Presentations
Monday November 4
Introduction (Ue-Li Pen)
Multiscreen Model for Intra-Day Variability (Carl Gwinn)
Multiscreen scattering provides insight into the conundrum of intra-day variability. A few compact extragalactic radio sources show such variability in extreme form, characterized by $\approx 100$\% intensity variations on lateral scales of $\approx 15$ Earth radii, traveling across Earth at $\approx 20\ \mathrm{km\ s}^{-1}$. One puzzle is that a simple coherence argument requires a single responsible scatterer to be closer than $\approx 15$ pc, whereas the path from the source extends hundreds of times this distance through the Milky Way, encountering many regions similar to our neighborhood. In contrast to a single-screen model, multiscreen models offer much more flexibility. A scattering screen can modify both the spatial coherence, and the spatial distribution of intensity, of incident radiation. Multiple screens can work together to control both. For similar reasons, telescopes, cameras, and semiconductor lithographs commonly include multiple lenses or mirrors. I will discuss multiscreen models from heuristic and formal standpoints, and how they can produce intra-day variability from scattering at greater distances.
Identifying Pulsar Candidates in Radio Images using Scintillation (Jitendra Salal)
Pulsars have been primarily detected by their narrow pulses or periodicity in time domain data. With a synthesis imaging telescope like the GMRT, typically pulsar surveys use wider incoherent beams to map large areas of the sky, but the incoherent beam is less sensitive compared to regular imaging data. Also, the detection sensitivity of time-domain searches for pulsars is affected by dispersion smearing, scattering, and rapid orbital motion of pulsars in binaries. Building on a variance imaging technique proposed by Dai et. al. (2016), we have developed a new technique that detects pulsar candidates in standard radio images by measuring their scintillation bandwidth and timescale from the cross-correlation of dynamic spectra of sources. The radio continuum images do not suffer from the limitations of time domain searches. They, therefore, are equally sensitive to all pulsars and allow us to find hard-to-detect pulsars, such as sub-millisecond pulsars, pulsar-black hole systems, and pulsars near the Galactic Centre. We use existing GMRT observations of PSR B1508+55 and its neighboring sources as a test case for our technique. We demonstrate that the technique correctly differentiates between the pulsar and other non-scintillating point sources and show that the extracted dynamical spectrum of the pulsar is equivalent to that extracted from the GMRT phased array beam. We developed an automated pipeline for processing archival data and testing it on GMRT and uGMRT data. We will show results from our analysis of known pulsar fields and challenges in dealing with interference and instrumental effects.
The Galactic Distribution of Pulsar Scattering and the τ-DM Relation (Qiuyi He)
A Cacophony of Echoes of the Crab Pulsar (Marten van Kerkwijk)
We present the evolution of the Crab giant-pulse profile as inferred from archival data from the 42-ft telescope at Jodrell Bank. We find these contain a cacaphony of echoes, additional pulse components where intervening material away from the line of sight has causes emission to be redirected towards the observer and thus arrive with a delay. All echoes are consistent with approaching zero-delay, indicating that the structures responsible for producing these events must be highly anisotropic, with typical lengths greater than 4 AU. Given the previously observed electron densities of the nebular filaments, we infer typical widths on the sky of 0.1 AU and typical depths of 5 AU. Overall, we thus infer sheet-like inhomogeneities seen at grazing incidence.
