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I want to show more results from our "Treasure Chest" of ~ 1500 dynamic spectra, many of which have never been shown/analyzed before. I want to use the meeting to continue to make progress on getting this data set ready for public release and publication.
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The evolution of scintillation characteristics with observing frequency is an important probe of different models of the underlying scattering material. The new ultra broad-band (UBB) receiver at the Effelsberg 100m telescope enables observations covering an almost continuous band between 1.3 and 6 GHz. Observing such a wide band simultaneously means to view the same physical structures under very different optical setups. I will present early results of the first scintillation studies using this instrument.
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I will be presenting work analyzing three of the scintillation arcs of PSR B1133+16 using VLBI techniques from an EVN observation. This highlights the usefulness of VLBI techniques for systems with more than one scintillation arc.
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As coherent radio emission from compact sources, such as pulsars, travels through the interstellar medium (ISM), a variety of physical processes can alter the phase and amplitude information of the signal received at Earth. By leveraging geometric arguments in the eikonal limit, coherent radio emission scattered by a thin screen of material --- as evidenced by discrete parabolic structures in the two-dimensional Fourier transform of the intensity as a function of time and frequency --- yields information about the structure and scale of semi-permanent formations in the ISM that are nigh-impossible to image directly. In this work, we examine a series of observations of PSR B0834+06 taken with the Arecibo Observatory 305-m dish that show lensed emission dominated by not one but two individual thin structures along the line of sight. We compare these observations with a recently-developed model (Jow et al. 2024) positing a universal explanation for the scintillation of radio emission in the ISM using corrugated plasma sheets with A3 cusp catastrophes in their density projections. We will end by discussing the open questions in our data, and the other potential optical regimes that may give rise to the observed scattering.
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Fast Radio Bursts (FRBs) are known to exhibit scintillation and scattering, often attributed to the interaction of screens in the Milky Way and the host galaxy. When two screens appear "point-like" to each other, they scintillate on both of their respective scintillation scales. In this talk, I will present my recent work, where we derive theoretical expectations for the spectral autocorrelation function (ACF) and modulation indices for a burst encountering two scattering screens. We investigate two regimes based on whether the screens resolve each other. By "resolving," I mean that the angular size of one screen is greater than the angular resolution of the other. In the unresolved regime, current literature considers the effect of screens in the ACF as additive. However, this work argues that the ACF contains both multiplicative and additive Lorentzian contributions, and the total modulation index is √3. For resolving screens, the relatively broad-scale scintillation in frequency is quenched, leaving only the narrow scintillation with a total modulation index of one. We validate these theoretical predictions through simulations within a cosmological two-screen scattering framework based on discrete images. Based on this study we advocate for a revision of two-screen or screen-emission region system studies in FRBs conducted so far. Through simulations, we argue for the importance of broadband scintillation studies of FRBs. For a point source, the scintillation bandwidth evolves with frequency as ∝ νobs^4. However, simulations show that in the resolving regime, the scintillation bandwidth evolves as ∝ νobs^2, contrary to the ∝ νobs^1 relationship expected for resolving an incoherent extended object that scales with frequency like a scatter disk. This finding demonstrates that scattering disks cannot be treated as incoherent sources. Additionally, this work introduces new observational techniques for studying and constraining the size of emission regions in FRBs and pulsars by using plasma screens as natural radio telescopes. FRBs encountering two screens, in the Milky Way and the host galaxy, can help us locate the host screen and probe turbulence on AU scales, opening a new avenue of research with FRBs. However, this result comes with the caveat that both this and previous formulas in the literature are only applicable to 2-D screens. These same techniques can be used to constrain scattering from intervening halos and the size of CGM cloudlets.
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Fast radio bursts (FRBs) are luminous extragalactic radio transients, with durations of milliseconds, most of which are seen only once and never again. The types of extreme astrophysical objects and physical processes responsible for the observed FRBs remains unknown. In this talk, I will give an overview of how scattering and scintillation have already given us insights into these important questions. I will also highlight the potential impact by scaling up these efforts in the future.
