A fast radio burst source at a complex magnetised site in a barred galaxy

Fast radio bursts (FRBs) are highly dispersed radio bursts prevailing in the universe. The recent detection of FRB~200428 from a Galactic magnetar suggested that at least some FRBs originate from magnetars, but it is unclear whether the majority of cosmological FRBs, especially the actively repeating ones, are produced from the magnetar channel. Here we report the detection of 1863 polarised bursts from the repeating source FRB~20201124A during a dedicated radio observational campaign of Five-hundred-meter Aperture Spherical radio Telescope (FAST). The large sample of radio bursts detected in 88 hr over 54 days indicate a significant, irregular, short-time variation of the Faraday rotation measure (RM) of the source during the first 36 days, followed by a constant RM during the later 18 days. Significant circular polarisation up to 75\% was observed in a good fraction of bursts. Evidence suggests that some low-level circular polarisation originates from the conversion from linear polarisation during the propagation of the radio waves, but an intrinsic radiation mechanism is required to produce the higher degree of circular polarisation. All of these features provide evidence for a more complicated, dynamically evolving, magnetised immediate environment around this FRB source. Its host galaxy was previously known. Our optical observations reveal that it is a Milky-Way-sized, metal-rich, barred-spiral galaxy at redshift z=0.09795+-0.00003, with the FRB residing in a low stellar density, interarm region at an intermediate galactocentric distance, an environment not directly expected for a young magnetar formed during an extreme explosion of a massive star.

Wideband Crescent-Shaped Slotted Printed Antenna with Radiant Circular Polarisation

Producing a suitable impedance matching between the radiating element and the feedline is the prior hurdle to overcome for a wideband antenna with circular polarisation designs. This study presents a novel antenna consisting of a defected ground structure (DGS) and a crescent-slot radiating patch for broad impedance bandwidth. In addition, a narrow rectangular slot was etched on the ground plane for antenna compactness and outcomes improvement. In order to examine the reliability, two different numerical softwares were compared based on the antenna’s basic structure. Apart from this, an equivalent circuit of the proposed prototype is modelled logically using ADS 2016. The numerical results demonstrate that the impedance bandwidth was about 74.6% for < −10 dB, while the 3 dB axial ratio bandwidth greater than 53% was achieved. In the operational bandwidth of the design, good impedance matching and high efficiency were seen, which shows that this design is appropriate for modern wireless communication systems in ISM and GSM bands.

Circularly Polarized Monopole Multiband Microstrip Patch Antenna for Multi-Band Operations

In this communication, an O-shape multi-band coordinated wideband monopole reception apparatus which supports present-day wireless communication is proposed. This proposed O- shaped antenna supports five different bandwidths in the -10db impedance bandwidth range. This antenna was constructed on a FR-4(Fire Retardant fibre glass) epoxy substrate having a dielectric constant (εr) of 4.4 within the dimensions 60x50x1.59.This antenna which at the same time have a circular polarisation radiation pattern, which allows the proposed antenna to function at different frequencies. To increase the number of bands and improve the bandwidth, a reverse replica of the O-shaped concentric ring is introduced in the main design. The proposed antenna is implemented and analysed on Electromagnetic simulation software (HFSS) This replicated antenna provides support to widely used bands such as Bluetooth, WLAN and Ultra-Wideband. An L-shaped split is introduced on the ground to obtain circular polarisation and for matching axial ratio with S11 bands to achieve circular polarisation. Here the H-plane (azimuthal plane) gives an omnidirectional radiation pattern and E-plane (elevation plane) figures patterns. This design has several advantages like low weight and volume, support linear and circular polarisation and capable of multi-band operations.