Connecting the Giant Pulses from J1823-3021A in Milkyway Globular Clusters and the M81 Fast Radio Burst (Simon Ho)
The progenitors of the Fast Radio Bursts (FRBs) are presently largely unknown, although magnetars are a leading candidate. Magnetars result from core-collapse supernova events in young high-mass stars, so the discovery of a repeating FRB (FRB20200120E) from a globular cluster (GC) associated within the nearby spiral galaxy M81 came as a complete surprise since most GCs are thought to be far too old to still host such young stars. However, GCs host substantial numbers of millisecond pulsars (MSPs), known to emit “Giant Pulses (GPs)”, which are remarkably intense bursts of radio emission that can exceed the typical pulse strength by many orders of magnitude. GPs from such millisecond pulsars and the FRBs from the GC in M81 occur on similar microsecond to millisecond timescales. There are several models suggesting GP-emitting pulsars as the source of the M81 FRB. Studying the properties of GC MSPs that emit GPs may give us clues to a connection to the GC FRBs. Abbate et al. 2020 studied the MSP J1823-3021A which is the most active GP emitter in the GC NGC6624 with MeerKAT L-band (856-1711 MHz). Here we present our observations of the same target, taken in the UHF band (580-1015 MHz) with two times better time resolution (5 μs/bin). Due to this advantageous setup, and the steep spectral index of the pulsar, we anticipate detecting a significantly higher number of GPs from this MSP than in previous studies. With a preliminary investigation on the first 23-min of the observation, we have already detected ~4000 GP candidates with signal-to-noise ratio > 10. The high time resolution of the data will allow us to explore the temporal structure of the GPs with unprecedented resolution and the baseband nature of our data will enable us to both coherently dedisperse and coherently descatter the GPs, leading to more meaningful comparisons of their properties with those from the FRB.
Slides
Dual Frequency Observation of Crab Giant Pulses (Sujin Eie)
The Crab pulsar is one of the best sources for studying giant pulses of radio pulsars. In this study, we investigate giant pulses from the Crab pulsar at frequencies of 2.3 GHz and 8.4 GHz through ~98-hr observations using Usuda 64-m and Kashima 34-m telescopes located in Japan. Our findings include significant numbers of giant pulses at different phases and frequencies, with negative spectral indices observed across the frequency range. Both the main pulses and interpulses at 2.3 and 8.4 GHz have power-law intensity distributions that are consistent with earlier studies. However, the power-law slopes of the intensity distributions are different between the main pulse and the interpulse at 8.4 GHz. Given that the interpulses at 8.4 GHz are mostly high-frequency interpulses, our results suggest the presence of a potential different emission mechanism for interpulses at different phases. Lastly, we present our preliminary results on the comparison of spectral indices and pulse luminosity for giant pulses, magnetar radio pulses, and fast radio bursts.
Can Scintillation Events be Predicted? (Tim Sprenger)
Scintillation at different frequencies probes different scales on scattering screens. Although moving from lower to higher frequencies means to zoom in on smaller regions within the screen, the locations of stationary phase points might also change with frequency. Past as well as recent observations of the pulsar B1508+55 show that scattering at ~1400 MHz gets stronger, when the pulsar is behind regions that were found to produce scattered images several years earlier at ~150 MHz. In observations of the pulsar B0355+54 with the Effelsberg 100-m telescope, we tested if this phenomenon is also visible when going from ~1400 MHz to ~6000 MHz, where the crossing time of scattered images is reduced to about one week. Instead, the result was refraction close to the crossing, which clearly shows that multi-frequency studies can advance our understanding of scintillation.
Tuesday November 5
Scintillation Arcs from Pulsars and Fast Radio Bursts (Weiwei Zhu)
Pulsars and Fast Radio Bursts (FRB) are some of the most energetic and "fast" astronomical phenomena. They are both possibly emitted by "neutron stars". I will introduce some of my group and collaborators' pulsars and FRB research using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in this talk. I will talk about the study of pulsar spin-kick alignment, the high Galactic altitude scattering screens and the first scintillation arcs discovered from FRBs. I will also present some FAST FRB key science project results.
