National Radio Astronomy Observatory
  • Charlottesville, United States
Recent publications
We report an ASKAP search for associated H i 21-cm absorption against bright radio sources from the Molonglo Reference Catalogue (MRC) 1-Jy sample. The search uses pilot survey data from the ASKAP First Large Absorption Survey in H i (FLASH) covering the redshift range 0.42 < z < 1.00. From a sample of 62 MRC 1-Jy radio galaxies and quasars, we report three new detections of associated H i 21-cm absorption, yielding an overall detection fraction of |$1.8~{{\%}}^{+4.0~{{\%}}}_{-1.5~{{\%}}}$|⁠. The detected systems comprise two radio galaxies (MRC 2216−281 at z = 0.657 and MRC 0531−237 at z = 0.851) and one quasar (MRC 2156−245 at z = 0.862). The MRC 0531−237 absorption system is the strongest found to date, with a velocity integrated optical depth of |$\rm 143.8 \pm 0.4 \ km \ s^{-1}$|⁠. All three objects with detected H i 21-cm absorption are peaked-spectrum or compact steep-spectrum (CSS) radio sources. Two of them show strong interplanetary scintillation at 162 MHz, implying that the radio continuum source is smaller than 1 arcsec in size even at low frequencies. Among the class of peaked-spectrum and compact steep-spectrum radio sources, the H i detection fraction is |$23~{{\%}}^{+22~{{\%}}}_{-13~{{\%}}}$|⁠. All three detections have a high 1.4 GHz radio luminosity, with MRC 0531−237 and MRC 2216−281 having the highest values in the sample, |$\rm > 27.5 \ W \ Hz^{-1}$|⁠. The preponderance of extended radio sources in our sample could partially explain the overall low detection fraction, while the effects of a redshift evolution in gas properties and AGN UV luminosity on the neutral gas absorption still need to be investigated.
In recent years, certain luminous extragalactic optical transients have been observed to last only a few days¹. Their short observed duration implies a different powering mechanism from the most common luminous extragalactic transients (supernovae), whose timescale is weeks². Some short-duration transients, most notably AT2018cow (ref. ³), show blue optical colours and bright radio and X-ray emission⁴. Several AT2018cow-like transients have shown hints of a long-lived embedded energy source⁵, such as X-ray variability6,7, prolonged ultraviolet emission⁸, a tentative X-ray quasiperiodic oscillation9,10 and large energies coupled to fast (but subrelativistic) radio-emitting ejecta11,12. Here we report observations of minutes-duration optical flares in the aftermath of an AT2018cow-like transient, AT2022tsd (the ‘Tasmanian Devil’). The flares occur over a period of months, are highly energetic and are probably nonthermal, implying that they arise from a near-relativistic outflow or jet. Our observations confirm that, in some AT2018cow-like transients, the embedded energy source is a compact object, either a magnetar or an accreting black hole.
Auroral radio emissions in planetary magnetospheres typically feature highly polarized, intense radio bursts, usually attributed to electron cyclotron maser emission from energetic electrons in the planetary polar region that features a converging magnetic field. Similar bursts have been observed in magnetically active low-mass stars and brown dwarfs, often prompting analogous interpretations. Here we report observations of long-lasting solar radio bursts with high brightness temperature, wide bandwidth and high circular polarization fraction akin to these auroral and exo-auroral radio emissions, albeit two to three orders of magnitude weaker than those on certain low-mass stars. Spatially, spectrally and temporally resolved analysis suggests that the source is located above a sunspot where a strong, converging magnetic field is present. The source morphology and frequency dispersion are consistent with electron cyclotron maser emission due to precipitating energetic electrons produced by recurring nearby flares. Our findings offer new insights into the origin of such intense solar radio bursts and may provide an alternative explanation for aurora-like radio emissions on other flare stars with large starspots.
We present the first sub-arcsecond localised Fast Radio Burst (FRB) detected using MeerKAT. FRB 20210405I was detected in the incoherent beam using the MeerTRAP pipeline on 2021 April 05 with a signal to noise ratio of 140.8 and a dispersion measure of 565.17 pc cm−3. It was detected while MeerTRAP was observing commensally with the ThunderKAT large survey project, and was sufficiently bright that we could use the ThunderKAT 8s images to localise the FRB. Two different models of the dispersion measure in the Milky Way and halo suggest that the source is either right at the edge of the Galaxy, or outside. This highlights the uncertainty in the Milky Way dispersion measure models, particularly in the Galactic Plane, and the uncertainty of Milky Way halo models. Further investigation and modelling of these uncertainties will be facilitated by future detections and localisations of nearby FRBs. We use the combined localisation, dispersion measure, scattering, specific luminosity and chance coincidence probability information to find that the origin is most likely extra-galactic and identify the likely host galaxy of the FRB: 2MASS J1701249−4932475. Using SALT spectroscopy and archival observations of the field, we find that the host is a disk/spiral galaxy at a redshift of z = 0.066.
