arXiv subjects

Robert Main

Publications and source records attributed to Robert Main.

At least 37 records · Page 2Linked to original sources

arXiv2024

Magnetospheric origin of a fast radio burst constrained using scintillation

Fast radio bursts (FRBs) are micro-to-millisecond duration radio transients that originate mostly from extragalactic distances. The emission mechanism responsible for these high luminosity, short duration transients remains debated. The models are broadly grouped into two classes: physical processes that occur within close proximity to a central engine; and central engines that release energy which moves to large radial distances and subsequently interacts with surrounding media producing radio waves. The expected emission region sizes are notably different between these two types of models. FRB emission size constraints can therefore be used to distinguish between these competing models and inform on the physics responsible. Here we present the measurement of two mutually coherent scintillation scales in the frequency spectrum of FRB 20221022A: one originating from a scattering screen located within the Milky Way, and the second originating from a scattering screen located within its host galaxy or local environment. We use the scattering media as an astrophysical lens to constrain the size of the lateral emission region, $R_{\star\mathrm{obs}} \lesssim 3\times10^{4}$ km. We find that this is inconsistent with the expected emission sizes for the large radial distance models, and is more naturally explained with an emission process that operates within or just beyond the magnetosphere of a central compact object. Recently, FRB 20221022A was found to exhibit an S-shaped polarisation angle swing, supporting a magnetospheric emission process. The scintillation results presented in this work independently support this conclusion, while highlighting scintillation as a useful tool in our understanding of FRB emission physics and progenitors.

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arXiv2024

A pulsar-like swing in the polarisation position angle of a nearby fast radio burst

Fast radio bursts (FRBs) last for milliseconds and arrive at Earth from cosmological distances. While their origin(s) and emission mechanism(s) are presently unknown, their signals bear similarities with the much less luminous radio emission generated by pulsars within our Galaxy and several lines of evidence point toward neutron star origins. For pulsars, the linear polarisation position angle (PA) often exhibits evolution over the pulse phase that is interpreted within a geometric framework known as the rotating vector model (RVM). Here, we report on a fast radio burst, FRB 20221022A, detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and localized to a nearby host galaxy ($\sim 65\; \rm{Mpc}$), MCG+14-02-011. This one-off FRB displays a $\sim 130$ degree rotation of its PA over its $\sim 2.5\; \rm{ms}$ burst duration, closely resembling the "S"-shaped PA evolution commonly seen from pulsars and some radio magnetars. The PA evolution disfavours emission models involving shocks far from the source and instead suggests magnetospheric origins for this source which places the emission region close to the FRB central engine, echoing similar conclusions drawn from tempo-polarimetric studies of some repeating sources. This FRB's PA evolution is remarkably well-described by the RVM and, although we cannot determine the inclination and magnetic obliquity due to the unknown period/duty cycle of the source, we can dismiss extremely short-period pulsars (e.g., recycled millisecond pulsars) as potential progenitors. RVM-fitting appears to favour a source occupying a unique position in the period/duty cycle phase space that implies tight opening angles for the beamed emission, significantly reducing burst energy requirements of the source.

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arXiv2023

DM-power: an algorithm for high precision dispersion measure with application to fast radio bursts

We present DM-power, a new method for precisely determining the dispersion measure (DM) of radio bursts, and apply it to the Fast Radio Burst (FRB) source FRB~20180916B. Motivated by the complex structure on multiple time scales seen in FRBs, DM-power optimizes the DM by combining measurements at multiple Fourier frequencies in the power spectrum of the burst. By optimally weighting the measurements at each Fourier frequency, DM-power finds a burst DM that effectively incorporates information on many different burst timescales. We validate this technique on simulated Gaussian pulse profiles with a precision down to $σ_{\rm DM} \sim 0.001~{\rm pc~cm}^{-3}$, and then apply it to bursts from pulsar B0329+54 and FRB~20180916B. The precision of these DM measurements are sufficient to measure a statistically significant variation in DM over a $\approx 2$ hr span. While this variation could be the result of electron density variations along the line of sight, it is more like that the observed variation is the result of intrinsic frequency-dependent burst structure that can mimic a dispersive delay.

