, and where the polarized emission originates from within the turbulent volume. From these simulated images we calculate the polarization diagnostics derived in Paper I, for different lines of sight relative to the mean magnetic field, and for a range of frequencies. For all of our simulations, we find that the polarization gradient is very similar to the generalized polarization gradient, and that both trace spatial variations in the magnetoionic medium for the case where emission originates within the turbulent volume, provided that the medium is not supersonic. We propose a method for distinguishing the cases of emission coming from behind or within a turbulent, Faraday rotating medium, and a method to partly map the rotation measure of the observed region. We also speculate on statistics of these diagnostics that may allow us to constrain the physical properties of an observed turbulent region.

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arXiv2018

Broadband radio polarimetry of Fornax A, I: Depolarized patches generated by advected thermal material from NGC 1316

We present observations and analysis of the polarized radio emission from the nearby radio galaxy Fornax A over 1.28--3.1 GHz, using data from the Australia Telescope Compact Array (ATCA). In this, the first of two associated papers, we use modern broadband polarimetric techniques to examine the nature and origin of conspicuous low-polarization (low-$p$) patches in the lobes. We resolve the low-$p$ patches, and find that their low fractional polarization is associated with complicated frequency-dependent interference in the polarized signal generated by Faraday effects along the line of sight. The low-$p$ patches are spatially correlated with interfaces in the magnetic structure of the lobe, across which the line-of-sight-projected magnetic field changes direction. Spatial correlations with the sky-projected magnetic field orientation and structure in total intensity are also identified and discussed. We argue that the low-$p$ patches, along with associated reversals in the line-of-sight magnetic field and other related phenomena, are best explained by the presence of $\mathcal{O}(10^9)$ $M_\odot$ of magnetized thermal plasma in the lobes, structured in shells or filaments, and likely advected from the ISM of NCG 1316 or its surrounding ICM. Our study underscores the power and utility of spatially-resolved, broadband, full-polarization radio observations to reveal new facets of flow behaviors and magneto-ionic structure in radio lobes and their interplay with the surrounding environment.

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arXiv2018

Revealing the Faraday Depth Structure of Radio Galaxy NGC 612 with Broad-Band Radio Polarimetric Observations

We present full-polarisation, broadband observations of the radio galaxy NGC 612 (PKS B0131-637) from 1.3 to 3.1 GHz using the Australia Telescope Compact Array. The relatively large angular scale of the radio galaxy makes it a good candidate with which to investigate the polarisation mechanisms responsible for the observed Faraday depth structure. By fitting complex polarisation models to the polarised spectrum of each pixel, we find that a single polarisation component can adequately describe the observed signal for the majority of the radio galaxy. While we cannot definitively rule out internal Faraday rotation, we argue that the bulk of the Faraday rotation is taking place in a thin skin that girts the polarised emission. Using minimum energy estimates, we find an implied total magnetic field strength of 4.2 microG.

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arXiv2017

Classifying Complex Faraday Spectra with Convolutional Neural Networks

Advances in radio spectro-polarimetry offer the possibility to disentangle complex regions where relativistic and thermal plasmas mix in the interstellar and intergalactic media. Recent work has shown that apparently simple Faraday Rotation Measure (RM) spectra can be generated by complex sources. This is true even when the distribution of RMs in the complex source greatly exceeds the errors associated with a single component fit to the peak of the Faraday spectrum. We present a convolutional neural network (CNN) that can differentiate between simple Faraday thin spectra and those that contain multiple or Faraday thick sources. We demonstrate that this CNN, trained for the upcoming Polarisation Sky Survey of the Universe's Magnetism (POSSUM) early science observations, can identify two component sources 99% of the time, provided that the sources are separated in Faraday depth by $>$10% of the FWHM of the Faraday Point Spread Function, the polarized flux ratio of the sources is $>$0.1, and that the Signal-to-Noise radio (S/N) of the primary component is $>$5. With this S/N cut-off, the false positive rate (simple sources mis-classified as complex) is $<$0.3%. Work is ongoing to include Faraday thick sources in the training and testing of the CNN.

