arXiv subjects

B. M. Gaensler

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

At least 109 records · Page 6Linked to original sources

arXiv2021

Late-Time Evolution and Modeling of the Off-Axis Gamma-ray Burst Candidate FIRST J141918.9+394036

We present new radio and optical data, including very long baseline interferometry, as well as archival data analysis, for the luminous decades-long radio transient FIRST J141918.9+394036. The radio data reveal a synchrotron self-absorption peak around 0.3 GHz and a radius of around 1.3 mas (0.5 pc) 26 years post-discovery, indicating a blastwave energy $\sim5 \times 10^{50}$ erg. The optical spectrum shows a broad [OIII]$λ$4959,5007 emission-line that may indicate collisional-excitation in the host galaxy, but its association with the transient cannot be ruled out. The properties of the host galaxy are suggestive of a massive stellar progenitor that formed at low metallicity. Based on the radio light curve, blastwave velocity, energetics, nature of the host galaxy and transient rates we find that the properties of FIRST J1419+39 are most consistent with long gamma-ray burst (LGRB) afterglows. Other classes of (optically-discovered) stellar explosions as well as neutron star mergers are disfavored, and invoking any exotic scenario may not be necessary. It is therefore likely that FIRST J1419+39 is an off-axis LGRB afterglow (as suggested by Law et al. and Marcote et al.), and under this premise the inverse beaming fraction is found to be $f_b^{-1}\simeq280^{+700}_{-200}$, corresponding to an average jet half-opening angle $<θ_j>\simeq5^{+4}_{-2}$ degrees (68% confidence), consistent with previous estimates. From the volumetric rate we predict that surveys with the VLA, ASKAP and MeerKAT will find a handful of FIRST J1419+39-like events over the coming years.

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arXiv2021

First discovery of new pulsars and RRATs with CHIME/FRB

We report the discovery of seven new Galactic pulsars with the Canadian Hydrogen Intensity Mapping Experiment's Fast Radio Burst backend (CHIME/FRB). These sources were first identified via single pulses in CHIME/FRB, then followed up with CHIME/Pulsar. Four sources appear to be rotating radio transients (RRATs), pulsar-like sources with occasional single pulse emission with an underlying periodicity. Of those four sources, three have detected periods ranging from 220 ms to 2.726 s. Three sources have more persistent but still intermittent emission and are likely intermittent or nulling pulsars. We have determined phase-coherent timing solutions for the latter three. These seven sources are the first discovery of previously unknown Galactic sources with CHIME/FRB and highlight the potential of fast radio burst detection instruments to search for intermittent Galactic radio sources.

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arXiv2021

Multi-scale feedback and feeding in the closest radio galaxy Centaurus A

Supermassive black holes and supernovae explosions at the centres of active galaxies power cycles of outflowing and inflowing gas that affect galactic evolution and the overall structure of the Universe. While simulations and observations show that this must be the case, the range of physical scales (over ten orders of magnitude) and paucity of available tracers, make both the simulation and observation of these effects difficult. By serendipity, there lies an active galaxy, Centaurus A (NGC 5128), at such a close proximity as to allow its observation over this entire range of scales and across the entire electromagnetic spectrum. In the radio band, however, details on scales of 10-100 kpc from the supermassive black hole have so far been obscured by instrumental limitations. Here we report low-frequency radio observations that overcome these limitations and show evidence for a broad, bipolar outflow with velocity 1100 km per s and mass outflow rate of 2.9 solar masses per year on these scales. We combine our data with the plethora of multi-scale, multi-wavelength historical observations of Centaurus A to probe a unified view of feeding and feedback, which we show to be consistent with the Chaotic Cold Accretion self-regulation scenario.