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arXiv2023

Resolving the Emission Regions of the Crab Pulsar's Giant Pulses II. Evidence for Relativistic Motion

The Crab Pulsar is the prime example of an emitter of giant pulses. These short, very bright pulses are thought to originate near the light cylinder, at $\sim\!1600{\rm\;km}$ from the pulsar. The pulsar's location inside the Crab Nebula offers an unusual opportunity to resolve the emission regions, using the nebula, which scatters radio waves, as a lens. We attempt to do this using a sample of 61998 giant pulses found in coherently combined European VLBI network observations at $18{\rm\;cm}$. These were taken at times of relatively strong scattering and hence good effective resolution, and from correlations between pulse spectra, we show that the giant pulse emission regions are indeed resolved. We infer apparent diameters of $\sim\!2000$ and $\sim\!2400{\rm\;km}$ for the main and interpulse components, respectively, and show that with these sizes the correlation amplitudes and decorrelation timescales and bandwidths can be understood quantitatively, both in our observations and in previous ones. Using pulse-spectra statistics and correlations between polarizations, we also show that the nebula resolves the nanoshots that comprise individual giant pulses. The implied diameters of $\sim\!1100{\rm\;km}$ far exceed light travel-time estimates, suggesting the emitting plasma is moving relativistically, with $γ\simeq10^{4}$, as inferred previously from drifting bands during the scattering tail of a giant pulse. If so, the emission happens over a region extended along the line of sight by $\sim\!10^{7}{\rm\;km}$. We conclude that relativistic motion likely is important for producing giant pulses, and may be similarly for other sources of short, bright radio emission, such as fast radio bursts.

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arXiv2023

Double-lens Scintillometry: The variable scintillation of pulsar B1508+55

We report on observations of PSR B1508+55's scintillation at the Effelsberg 100-m telescope spanning from early 2020 to early 2022. In the autumn of 2020, close to the time the pulsar was predicted to cross echoes in its pulse profile, a sudden transition in the scintillation arcs from peculiar stripe-like features to parabolic arclets was observed. To infer a geometric model of the scattering we measure the effects of the annual velocity curve of Earth, of the relative movement of the line of sight, and of the projection of points on a second scattering screen and develop novel methods to do so. The latter phenomenon was discovered by this study and strongly indicates a two-screen scattering geometry. We derive an analytical two-screen model and demonstrate in a Markov Chain Monte Carlo analysis as well as simulations that it can be successfully applied to explain the observations by interpreting the transition as a change of relative amplitudes of images as well as a shift in the orientation of anisotropy. The collection of methods we demonstrate here is transferable to other pulsars with the potential to strongly improve constraints on scattering models.

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arXiv2022

Plasma lensing near the eclipses of the Black Widow pulsar B1957+20

Recently, several eclipsing millisecond pulsars have been shown to experience strong and apparent weak lensing from the outflow of their ionized companions. Lensing can be a powerful probe of the ionized plasma, with the strongest lenses potentially resolving emission regions of pulsars. Understanding lensing in the `laboratory-like' conditions of an eclipsing pulsar may be analogously applied to fast radio bursts, many of which reside in dense, magnetized environments. We examined variable dispersion measure (DM), absorption, scattering, and flux density in the original Black Widow pulsar PSR B1957+20 through an eclipse at the Arecibo Observatory at 327 MHz. We discovered clear evidence of the two regimes of lensing, strong and apparent weak. We show that the flux density variations in the apparently weak lensing regime can be modeled directly from variations of DM, using geometric optics. The mean effective velocities in the ingress, $954\pm 99$ km/s, and egress $604\pm 47$ km/s cannot be explained by orbital motions alone, but are consistent with significant outflow velocity of material from the companion. We also show that geometric optics can predict when and where the lensing regime-change between weak and strong occurs, and argue that the apparent weak lensing is due to averaging many images. Our framework can be applied in any source with variable electron columns, measuring their relative velocities and distances. In other eclipsing pulsars, this provides a unique opportunity to measure companion outflow velocity, predict regions of weak and strong lensing, and in principle independently constrain orbital inclinations.

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arXiv2022

A Highly Variable Magnetized Environment in a Pulsar Binary resembling Fast Radio Bursts

Fast radio bursts (FRBs) are short, intense extragalactic radio bursts of unknown origin. Recent polarimetric studies have shown that a noticeable fraction of the repeating FRBs display irregular, short-time variations of the Faraday rotation measure (RM). Moreover, evidence for rare propagation effects such as Faraday conversion and polarized attenuation is seen in at least one FRB repeater. Together, they suggest a highly variable magneto-active circum-burst environment. In this paper, we report similar behavior in a globular cluster pulsar binary system PSR B1744-24A. We observe irregular fast changes of RM with both signs at random orbital phases as well as profile changes of the circular polarization when the pulsar emission passes close to the companion. The latter provides strong evidence for Faraday conversion and circularly polarized attenuation. These similarities between PSR B1744-24A and some FRB repeaters, as well as the possible binary-produced long-term periodicity of two active repeaters, and the discovery of a nearby FRB in a globular cluster, where pulsar binaries are common, all suggest that some fraction of FRBs have binary companions.