Slides
LPDA Arrays for Localizing Bright Nearby FRBs from CHIME Sidelobes (Nina Gusinskaia)
Over the past 4 years, 10 very bright but extremely rare FRBs were detected in the CHIME far side lobes. These FRBs are statistically ~20 times closer than the typical FRBs detected in CHIME's main lobe. Localizing them to a Galactic source or pinpointing their exact locations in nearby galaxies will be invaluable for unraveling the nature of FRBs. However, achieving this requires VLBI-quality localisation precision, which CHIME lacks. To address this limitation we built Log-Periodic Dipole Antenna (LPDA) arrays in two locations: the Algonquin Radio Observatory (3000km west of CHIME) and near Hilo, Hawaii (5000km east-south of CHIME). Each array has 8 small antennas, with a wide field of view covering half of the sky, matching CHIME's side-lobes. The combined sensitivity of each array is enough to detect the brightest of CHIME's side-lobe FRBs through cross-correlation with CHIME. These arrays serve as testbeds and demonstrators of the technology and analysis techniques for BURSTT, a large LPDA array being constructed in Taiwan, optimized to discover and localize a significant number of rare, high-fluence, nearby FRBs. In my talk, I will present the development and commissioning of these LPDA arrays, as well as our early results.
Two-Screen Scattering in CRAFT FRBs (Mawson Sammons)
Temporal broadening is a commonly observed property of fast radio bursts (FRBs), associated with turbulent media which cause radiowave scattering. Similarly to dispersion, scattering is an important probe of the media along the line of sight to an FRB source, such as the circum-burst or circum-galactic mediums (CGM). Measurements of characteristic scattering times alone are insufficient to constrain the position of the dominant scattering media along the line of sight. However, where more than one scattering screen exists, Galactic scintillation can be leveraged to form strong constraints. We quantify the scattering and scintillation in 10 FRBs with 1) known host galaxies and redshifts and 2) captured voltage data enabling high time resolution analysis, obtained from the Commensal Real-time ASKAP (Australian Square Kilometre Array Pathfinder) Fast Transient survey science project (CRAFT). In this talk I will demonstrate the potential of high resolution CRAFT FRBs as probes of Galactic and extragalactic scattering media, presenting three cases where we find strong evidence for scattering by two screens and four cases displaying a lack on scintillation in tension with current models.
(Kejia Lee)
Ocean-Surface Wave Measurements using Scintillation Theories on Seaborne Software Defined GPS and SBAS Reflectometry Observations (Lung-Chih Tsai)
Fast Radio Bursts: Nature's Latest Cosmic Mystery (Duncan Lorimer)
Fast radio bursts are millisecond-duration pulses of cosmological origin that were discovered by an undergraduate student at West Virginia University in 2007 in data collected by the Parkes radio telescope in 2001. They show amazing promise as probes of the large-scale structure of the Universe and provide a new window into the population(s) of compact objects at vast distances. Although much of the details as to their origins remain to be discovered, in this talk I will attempt to give an account of their discovery and what we have learned in the past sixteen years.
Wednesday November 8
Scintillation Arcs at Age 25 (or so): What's New? What Does the Future Hold? (Dan Stinebring)
More than twenty years ago, the pulsar scintillation “fringing patterns” that Cordes, Wolszczan, Rickett, and others had studied were shown to be part of a larger phenomenon, what we now call scintillation arcs. Due in large part to the organizers of this meeting, scintillation arcs + interferometry have produced the high-precision technique of scintillometry. Whether studied with space-based interferometers, dense Earth-based arrays, or giant single dishes, precision measurements have become a hallmark of scintillometry. What are we to make of a profusion of (e.g. more than 20) sharply delineated arcs along the line of sight to a pulsar only 160 pc away!? What do new one-dimensional (theta-theta) analyses of scintillation arcs tell us about the physical properties of the scattering material? Does the wealth of multi-frequency scintillation arc observations help us to distinguish between blobs, filaments, or planes of scattering material? I will try and frame some of the basics of this subject and ask some of the questions that seem most promising for future progress. It will be up to the rest of you to answer the questions!