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arXiv2021

The Galactic Faraday rotation sky 2020

This work gives an update to existing reconstructions of the Galactic Faraday rotation sky by processing almost all Faraday rotation data sets available at the end of the year 2020. Observations of extra-Galactic sources in recent years have, among other regions, further illuminated the previously under-constrained southern celestial sky, as well as parts of the inner disc of the Milky Way. This has culminated in an all-sky data set of 55,190 data points, which is a significant expansion on the 41,330 used in previous works, hence making an updated separation of the Galactic component a promising venture. The increased source density allows us to present our results in a resolution of about $1.3\cdot 10^{-2}\, \mathrm{deg}^2$ ($46.8\,\mathrm{arcmin}^2$), which is a twofold increase compared to previous works. As for previous Faraday rotation sky reconstructions, this work is based on information field theory, a Bayesian inference scheme for field-like quantities which handles noisy and incomplete data. In contrast to previous reconstructions, we find a significantly thinner and pronounced Galactic disc with small-scale structures exceeding values of several thousand $\mathrm{rad}\,\mathrm{m}^{-2}$. The improvements can mainly be attributed to the new catalog of Faraday data, but are also supported by advances in correlation structure modeling within numerical information field theory. We furthermore give a detailed discussion on statistical properties of the Faraday rotation sky and investigate correlations to other data sets.

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arXiv2021

A repeating fast radio burst source in a globular cluster

Fast radio bursts (FRBs) are exceptionally luminous flashes of unknown physical origin, reaching us from other galaxies (Petroff et al. 2019). Most FRBs have only ever been seen once, while others flash repeatedly, though sporadically (Spitler et al. 2016, CHIME/FRB Collaboration et al. 2021). Many models invoke magnetically powered neutron stars (magnetars) as the engines producing FRB emission (Margalit & Metzger 2018, CHIME/FRB Collaboration et al. 2020). Recently, CHIME/FRB announced the discovery (Bhardwaj et al. 2021) of the repeating FRB 20200120E, coming from the direction of the nearby grand design spiral galaxy M81. Four potential counterparts at other observing wavelengths were identified (Bhardwaj et al. 2021) but no definitive association with these sources, or M81, could be made. Here we report an extremely precise localisation of FRB 20200120E, which allows us to associate it with a globular cluster (GC) in the M81 galactic system and to place it ~2pc offset from the optical center of light of the GC. This confirms (Bhardwaj et al. 2021) that FRB 20200120E is 40 times closer than any other known extragalactic FRB. Because such GCs host old stellar populations, this association strongly challenges FRB models that invoke young magnetars formed in a core-collapse supernova as powering FRB emission. We propose, instead, that FRB 20200120E is a highly magnetised neutron star formed via either accretion-induced collapse of a white dwarf or via merger of compact stars in a binary system (Margalit et al. 2019). Alternative scenarios involving compact binary systems, efficiently formed inside globular clusters, could also be responsible for the observed bursts.

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arXiv2021

A Unified Model for the Fan Region and the North Polar Spur: A bundle of filaments in the Local Galaxy

We present a simple, unified model that can explain two of the brightest, large-scale, diffuse, polarized radio features in the sky, the North Polar Spur (NPS) and the Fan Region, along with several other prominent loops. We suggest that they are long, magnetized, and parallel filamentary structures that surround the Local arm and/or Local Bubble, in which the Sun is embedded. We show this model is consistent with the large number of observational studies on these regions, and is able to resolve an apparent contradiction in the literature that suggests the high latitude portion of the NPS is nearby, while lower latitude portions are more distant. Understanding the contributions of this local emission is critical to developing a complete model of the Galactic magnetic field. These very nearby structures also provide context to help understand similar non-thermal, filamentary structures that are increasingly being observed with modern radio telescopes.

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arXiv2021

Discovery of ASKAP J173608.2-321635 as a Highly-Polarized Transient Point Source with the Australian SKA Pathfinder

We report the discovery of a highly-polarized, highly-variable, steep-spectrum radio source, ASKAP J173608.2-321635, located $\sim$4\,deg from the Galactic center in the Galactic plane. The source was detected six times between 2020 January and 2020 September as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey at 888\,MHz. It exhibited a high degree ($\sim 25$\%) of circular polarization when it was visible. We monitored the source with the MeerKAT telescope from 2020 November to 2021 February on a 2--4 week cadence. The source was not detected with MeerKAT before 2021 February 07 when it appeared and reached a peak flux density of 5.6\,mJy. The source was still highly circularly polarized, but also showed up to 80\% linear polarization, and then faded rapidly with a timescale of one day. The rotation measure of the source varied significantly, from $-11.8\pm0.8$\,rad\,m$^{-2}$ to $-64.0\pm1.5$\,rad\,m$^{-2}$, over three days. No X-ray counterpart was found in follow-up \textit{Swift} or \textit{Chandra} observations about a week after the first MeerKAT detection, with upper limits of $\sim 5.0\times10^{31}$\,erg\,s$^{-1}$ (0.3--8\,keV, assuming a distance $\sim10$ kpc). No counterpart is seen in new or archival near-infrared observations down to $J=20.8$\,mag. We discuss possible identifications for ASKAP J173608.2-321635 including a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center Radio Transient: none of these fully explains the observations, which suggests that ASKAP J173608.2-321635 may represent part of a new class of objects being discovered through radio imaging surveys.