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Searching for time-lag phase correlations in the electric field of an FRB provides a robust way to detect astrophysical lenses. The radio emission produced by FRB sources is extremely point-like such that some lensing systems can have the emission be spatially coherent even after being bent through multiple lensing paths. We previously reported evidence for one such diffractive scattering system through the detection of coherent phase correlations that were present in the baseband data of an FRB event detected by CHIME/FRB. To study systems like these, we have developed a new simulation toolset using phase preserving ray optics to model FRB morphologies and time-lag correlation signatures from propagating through multiple thin lensing screens. In this talk, I will highlight how we see evidence for coherent phase correlations for a new, possibly plasma lensed event detected by CHIME/FRB and, for this FRB, show how our simulation toolset can be used to both model the time-lag correlations and qualitatively reproduce the complex spectro-temporal morphology of the burst from propagating through a small number of dispersive lensed paths.
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FRB 20200723B is the most highly scattered single-component fast radio burst observed thus far by CHIME/FRB. Its most probable host galaxy is NGC 4602, which is only ~32.5 Mpc away and resides in a filamentary sheet structure on the outskirts of the Virgo Cluster. We investigate the sightline of this highly scattered FRB and find that its DM directly constrains the free electron density of the cosmic filament it traverses. We also investigate various possible sources of scattering and suggest the scattering may originate from within the host galaxy.
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The repeating FRB 20200120E is associated with a globular cluster outside the optical disk but within the extended HI region of the M81 spiral galaxy, indicating that bursts propagate through the M81 galactic halo, intergalactic medium, Milky Way (MW) halo, and MW interstellar medium (ISM). We search for evidence of scattering in the temporal and spectral structure of five bursts detected February 28 - April 20, 2021, in the context of the scintillated amplitude modulated polarized shot noise (SAMPSN) model. We constrain the level of turbulence present within both galactic halos by accounting for scattering within the MW ISM using Galactic electron density models.
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The Crab pulsar emits so-called giant pulses, which are intense and short, about a microsecond, burst of radio emission. Giant pulses are believed to form close to the pulsar's light cylinder, where the plasma rotating with the pulsar reaches velocities close to the speed of light. We present a study of giant pulses with LOFAR, at ~150 MHz, which aims at confirming that plasma emitting giant pulses moves at highly relativistic speeds. We hope to do so by examining in detail the ~0.1 s long scattering tails, during which one sees the emission at slightly different angles, and look for differences between pulses related to differences in orientation, Doppler shift and Doppler boosting. If found, these changes will allow us to constrain the range in velocity with which the giant-pulse emitting material is ejected from the neutron star, and thus provide insight into how giant pulses are created.
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Giant pulses (GPs) occur in high magnetic-field millisecond pulsars (MSPs) and young Crab-like pulsars. Motivated by the fast radio bursts (FRBs) discovered in a globular cluster (GC) in the M81, we undertook baseband observations of J1823−3021A, the most active GP emitter in a GC with the MeerKAT UHF band receiver (544-1088 MHz). The steep spectral index of the pulsar yields a GP rate of over 37,000 GPs/hr with 𝑆/𝑁 > 7. This is significantly higher than the 3000 GPs/hr rate detected by Abbate et al. 2020 with the L-band (856-1712 MHz) receiver. Similarly to Abbate et al 2020, we find that the GPs are (1) strongly clustered in 2 particular phases of its rotation, (2) well described by a power-law in terms of energies, (3) typically broadband, and have steep spectral indices of ∼ −3. Although the integrated pulse profile is not significantly polarised (< 1% linear and < 3% circular), one of the brightest GPs displays notable polarisation of 7% (linear) and 8% (circular). The high-time resolution data reveals the GPs have a range of single-peak and multi-peak morphologies. For the first time, we measured the temporal scattering of the pulsar using 10 bright, narrow, single-pulse GPs, obtaining a weighted mean value of 5.3 ± 0.6 𝜇s at 1 GHz. The distinct periodicity and low polarisation of GPs differentiate them from typical FRBs, although potential quasi-periodicity substructures in some GPs may suggest a connection to magnetars/FRBs.
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In engineering and radar science, the map of observed electric field components as a function of time delay and doppler shift, analaogous to the conjugate wavefield in scintillometry, is known as the Doppler-Delay Map (DDM). I will be discussing the use of these DDMs from reflected GPS signals to study ocean winds and the surprising, or perhaps not so surprising, link to pulsar scintillometry.