This paper presents the results of 475 hours of interferometric observations with the Australia Telescope Compact Array towards the Spiderweb protocluster at (z=2.16). We search for large, extended molecular gas reservoirs among 46 previously detected CO(1-0) emitters, employing a customised method we developed. Based on the CO emission images and position-velocity diagrams, as well as the ranking of sources using a binary weighting of six different criteria, we have identified 14 robust and 7 tentative candidates that exhibit large extended molecular gas reservoirs. These extended reservoirs are defined as having sizes greater than 40 kpc or super-galactic scale. This result suggests a high frequency of extended gas reservoirs, comprising at least (30 %) of our CO-selected sample. An environmental study of the candidates is carried out based on N-th nearest neighbour and we find that the large molecular gas reservoirs tend to exist in denser regions. The spatial distribution of our candidates is mainly centred on the core region of the Spiderweb protocluster. The performance and adaptability of our method are discussed. We found 13 (potentially) extended gas reservoirs located in nine galaxy (proto)clusters from the literature. We noticed that large extended molecular gas reservoirs surrounding (normal) star-forming galaxies in protoclusters are rare. This may be attributable to the lack of observations low-J CO transitions and the lack of quantitative analyses of molecular gas morphologies. The large gas reservoirs in the Spiderweb protocluster are a potential source of the intracluster medium seen in low redshift Virgo- or Coma-like galaxy clusters.
Solar observations with the Atacama Large Millimeter-Submillimeter Array (ALMA) became available to the community in late-2016. For the first time, high angular resolution (sub-arcsec) and high-time-resolution (1 s) observations of the Sun became possible at millimeter wavelengths, providing observations of the solar chromosphere that are uniquely complementary to those in O/IR and UV wavelengths. Here, an overview of current ALMA capabilities is provided, selected recent results of ALMA observations of the Sun are highlighted, and future capabilities are outlined.
This open case presents the dilemma of assessing students’ engineering design practices and science learning. Mr. Jensen, a beginning fifth-grade science teacher, hopes that the integrated STEM unit in which students design a natural sunscreen, will keep them engaged, help them learn about engineering design practices, and enhance their understanding of cell cycles and skin cancer. Mr. Jensen decides to use the final project, a “Shark -Tank” style class presentation of students’ engineering design products, as an assessment tool. However, he struggles with effectively assessing students’ engineering practices and science learning . Mr. Jensen desperately needs more effective strategies to help him evaluate students’ understanding of both engineering practices and life science content.
The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole1–4. The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from general relativity⁵. With a baseline of 17 years of observations, there was a shift in the jet’s transverse position, possibly arising from an 8- to 10-year quasi-periodicity³. However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years for the variation in the position angle of the jet. We infer that we are seeing a spinning black hole that induces the Lense–Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation.
We present detailed modelling of periodic flaring events in the 6.7 GHz and 12.2 GHz methanol lines as well as the OH 1665 MHz and 1667 MHz transitions observed in the G9.62+0.20E star-forming region. Our analysis is performed within the framework of the one-dimensional Maxwell–Bloch equations, which intrinsically cover the complementary quasi-steady state maser and transient superradiance regimes. We find that the variations in flaring time-scales measured for the different species/transitions, and sometimes even for a single spectral line, are manifestations of and are best modelled with Dicke’s superradiance, which naturally accounts for a modulation in the duration of flares through corresponding changes in the inversion pump. In particular, it can explain the peculiar behaviour observed for some features, such as the previously published result for the OH 1667 MHz transition at vlsr = +1.7 km s−1 as well as the methanol 6.7 GHz line at vlsr = −1.8 km s−1, through a partial quenching of the population inversion during flaring events.
Classical Be stars are fast rotating, near main sequence B-type stars. The rotation and the presence of circumstellar discs profoundly modify the observables of active Be stars. Our goal is to infer stellar and disc parameters, as well as distance and interstellar extinction, using the currently most favoured physical models for these objects. We present BeAtlas, a grid of 61 600 NLTE radiative transfer models for Be stars, calculated with the hdust code. The grid was coupled with a Monte Carlo Markov chain code to sample the posterior distribution. We test our method on two well-studied Be stars, α Eri and β CMi, using photometric, polarimetric and spectroscopic data as input to the code. We recover literature determinations for most of the parameters of the targets, in particular the mass and age of α Eri, the disc parameters of β CMi, and their distances and inclinations. The main discrepancy is that we estimate lower rotational rates than previous works. We confirm previously detected signs of disc truncation in β CMi and note that its inner disc seems to have a flatter density slope than its outer disc. The correlations between the parameters are complex, further indicating that exploring the entire parameter space simultaneously is a more robust approach, statistically. The combination of BeAtlas and Bayesian-MCMC techniques proves successful, and a powerful new tool for the field: the fundamental parameters of any Be star can now be estimated in a matter of hours or days.