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arXiv2021

A Local Universe Host for the Repeating Fast Radio Burst FRB 20181030A

We report on the host association of FRB 20181030A, a repeating fast radio burst (FRB) with a low dispersion measure (DM, 103.5 pc cm$^{-3}$) discovered by CHIME/FRB Collaboration et al. (2019a). Using baseband voltage data saved for its repeat bursts, we localize the FRB to a sky area of 5.3 sq. arcmin (90% confidence). Within the FRB localization region, we identify NGC 3252 as the most promising host, with an estimated chance coincidence probability $< 2.5 \times 10^{-3}$. Moreover, we do not find any other galaxy with M$_{r} < -15$ AB mag within the localization region to the maximum estimated FRB redshift of 0.05. This rules out a dwarf host 5 times less luminous than any FRB host discovered to date. NGC 3252 is a star-forming spiral galaxy, and at a distance of $\approx$ 20 Mpc, it is one of the closest FRB hosts discovered thus far. From our archival radio data search, we estimate a 3$σ$ upper limit on the luminosity of a persistent compact radio source (source size $<$ 0.3 kpc at 20 Mpc) at 3 GHz to be ${\rm 2 \times 10^{26} erg~s^{-1} Hz^{-1}}$, at least 1500 times smaller than that of the FRB 20121102A persistent radio source. We also argue that a population of young millisecond magnetars alone cannot explain the observed volumetric rate of repeating FRBs. Finally, FRB 20181030A is a promising source for constraining FRB emission models due to its proximity, and we strongly encourage its multi-wavelength follow-up.

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arXiv2021

The ASKAP Variables and Slow Transients (VAST) Pilot Survey

The Variables and Slow Transients Survey (VAST) on the Australian Square Kilometre Array Pathfinder (ASKAP) is designed to detect highly variable and transient radio sources on timescales from 5 seconds to $\sim 5$ years. In this paper, we present the survey description, observation strategy and initial results from the VAST Phase I Pilot Survey. This pilot survey consists of $\sim 162$ hours of observations conducted at a central frequency of 888~MHz between 2019 August and 2020 August, with a typical rms sensitivity of 0.24~mJy~beam$^{-1}$ and angular resolution of $12-20$ arcseconds. There are 113 fields, \red{each of which was observed for 12 minutes integration time}, with between 5 and 13 repeats, with cadences between 1 day and 8 months. The total area of the pilot survey footprint is 5\,131 square degrees, covering six distinct regions of the sky. An initial search of two of these regions, totalling 1\,646 square degrees, revealed 28 highly variable and/or transient sources. Seven of these are known pulsars, including the millisecond pulsar J2039--5617. Another seven are stars, four of which have no previously reported radio detection (SCR~J0533--4257, LEHPM~2-783, UCAC3~89--412162 and 2MASS J22414436--6119311). Of the remaining 14 sources, two are active galactic nuclei, six are associated with galaxies and the other six have no multiwavelength counterparts and are yet to be identified.

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arXiv2021

The Global Magneto-Ionic Medium Survey: A Faraday Depth Survey of the Northern Sky Covering 1280-1750 MHz

