Scintillation can explain the spectral structure of the bright radio burst from SGR 1935+2154

The discovery of a fast radio burst (FRB) associated with a magnetar in the Milky Way by the Canadian Hydrogen Intensity Mapping Experiment FRB collaboration (CHIME/FRB) and the Survey for Transient Astronomical Radio Emission 2 (STARE2) has provided an unprecedented opportunity to refine FRB emission models. The burst discovered by CHIME/FRB shows two components with different spectra. We explore interstellar scintillation as the origin for this variation in spectral structure. Modeling a weak scattering screen in the supernova remnant associated with the magnetar, we find that a superluminal apparent transverse velocity of the emission region of $>9.5\,c$ is needed to explain the spectral variation. Alternatively, the two components could have originated from independent emission regions spaced by $>8.3\times10^4\,$km. These scenarios may arise in "far-away" models where the emission originates from well beyond the magnetosphere of the magnetar (for example through a synchrotron-maser mechanism set up by an ultra-relativistic radiative shock), but not in "close-in" models of emission from within the magnetosphere. If further radio observations of the magnetar confirm scintillation as the source for the observed variation in spectral structure, this scattering model thus constrains the location of the emission region.Comment: 12 pages, 1 figure, 1 table. Submitted to ApJ

Disentangling interstellar plasma screens with pulsar VLBI: Combining auto- and cross-correlations

Pulsar scintillation allows a glimpse into small-scale plasma structures in the interstellar medium, if we can infer their properties from the scintillation pattern. With Very Long Baseline Interferometry and working in delay-delay rate space, where the contributions of pairs of images to the interference pattern become localized, the scattering geometry and distribution of scattered images on the sky can be determined if a single, highly-anisotropic scattering screen is responsible for the scintillation. However, many pulsars are subject to much more complex scattering environments where this method cannot be used. We present a novel technique to reconstruct the scattered flux of the pulsar and solve for the scattering geometry in these cases by combining interferometric visibilities with cross-correlations of single-station intensities. This takes advantage of the fact that, considering a single image pair in delay-delay rate space, the visibilities are sensitive to the sum of the image angular displacements, while the cross-correlated intensities are sensitive to the difference, so that their combination can be used to localize both images of the pair. We show that this technique is able to reconstruct the published scattering geometry of PSR B0834+06, then apply it to simulations of more complicated scattering systems, where we find that it can distinguish features from different scattering screens even when the presence of multiple screens is not obvious in the Fourier transform of the dynamic spectrum. This technique will allow us to both better understand the distribution of scattering within the interstellar medium and to apply current scintillometry techniques, such as modelling scintillation and constraining the location of pulsar emission, to sources for which a current lack of understanding of the scattering environment precludes the use of these techniques. (abridged)Comment: Submitted to MNRAS; comments welcom

Deep Synoptic Array Science: Polarimetry of 25 New Fast Radio Bursts Provides Insights into their Origins

We report on a full-polarization analysis of the first 25 as yet non-repeating FRBs detected at 1.4 GHz by the 110-antenna Deep Synoptic Array (DSA-110) during commissioning observations. We present details of the data reduction, calibration, and analysis procedures developed for this novel instrument. The data have 32 $\mu$s time resolution and sensitivity to Faraday rotation measures (RMs) between $\pm10^{6}$rad m$^{-2}$. RMs are detected for 20 FRBs with magnitudes ranging from $4-4670$ rad m$^{-2}$. $9/25$ FRBs are found to have high ($\ge 70\%$) linear-polarization fractions. The remaining FRBs exhibit significant circular polarization ($3/25$), or are either partially depolarized ($8/25$) or unpolarized ($5/25$). We investigate the mechanism of depolarization, disfavoring stochastic RM variations within a scattering screen as a dominant cause. Polarization-state and possible RM variations are observed in the four FRBs with multiple sub-components, but only one other FRB shows a change in polarization state. We combine the DSA-110 sample with polarimetry of previously published FRBs, and compare the polarization properties of FRB sub-populations and FRBs with Galactic pulsars. Although FRBs are typically far more polarized than the average profiles of Galactic pulsars, and exhibit greater spread in polarization fractions than pulsar single pulses, we find a remarkable similarity between FRB polarization fractions and the youngest (characteristic ages $<10^{5}$ yr) pulsars. Our results support a scenario wherein FRB emission is intrinsically highly linearly polarized, and where propagation effects within progenitor magnetospheres can result in conversion to circular polarization and depolarization. Young pulsar emission and magnetospheric-propagation geometries may form a useful analogy for the origin of FRB polarization.Comment: 43 pages, 17 figure

Deep Synoptic Array Science: Implications of Faraday Rotation Measures of Localized Fast Radio Bursts

Faraday rotation measures (RMs) of fast radio bursts (FRBs) offer the prospect of directly measuring extragalactic magnetic fields. We present an analysis of the RMs of ten as yet non-repeating FRBs detected and localized to host galaxies by the 110-antenna Deep Synoptic Array (DSA-110). We combine this sample with published RMs of 15 localized FRBs, nine of which are repeating sources. For each FRB in the combined sample, we estimate the host-galaxy dispersion measure (DM) contributions and extragalactic RM. We find compelling evidence that the extragalactic components of FRB RMs are often dominated by contributions from the host-galaxy interstellar medium (ISM). Specifically, we find that both repeating and as yet non-repeating FRBs show a correlation between the host-DM and host-RM in the rest frame, and we find an anti-correlation between extragalactic RM (in the observer frame) and redshift for non-repeaters, as expected if the magnetized plasma is in the host galaxy. Important exceptions to the ISM origin include a dense, magnetized circum-burst medium in some repeating FRBs, and the intra-cluster medium (ICM) of host or intervening galaxy clusters. We find that the estimated ISM magnetic-field strengths, $B_{||}$, are characteristically larger than those inferred from Galactic radio pulsars. This suggests either increased ISM magnetization in FRB hosts in comparison with the Milky Way, or that FRBs preferentially reside in regions of increased magnetic-field strength within their hosts

Color-to-Grayscale: Does the Method Matter in Image Recognition?

In image recognition it is often assumed the method used to convert color images to grayscale has little impact on recognition performance. We compare thirteen different grayscale algorithms with four types of image descriptors and demonstrate that this assumption is wrong: not all color-to-grayscale algorithms work equally well, even when using descriptors that are robust to changes in illumination. These methods are tested using a modern descriptor-based image recognition framework, on face, object, and texture datasets, with relatively few training instances. We identify a simple method that generally works best for face and object recognition, and two that work well for recognizing textures