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arXiv2021

Interstellar Interferometry: Precise Curvature Measurement from Pulsar Secondary Spectra

The parabolic structure of the secondary or conjugate spectra of pulsars is often the result of isolated one-dimensional (or at least highly anisotropic) lenses in the ISM. The curvature of these features contains information about the velocities of the Earth, ISM, and pulsar along the primary axis of the lens. As a result, measuring variations in the curvature over the course of a year, or the orbital period for pulsars in binaries, can constrain properties of the screen and pulsar. In particular the pulsar distance and orbital inclination for binary systems can be found for multiple screens or systems with prior information on $\sin(i)$. By mapping the conjugate spectra into a space where the main arc and inverted arclets are straight lines, we are able to make use of the full information content from the inverted arclet curvatures, amplitudes, and phases using eigenvectors to uniquely and optimally retrieve phase information. This allows for a higher precision measurement than the standard Hough transform for systems where these features are available. Our technique also directly yields the best fit 1D impulse response function for the interstellar lens given in terms of the Doppler shift, time delay, and magnification of images on the sky as seen from a single observatory. This can be extended for use in holographic imaging of the lens by combining multiple telescopes. We present examples of this new method for both simulated data and actual observations of PSR B0834+06.

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arXiv2021

Resolving the emission regions of the Crab pulsar's giant pulses

The Crab pulsar has striking radio emission properties, with the two dominant pulse components -- the main pulse and the interpulse -- consisting entirely of giant pulses. The emission is scattered in both the Crab nebula and the interstellar medium, causing multi-path propagation and thus scintillation. We study the scintillation of the Crab's giant pulses using phased Westerbork Synthesis Radio Telescope data at 1668\,MHz. We find that giant pulse spectra correlate at only $\sim 2 \%$, much lower than the $1/3$ correlation expected from a randomized signal imparted with the same impulse response function. In addition, we find that the main pulse and the interpulse appear to scintillate differently; the 2D cross-correlation of scintillation between the interpulse and main pulse has a lower amplitude, and is wider in time and frequency delay than the 2D autocorrelation of main pulses. These lines of evidence suggest that the giant pulse emission regions are extended, and that the main pulse and interpulse arise in physically distinct regions which are resolved by the scattering screen. Assuming the scattering takes place in the nebular filaments, the emission regions are of order a light cylinder radius, as projected on the sky. With further VLBI and multi-frequency data, it may be possible to measure the distance to the scattering screens, the size of giant pulse emission regions, and the physical separation between the pulse components.

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arXiv2021

Profile changes associated with DM events in PSR J1713+0747

Propagation effects in the interstellar medium and intrinsic profile changes can cause variability in the timing of pulsars, which limits the accuracy of fundamental science done via pulsar timing. One of the best timing pulsars, PSR J1713+0747, has gone through two `dip' events in its dispersion measure (DM) time series. If these events reflect real changes in electron column density, they should lead to multiple imaging. We show that the events are are well fitted by an underdense corrugated sheet model, and look for associated variability in the pulse profile using principal component analysis. We find that there are transient pulse profile variations, but they vary in concert with the dispersion measure, unlike what is expected from lensing due to a corrugated sheet. The change is consistent in shape across profiles from both the Greenbank and Arecibo radio observatories, and its amplitude appears to be achromatic across the 820-MHz, 1.4-GHz, and 2.3-GHz bands, again unlike expected from interference between lensed images. This result is puzzling. We note that some of the predicted lensing effects would need higher time and frequency resolution data than used in this analysis. Future events appear likely, and storing baseband data or keeping multiple time-frequency resolutions will allow more in-depth study of propagation effects and hence improvements to pulsar timing accuracy.

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arXiv2021

Kinematics of Crab Giant Pulses

The Crab Pulsar's radio emission is unusual, consisting predominantly of giant pulses, with durations of about a micro-second but structure down to the nano-second level, and brightness temperatures of up to $10^{37}\,$K. It is unclear how giant pulses are produced, but they likely originate near the pulsar's light cylinder, where corotating plasma approaches the speed of light. We report observations in the 400-800 MHz frequency band, where the pulses are broadened by scattering in the surrounding Crab nebula. We find that some pulse frequency spectra show strong bands, which vary during the scattering tail, in one case showing a smooth upward drift. While the banding may simply reflect interference between nano-second scale pulse components, the variation is surprising, as in the scattering tail the only difference is that the source is observed via slightly longer paths, bent by about an arcsecond in the nebula. The corresponding small change in viewing angle could nevertheless reproduce the observed drift by a change in Doppler shift, if the plasma that emitted the giant pulses moved highly relativistically, with a Lorentz factor $γ\sim10^4$ (and without much spread in $γ$). If so, this would support models that appeal to highly relativistic plasma to transform ambient magnetic structures to coherent GHz radio emission, be it for giant pulses or for potentially related sources, such as fast radio bursts.

