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S. K. Ocker

Publications and source records attributed to S. K. Ocker.

16 recordsLinked to original sources

Exploring the Galactic plasma with pulsars in the SKA Era

The ionised media that permeate the Milky Way have been active topics of research since the discovery of pulsars in 1967. In fact, pulsars allow one to study several aspects of said plasma, such as their column density, turbulence, scattering measures, and discrete, intervening structures between the neutron star and the observer, and aspects of the magnetic field throughout. Such sources of information allow us to characterise the electron distribution in the terrestrial ionosphere, the Solar Wind, and our Galaxy and have an important impact on other experiments involving pulsars such as Pulsar Timing Arrays. In this article, we review the state-of-the-art of plasma research using pulsars, the aspects that should be taken into consideration for optimal plasma studies, and we provide future perspectives on improvements to those enabled by the SKA.

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Characterizing Pulsar Distances Using HI Kinematics

Distance measurements are fundamental to radio pulsars' use as astrophysical probes of General Relativity and the interstellar medium. One of the primary methods for determining pulsar distances is HI kinematics, which leverages the radial velocities of HI absorption and emission features detected along pulsar lines-of-sight. This method necessarily assumes a model for Galactic rotation, our knowledge of which continues to evolve in both accuracy and precision. In this research note, we derive kinematic distances for 66 pulsars with archival HI radial velocity measurements using a state-of-the-art Galactic rotation curve. The results and software are provided in an online repository. Our kinematic distances differ by $<1σ$ from published parallaxes for nearly all pulsars in the sample that have both types of distance measurement available. Comparison to the NE2025 Galactic electron density model shows general consistency between measured and predicted distances.

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NE2025: An Updated Electron Density Model for the Galactic Interstellar Medium

Free electrons in the Galactic interstellar medium (ISM) disperse and scatter coherent radio waves, by amounts that depend on the distance to the radio source. Models of the Galactic electron density are thus widely used to predict distances and scattering of compact radio sources (including pulsars, fast radio bursts (FRBs), and long-period transients), in addition to mitigating ISM foregrounds in Galactic and extragalactic studies. We use a sample of 171 precise pulsar distances, based entirely on parallaxes and globular cluster associations, as well as scattering measurements of 568 pulsars, active galactic nuclei, and masers, to update the NE2001 Galactic electron density model. We refit the thick and thin disks and three of the spiral arms. The new parameters for these large-scale components significantly repartition free electrons between the thick disk and spiral arms, thereby correcting NE2001's systematic underestimation of pulsar distance and scattering. Sightlines with excessive dispersion and scattering are used to identify new clumps that are added to the model, in addition to refining clumps that were already included (e.g., Cygnus, Vela, and Gum). The Galactic Center component is revised, yielding scattering time predictions that are $10^3$ times smaller than the Galactic Center in NE2001. The updated model, NE2025, provides a factor of $20\times$ improvement in median distance prediction accuracy and $100\%$ median improvement in scattering predictions based on DM, relative to NE2001. There is a $15\times$ improvement in median distance prediction accuracy relative to YMW16. NE2025 is available on Github and the Python Package Interface.

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A HyperFlash and ÉCLAT view of the local environment and energetics of the repeating FRB 20240619D

Time-variable propagation effects provide a window into the local plasma environments of repeating fast radio burst (FRB) sources. Here we report high-cadence observations of FRB 20240619D, as part of the HyperFlash and ÉCLAT programs. We observed for $500$h and detected $217$ bursts, including $10$ bursts with high fluence ($>25$ Jy ms) and implied energy. We track burst-to-burst variations in dispersion measure (DM) and rotation measure (RM), from which we constrain the parallel magnetic field strength in the source's local environment: $0.27\pm0.13$ mG. Apparent DM variations between sub-bursts in a single bright event are interpreted as coming from plasma lensing or variable emission height. We also identify two distinct scintillation screens along the line of sight, one associated with the Milky Way and the other likely located in the FRB's host galaxy or local environment. Together, these (time-variable) propagation effects reveal that FRB 20240619D is embedded in a dense, turbulent and highly magnetised plasma. The source's environment is more dynamic than that measured for many other (repeating) FRB sources, but less extreme compared to several repeaters that are associated with a compact, persistent radio source. FRB 20240619D's cumulative burst fluence distribution shows a power-law break, with a flat tail at high energies. Along with previous studies, this emphasises a common feature in the burst energy distribution of hyperactive repeaters. Using the break in the burst fluence distribution, we estimate a source redshift of $z=0.042$-$0.240$. We discuss FRB 20240619D's nature in the context of similar studies of other repeating FRBs.