The Galactic interstellar medium hosts a significant magnetic field, which can be probed through the synchrotron emission produced from its interaction with relativistic electrons. Linearly polarized synchrotron emission is generated throughout the Galaxy, and at longer wavelengths, modified along nearly every path by Faraday rotation in the intervening magneto-ionic medium. Full characterization of the polarized emission requires wideband observations with many frequency channels. We have surveyed polarized radio emission from the Northern sky over the the range 1280-1750 MHz, with channel width 236.8 kHz, using the John A. Galt Telescope (diameter 25.6 m) at the Dominion Radio Astrophysical Observatory, as part of the Global Magneto-Ionic Medium Survey. The survey covered 72% of the sky, declinations -30 to +87 degrees at all right ascensions. The intensity scale was absolutely calibrated, based on the flux density and spectral index of Cygnus A. Polarization angle was calibrated using the extended polarized emission of the Fan Region. Data are presented as brightness temperatures with angular resolution 40'. Sensitivity in Stokes Q and U is 45 mK rms in a 1.18 MHz band. We have applied rotation measure synthesis to the data to obtain a Faraday depth cube of resolution 150 radians per square metre and sensitivity 3 mK rms of polarized intensity. Features in Faraday depth up to a width of 110 radians per square metre are represented. The maximum detectable Faraday depth is +/- 20,000 radians per square metre. The survey data are available at the Canadian Astronomy Data Centre.

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arXiv2021

The GLEAM 200 MHz Local Radio Luminosity Function for AGN and Star-forming Galaxies

The GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) is a radio continuum survey at 76-227 MHz of the entire southern sky (Declination $<+30°$) with an angular resolution of $\approx 2$ arcmin. In this paper, we combine GLEAM data with optical spectroscopy from the 6dF Galaxy Survey to construct a sample of 1,590 local (median $z \approx 0.064$) radio sources with $S_{200\,\mathrm{MHz}} > 55$ mJy across an area of $\approx 16,700~\mathrm{deg}^{2}$. From the optical spectra, we identify the dominant physical process responsible for the radio emission from each galaxy: 73 per cent are fuelled by an active galactic nucleus (AGN) and 27 per cent by star formation. We present the local radio luminosity function for AGN and star-forming galaxies at 200 MHz and characterise the typical radio spectra of these two populations between 76 MHz and $\sim 1$ GHz. For the AGN, the median spectral index between 200 MHz and $\sim 1$ GHz, $α_{\mathrm{high}}$, is $-0.600 \pm 0.010$ (where $S \propto ν^α$) and the median spectral index within the GLEAM band, $α_{\mathrm{low}}$, is $-0.704 \pm 0.011$. For the star-forming galaxies, the median value of $α_{\mathrm{high}}$ is $-0.650 \pm 0.010$ and the median value of $α_{\mathrm{low}}$ is $-0.596 \pm 0.015$. Among the AGN population, flat-spectrum sources are more common at lower radio luminosity, suggesting the existence of a significant population of weak radio AGN that remain core-dominated even at low frequencies. However, around 4 per cent of local radio AGN have ultra-steep radio spectra at low frequencies ($α_{\mathrm{low}} < -1.2$). These ultra-steep-spectrum sources span a wide range in radio luminosity, and further work is needed to clarify their nature.

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arXiv2021

A Polarization Pipeline for Fast Radio Bursts Detected by CHIME/FRB

Polarimetric observations of Fast Radio Bursts (FRBs) are a powerful resource for better understanding these mysterious sources by directly probing the emission mechanism of the source and the magneto-ionic properties of its environment. We present a pipeline for analysing the polarized signal of FRBs captured by the triggered baseband recording system operating on the FRB survey of The Canadian Hydrogen Intensity Mapping Experiment (CHIME/FRB). Using a combination of simulated and real FRB events, we summarize the main features of the pipeline and highlight the dominant systematics affecting the polarized signal. We compare parametric (QU-fitting) and non-parametric (rotation measure synthesis) methods for determining the Faraday rotation measure (RM) and find the latter method susceptible to systematic errors from known instrumental effects of CHIME/FRB observations. These errors include a leakage artefact that appears as polarized signal near $\rm{RM\sim 0 \; rad \, m^{-2}}$ and an RM sign ambiguity introduced by path length differences in the system's electronics. We apply the pipeline to a bright burst previously reported by \citet[FRB 20191219F;][]{Leung2021}, detecting an $\mathrm{RM}$ of $\rm{+6.074 \pm 0.006 \pm 0.050 \; rad \, m^{-2}}$ with a significant linear polarized fraction ($\gtrsim0.87$) and strong evidence for a non-negligible circularly polarized component. Finally, we introduce an RM search method that employs a phase-coherent de-rotation algorithm to correct for intra-channel depolarization in data that retain electric field phase information, and successfully apply it to an unpublished FRB, FRB 20200917A, measuring an $\mathrm{RM}$ of $\rm{-1294.47 \pm 0.10 \pm 0.05 \; rad \, m^{-2}}$ (the second largest unambiguous RM detection from any FRB source observed to date).