Unravelling Interstellar Scintillation of the Double Pulsar with MeerKAT (Jacob Askew)
One of the most important astrophysical objects is the double pulsar. The short 2.45-hour orbital period and the presence of two neutron stars observed as radio pulsars have enabled some of the most stringent tests of relativistic gravity and have the potential to probe nuclear equations of state. However, to make more precise tests, it is necessary to improve on the distance measurement. The distance to the system has been measured using very long baseline interferometry and through pulsar timing. There is growing evidence of systematic discrepancies between the two measurements. It is possible to measure the distance independently using interstellar scintillation. Here we present a scintillation analysis of the double pulsar from a year-long campaign with the MeerKAT radio telescope. We offer a model for the interstellar medium along the line of sight to the pulsar. We use this model to infer an independent distance to the pulsar. We explore the anisotropy and phase gradients across the orbital phase and expand upon the potential of the scintillation velocity method. We conclude by discussing how revised distance estimates will impact future tests of relativity with the system.
Inferring Binary Parameters Using Scintillometry: The Double Pulsar (Diego Montalvo)
I will present our work regarding the scintillation properties of the double pulsar J0737-3039. The double pulsar’s orbital motion distorts scintillation patterns on the dynamic spectrum, which in turn yields any parabolic arc analysis on the secondary spectra impractical. We take advantage of the numerous times the double pulsar sweeps through the same regions of the anisotropic scintillating screen in order to model its motion relative to the pulsar. We use the corrective effect this modeling has on the secondary spectra in the hopes of improving the measurements of the orientation of the double pulsar relative to the screen, as well as ISM properties.
Double Lensed Emission of PSR B0834+06 (Lydia Guertin)
Using archival Arecibo observations of PSR B0834+06, we identify and track the motion of a doubly-lensed group of images in the two-dimensional Fourier transform of the dynamic spectrum. Following basic scintillometry techniques, we use the theta-theta transform to phase-connect frequency and time chunks of each observation, allowing us to extract parameters of the physical screen causing the double-lensing of the pulsar, including its distance and orientation relative to the line of sight. By tracing the group of doubly-lensed images, we are able to determine the geometry of the lensing event and compare this event to the literature of proposed mechanisms. Interestingly, we find a distinct asymmetry in the secondary spectra that does not appear in previous literature. The timing of this asymmetry aligns with a point of crossing in the double-lensed scenario, but the precise origin of this asymmetry remains for further investigation.
Scintillation Arcs Unveil Inner Structures of Pulsar Bow Shock and the Local Bubble (Daniel Reardon)
The MeerKAT radio telescope has provided an incredible tomographic view of the plasma along the line of sight to the nearest and brightest millisecond pulsar, J0437-4715. We observe at least 25 scintillation arcs, of which 21 originate from the interstellar plasma and four low-curvature arcs from structures within the pulsar bow shock. The pulsar lies within the Local Bubble, as traced by neutral gas and dust, which makes the large number density of scattering screens remarkable. In this talk, I present an update on our detailed study of these scintillation arcs and our models for the scattering screen distances and velocities. The bow shock arcs all originate from within 5000 au of the pulsar. The measured radial distance to the shocks solves the shock geometry and the space velocity of the pulsar in three dimensions, while their velocities identify one as originating from an anisotropic inflow from the direction of the shock or pulsar-wind tail. The interstellar scintillation arcs show that while the Local Bubble is thought to be filled with hot gas as remnants of supernovae over the past 14 Myrs, it also sustains a large number of turbulent structures down to the AU scales probed by scintillation.
A High-Sensitivity Study of Pulsar Scintillation Arcs: The Roles of of Bow Shocks, Bubbles, and the Broader Interstellar Medium (Stella Ocker)
I will present a survey of eight pulsars conducted at the Five-hundred-meter Aperture Spherical Telescope (FAST), revealing a diverse range of scintillation arc characteristics at high sensitivity. These observations reveal more arcs than measured previously for our sample. At least nine arcs are observed toward B1929+10 at screen distances spanning 90% of the pulsar's 361 pc path-length to the observer. Four arcs are observed toward B0355+54, with one arc yielding a screen distance <1 pc from the pulsar, or <2 pc from the observer. Several pulsars show highly truncated, low-curvature arcs that may be attributable to scattering near the pulsar. We have synthesized the scattering screen constraints with maps of the local ISM and other well-characterized pulsar scintillation arcs, yielding a three-dimensional view of the scattering media in context.
