Pulsar Dispersion Measures Research Articles

Metallicity Mapping of the Ionized Diffuse Gas at the Milky Way Disk–Halo Interface

Metals in the diffuse, ionized gas at the boundary between the Milky Way’s interstellar medium (ISM) and circumgalactic medium, known as the disk–halo interface (DHI), are valuable tracers of the feedback processes that drive the Galactic fountain. However, metallicity measurements in this region are challenging due to obscuration by the Milky Way ISM and uncertain ionization corrections that affect the total hydrogen column density. In this work, we constrain ionization corrections to neutral hydrogen column densities using precisely measured electron column densities from the dispersion measures of pulsars that lie in the same globular clusters as UV-bright targets with high-resolution absorption spectroscopy. We address the blending of absorption lines with the ISM by jointly fitting Voigt profiles to all absorption components. We present our metallicity estimates for the DHI of the Milky Way based on detailed photoionization modeling of the absorption from ionized metal lines and ionization-corrected total hydrogen columns. Generally, the gas clouds show a large scatter in metallicity, ranging between 0.04 and 3.2 Z ⊙, implying that the DHI consists of a mixture of gaseous structures having multiple origins. We estimate the inflow and outflow timescales of the DHI ionized clouds to be 6–35 Myr. We report the detection of an infalling cloud with supersolar metallicity that suggests a Galactic fountain mechanism, whereas at least one low-metallicity outflowing cloud (Z < 0.1 Z ⊙) poses a challenge for Galactic fountain and feedback models.

Statistical research on Megahertz-peaked spectra pulsars

Abstract Megahertz-peaked spectra (MPS) pulsars are referred as whose radio spectra turn over around 100 MHz. We identified 53 MPS pulsars based on the spectral data from the literature, and statistically analyzed the spatial location distribution, the magnetic field-period distribution, peak frequencies, spectral indices, and dispersion measures of these MPS pulsars. We found that there is a strong positive correlation between the dispersion measures and the peak frequencies of MPS pulsars, and the negative correlations of the dispersion measures with spectral indices and the ages are also found. Such correlations suggest that the interstel lar medium is an important factor that affects observational properties of MPS pulsars.

Disentangling the Faraday rotation sky

Context. Magnetic fields permeate the diffuse interstellar medium (ISM) of the Milky Way, and are essential to explain the dynamical evolution and current shape of the Galaxy. Magnetic fields reveal themselves via their influence on the surrounding matter, and as such are notoriously hard to measure independently of other tracers. Aims. In this work, we attempt to disentangle an all-sky map of the line-of-sight (LoS)-parallel component of the Galactic magnetic field from the Faraday effect, utilizing several tracers of the Galactic electron density, ne. Additionally, we aim to produce a Galactic electron dispersion measure map and quantify several tracers of the structure of the ionized medium of the Milky Way. Methods. The method developed to reach these aims is based on information field theory, a Bayesian inference framework for fields, which performs well when handling noisy and incomplete data and constraining high-dimensional-parameter spaces. We rely on compiled catalogs of extragalactic Faraday rotation measures and Galactic pulsar dispersion measures, a well as data on bremsstrahlung and the hydrogen α spectral line to trace the ionized medium of the Milky Way. Results. We present the first full sky map of the LoS-averaged Galactic magnetic field. Within this map, we find LoS-parallel and LoS-averaged magnetic field strengths of up to 4 µG, with an all-sky root mean square of 1.1 µG, which is consistent with previous local measurements and global magnetic field models. Additionally, we produce a detailed electron dispersion measure map that agrees with existing parametric models at high latitudes but suffers from systematic effects in the disk. Further analysis of our results with regard to the 3D structure of ne reveals that it follows a Kolmogorov-type turbulence for most of the sky. From the reconstructed dispersion measure and emission measure maps, we construct several tracers of variability in ne along the LoS. Conclusions. This work demonstrates the power of consistent joint statistical analysis including multiple datasets and physical quantities and defines a road map toward a full three-dimensional joint reconstruction of the Galactic magnetic field and the ionized ISM.

