- ID:
- ivo://CDS.VizieR/J/MNRAS/467/1776
- Title:
- Faraday rotation in Magellanic Bridge
- Short Name:
- J/MNRAS/467/1776
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an investigation into the magnetism of the Magellanic Bridge, carried out through the observation of Faraday rotation towards 167 polarized extragalactic radio sources spanning the continuous frequency range of 1.3-3.1GHz with the Australia Telescope Compact Array. Comparing measured Faraday depth values of sources 'on' and 'off' the Bridge, we find that the two populations are implicitly different. Assuming that this difference in populations is due to a coherent field in the Magellanic Bridge, the observed Faraday depths indicate a median line-of-sight coherent magnetic-field strength of B||~=0.3uG directed uniformly away from us. Motivated by the varying magnitude of Faraday depths of sources on the Bridge, we speculate that the coherent field observed in the Bridge is a consequence of the coherent magnetic fields from the Large and Small Magellanic Clouds being pulled into the tidal feature. This is the first observation of a coherent magnetic field spanning the entirety of the Magellanic Bridge and we argue that this is a direct probe of a 'pan-Magellanic' field.
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- ID:
- ivo://CDS.VizieR/J/other/Galax/7.43
- Title:
- Faraday rotation of extended emission
- Short Name:
- J/other/Galax/7.
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Galactic magnetic field is an integral constituent of the interstellar medium (ISM), and knowledge of its structure is crucial to understanding Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources have been the basis of comprehensive Galactic magnetic field models. Polarised extended emission (XE) is also seen along lines of sight through the Galactic disk, and also displays the effects of Faraday rotation. Our aim is to investigate and understand the relationship between EG and XE RMs near the Galactic plane, and to determine how the XE RMs, a hitherto unused resource, can be used as a probe of the large-scale Galactic magnetic field. We used polarisation data from the Canadian Galactic Plane Survey (CGPS), observed near 1420 MHz with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope. We calculated RMs from a linear fit to the polarisation angles as a function of wavelength squared in four frequency channels, for both the EG sources and the XE. Across the CGPS area, l=55 to l=193 degrees, b=-3 to b=5 degrees, the RMs of the XE closely track the RMs of the EG sources, with XE RMs about half the value of EG-source RMs. The exceptions are places where large local HII complexes heavily depolarise more distant emission. We conclude that there is valuable information in the XE RM dataset. The factor of 2 between the two types of RM values is close to that expected from a Burn slab model of the ISM. This result indicates that, at least in the outer Galaxy, the EG and XE sources are likely probing similar depths, and that the Faraday rotating medium and the synchrotron emitting medium have similar variation with galactocentric distance.
- ID:
- ivo://CDS.VizieR/J/ApJ/887/L7
- Title:
- Faraday rotation of radio sources from THOR
- Short Name:
- J/ApJ/887/L7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present first results for Faraday rotation of compact polarized sources (1-2GHz continuum) in The HI/OH/Recombination line (THOR) survey of the inner Galaxy. In the Galactic longitude range 39{deg}<l<52{deg}, we find rotation measures (RMs) in the range -310rad/m^2^<=RM<=+4219rad/m^2^, with the highest values concentrated within a degree of l=48{deg} at the Sagittarius arm tangent. Most of the high RMs arise in diffuse plasma, along lines of sight that do not intersect HII regions. For l>49{deg}, RM drops off rapidly, while at l<47{deg}, the mean RM is higher with a larger standard deviation than at l>49{deg}. We attribute the RM structure to the compressed diffuse warm ionized medium in the spiral arm, upstream of the major star formation regions. The Sagittarius arm acts as a significant Faraday screen inside the Galaxy. This has implications for models of the Galactic magnetic field and the expected amount of Faraday rotation of fast radio bursts from their host galaxies. We emphasize the importance of sensitivity to high Faraday depth in future polarization surveys.