Scintillation and Hydrogen Filaments (Ashley Stock)
Little is known about the interstellar medium features that cause scintillation. I compared the screens from twelve pulsars with maps of the interstellar medium in search of potential associations. In this search I found that the phase gradient of the screens was often well aligned with filamentary structures commonly seen in neutral hydrogen maps. Additionally, nearby screens were found to have similar distance to the Local Bubble. In this presentation I will give an update on these results and future steps with higher resolution neutral hydrogen maps and more screens.
Bustling Universe Radio Survey Telecope in Taiwan (Kai-yang Lin)
Magnetism & Morphology (Susan Clark)
Thursday November 9
Observations of Fast Radio Bursts with CHIME/FRB (Kaitlyn Shin)
The Highs and Lows of Earth-Space VLBI for Scintillometry (Daniel Baker)
The appearance of clear scintillation arcs and inverted arclets in pulsar secondary spectra suggests that the scattering is dominated by a single thin region producing images along a straight line. For PSR B0834+06 this line has been measured directly using VLBI on ground based baselines and shown to be very nearly one dimensional. However, there are indications of small scale deviations from the line just below the level detectable on these baselines. I will present recent attempts to use Ground-Space baselines to resolve these tiny wiggles.
Using Cyclic Spectroscopy in High-Accuracy Pulsar Timing Efforts (Jacob Turner)
Cyclic spectroscopy is a powerful technique that takes advantage of the periodic nature of pulsar signals to yield massive improvements in both frequency and pulse-phase resolution. In this talk we demonstrate via simulation the effectiveness of this approach for measuring scattering delays by way of impulse response function recovery compared with traditional dynamic spectrum-based autocorrelation function methods. We also show the potential for this technique in studies of the interstellar medium through analysis of L-band baseband data of PSR B1937+21, resulting in the resolution of fine details in the dynamic and secondary spectra unobtainable through the original filterbank data.
Probing the Orbit of PSR J0621+1002 (Geetam Mall)
Atomic Microstructures in the Diffuse Interstellar Medium (Snežana Stanimirović)
Depolarisation as a Probe of the Intracluster Medium (Alvina On)
It is important to understand the effects of scintillation and polarisation in order to correctly interpret the magneto-ionic properties of the intracluster medium (ICM). Depolarisation effects due to Faraday rotation are often significant at low radio frequencies. In particular, the observed degrees of linear polarisation of radio point sources in the field of a galaxy cluster may not always truly indicate the intrinsic degrees of linear polarisation. It is also possible for some point sources to appear as unpolarised in an observation, even if they are intrinsically polarised. In this talk, I will present some of our recent results, where we investigate the effects on linear polarisation of radio point sources in various models of the ICM. We find that, in general, bright sources do not experience any significant changes in polarisation, whereas faint sources either get depolarised or repolarised as their radiation propagates through the ICM. I will highlight the physical conditions under which polarisation may change significantly along the line-of-sight and discuss how these may impact the interpretations of radio observations.
Measurement of Magnetic Helicity of Diffuse Medium (Jennifer Chan)
How to tell FRBs from Quakes? (Di Li)
The search for precise periodicity in FRBs has largely failed. This present a challenge (regardless of how easy to explain away the non-detection) for compact stars' being the nature central engine. We studied the dynamic trajectory of repeating pulses on the Energy-Time (ET) phase plane. Through mathematical physics tools such as a modified Pincus index and the Lyapunov exponent, we found the repeating FRBs to be less chaotic and more stochastic than Earthquakes and solar flares. The key distinction lies in the fact that the arrival time of FRBs seem to be independent of their energy, either absolute or relative. Such brownian-motion type of behavior deepens the mystery of how to generate FRBs from a quake-like energy release, such as those in the magnetar's crust.
