74 research outputs found
Broadband radio polarimetry and Faraday rotation of 563 extragalactic radio sources
We present a broadband spectropolarimetric survey of 563 discrete, mostly unresolved radio sources between 1.3 and 2.0 GHz using data taken with the Australia Telescope Compact Array. We have used rotation-measure synthesis to identify Faraday-complex polarized sources, those objects whose frequency-dependent polarization behavior indicates the presence of material possessing complicated magnetoionic structure along the line of sight (LOS). For sources classified as Faraday-complex, we have analyzed a number of their radio and multiwavelength properties to determine whether they differ from Faraday-simple polarized sources (sources for which LOS magnetoionic structures are comparatively simple) in these properties. We use this information to constrain the physical nature of the magnetoionic structures responsible for generating the observed complexity. We detect Faraday complexity in 12% of polarized sources at ~1' resolution, but we demonstrate that underlying signal-to-noise limitations mean the true percentage is likely to be significantly higher in the polarized radio source population. We find that the properties of Faraday-complex objects are diverse, but that complexity is most often associated with depolarization of extended radio sources possessing a relatively steep total intensity spectrum. We find an association between Faraday complexity and LOS structure in the Galactic interstellar medium (ISM) and claim that a significant proportion of the Faraday complexity we observe may be generated at interfaces of the ISM associated with ionization fronts near neutral hydrogen structures. Galaxy cluster environments and internally generated Faraday complexity provide possible alternative explanations in some cases
On the reliability of polarization estimation using Rotation Measure Synthesis
We benchmark the reliability of the Rotation Measure (RM) synthesis algorithm
using the 1005 Centaurus A field sources of Feain et al. (2009). The RM
synthesis solutions are compared with estimates of the polarization parameters
using traditional methods. This analysis provides verification of the
reliability of RM synthesis estimates. We show that estimates of the
polarization parameters can be made at lower S/N if the range of RMs is
bounded, but reliable estimates of individual sources with unusual RMs require
unconstrainted solutions and higher S/N.
We derive from first principles the statistical properties of the
polarization amplitude associated with RM synthesis in the presence of noise.
The amplitude distribution depends explicitly on the amplitude of the
underlying (intrinsic) polarization signal. Hence it is necessary to model the
underlying polarization signal distribution in order to estimate the
reliability and errors in polarization parameter estimates. We introduce a
Bayesian method to derive the distribution of intrinsic amplitudes based on the
distribution of measured amplitudes.
The theoretically-derived distribution is compared with the empirical data to
provide quantitative estimates of the probability that an RM synthesis solution
is correct as a function of S/N. We provide quantitative estimates of the
probability that any given RM synthesis solution is correct as a function of
measured polarized amplitude and the intrinsic polarization amplitude compared
to the noise.Comment: accepted for publication in the Astrophysical Journa
Technical Note: The design and function of a horizontal patient rotation system for the purposes of fixed-beam cancer radiotherapy.
PURPOSE: Cancer radiation therapy treatment is performed by delivering a 3D dose distribution to the tumor via the relative rotation between beam and patient. While most modern machines rotate the radiation beam around a still patient, the treatment can also be delivered by rotating the patient relative to a fixed beam. Fixed-beam, patient rotation radiotherapy machines show promise for reducing the size, surface area footprint, and shielding requirements compared with rotating gantry machines. In this Technical Note, we describe the development of a bespoke horizontal patient rotation system for the purposes of a fixed-beam cancer radiotherapy architecture. METHODS: A horizontal Patient Rotation System was designed in accordance with the appropriate standards pertaining to performance and safety of medical electrical equipment and medical linear accelerators (ISO 9001, IEC 60601-1, IEC 60601-2-1, ISO 14971, ISO 13485, 21CFR820, IEC 62304, Machinery Directive 98/37/EC). The principal criteria for the design were safety, patient comfort, real-time control and the ability to be integrated with other radiation therapy componentry (including a linear accelerator and kV imaging systems). RESULTS: A first of its kind device for securing, immobilizing, translating, and rotating patients has been designed and built and tested against 161 different design, safety, and usability specifications. The device has real-time control for all critical applications. CONCLUSIONS: We designed and built a bespoke device which can translate and rotate patients 360° around a horizontal axis. The device meets all design and safety criteria with early usability tests indicating a high degree of comfort and utility. The system has been installed in a clinical bunker, integrated with a fixed-beam linear accelerator and is currently being commissioned for the purposes of cancer radiotherapy treatment
Broad-band radio circular polarization spectrum of the relativistic jet in PKS B2126-158
We present full Stokes radio polarization observations of the quasar PKS B2126â158 (z = 3.268) from 1 to 10 GHz using the Australia Telescope Compact Array. The source
has large fractional circular polarization (CP), mc ⥠|V|/I, detected at high significance across the entire band (from 15 to 90Ï per 128 MHz subband). This allows us to construct the most robust CP spectrum of an active galactic nucleus (AGN) jet to date. We find mc â Îœ+0.60 ± 0.03 from 1.5 to 6.5 GHz, with a peak of mc ⌠1 per cent before the spectrum turns over somewhere between 6.5 and 8 GHz, above which mc â Îœâ3.0 ± 0.4. The fractional linear polarization (LP;p) varies from 0.2 to âŒ1 per cent across our frequency range and is strongly anticorrelated with the fractional CP, with a best-fitting power law giving mc â pâ0.24 ± 0.03. This is the
first clear relation between the observed LP and CP of an AGN jet, revealing the action of Faraday conversion of LP to CP within the jet. More detailed modelling in conjunction with high spatial resolution observations are required to determine the true driving force behind the conversion (i.e. magnetic twist or internal Faraday rotation). In particular determining whether the observed Faraday rotation is internal or entirely external to the jet is key to this goal. The simplest interpretation of our observations favours some internal Faraday rotation, implying that Faraday rotation-driven conversion of LP to CP is the dominant CP generation mechanism. In this case, a small amount of vector-ordered magnetic field along the jet axis
is required, along with internal Faraday rotation from the low-energy end of the relativistic electron energy spectrum in an electronâproton-dominated jet
Unveiling the mask on the ULIRG-to-QSO transition object [H89]1821+643 at z=0.3: a gas-poor/gas-rich galaxy merger and the implications for CO-based dynamical mass estimates
We report the detection of the CO J=1-0 emission line in [H89]1821+643, one
of the most optically luminous QSOs in the local Universe and a template
ULIRG-to-QSO transition object, located in a rich cool-core cluster at z=0.297.