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Microphysics of Circumgalactic Turbulence Probed by Fast Radio Bursts and Quasars

The circumgalactic medium (CGM) is poorly constrained at the sub-parsec scales relevant to turbulent energy dissipation and regulation of multi-phase structure. Fast radio bursts (FRBs) are sensitive to small-scale plasma density fluctuations, which can induce multipath propagation (scattering). The amount of scattering depends on the density fluctuation spectrum, including its amplitude $C_{\rm n}^2$, spectral index $β$, and dissipation scale $l_{\rm i}$. We use quasar observations of CGM turbulence at $\gtrsim$ pc scales to infer $C_{\rm n}^2$, finding it to be $10^{-16}\lesssim C_{\rm n}^2\lesssim 10^{-9}$ m$^{-20/3}$ for hot ($T>10^6$ K) gas and $10^{-8}\lesssim C_{\rm n}^2\lesssim 10^{-4}$ m$^{-20/3}$ for cool ($10^4\lesssim T\lesssim 10^5$ K) gas, depending on the gas sound speed and density. These values of $C_{\rm n}^2$ are much smaller than those inferred in the interstellar medium at similar physical scales. The resulting scattering delays from the hot CGM are negligible ($\ll1$ $μ$s at 1 GHz), but are more detectable from the cool gas as either radio pulse broadening or scintillation, depending on the observing frequency and sightline geometry. Joint quasar-FRB observations of individual galaxies can yield lower limits on $l_{\rm i}$, even if the CGM is not a significant scattering site. An initial comparison between quasar and FRB observations (albeit for different systems) suggests $l_{\rm i}\gtrsim750$ km in $\sim10^4$ K gas in order for the quasar and FRB constraints to be consistent. If a foreground CGM is completely ruled out as a source of scattering along an FRB sightline then $l_{\rm i}$ may be comparable to the smallest cloud sizes ($\lesssim$ pc) inferred from photoionization modeling of quasar absorption lines.

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The emission of interpulses by a 6.45-hour period coherent radio transient

Long-period radio transients are a novel class of astronomical objects characterised by prolonged periods ranging from 18 minutes to 54 minutes. They exhibit highly polarised, coherent, beamed radio emission lasting only 10--100 seconds. The intrinsic nature of these objects is subject to speculation, with highly magnetised white dwarfs and neutron stars being the prevailing candidates. Here we present ASKAP J183950.5-075635.0 (hereafter, ASKAP J1839-0756), boasting the longest known period of this class at 6.45 hours. It exhibits emission characteristics of an ordered dipolar magnetic field, with pulsar-like bright main pulses and weaker interpulses offset by about half a period are indicative of an oblique or orthogonal rotator. This phenomenon, observed for the first time in a long-period radio transient, confirms that the radio emission originates from both magnetic poles and that the observed period corresponds to the rotation period. The spectroscopic and polarimetric properties of ASKAP J1839-0756 are consistent with a neutron star origin, and this object is a crucial piece of evidence in our understanding of long-period radio sources and their links to neutron stars.

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Probing the low-velocity regime of non-radiative shocks with neutron star bow shocks