- ID:
- ivo://CDS.VizieR/J/A+A/597/A98
- Title:
- Faraday tomography of foreground towards IC342
- Short Name:
- J/A+A/597/A98
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Magnetic fields pervade the interstellar medium (ISM), but are difficult to detect and characterize. The new generation of low-frequency radio telescopes, such as the Low Frequency Array (LOFAR: a Square Kilometre Array-low pathfinder), provides advancements in our capability of probing Galactic magnetism through low-frequency polarimetry. Maps of diffuse polarized radio emission and the associated Faraday rotation can be used to infer properties of, and trace structure in, the magnetic fields in the ISM. However, to date very little of the sky has been probed at high angular and Faraday depth resolution. We observed a 5 by 5 degree region centred on the nearby galaxy IC 342 (l=138.2, b=+10.6) using the LOFAR High Band Antennas in the frequency range 115-178MHz. We imaged this region at 4.5x3.8arcmin^2^ resolution and performed Faraday tomography to detect foreground Galactic polarized synchrotron emission separated by Faraday depth (different amounts of Faraday rotation). Our Faraday depth cube shows rich polarized structure, with up to 30K of polarized emission at 150MHz. We clearly detect two polarized features that extend over most of the field but are clearly separated in Faraday depth. Simulations of the behaviour of the depolarization of Faraday-thick structures at such low frequencies show that such structures would be too strongly depolarized to explain the observations. These structures are therefore rejected as the source of the observed polarized features. Only Faraday thin structures will not be strongly depolarized at low frequencies; producing such structures requires localized variations in the ratio of synchrotron emissivity to Faraday depth per unit distance. Such variations can arise from several physical phenomena, such as a transition between regions of ionized and (mostly) neutral gas. We conclude that the observed polarized emission is Faraday thin, and propose that the emission originates from two mostly neutral clouds in the local ISM. Using maps of the local ISM to estimate distances to these clouds, we have modelled the Faraday rotation for this line of sight and estimated that the strength of the line of sight component of magnetic field of the local ISM for this direction varies between -0.86 and +0.12uG (where positive is towards the Earth). We propose that this may be a useful method for mapping magnetic fields within the local ISM in all directions towards nearby neutral clouds.
- ID:
- ivo://CDS.VizieR/J/ApJS/234/27
- Title:
- FeXIX-XXV recombination lines
- Short Name:
- J/ApJS/234/27
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a systematic measurement of the X-ray emission asymmetries in the K-shell dielectronic, trielectronic, and quadruelectronic recombination of free electrons into highly charged ions. Iron ions in He-like through O-like charge states were produced in an electron beam ion trap, and the electron-ion collision energy was scanned over the recombination resonances. Two identical X-ray detectors mounted head-on and side-on with respect to the electron beam propagation recorded X-rays emitted in the decay of resonantly populated states. The degrees of linear polarization of X-rays inferred from observed emission asymmetries benchmark distorted-wave predictions of the Flexible Atomic Code for several dielectronic recombination satellite lines. The present method also demonstrates its applicability for diagnostics of energy and direction of electron beams inside hot anisotropic plasmas. Both experimental and theoretical data can be used for modeling of hot astrophysical and fusion plasmas.
- ID:
- ivo://CDS.VizieR/J/ApJ/859/4
- Title:
- First-look analysis of {rho} Oph-A at 850um
- Short Name:
- J/ApJ/859/4
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present 850{mu}m imaging polarimetry data of the {rho} Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2) as part of our ongoing survey project, B-fields In STar forming RegiOns (BISTRO). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2pc {rho} Oph-A core; some of them can be part of a coherent magnetic field structure in the {rho} Oph region. The results are consistent with previous observations of the brightest regions of {rho} Oph-A, where the degrees of polarization are at a level of a few percent, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher (>5%). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components that are consistent at near-infrared and submillimeter wavelengths. Using the Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in several subcore regions, which range from approximately 0.2 to 5mG. We also find a correlation between the magnetic field orientations projected on the sky and the core centroid velocity components.
- ID:
- ivo://CDS.VizieR/J/other/PASA/38.20
- Title:
- Fornax cluster magnetic fields
- Short Name:
- J/other/PASA/38.
- Date:
- 17 Jan 2022 00:49:54
- Publisher:
- CDS
- Description:
- 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 ~34 square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of ~25 RMs per square degree from a 280MHz band centred at 887MHz, which is similar to expectations for forthcoming GHz-frequency 3-steradian sky surveys. These data allow us to probe the extended magnetoionic structure of the cluster and its surroundings in unprecedented detail. We find that the scatter in the Faraday RM of confirmed background sources is increased by 16.8+/-2.4rad/m^2^ within 1 degree (360kpc) projected distance to the cluster centre, which is 2-4 times larger than the spatial extent of the presently-detectable X-ray-emitting intracluster medium (ICM). The mass of the Faraday-active plasma is larger than that of the X-ray-emitting ICM, and exists in a density regime that broadly matches expectations for moderately-dense components of the Warm-Hot Intergalactic Medium. We argue that forthcoming RM grids from both targeted and survey observations may be a singular probe of cosmic plasma in this regime. The morphology of the global Faraday depth enhancement is not uniform and isotropic, but rather 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 for these phenomena is an ongoing merger between the main cluster and a sub-cluster to the southwest. The shock's Mach angle and stand-o distance lead to a self-consistent transonic merger speed with Mach 1.06. The region hosting the Faraday depth enhancement also appears to show a decrement in both total and polarised radio emission compared to the broader field. We evaluate cosmic variance and free-free absorption by a pervasive cold dense gas surrounding NGC 1399 as possible causes, but find both explanations unsatisfactory, warranting further observations. 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.