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arXiv2017

The jet/wind outflow in Centaurus A: a local laboratory for AGN feedback

We present new radio and optical images of the nearest radio galaxy Centaurus A and its host galaxy NGC 5128. We focus our investigation on the northern transition region, where energy is transported from the ~5 kpc (~5 arcmin) scales of the Northern Inner Lobe (NIL) to the ~30 kpc (~30 arcmin) scales of the Northern Middle Lobe (NML). Our Murchison Widefield Array observations at 154 MHz and our Parkes radio telescope observations at 2.3 GHz show diffuse radio emission connecting the NIL to the NML, in agreement with previous Australia Telescope Compact Array observations at 1.4 GHz. Comparison of these radio data with our widefield optical emission line images show the relationship between the NML radio emission and the ionised filaments that extend north from the NIL, and reveal a new ionised filament to the east, possibly associated with a galactic wind. Our deep optical images show clear evidence for a bipolar outflow from the central galaxy extending to intermediate scales, despite the non-detection of a southern radio counterpart to the NML. Thus, our observational overview of Centaurus A reveals a number of features proposed to be associated with AGN feedback mechanisms, often cited as likely to have significant effects in galaxy evolution models. As one of the closest galaxies to us, Centaurus A therefore provides a unique laboratory to examine feedback mechanisms in detail.

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arXiv2017

When disorder looks like order: a new model to explain radial magnetic fields in young supernova remnants

Radial magnetic fields are observed in all known young, shell-type supernova remnants (SNRs) in our Galaxy, including Cas A, Tycho, Kepler, and SN1006 and yet the nature of these radial fields has not been thoroughly explored. Using a 3D model, we consider the existence and observational implications of an intrinsically radial field. We also present a new explanation of the origin of the radial pattern observed from polarization data as resulting from a selection effect due to the distribution of cosmic-ray electrons (CREs). We show that quasi-parallel acceleration can concentrate CREs at regions where the magnetic field is radial, making a completely turbulent field appear ordered, when it is in fact disordered. We discuss observational properties that may help distinguish between an intrinsically radial magnetic field and the case where it only appears radial due to the CRE distribution. We also show that the case of an intrinsically radial field with a quasi-perpendicular CRE acceleration mechanism has intriguing similarities to the observed polarization properties of SN1006.

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arXiv2017

Calibration and Stokes Imaging with Full Embedded Element Primary Beam Model for the Murchison Widefield Array

The Murchison Widefield Array (MWA), located in Western Australia, is one of the low-frequency precursors of the international Square Kilometre Array (SKA) project. In addition to pursuing its own ambitious science program, it is also a testbed for wide range of future SKA activities ranging from hardware, software to data analysis. The key science programs for the MWA and SKA require very high dynamic ranges, which challenges calibration and imaging systems. Correct calibration of the instrument and accurate measurements of source flux densities and polarisations require precise characterisation of the telescope's primary beam. Recent results from the MWA GaLactic Extragalactic All-sky MWA (GLEAM) survey show that the previously implemented Average Embedded Element (AEE) model still leaves residual polarisations errors of up to 10-20 % in Stokes Q. We present a new simulation-based Full Embedded Element (FEE) model which is the most rigorous realisation yet of the MWA's primary beam model. It enables efficient calculation of the MWA beam response in arbitrary directions without necessity of spatial interpolation. In the new model, every dipole in the MWA tile (4 x 4 bow-tie dipoles) is simulated separately, taking into account all mutual coupling, ground screen and soil effects, and therefore accounts for the different properties of the individual dipoles within a tile. We have applied the FEE beam model to GLEAM observations at 200 - 231 MHz and used false Stokes parameter leakage as a metric to compare the models. We have determined that the FEE model reduced the magnitude and declination-dependent behaviour of false polarisation in Stokes Q and V while retaining low levels of false polarisation in Stokes U.

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arXiv2017

Advanced Diagnostics for the Study of Linearly Polarized Emission. I: Derivation

Linearly polarized emission is described, in general, in terms of the Stokes parameters $Q$ and $U$, from which the polarization intensity and polarization angle can be determined. Although the polarization intensity and polarization angle provide an intuitive description of the polarization, they are affected by the limitations of interferometric data, such as missing single-dish data in the u-v plane, from which radio frequency interferometric data is visualized. To negate the effects of these artefacts, it is desirable for polarization diagnostics to be rotationally and translationally invariant in the $Q$-$U$ plane. One rotationally and translationally invariant quantity, the polarization gradient, has been shown to provide a unique view of spatial variations in the turbulent interstellar medium when applied to diffuse radio frequency synchrotron emission. In this paper we develop a formalism to derive additional rotationally and translationally invariant quantities. We present new diagnostics that can be applied to diffuse or point-like polarized emission in any waveband, including a generalization of the polarization gradient, the polarization directional curvature, polarization wavelength derivative, and polarization wavelength curvature. In Paper II we will apply these diagnostics to observed and simulated images of diffuse radio frequency synchrotron emission.

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arXiv2017

Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programs

The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source afterglow cooled from approximately 6400K to 2100K over a 7-day period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (about 2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.

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