Non-radiative shocks accelerate particles and heat astrophysical plasmas. While supernova remnants are the most well-studied example, neutron star (NS) bow shocks are also non-radiative and Balmer-dominated. NS bow shocks are likely ubiquitous in the interstellar medium due to their large speeds imparted at birth, and they are thought to be a discrete source population contributing to the Galactic cosmic ray spectrum. To date, nine NS bow shocks have been directly observed in H$α$ images. Most of these shocks have been characterized using narrowband H$α$ imaging and slit spectroscopy, which do not resolve the multi-component velocity structure of the shocks and their spatial geometry. Here we present integral field spectroscopy of three NS bow shocks: J0742$-$2822, J1741$-$2054, and J2225$+$6535 (the Guitar Nebula). We measure the shock properties simultaneously in four dimensions: the 2D projected shock morphology, the radial velocity structure, and the H$α$ flux. The broad-to-narrow line ratio ($I_{\rm b}/I_{\rm n}$) is inferred from radial velocity profiles, and for J1741$-$2054 the narrow line is detected in multiple regions of the shock. The inferred line ratios and widths suggest that NS bow shocks represent a low shock velocity regime ($V \lesssim 200$ km/s) in which $I_{\rm b}/I_{\rm n}$ is high, distinct from the shock regime probed by supernova remnants. Our results illustrate a need for non-radiative shock models at velocities lower than previously considered, which will reveal the electron-ion temperature ratios and particle acceleration efficiencies of these bow shocks.

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Implications for Galactic Electron Density Structure from Pulsar Sightlines Intersecting HII Regions

Recent radio surveys have revealed pulsars with dispersion and scattering delays induced by ionized gas that are larger than the rest of the observed pulsar population, in some cases with electron column densities (or dispersion measures; DMs) larger than the maximum predictions of Galactic electron density models. By cross-matching the observed pulsar population against HII region catalogs, we show that the majority of pulsars with $\rm DM > 600$ pc cm$^{-3}$ and scattering delays $τ(1\ {\rm GHz}) > 10$ ms lie behind HII regions, and that HII region intersections may be relevant to as much as a third of the observed pulsar population. The fraction of the full pulsar population with sightlines intersecting HII regions is likely larger. Accounting for HII regions resolves apparent discrepancies where Galactic electron density models place high-DM pulsars beyond the Galactic disk. By comparing emission measures (EMs) inferred from recombination line observations to pulsar DMs, we show that HII regions can contribute tens to hundreds of pc cm$^{-3}$ in electron column density along a pulsar LOS. We find that nearly all pulsars with significant excess (and deficit) scattering from the mean $τ$-DM relation are spatially coincident with known discrete ionized gas structures, including HII regions. Accounting for HII regions is critical to the interpretation of radio dispersion and scattering measurements as electron density tracers, both in the Milky Way and in other galaxies.

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NE2001p: A Native Python Implementation of the NE2001 Galactic Electron Density Model

The Galactic electron density model NE2001 describes the multicomponent ionized structure of the Milky Way interstellar medium. NE2001 forward models the dispersion and scattering of compact radio sources, including pulsars, fast radio bursts, AGNs, and masers, and the model is routinely used to predict the distances of radio sources lacking independent distance measures. Here we present the open-source package NE2001p, a fully Python implementation of NE2001. The model parameters are identical to NE2001 but the computational architecture is optimized for Python, yielding small (<1%) numerical differences between NE2001p and the Fortran code. NE2001p can be used on the command-line and through Python scripts available on PyPI. Future package releases will include modular extensions aimed at providing short-term improvements to model accuracy, including a modified thick disk scale height and additional clumps and voids. This implementation of NE2001 is a springboard to a next-generation Galactic electron density model now in development.

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Pulsar Scintillation through Thick and Thin: Bow Shocks, Bubbles, and the Broader Interstellar Medium

Observations of pulsar scintillation are among the few astrophysical probes of very small-scale ($\lesssim$ au) phenomena in the interstellar medium (ISM). In particular, characterization of scintillation arcs, including their curvature and intensity distributions, can be related to interstellar turbulence and potentially over-pressurized plasma in local ISM inhomogeneities, such as supernova remnants, HII regions, and bow shocks. Here we 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 $\sim 90\%$ 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 as close as $\sim10^5$ au ($<1$ pc) from either the pulsar or the observer. Several pulsars show highly truncated, low-curvature arcs that may be attributable to scattering near the pulsar. The scattering screen constraints are synthesized with continuum maps of the local ISM and other well-characterized pulsar scintillation arcs, yielding a three-dimensional view of the scattering media in context.