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arXiv2021

The Thousand-Pulsar-Array programme on MeerKAT III: Giant pulse characteristics of PSR J0540$-$6919

PSR J0540$-$6919 is the second-most energetic radio pulsar known and resides in the Large Magellanic Cloud. Like the Crab pulsar it is observed to emit giant radio pulses (GPs). We used the newly-commissioned PTUSE instrument on the MeerKAT radio telescope to search for GPs across three observations. In a total integration time of 5.7 hrs we detected 865 pulses above our 7$σ$ threshold. With full polarisation information for a subset of the data, we estimated the Faraday rotation measure, $\rm{RM}=-245.8 \pm 1.0$ rad m$^{-2}$ toward the pulsar. The brightest of these pulses is $\sim$ 60% linearly polarised but the pulse-to-pulse variability in the polarisation fraction is significant. We find that the cumulative GP flux distribution follows a power law distribution with index $-2.75 \pm 0.02$. Although the detected GPs make up only $\sim$ 10% of the mean flux, their average pulse shape is indistinguishable from the integrated pulse profile, and we postulate that there is no underlying emission. The pulses are scattered at L-band frequencies with the brightest pulse exhibiting a scattering time-scale of $τ= 0.92 \pm 0.02$ ms at 1.2 GHz. We find several of the giants display very narrow-band "flux knots" similar to those seen in many Fast Radio Bursts, which we assert cannot be due to scintillation or plasma lensing. The GP time-of-arrival distribution is found to be Poissonian on all but the shortest time-scales where we find four GPs in six rotations, which if GPs are statistically independent is expected to occur in only 1 of 7000 observations equivalent to our data.

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arXiv2020

The $θ$-$θ$ Diagram: Transforming pulsar scintillation spectra to coordinates on highly anisotropic interstellar scattering screens

We introduce a novel analysis technique for pulsar secondary spectra. The power spectrum of pulsar scintillation (referred to as the "secondary spectrum") shows differential delays and Doppler shifts due to interference from multi-path propagation through the interstellar medium. We develop a transformation which maps these observables to angular coordinates on a single thin screen of phase-changing material. This transformation is possible without degeneracies in the case of a one-dimensional distribution of images on this screen, which is often a successful description of the phenomenon. The double parabolic features of secondary spectra are transformed into parallel linear features, whose properties we describe in detail. Furthermore, we introduce methods to measure the curvature parameter and the field amplitude distribution of images by applying them to observations of PSR B0834+06. Finally, we extend this formalism to two-dimensional distributions of images on the interstellar screen.

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arXiv2020

Detection of 15 bursts from FRB 180916.J0158+65 with the uGMRT

We report the findings of a uGMRT observing campaign on FRB 180916.J0158+65, discovered recently to show a 16.35-day periodicity of its active cycle. We observed the source at 550-750 MHz for $\sim 2$ hours each during three successive cycles at the peak of its expected active period. We find 0, 12, and 3 bursts respectively, implying a highly variable bursting rate even within the active phase. We consistently detect faint bursts with spectral energies only an order of magnitude higher than the Galactic burst source SGR~1935+2154. The times of arrival of the detected bursts rule out many possible aliased solutions, strengthening the findings of the 16.35-day periodicity. A short-timescale periodicity search returned no highly significant candidates. Two of the beamformer-detected bursts were bright enough to be clearly detected in the imaging data, achieving sub-arcsecond localization, and proving as a proof-of-concept for FRB imaging with the GMRT. We provide a $3σ$ upper limit of the persistent radio flux density at 650 MHz of $66~μ{\rm Jy}$ which, combined with the EVN and VLA limits at 1.6~GHz, further constrains any potential radio counterpart. These results demonstrate the power of uGMRT for targeted observations to detect and localize known repeating FRBs.