This study uses multifrequency very long baseline interferometry (VLBI) to study the radio emission from 10 radio-quiet quasars (RQQs) and four radio-loud quasars (RLQs). The diverse morphologies, radio spectra, and brightness temperatures observed in the VLBI images of these RQQs, together with the variability in their GHz spectra and VLBI flux densities, shed light on the origins of their nuclear radio emission. The total radio emission of RQQs appears to originate from non-thermal synchrotron radiation due to a combination of active galactic nuclei and star formation activities. However, our data suggest that the VLBI-detected radio emission from these RQQs is primarily associated with compact jets or corona, with extended emissions such as star formation and large-scale jets being resolved by the high resolution of the VLBI images. Wind emission models are not in complete agreement the VLBI observations. Unlike RLQs, where the parsec-scale radio emission is dominated by a relativistically boosted core, the radio cores of RQQs are either not dominant or are mixed with significant jet emission. RQQs with compact cores or core-jet structures typically have more pronounced variability, with flat or inverted spectra, whereas jet-dominated RQQs have steep spectra and unremarkable variability. Future high-resolution observations of more RQQs could help to determine the fraction of different emission sources and their associated physical mechanisms.
The Australian SKA Pathfinder (ASKAP) radio telescope has carried out a survey of the entire Southern Sky at 887.5 MHz. The wide area, high angular resolution, and broad bandwidth provided by the low-band Rapid ASKAP Continuum Survey (RACS-low) allow the production of a next-generation rotation measure (RM) grid across the entire Southern Sky. Here we introduce this project as Spectral and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). In our first data release, we image 30 RACS-low fields in Stokes I , Q , U at 25 $^{\prime\prime}$ angular resolution, across 744–1032 MHz with 1 MHz spectral resolution. Using a bespoke, highly parallelised, software pipeline we are able to rapidly process wide-area spectro-polarimetric ASKAP observations. Notably, we use ‘postage stamp’ cutouts to assess the polarisation properties of 105912 radio components detected in total intensity. We find that our Stokes Q and U images have an rms noise of $\sim$ 80 $\unicode{x03BC}$ Jy PSF $^{-1}$ , and our correction for instrumental polarisation leakage allows us to characterise components with $\gtrsim$ 1% polarisation fraction over most of the field of view. We produce a broadband polarised radio component catalogue that contains 5818 RM measurements over an area of $\sim$ 1300 deg $^{2}$ with an average error in RM of $1.6^{+1.1}_{-1.0}$ rad m $^{-2}$ , and an average linear polarisation fraction $3.4^{+3.0}_{-1.6}$ %. We determine this subset of components using the conditions that the polarised signal-to-noise ratio is $>$ 8, the polarisation fraction is above our estimated polarised leakage, and the Stokes I spectrum has a reliable model. Our catalogue provides an areal density of $4\pm2$ RMs deg $^{-2}$ ; an increase of $\sim$ 4 times over the previous state-of-the-art (Taylor, Stil, Sunstrum 2009, ApJ, 702, 1230). Meaning that, having used just 3% of the RACS-low sky area, we have produced the 3rd largest RM catalogue to date. This catalogue has broad applications for studying astrophysical magnetic fields; notably revealing remarkable structure in the Galactic RM sky. We will explore this Galactic structure in a follow-up paper. We will also apply the techniques described here to produce an all-Southern-sky RM catalogue from RACS observations. Finally, we make our catalogue, spectra, images, and processing pipeline publicly available.
The increment in the demand for high-frequency monolithic integrated circuits has driven the development of waveguide-to-microstrip transition that allow their characterization and integration with waveguide components. Unfortunately, when return losses are taken into account, these transitions feature a rather narrow bandwidth, especially when the substrate is inserted transversal to the propagation direction of the waveguide. Here, we present a new scalable design that overcomes this problem. The transition was originally designed for a bandwidth of 33.5–60 GHz (extended V-band) with the simulation results showing reflections below − 19.5 dB in a fractional bandwidth of 55%. To validate the design, we show the scaling, construction, and measurement in an extended Q-band (27–50 GHz) with the additional advantage of having a standard impedance of 50 Ω. The most novel feature is a staggered air cavity for the microstrip and planar probe, which generates return losses better than 20 dB in a fractional bandwidth of 53%.