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Scattering variability detected from the circumsource medium of FRB 20190520B

Fast radio bursts (FRBs) are millisecond-timescale radio transients, the origins of which are predominantly extragalactic and likely involve highly magnetized compact objects. FRBs undergo multipath propagation, or scattering, from electron density fluctuations on sub-parsec scales in ionized gas along the line-of-sight. Scattering observations have located plasma structures within FRB host galaxies, probed Galactic and extragalactic turbulence, and constrained FRB redshifts. Scattering also inhibits FRB detection and biases the observed FRB population. We report the detection of scattering times from the repeating FRB 20190520B that vary by up to a factor of two or more on minutes to days-long timescales. In one notable case, the scattering time varied from $7.9\pm0.4$ ms to less than 3.1 ms ($95\%$ confidence) over 2.9 minutes at 1.45 GHz. The scattering times appear to be uncorrelated between bursts or with dispersion and rotation measure variations. Scattering variations are attributable to dynamic, inhomogeneous plasma in the circumsource medium, and analogous variations have been observed from the Crab pulsar. Under such circumstances, the frequency dependence of scattering can deviate from the typical power-law used to measure scattering. Similar variations may therefore be detectable from other FRBs, even those with inconspicuous scattering, providing a unique probe of small-scale processes within FRB environments.

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Radio Scattering Horizons for Galactic and Extragalactic Transients

Radio wave scattering can cause severe reductions in detection sensitivity for surveys of Galactic and extragalactic fast ($\sim$ms duration) transients. While Galactic sources like pulsars undergo scattering in the Milky Way interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also experience scattering in their host galaxies and other galaxies intervening their lines-of-sight. We assess Galactic and extragalactic scattering horizons for fast radio transients using a combination of NE2001 to model the dispersion measure (DM) and scattering time ($τ$) contributed by the Galactic disk, and independently constructed electron density models for the Galactic halo and other galaxies' ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For source redshifts $0.5\leq z_{\rm s}\leq1$, an all-sky, isotropic FRB population has simulated values of $τ(1\rm~GHz)$ ranging from $\sim1~μ$s to $\sim2$ ms ($90\%$ confidence, observer frame) that are dominated by host galaxies, although $τ$ can be $\gg2$ ms at low Galactic latitudes. A population at $z_{\rm s}=5$ has $0.01\lesssimτ\lesssim300$ ms at 1 GHz ($90\%$ confidence), dominated by intervening galaxies. About $20\%$ of these high-redshift FRBs are predicted to have $τ>5$ ms at 1 GHz (observer frame), and $\gtrsim40\%$ of FRBs between $z_{\rm s}\sim0.5-5$ have $τ\gtrsim1$ ms for $ν\leq 800$ MHz. Our scattering predictions may be conservative if scattering from circumsource environments is significant, which is possible under specific conditions. The percentage of FRBs selected against from scattering could also be substantially larger than we predict if circumgalactic turbulence causes more small-scale ($\ll1$ au) density fluctuations than observed from nearby halos.

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Constraining Galaxy Haloes from the Dispersion and Scattering of Fast Radio Bursts and Pulsars

Fast radio bursts (FRBs) can be scattered by ionized gas in their local environments, host galaxies, intervening galaxies along their lines-of-sight, the intergalactic medium, and the Milky Way. The relative contributions of these different media depend on their geometric configuration and the internal properties of the gas. When these relative contributions are well understood, FRB scattering is a powerful probe of density fluctuations along the line-of-sight. The precise scattering measurements for FRB 121102 and FRB 180916 allow us to place an upper limit on the amount of scattering contributed by the Milky Way halo to these FRBs. The scattering time $τ\propto(\tilde{F} \times {\rm DM}^2) A_τ$, where ${\rm DM}$ is the dispersion measure, $\tilde{F}$ quantifies electron density variations with $\tilde{F}=0$ for a smooth medium, and the dimensionless constant $A_τ$ quantifies the difference between the mean scattering delay and the $1/e$ scattering time typically measured. A likelihood analysis of the observed scattering and halo DM constraints finds that $\tilde{F}$ is at least an order of magnitude smaller in the halo than in the Galactic disk. The maximum pulse broadening from the halo is $τ\lesssim12$ $μ$s at 1 GHz. We compare our analysis of the Milky Way halo with other galaxy haloes by placing limits on the scattering contributions from haloes intersecting the lines-of-sight to FRB 181112 and FRB 191108. Our results are consistent with haloes making negligible or very small contributions to the scattering times of these FRBs.