A Millisecond Pulsar Binary Embedded in a Galactic Center Radio Filament

The Galactic center is host to a population of extraordinary radio filaments, thin linear structures that trace out magnetic field lines running perpendicular to the Galactic plane. Using Murriyang, the 64 m Parkes radio telescope, we conducted a search for pulsars centered on the position of a compact source in the filament G359.0−0.2. We discovered a millisecond pulsar (MSP), PSR J1744−2946, with a period P = 8.4 ms, that is bound in a 4.8 hr circular orbit around an M c > 0.05 M ⊙ companion. The pulsar dispersion measure of 673.7 ± 0.1 pc cm−3 and Faraday rotation measure of 3011 ± 3 rad m−2 are the largest of any known MSP. Its radio pulses are moderately scattered due to multipath propagation through the interstellar medium, with a scattering timescale of 0.87 ± 0.08 ms at 2.6 GHz. Using MeerKAT, we localized the pulsar to a point source embedded in a low-luminosity radio filament, the “Sunfish”, that is unrelated to G359.0−0.2. Our discovery of the first MSP within 1° of the Galactic center hints at a large population of these objects detectable via high-frequency surveys. The association with a filament points to pulsars as the energy source responsible for illuminating the Galactic center radio filaments.

Green Bank Telescope Discovery of the Redback Binary Millisecond Pulsar PSR J0212+5321

We report the discovery of a 2.11 ms binary millisecond pulsar during a targeted search of the redback optical candidate coincident with the γ-ray source 3FGL J0212.5+5320 using the Robert C. Byrd Green Bank Telescope (GBT) with the Breakthrough Listen backend at L band. Over a seven month period, five pointings were made near inferior conjunction of the pulsar in its 20.9 hr orbit, resulting in two detections, lasting 12 and 42 minutes. The pulsar dispersion measure (DM) of 25.7 pc cm−3 corresponds to a distance of 1.15 kpc in the NE2001 Galactic electron density model, consistent with the Gaia parallax distance of 1.16 ± 0.03 kpc for the companion star. We suspect the pulsar experiences wide orbital eclipses, similar to other redbacks, as well as scintillation and DM delays caused by its interaction with its companion and surroundings. Although the pulsar was only detected over ≈3.7% of the orbit, its measured acceleration is consistent with published binary parameters from optical radial velocity spectroscopy and light-curve modeling of the companion star, and it provides a more precise mass ratio and a projected semimajor axis for the pulsar orbit. We also obtained a refined optical photometric orbit ephemeris, and observed variability of the tidally distorted companion over 7 yr. A hard X-ray light curve from NuSTAR shows expected orbit-modulated emission from the intrabinary shock. The pulsar parameters and photometric ephemeris greatly restrict the parameter space required to search for a coherent timing solution including pulsar spin-down rate, either using Fermi γ-rays or further radio pulse detections.

Do pulsar and Fast Radio Burst dispersion measures obey Benford’s law?

We check if the first significant digit of the dispersion measure of pulsars and Fast Radio Bursts (using the CHIME catalog) is consistent with the Benford distribution. We find a large disagreement with Benford’s law with χ2 close to 80 for 8 degrees of freedom for both these aforementioned datasets. This corresponds to a discrepancy of about 7σ. Therefore, we conclude that the dispersion measures of pulsars and FRBs do not obey Benford’s law.

Detection of Frequency-dependent Dispersion Measure toward the Millisecond Pulsar J2241–5236 from Contemporaneous Wideband Observations

Making precise measurements of pulsar dispersion measures (DMs) and applying suitable corrections for them is among the major challenges in high-precision timing programs such as pulsar timing arrays (PTAs). While the advent of wideband pulsar instrumentation can enable more precise DM measurements and thence improved timing precision, it also necessitates doing careful assessments of frequency-dependent (chromatic) DMs that were theorized by Cordes et al (2016). Here we report the detection of such an effect in broadband observations of the millisecond pulsar PSR J2241−5236, a high-priority target for current and future PTAs. The observations were made contemporaneously using the wideband receivers and capabilities now available at the Murchison Widefield Array, the upgraded Giant Metrewave Radio Telescope, and the Parkes telescopes, thus providing an unprecedentedly large frequency coverage from 80 MHz to 4 GHz. Our analysis shows the measurable changes in DM that scale with the observing frequency (ν) as δDM ∝ ν 2.5±0.1. We discuss the potential implications of such a frequency dependence in the measured DMs and the likely impact on the timing noise budget and comment on the usefulness of low-frequency observations in advancing PTA efforts.