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arXiv2021

No Evidence for Galactic Latitude Dependence of the Fast Radio Burst Sky Distribution

We investigate whether the sky rate of Fast Radio Bursts depends on Galactic latitude using the first catalog of Fast Radio Bursts (FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) Project. We first select CHIME/FRB events above a specified sensitivity threshold in consideration of the radiometer equation, and then compare these detections with the expected cumulative time-weighted exposure using Anderson-Darling and Kolmogrov-Smirnov tests. These tests are consistent with the null hypothesis that FRBs are distributed without Galactic latitude dependence ($p$-values distributed from 0.05 to 0.99, depending on completeness threshold). Additionally, we compare rates in intermediate latitudes ($|b| < 15^\circ$) with high latitudes using a Bayesian framework, treating the question as a biased coin-flipping experiment -- again for a range of completeness thresholds. In these tests the isotropic model is significantly favored (Bayes factors ranging from 3.3 to 14.2). Our results are consistent with FRBs originating from an isotropic population of extragalactic sources.

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arXiv2021

The Global Magneto-Ionic Medium Survey (GMIMS): The brightest polarized region in the Southern sky at 75cm and its implications for Radio Loop II

Using the Global Magneto-Ionic Medium Survey (GMIMS) Low-Band South (LBS) southern sky polarization survey, covering 300 to 480 MHz at 81 arcmin resolution, we reveal the brightest region in the Southern polarized sky at these frequencies. The region, G150-50, covers nearly 20deg$^2$, near (l,b)~(150 deg,-50 deg). Using GMIMS-LBS and complementary data at higher frequencies (~0.6--30 GHz), we apply Faraday tomography and Stokes QU-fitting techniques. We find that the magnetic field associated with G150-50 is both coherent and primarily in the plane of the sky, and indications that the region is associated with Radio Loop II. The Faraday depth spectra across G150-50 are broad and contain a large-scale spatial gradient. We model the magnetic field in the region as an expanding shell, and we can reproduce both the observed Faraday rotation and the synchrotron emission in the GMIMS-LBS band. Using QU-fitting, we find that the Faraday spectra are produced by several Faraday dispersive sources along the line-of-sight. Alternatively, polarization horizon effects that we cannot model are adding complexity to the high-frequency polarized spectra. The magnetic field structure of Loop II dominates a large fraction of the sky, and studies of the large-scale polarized sky will need to account for this object. Studies of G150-50 with high angular resolution could mitigate polarization horizon effects, and clarify the nature of G150-50.

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arXiv2021

Fast Radio Burst Morphology in the First CHIME/FRB Catalog

We present a synthesis of fast radio burst (FRB) morphology (the change in flux as a function of time and frequency) as detected in the 400-800 MHz octave by the FRB project on the Canadian Hydrogen Intensity Mapping Experiment (CHIME/FRB), using events from the first CHIME/FRB catalog. The catalog consists of 61 bursts from 18 repeating sources, plus 474 one-off FRBs, detected between 2018 July 25 and 2019 July 2. We identify four observed archetypes of burst morphology ("simple broadband," "simple narrowband," "temporally complex" and "downward drifting") and describe relevant instrumental biases that are essential for interpreting the observed morphologies. Using the catalog properties of the FRBs, we confirm that bursts from repeating sources, on average, have larger widths and we show, for the first time, that bursts from repeating sources, on average, are narrower in bandwidth. This difference could be due to a beaming or propagation effects, or it could be intrinsic to the populations. We discuss potential implications of these morphological differences for using FRBs as astrophysical tools.