We present observations of the Mopra carbon monoxide (CO) survey of the Southern Galactic Plane, covering Galactic longitudes spanning l = 250 ○ (–110 ○ ) to l = 355 ○ (–5 ○ ), with a latitudinal coverage of at least | b | < 1 ○ , totalling an area of >210 deg ² . These data have been taken at 0.6 arcminute spatial resolution and 0.1kms –1 spectral resolution, providing an unprecedented view of the molecular gas clouds of the Southern Galactic Plane in the 109-115 GHz J = 1-0 transitions of ¹² CO, ¹³ CO, C ¹⁸ O and C ¹⁷ O.
Planetary-scale giant storms erupt on Saturn quasiperiodically. There have been at least six recorded occurrences of past eruptions, and the most recent one was in 2010, with its whole life span captured by the Cassini mission. In 2015, we used the Very Large Array to probe the deep response of Saturn's troposphere to the giant storms. In addition to the remnant effect of the storm in 2010, we have found long-lasting signatures of all mid-latitude giant storms, a mixture of equatorial storms up to hundreds of years old, and potentially an unreported older storm at 70°N. We derive an ammonia anomaly map that shows an extended meridional migration of the storm's aftermath and vertical transport of ammonia vapor by storm dynamics. Intriguingly, the last storm in 2010 splits into two distinct components that propagate in opposite meridional directions, leaving a gap at 43°N planetographic latitude.
One of the most fundamental questions in galaxy evolution is how galaxies regulate nuclear growth and accretion onto supermassive black holes. One potential way to do this is through a galactic wind which removes gas from the nucleus. It is unclear whether galactic winds are powered by jets, mechanical winds, radiation, or via magnetohydrodynamic processes. Compact obscured nuclei (CONs) represent a significant phase of galactic nuclear growth. These galaxies hide growing supermassive black holes or unusual starbursts in their very opaque, extremely compact (r < 100 pc) centres. They are found in approximately 30% of the luminous and ultra-luminous infrared galaxy (LIRG and ULIRG) population. Here, we present high resolution ALMA observations (~30mas, ~5pc) of ground-state and vibrationally excited HCN towards ESO 320-G030 (IRAS 11506−3851). ESO 320-G030 is an isolated luminous infrared galaxy known to host a compact obscured nucleus and a kiloparsec-scale molecular wind. Our analysis of these high-resolution observations excludes the possibility of a starburst driven wind, a mechanically or energy driven AGN wind, and exposes a molecular magnetohydrodynamic wind. If magnetohydrodynamic winds are intrinsic to CONs, these results imply that nuclear evolution of galaxies and growth of SMBHs is similar to the growth of hot cores or protostars, and that star formation or active galactic nuclei feedback may not be necessary to drive galactic winds.
We propose compact bandpass filters (BPFs) based on a dual-mode slow wave (DMSW) substrate integrated waveguide (SIW) cavity, which consists of periodically arranged blind via holes. The ultracompact size ( $\sim$ 0.6 $\times$ $\uplambda _{g}^{2})$ and high unloaded quality factor ( $\sim$ 465) bring advantages to BPFs. $^{^{^{^{}}}}$ Two-pole BPFs based on a single DMSW SIW cavity are designed with a transmission zero (TZ) appearing in the lower or upper stopband. To realize more complex BPF topology, we design four-pole BPFs based on two traditional SIW cavities and one DMSW SIW cavity. It is the first time that the DMSW SIW cavity is used to implement compact higher order BPFs with the parallel-coupling topology. Results show that the proposed BPFs have competitive performance with significantly small size, which shows potential on microwave circuit designs.
The recent development of θ − θ techniques in pulsar scintillometry has opened the door for new high resolution imaging techniques of the scattering medium. By solving the phase retrieval problem and recovering the wavefield from a pulsar dynamic spectrum, the Doppler shift, time delay, and phase offset of individual images can be determined. However, the results of phase retrieval from a single dish are only known up to a constant phase rotation, which introduces extra parameters when doing astrometry using Very Long Baseline Interferometry. We present an extension to previous θ − θ methods using the interferometric visibilities between multiple stations to calibrate the wavefields. When applied to existing data for PSR B0834+06 we measure the effective screen distance and lens orientation with five times greater precision than was possible in previous work.
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107 members
Richard Bradley
  • NRAO Technology Center (NTC)
Todd Russell Hunter
  • Atacama Large Millimeter/submillimeter Array (ALMA)
Alwyn Wootten
  • Atacama Large Millimeter/submillimeter Array (ALMA)
Jeff Mangum
  • Atacama Large Millimeter/submillimeter Array (ALMA)
Carol J. Lonsdale
  • Atacama Large Millimeter/submillimeter Array (ALMA)
Charlottesville, United States