The CO emission is likely to be extended, highly asymmetric with respect to the
center of the host elliptical where the QSO resides, and corresponds to a
molecular gas mass of ~ 8.0 x 10^{9} M_sun. The dynamical mass enclosed by the
CO line emission region could amount up to ~ 1.7x10^{12} M_sun (80% of the
total mass of the elliptical host). The bulk of the CO emission is located at
~9 kpc south-east from the nuclei position, and close to a faint optical
structure, suggesting that the CO emission could either represent a gas-rich
companion galaxy merging with the elliptical host or a tail-like structure
reminiscent of a previous interaction. We argue that the first scenario is more
likely given the large masses implied for the CO source, which would imply a
highly asymmetrical elliptical host. The close alignment between the CO
emission major axis and the radio-plume suggests a possible role of the latter
excitation of the ambient gas reservoir. The stacking technique was used to
search for CO emission and 3mm continuum emission from galaxies in the
surrounding cluster, however no detection was found either toward individual
galaxies or the stacked ensemble of galaxies, with a 3 sigma limit of <
1.1x10^9 M_sun for the molecular gas.Comment: Accepted for publication in the Astrophysical Journal; replaced title
revise
Complex Faraday depth structure of active galactic nuclei as revealed by broad-band radio polarimetry
We present a detailed study of the Faraday depth structure of four bright (>1 Jy), strongly polarized, unresolved radio-loud quasars. The Australia Telescope Compact Array (ATCA) was used to observe these sources with 2 GHz of instantaneous bandwidth from 1.1 to 3.1 GHz. This allowed us to spectrally resolve the polarization structure of spatially unresolved radio sources, and by fitting various Faraday rotation models to the data, we conclusively demonstrate that two of the sources cannot be described by a simple rotation measure (RM) component modified by depolarization from a foreground Faraday screen. Our results have important implications for using background extragalactic radio sources as probes of the Galactic and intergalactic magneto-ionic media as we show how RM estimations from narrow-bandwidth observations can give erroneous results in the presence of multiple interfering Faraday components. We postulate that the additional RM components arise from polarized structure in the compact inner regions of the radio source itself and not from polarized emission from galactic or intergalactic foreground regions. We further suggest that this may contribute significantly to any RM time variability seen in RM studies on these angular scales. Follow-up, high-sensitivity very long baseline interferometry (VLBI) observations of these sources will directly test our predictions
Cone-beam CT reconstruction with gravity-induced motion.
Fixed-gantry cone-beam computed tomography (CBCT), where the imaging hardware is fixed while the subject is continuously rotated 360° in the horizontal position, has implications for building compact and affordable fixed-gantry linear accelerators (linacs). Fixed-gantry imaging with a rotating subject presents a challenging image reconstruction problem where the gravity-induced motion is coupled to the subject's rotation angle. This study is the first to investigate the feasibility of fixed-gantry CBCT using imaging data of three live rabbits in an ethics-approved study. A novel data-driven motion correction method that combines partial-view reconstruction and motion compensation was developed to overcome this challenge. Fixed-gantry CBCT scans of three live rabbits were acquired on a standard radiotherapy system with the imaging beam fixed and the rabbits continuously rotated using an in-house programmable rotation cradle. The reconstructed images of the thoracic region were validated against conventional CBCT scans acquired at different cradle rotation angles. Results showed that gravity-induced motion caused severe motion blur in all of the cases if unaccounted for. The proposed motion correction method yielded clinically usable image quality withââ<1âmm gravity-induced motion blur for rabbits that were securely immobilized on the rotation cradle. Shapes of the anatomic structures were correctly reconstructed withââ<0.5âmm accuracy. Translational motion accounted for the majority of gravity-induced motion. The motion-corrected reconstruction represented the time-averaged location of the thoracic region over a 360° rotation. The feasibility of fixed-gantry CBCT has been demonstrated. Future work involves the validation of imaging accuracy for human subjects, which will be useful for emerging compact fixed-gantry radiotherapy systems
- âŠ