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arXiv2021

A nearby repeating fast radio burst in the direction of M81

We report on the discovery of FRB 20200120E, a repeating fast radio burst (FRB) with low dispersion measure (DM), detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB project. The source DM of 87.82 pc cm$^{-3}$ is the lowest recorded from an FRB to date, yet is significantly higher than the maximum expected from the Milky Way interstellar medium in this direction (~ 50 pc cm$^{-3}$). We have detected three bursts and one candidate burst from the source over the period 2020 January-November. The baseband voltage data for the event on 2020 January 20 enabled a sky localization of the source to within $\simeq$ 14 sq. arcmin (90% confidence). The FRB localization is close to M81, a spiral galaxy at a distance of 3.6 Mpc. The FRB appears on the outskirts of M81 (projected offset $\sim$ 20 kpc) but well inside its extended HI and thick disks. We empirically estimate the probability of chance coincidence with M81 to be $< 10^{-2}$. However, we cannot reject a Milky Way halo origin for the FRB. Within the FRB localization region, we find several interesting cataloged M81 sources and a radio point source detected in the Very Large Array Sky Survey (VLASS). We searched for prompt X-ray counterparts in Swift/BAT and Fermi/GBM data, and for two of the FRB 20200120E bursts, we rule out coincident SGR 1806$-$20-like X-ray bursts. Due to the proximity of FRB 20200120E, future follow-up for prompt multi-wavelength counterparts and sub-arcsecond localization could be constraining of proposed FRB models.

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arXiv2021

Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

We present the first Faraday rotation measure (RM) grid study of an individual low-mass cluster -- the Fornax cluster -- which is presently undergoing a series of mergers. Exploiting commissioning data for the POlarisation Sky Survey of the Universe's Magnetism (POSSUM) covering a $\sim34$ square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of $\sim25$ RMs per square degree from a 280 MHz band centred at 887 MHz, which is similar to expectations for forthcoming GHz-frequency all-sky surveys. We thereby probe the extended magnetoionic structure of the cluster in unprecedented detail. We find that the scatter in the Faraday RM of confirmed background sources is increased by $16.8\pm2.4$ rad m$^{-2}$ within 1 degree (360 kpc) projected distance to the cluster centre, which is 2--4 times more extended than the presently-detectable X-ray-emitting intracluster medium (ICM). The Faraday-active plasma is more massive than the X-ray-emitting ICM, with an average density that broadly matches expectations for the Warm-Hot Intergalactic Medium. The morphology of the Faraday depth enhancement exhibits the classic morphology of an astrophysical bow shock on the southwest side of the main Fornax cluster, and an extended, swept-back wake on the northeastern side. Our favoured explanation is an ongoing merger between the main cluster and a sub-cluster to the southwest. The shock's Mach angle and stand-off distance lead to a self-consistent transonic merger speed with Mach 1.06. The region hosting the Faraday depth enhancement shows a decrement in both total and polarised intensity. We fail to identify a satisfactory explanation for this; further observations are warranted. Generally, our study illustrates the scientific returns that can be expected from all-sky grids of discrete sources generated by forthcoming all-sky radio surveys.

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arXiv2021

Heightened Faraday Complexity in the inner 1 kpc of the Galactic Centre

We have measured the Faraday rotation of 62 extra-galactic background sources in 58 fields using the CSIRO Australia Telescope Compact Array (ATCA) with a frequency range of 1.1 - 3.1 GHz with 2048 channels. Our sources cover a region $\sim 12\, \mathrm{deg}\, \times 12\,\mathrm{deg}$ ($\sim 1 $kpc) around the Galactic Centre region. We show that the Galactic Plane for $|l| < 10^\circ$ exhibits large Rotation Measures (RMs) with a maximum |RM| of $1691.2 \pm 4.9\, \mathrm{rad}\,\mathrm{m}^{-2}$ and a mean $|\mathrm{RM}| = 219 \pm 42\,\mathrm{rad}\,\mathrm{m}^{-2}$. The RMs decrease in magnitude with increasing projected distance from the Galactic Plane, broadly consistent with previous findings. We find an unusually high fraction (95\%) of the sources show Faraday complexity consistent with multiple Faraday components. We attribute the presences of multiple Faraday rotating screens with widely separated Faraday depths to small-scale turbulent RM structure in the Galactic Centre region. The second order structure function of the RM in the Galactic Centre displays a line with a gradient of zero for angular separations spanning $0.83^\circ - 11^\circ$ ($\sim 120 - 1500$ pc), which is expected for scales larger than the outer scale (or driving scale) of magneto-ionic turbulence. We place an upper limit on any break in the SF gradient of 66'', corresponding to an inferred upper limit to the outer scale of turbulence in the inner 1 kpc of the Galactic Centre of $3$ pc. We propose stellar feedback as the probable driver of this small-scale turbulence.

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