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Radar observations make it possible to obtain spatially resolved images of near-Earth asteroids (NEAs) from ground-based telescopes. A powerful series of radio waves is transmitted toward the asteroid, and the recorded echoes can be decoded in time (delay) and Doppler frequency to produce two-dimensional images of the target. I will discuss how radar images can be combined with other observational data to determine an asteroid's size, shape, rotation state, and other properties.
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In this talk, I will describe how strong gravitational lenses observed with very long baseline interferometry (VLBI) are contributing to our understanding of some small-scale cosmological issues up to high redshift. Radio observations at milliarcsec angular resolution can also be used to study the interstellar medium. In this context, I will show some of the most promising systems for this scientific application of strong lensing and VLBI.
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Recent evidence of detection for nanohertz gravitational waves using pulsar timing arrays measure an amplitude significantly higher than expected for an astrophysical gravitational wave background (GWB). One explanation for this high amplitude is a signal dominated by a small handful of strong GW sources. We introduce a polarized map making method which characterizes and localizes individual GWs which we hope to test on current PTA data.
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Scintillation of radiation is a common feature of lensing in wave optics. As coherent radiation propagates through space, it is lensed by the curvature of spacetime, yielding interference patterns that can be observed through scintillations. In this presentation, I show how the frame-dragging of a rotating star in general relativity unfolds the degenerate caustic of the pointed lens into a caustic pattern where the radiation is amplified. I demonstrate how frame-dragging changes the interference pattern of the rotating star, and how this can be efficiently evaluated with Picard-Lefschetz theory. Contrary to what was previously concluded, frame-dragging leads to a systematic change of the interference pattern and measurements of the interference pattern can, in principle, be used to estimate the spin of the deflector. Finally, I will relate this calculation to lensing by a rotating Kerr black hole.
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Compact radio sources scintillate due to plasma density fluctuations in the interstellar medium. The exact origin and nature of these density fluctuations remains debated. In this talk I will synthesize our current state of knowledge on the origin of pulsar scintillation arcs, in terms of associations or lack thereof with known structures, and contextualize these results using complementary analysis of the large-scale scattering distribution in the Galaxy. Cross-matching pulsar sightlines with HII region catalogs suggests that HII region intersections are more common than previously thought, and the majority of pulsars with significant excess or deficit of scattering appear to be spatially coincident with discrete plasma structures.
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We apply the volume Green's function method [1] for pulsar scintillation arcs to different pulsars, including binary systems and multiple intervening ISM concentrations and compare to observational data. Our analysis provides insights into the ISM cloud sizes.
[1] Tobias Kramer, Daniel Waltner, Eric J Heller, Dan R Stinebring, Scattering model of scintillation arcs in pulsar secondary spectra, Monthly Notices of the Royal Astronomical Society, Volume 531, Issue 4, July 2024, Pages 3950–3960, https://doi.org/10.1093/mnras/stae1342
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The Crab pulses often show echoes, additional pulse components where intervening material away from the line of sight has caused emission to be redirected towards the observer and thus arrive with a delay. We show that the structures responsible for these events must be highly anisotropic, with typical lengths greater than 4 AU, typical widths on the sky of 0.1 AU and typical depths of 5 AU. We find that with a sheet-like lens, we can reproduce the echo features, matching their evolution with frequency and time. The magnifications are less well matched, and we suggest the sheet has substructure and that therefore it behaves as an imperfect lens, which in which many sub-images are formed.
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This is only a preliminary summary but what I want to talk about is the scintillation properties of a potential new inhabitant of the ISM that might naturally explain highly anisotropic scattering patterns with essentially arbitrary orientation relative to the anisotropy of the actual physical cloud. The background is a spectacular straight filament discovered by Yuanming Wang and colleagues in 2021 in ASKAP/VAST follow-up of a Galactic South pole field. Our modelling of the annual cycles seen in the scintillation timescales of compact sources behind the filament revealed a structure that is largely across the filament - like pattern on a cat (or racoon) tail rather than a skunk tail that might have been expected. This is puzzling (as is the existence of a straight and narrow filament in the first place, admittedly). What we realised recently is that if a cold self-gravitation cloud was to be tidally disrupted by a star, much of its hydrogen would precipitate into a large number of big (of order metre, give or take) solid lumps. When such a tidal tail eventually runs into the general ISM and gets shocked by it, the lumps will evaporate and leave multiple ionised trails as they coast through. The orientation of the structure in this case depends on the relative velocity between the disrupted cloud and the ISM, so does not have to align with the shape of the cloud. The talk would explain these settings and present the expected scattering properties of such structures and how they align (or where they don't) with what we know about the medium responsible for scintillation.