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arXiv2018

Mode changing and giant pulses in the millisecond pulsar PSR B1957+20

Many radio pulsars have stable pulse profiles, but some exhibit mode changing where the profile switches between two or more quasi-stable modes of emission. So far, these effects had only been seen in relatively slow pulsars, but we show here that the pulse profile of PSR B1957+20, a millisecond pulsar, switches between two modes, with a typical time between mode changes of only $1.7$ s (or $\sim\!1000$ rotations), the shortest observed so far. The two modes differ in both intensity and polarization, with relatively large differences in the interpulse and much more modest ones in the main pulse. We find that the changes in the interpulse precede those in the main pulse by $\sim\!25$ ms, placing an empirical constraint on the timescale over which mode changes occurs. We also find that the properties of the giant pulses emitted by PSR B1957+20 are correlated with the mode of the regular emission: their rate and the rotational phase at which they are emitted both depend on mode. Furthermore, the energy distribution of the giant pulses emitted near the main pulse depends on mode as well. We discuss the ramifications for our understanding of the radio emission mechanisms as well as for pulsar timing experiments.

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arXiv2018

Constraining small scale magnetic fields through plasma lensing: Application to the Black widow eclipsing pulsar binary

In regions with strongly varying electron density, radio emission can be magnified significantly by plasma lensing. In the presence of magnetic fields, magnification in time and frequency will be different for two circular polarizations. We show how these effects can be used to measure or constrain the magnetic field parallel to the line of sight, $B_\parallel$, as well as its spatial structure, $σ_{B_\parallel}$, in the lensing region. In addition, we discuss how generalized Faraday rotation can constrain the strength of the perpendicular field, $B_\perp$. We attempt to make such measurements for the Black Widow pulsar, PSR~B1957+20, in which plasma lensing was recently discovered. For this system, pressure equilibrium suggests $B\gtrsim 20\,$G at the interface between the pulsar and companion winds, where the radio eclipse starts and ends, and where most lensing occurs. We find no evidence for large-scale magnetic fields, with, on average, $B_\parallel=0.02\pm0.09\,$G over the egress lensing region. From individual lensing events, we strongly constrain small scale magnetic structure to $σ_B<10\,$mG, thus excluding scenarios with a strong but rapidly varying field. Finally, from the lack of reduction of average circular polarization in the same region, we rule out a strong, quasi-transverse field. We cannot identify any plausible scenario in which a large magnetic field in this system is concealed, leaving the nature of the interface between the pulsar and companion winds an enigma. Our method can be applied to other sources showing plasma lensing, including other eclipsing pulsars and fast radio bursts, to study the local properties of the magnetic field.

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arXiv2018

Pulsar emission amplified and resolved by plasma lensing in an eclipsing binary

Radio pulsars scintillate because their emission travels through the ionized interstellar medium via multiple paths, which interfere with each other. It has long been realized that the scattering screens responsible for the scintillation could be used as `interstellar lenses' to localize pulsar emission regions. Most scattering screens, however, only marginally resolve emission components, limiting results to statistical inferences and detections of small positional shifts. Since screens situated close to the source have better resolution, it should be easier to resolve emission regions of pulsars located in high density environments such as supernova remnants or binaries in which the pulsar's companion has an ionized outflow. Here, we report events of extreme plasma lensing in the `Black Widow' pulsar, PSR~B1957+20, near the phase in its 9.2 hour orbit in which its emission is eclipsed by its companion's outflow. During the lensing events, the flux is enhanced by factors of up to 70--80 at specific frequencies. The strongest events clearly resolve the emission regions: they affect the narrow main pulse and parts of the wider interpulse differently. We show that the events arise naturally from density fluctuations in the outer regions of the outflow, and infer a resolution of our lenses comparable to the pulsar's radius, about 10\,km. Furthermore, the distinct frequency structures imparted by the lensing are reminiscent of what is observed for the repeating fast radio burst FRB 121102, providing observational support for the idea that this source is observed through, and thus at times strongly magnified by, plasma lenses.

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arXiv2017

Descattering of Giant Pulses in PSR B1957+20

The interstellar medium scatters radio waves which causes pulsars to scintillate. For intrinsically short bursts of emission, the observed signal should be a direct measurement of the impulse response function. We show that this is indeed the case for giant pulses from PSR B1957+20: from baseband observations at 327 MHz, we demonstrate that the observed voltages of a bright pulse allow one to coherently descatter nearby ones. We find that while the scattering timescale is $12.3\,μ$s, the power in the descattered pulses is concentrated within a span almost two orders of magnitude shorter, of $\lesssim\!200\,$ns. This sets an upper limit to the intrinsic duration of the giant pulses. We verify that the response inferred from the giant pulses is consistent with the scintillation pattern obtained by folding the regular pulsed emission, and that it decorrelates on the same timescale, of~$84\,$s. In principle, with large sets of giant pulses, it should be possible to constrain the structure of the scattering screen much more directly than with other current techniques, such as holography on the dynamic spectrum and cyclic spectroscopy.

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