Exploring the circular polarisation of low-frequency solar radio bursts with LOFAR and estimating the coronal magnetic field

&lt;p&gt;The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. In particular, low frequency (&lt; 150 MHz) &amp;#160;radio bursts have recently been brought back to light with the advancement of novel radio interferometric arrays. However, the polarisation properties of solar radio bursts have not yet been explored in detail, especially with the Low Frequency Array (LOFAR). Here, we explore the circular polarisation of type III radio bursts and a type I noise storm and present the first Stokes V low frequency radio images of the Sun with LOFAR in tied array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental plasma emission, while this trend is either not clear or absent for harmonic plasma emission. In the case of type III bursts, we also find that the sense of circular polarisation varies with each burst, most likely due to their different propagation directions, despite all of these bursts being part of a long-lasting type III storm. Furthermore, we use the degree of circular polarisation of the harmonic emission of type III bursts to estimate the coronal magnetic field at distances of 1.4 to 4 solar radii from the centre of the Sun. We found that the magnetic field has a power law variation with a power index in the range 2.4-3.6, depending on the individual type III burst observed.&lt;/p&gt;

Polarisation properties of methanol masers

Context. Astronomical masers have been effective tools in the study of magnetic fields for years. Observations of the linear and circular polarisation of different maser species allow for the determination of magnetic field properties, such as morphology and strength. In particular, methanol can be used to probe different parts of protostars, such as accretion discs and outflows, since it produces one of the strongest and the most commonly observed masers in massive star-forming regions. Aims. We investigate the polarisation properties of selected methanol maser transitions in light of newly calculated methanol Landé g-factors and in consideration of hyperfine components. We compare our results with previous observations and evaluate the effect of preferred hyperfine pumping and non-Zeeman effects. Methods. We ran simulations using the radiative transfer code, CHAMP, for different magnetic field values, hyperfine components, and pumping efficiencies. Results. We find a dependence between the linear polarisation fraction and the magnetic field strength as well as the hyperfine transitions. The circular polarisation fraction also shows a dependence on the hyperfine transitions. Preferred hyperfine pumping can explain some high levels of linear and circular polarisation and some of the peculiar features seen in the S-shape of observed V-profiles. By comparing a number of methanol maser observations taken from the literature with our simulations, we find that the observed methanol masers are not significantly affected by non-Zeeman effects related to the competition between stimulated emission rates and Zeeman rates, such as the rotation of the symmetry axis. We also consider the relevance of other non-Zeeman effects that are likely to be at work for modest saturation levels, such as the effect of magnetic field changes along the maser path and anisotropic resonant scattering. Conclusions. Our models show that for methanol maser emission, both the linear and circular polarisation percentages depend on which hyperfine transition is masing and the degree to which it is being pumped. Since non-Zeeman effects become more relevant at high values of brightness temperatures, it is important to obtain good estimates of these quantities and the maser beaming angles. Better constraints on the brightness temperature will help improve our understanding of the extent to which non-Zeeman effects contribute to the observed polarisation percentages. In order to detect separate hyperfine components, an intrinsic thermal line width that is significantly smaller than the hyperfine separation is required.

Discovery of six new strongly magnetic white dwarfs in the 20 pc local population

The sample of white dwarfs included in the local 20 pc volume documents, fairly accurately, the total production of white dwarfs over roughly 10 Gyr of stellar evolution in this part of the Milky Way Galaxy. In this sample, we have been systematically searching for magnetic white dwarfs. Here we report the discovery of six new magnetic white dwarfs, with a field strength from a few MG to about 200 MG. Two of these stars show H lines that are split and polarised by the magnetic field. One star shows extremely weak spectral lines in intensity, to which highly polarised narrow features correspond. The three other stars have featureless flux spectra, but show continuum polarisation. These new discoveries support the view that at least 20% of all white dwarfs in the local 20 pc volume have magnetic fields, and they fully confirm the suspicion that magnetism is a common rather than a rare characteristic of white dwarfs. We discuss the level and the handedness of the continuum polarisation in the presence of a magnetic field in cool white dwarfs. We suggest that a magnetic field with a 15 MG longitudinal component produces 1% of continuum circular polarisation. We have also shown that the problem of cross-talk from linear to circular polarisation of the FORS2 instrument, used in our survey, represents an obstacle to accurate measurements of the circular polarisation of faint white dwarfs when the background is illuminated, and polarised, by the moon.