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We revisit constraints on the size of gas clouds populating the circumgalactic medium (CGM) of galaxies. These are obtained by modeling the refractive scattering induced by halos along the sightline of scintillating Fast Radio Bursts (FRBs). Scintillation sets an upper limit on the refractive scattering time delay produced by the CGM of these halos that translates into the minimum size of gas clouds. Considering typical low-redshift CGM parameter values (density, volume filling fraction, etc.) and their uncertainties, we derive minimum cloud sizes in the range 0.1 - 10 pc for the average scintillation from the Milky Way. The same results arise when adopting empirical values for the dispersion measure inferred from a number of CGM models, as well as from analyzing two real FRBs for which detailed information on scintillation and their foreground density fields exists. Our findings are in agreement with constraints from CGM observations and with the shattering scales derived from theoretical work and simulations. Finally, we discuss the impact of a physically-motivated distribution of cloud sizes in the CGM, the case of high-redshift halos, and further constraints from the presence of FRB pulse broadening.
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Interstellar scintillation has only previously been observed from compact radio objects, such as pulsars and quasars. Here, we report the first detection of scintillation-like patterns in the radio emission from an M-dwarf. In the high-resolution radio dynamic spectra of M-dwarf AD Leo, we have discovered drifting structures that can not be explained by the source properties alone. A closer inspection reveals some broadband modulation lanes, which share some characteristics to scintillation caustics. There are a vast number of finer lanes inside these broad lanes, which we analyzed using the secondary spectra. We identify curved splines in the secondary spectra and associate them with parabolic arcs commonly seen in pulsar scintillations. These findings suggest the presence of a serendipitous plasma screen within the magnetosphere of the star, representing a new method for determining plasma structures in stellar magnetospheres and tracing individual charge bunches creating the maser radiation.
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Highly relativistic or fast pulsar binaries offer unique opportunities for studies of gravity and matter at extreme densities. On the other hand, the study of scintillation arcs has enabled us to probe various facets of the ISM and structures responsible for scintillation. It could be expected that fast binaries can offer opportunities for unique studies in both worlds, however, their short orbital period yields the standard arc analysis inadequate. We propose a set of techniques to amend the study of scintillation arcs with the use of the scintillation velocity, as well as the prospect of improving the measurement accuracy of physical parameters. In addition, we also present the analysis of these procedures in the presence of a second scintillation screen. We report on the application of these methods to the double pulsar.
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Interaction of the radio signal with the ISM is a significant source of noise in the pulse arrival times for pulsar timing array (PTA) experiments. Appropriate modelling of these noise signals not only ensures robust detection of stochastic gravitational wave background (GWB), but also provides an excellent opportunity to study the properties of ISM along different lines of sight. In this work we examine the ability of the current noise modelling strategies to accurately capture the noise due to ISM. Our analysis is based on the data from the MeerKAT pulsar timing array (MPTA). We also present the results obtained from the simulations to access the role of anisotropy on the determination of the statistical properties of the noise due to refractive scattering of radio waves. We also investigate the impact of scattering noise model misspecification on the inference of the GWB parameters for the recent MPTA data release.
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Free electrons in the interstellar medium refract and diffract radio waves along multiple paths, resulting in distortions of radio pulses. Pulsar J1903+0327, the pulsar observed by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) with the highest dispersion measure (integrated line-of-sight electron density), is one of several pulsars now shown to exhibit peculiar signatures of scattering: not only does the characteristic scattering timescale change with time but the frequency dependence of scattering changes with time as well. We find that the scaling law for the scattering timescale versus radio frequency is strongly affected by, for example, any mismatch between the true and assumed pulse broadening function (PBF), which quantifies the changes to the intrinsic pulse shape as it propagates through the interstellar medium. We show using simulations that refraction manifests as changes in the PBF shape and is a plausible cause of the epoch dependence of the scattering timescale and spectral indices. I will briefly discuss the implications for precision pulsar timing and studies of the interstellar medium.