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The Large Dispersion and Scattering of FRB 20190520B are Dominated by the Host Galaxy

The repeating FRB 20190520B is localized to a galaxy at $z=0.241$, much closer than expected given its dispersion measure $\rm DM=1205\pm4\ pc\ cm^{-3}$. Here we assess implications of the large DM and scattering observed from FRB 20190520B for the host galaxy's plasma properties. A sample of 75 bursts detected with the Five-hundred-meter Aperture Spherical radio Telescope shows scattering on two scales: a mean temporal delay $τ(1.41\ {\rm GHz})=10.9\pm1.5$ ms, which is attributed to the host galaxy, and a mean scintillation bandwidth $ν_{\rm d}(1.41\ {\rm GHz})=0.21\pm0.01$ MHz, which is attributed to the Milky Way. Balmer line measurements for the host imply an H$α$ emission measure (galaxy frame) $\rm EM_s=620$ pc cm$^{-6} \times (T/10^4 {\rm K})^{0.9}$, implying $\rm DM_{\rm Hα}$ of order the value inferred from the FRB DM budget, $\rm DM_h=1121^{+89}_{-138}$ pc cm$^{-3}$ for plasma temperatures greater than the typical value $10^4$ K. Combining $τ$ and $\rm DM_h$ yields a nominal constraint on the scattering amplification from the host galaxy $\tilde{F} G=1.5^{+0.8}_{-0.3}$ (pc$^2$ km)$^{-1/3}$, where $\tilde{F}$ describes turbulent density fluctuations and $G$ represents the geometric leverage to scattering that depends on the location of the scattering material. For a two-screen scattering geometry where $τ$ arises from the host galaxy and $Δν_{\rm d}$ from the Milky Way, the implied distance between the FRB source and dominant scattering material is $\lesssim100$ pc. The host galaxy scattering and DM contributions support a novel technique for estimating FRB redshifts using the $τ-\rm DM$ relation, and are consistent with previous findings that scattering of localized FRBs is largely dominated by plasma within host galaxies and the Milky Way.

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Persistent Plasma Waves in Interstellar Space Detected by Voyager 1

In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 astronomical units (au) of interstellar space, revealing a large-scale density gradient and compressive turbulence outside the heliopause. Previous studies of the plasma density relied exclusively on the detection of discrete plasma oscillation events that are triggered ahead of shocks propagating outwards from the Sun, which were used to infer the plasma frequency and hence density. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 Plasma Wave System data that persists from 2017 onwards and enables a steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling time of 3 days, or 0.03 au. We find au-scale density fluctuations that trace turbulence in the interstellar medium between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and could be clarified by new findings from Voyager or a future interstellar mission. The persistence of the emission suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.

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An In Situ Study of Turbulence Near Stellar Bow Shocks

Stellar bow shocks are observed in a variety of interstellar environments and are shaped by the conditions of gas in the interstellar medium (ISM). In situ measurements of turbulent density fluctuations near stellar bow shocks are only achievable with a few observational probes, including H$α$ emitting bow shocks and the Voyager Interstellar Mission (VIM). In this paper, we examine density variations around the Guitar Nebula, an H$α$ bow shock associated with PSR B2224$+$65, in tandem with density variations probed by VIM near the boundary of the solar wind and ISM. High-resolution Hubble Space Telescope observations of the Guitar Nebula taken between 1994 and 2006 trace density variations over scales from 100s to 1000s of au, while VIM density measurements made with the Voyager 1 Plasma Wave System constrain variations from 1000s of meters to 10s of au. The power spectrum of density fluctuations constrains the amplitude of the turbulence wavenumber spectrum near the Guitar Nebula to ${\rm log}_{10}C_{\rm n}^2 = -0.8\pm0.2$ m$^{-20/3}$ and for the very local ISM probed by Voyager ${\rm log}_{10}C_{\rm n}^2 = -1.57\pm0.02$ m$^{-20/3}$. Spectral amplitudes obtained from multi-epoch observations of four other H$α$ bow shocks also show significant enhancements in $C_{\rm n}^2$ from values that are considered typical for the diffuse, warm ionized medium, suggesting that density fluctuations near these bow shocks may be amplified by shock interactions with the surrounding medium, or by selection effects that favor H$α$ emission from bow shocks embedded in denser media.

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