- ID:
- ivo://CDS.VizieR/J/A+A/583/A115
- Title:
- FORS1 catalogue of stellar magnetic fields
- Short Name:
- J/A+A/583/A115
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The FORS1 instrument of the ESO Very Large Telescope was used to obtain low resolution circular polarized spectra of nearly a thousand of different stars, with the aim of measuring their mean longitudinal magnetic fields. Magnetic fields were measured by different authors, and using different methods and software tools. A catalogue of FORS1 magnetic measurements would provide a valuable resource with which to better understand the strengths and limitations of this instrument and of similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data reduction has been carried out by a number of different groups using a variety of reduction and analysis techniques. Both our understanding of the instrument and our data reduction techniques have improved over time. A full re-analysis of FORS1 archive data using a consistent and fully documented algorithm would optimise the accuracy and usefulness of a catalogue of field measurements. Based on ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determinations, which includes self-consistent checks for field detection reliability. We have applied our procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive. We have produced a catalogue of spectro-polarimetric observations and magnetic field measurements for 1400 observations of 850 different objects. The spectral type of each object has been accurately classified. We have also been able to test different methods for data reduction is a systematic way. The resulting catalogue has been used to produce an estimator for an upper limit to the uncertainty in a field strength measurement of an early type star as a function of the signal-to-noise ratio of the observation. While FORS1 is not necessarily an optimal instrument for the discovery of weak magnetic fields, it is very useful for the systematic study of larger fields, such as those found in Ap/Bp stars and in white dwarfs.
- ID:
- ivo://CDS.VizieR/J/ApJS/160/220
- Title:
- Full-polarization observations of OH masers
- Short Name:
- J/ApJS/160/220
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present full-polarization VLBA maps of the ground-state, main-line, ^2^{Pi}_3/2_, J=3/2 OH masers in 18 Galactic massive star-forming regions. This is the first large polarization survey of interstellar hydroxyl masers at VLBI resolution. A total of 184 Zeeman pairs are identified, and the corresponding magnetic field strengths are indicated. We also present spectra of the NH_3_ emission or absorption in these star-forming regions.
- ID:
- ivo://CDS.VizieR/J/A+A/609/A68
- Title:
- Full-Stokes polarimetry of 5 radio sources
- Short Name:
- J/A+A/609/A68
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an analysis pipeline that enables the recovery of reliable information for all four Stokes parameters with high accuracy. Its novelty relies on the effective treatment of the instrumental effects even before the computation of the Stokes parameters, contrary to conventionally used methods such as that based on the Mueller matrix. For instance, instrumental linear polarization is corrected across the whole telescope beam and significant Stokes Q and U can be recovered even when the recorded signals are severely corrupted by instrumental effects. The accuracy we reach in terms of polarization degree is of the order of 0.1-0.2%. The polarization angles are determined with an accuracy of almost 1{def}. The presented methodology was applied to recover the linear and circular polarization of around 150 active galactic nuclei, which were monitored between July 2010 and April 2016 with the Effelsberg 100-m telescope at 4.85GHz and 8.35GHz with a median cadence of 1.2 months. The polarized emission of the Moon was used to calibrate the polarization angle measurements. Our analysis showed a small system-induced rotation of about 1{deg} at both observing frequencies. Over the examined period, five sources have significant and stable linear polarization; three sources remain constantly linearly unpolarized; and a total of 11 sources have stable circular polarization degree m_c_, four of them with non-zero m_c_. We also identify eight sources that maintain a stable polarization angle. All this is provided to the community for future polarization observations reference. We finally show that our analysis method is conceptually different from those traditionally used and performs better than the Mueller matrix method. Although it has been developed for a system equipped with circularly polarized feeds, it can easily be generalized to systems with linearly polarized feeds as well.