Slides
Scintillated Microlensing: Measuring Cosmic Distances with Fast Radio Bursts (Anna Tsai)
Does the Magnetar XTE J1810-197 Scintillate at Low Frequencies? (Visweshwar Ram Marthi)
The magnetar XTE J1810-197 was discovered in X-ray, with the distinction that it is the first transient Anoma- lous X-ray Pulsar (AXP). Subsequently it was detected in radio [2] and observed to have highly variable radio pulsations at the same period as in X-ray, 5.54 s and a remarkably flat radio spectrum S ∝ ν −0.5 [3]. It has a DM of 178 pc cm−3 and a period of ∼ 5.54 s. Although a scintillation bandwidth of ∼ 320 MHz and a scintillation timescale of 10 min is measured at 15 GHz (Ibrahim et al. 2004), no direct low frequency measurement exists. Spectral characteristics that resemble interstellar scintillation are reported (Maan et al. 2019), but the characteristic bandwidths (visually, ∼ 20 − 40 MHz) of these spectral features are inconsistent with the expected scintillation bandwidth (Lazaridis et al. 2008) derived either from the NE2001 model or from scaling the 15-GHz measurement (Ibrahim et al. 2004) and hence conclude the features are likely intrinsic. A scattering timescale of (unpublished) τd ∼ 6 ms is measured at 400 MHz by Maan: using a scaling index of β ∼ 4 for the frequency dependence of scattering, the expected scattering time at 650 MHz is τd ∼ 0.86 ms. This translates roughly into a scintillation bandwidth of the order of 200 Hz at 650MHz. Man et al. (2019) report a τd ∼1.05−1.3ms, which translates into an even smaller ∆νd ∼ 140 − 175 Hz. If the scintillation bandwidth of ∆νd = 320 MHz [4] is extrapolated with β ∼ 4 to 650 MHz, the expected ∆νd ∼ 1.2 kHz, which is ∼ 6 × −10× more compared to the expectation from the τd measurement. To settle the conundrum, we have carried out 200-MHz observations with the GMRT, using a newly enabled phased-array baseband beam- former mode. The baseband beamformer data allows us to channelize the data very finely 2M / 200 MHz) inorder to resolve the scintillation. However, because of the poor S/N in each channel, we apply a sub- banding and autocorrelation technique to successfully detect scintillation. We are able to constrain the scintillation bandwidth at 650 MHz to < 150 Hz and also measure a scattering timescale of ~ 1 ms, effectively ruling out a two-screen scattering scenario. Other pulsars located in a 5 deg radius are seen to agree with the NE2001 model just as well as the magnetar. The question remains if the diffuse volume filling medium is equivalent to a series of thin screens, but such observations provide an independent tomographic picture of the electron distribution in Milky Way.
Cusps of Cusps: a Universal Model for Extreme Scattering Events in the ISM (Dylan Jow)
Since the 80s, radio sources have been observed to undergo extreme scattering events (ESEs): large, frequency dependent flux modulations due to scattering off the ISM. Recently, the study of these events has undergone a revived interest due to the increase in pulsar timing data, as well as the realization that FRBs will be scattered by the same structures in the ISM. Models of the structures responsible for ESEs range from spherical-cow approximations (e.g. simple Gaussian profiles) to more exotic models (e.g. plasma shells around compact dark matter). Here we present a new model in which ESEs are produced by corrugated sheets in the ISM, which, when projected onto the plane of the sky, generically form A3 cusp catastrophes. We will argue that this model naturally explains several features in scattering data, including observations of PSR 0834+06 and the original Fiedler et al. 0954+658 ESE. Moreover, this model is a natural consequence of existing models of pulsar scintillation and does not require exotic physics to explain.