Low-frequency wideband timing of InPTA pulsars observed with the uGMRT

ABSTRACT High-precision measurements of the pulsar dispersion measure (DM) are possible using telescopes with low-frequency wideband receivers. We present an initial study of the application of the wideband timing technique, which can simultaneously measure the pulsar times of arrivals (ToAs) and DMs, for a set of five pulsars observed with the upgraded Giant Metrewave Radio Telescope (uGMRT) as part of the Indian Pulsar Timing Array (InPTA) campaign. We have used the observations with the 300–500 MHz band of the uGMRT for this purpose. We obtain high precision in DM measurements with precisions of the order 10−6 cm−3 pc. The ToAs obtained have sub-μs precision and the rms of the post-fit ToA residuals are in the sub-μs range. We find that the uncertainties in the DMs and ToAs obtained with this wideband technique, applied to low-frequency data, are consistent with the results obtained with traditional pulsar timing techniques and comparable to high-frequency results from other PTAs. This work opens up an interesting possibility of using low-frequency wideband observations for precision pulsar timing and gravitational wave detection with similar precision as high-frequency observations used conventionally.

The thousand-pulsar-array programme on MeerKAT VII: polarisation properties of pulsars in the Magellanic Clouds

ABSTRACT The Magellanic Clouds are the only external galaxies known to host radio pulsars. The dispersion and rotation measures of pulsars in the Clouds can aid in understanding their structure, and studies of the pulsars themselves can point to potential differences between them and their Galactic counterparts. We use the high sensitivity of the MeerKAT telescope to observe 17 pulsars in the Small and Large Magellanic Clouds in addition to five foreground (Galactic) pulsars. We provide polarisation profiles for 18 of these pulsars, improved measurements of their dispersion and rotation measures, and derive the mean parallel magnetic field along the lines of sight. The results are broadly in agreement with expectations for the structure and strength of the magnetic field in the Large and Small Magellanic Clouds. The Magellanic Cloud pulsars have profiles which are narrower than expected from the period-width relationship and we show this is due to selection effects in pulsar surveys rather than any intrinsic difference between the population of Galactic and Magellanic objects.

Could the Local Cavity be an Irregularly Shaped Strömgren Sphere?*

Abstract Extending for 50–200 pc in all directions from the Sun, the Local Cavity has been characterized as an old supernova bubble consisting of low-density million-degree plasma heated by supernova shocks. We summarize the arguments for and against this model and conclude that hydrogen in the Local Cavity is fully ionized, and the plasma near the Galactic plane is mostly warm (10,000–20,000 K) rather than hot (106 K). The brightest extreme-ultraviolet source detected in the EUVE all-sky survey is the star ϵ CMa. Its EUV radiation photoionizes the outer layers of the Local Interstellar Cloud and other nearby warm interstellar clouds despite the star’s 124 pc distance. Pulsar dispersion measures indicate an electron density of 0.012 cm−3 in the Local Cavity itself. At this density the Strömgren sphere of ϵ CMa is as large as the Local Cavity. We propose that the Local Cavity is an irregularly shaped Strömgren sphere containing a small percentage of hot gas likely in many filamentary structures. We also propose that shocks from recent supernovae encountered pre-existing Strömgren sphere gas, and that the partially ionized Local Interstellar Cloud and other nearby clouds could have been formed when supernova shocks encountered regions with relatively weak magnetic fields producing compression, higher density, and recombining hydrogen.

λ21-cm Interstellar HI Profiles, Critical Ionization Velocities, and Derived Electron Densities

Analysis of neutral hydrogen λ21-cm profiles at high northern and southern Galactic latitudes reveals unexpected properties, connects unanticipated associations, and confirms the fundamental parameters of the interstellar medium. Gaussian decomposition of profiles from the second data release of the Galactic Arecibo L-Band Feed Array (GALFA) confirms earlier results of a pervasive component with a linewidth of order 34 km s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , consistent with the critical ionization velocity (CIV) of helium. The ratio of the column density of this component to the entire profile is proportional to the interstellar helium abundance. The results are comparable to the accepted value of the cosmic abundance, 0.085, in the southern Galactic hemisphere, but different for directions in the northern Galactic hemisphere. Normalizing to the cosmic abundance allows us to estimate how much HI is “missing” from the λ21-cm observations in the northern sky, confirming an asymmetry that has been known for decades. These CIV results also predict the volume density of interstellar electrons along different lines of site, which match values derived from pulsar dispersion measures and Hα observations. These results provide strong support for the hypothesis that interstellar λ21-cm HI profile linewidths are modulated by the CIV plasma phenomenon.

Nonuniversal Interstellar Density Spectra Probed by Pulsars

Abstract Galactic interstellar turbulence affects density distribution and star formation. We introduce a new method of measuring interstellar turbulent density spectra by using the dispersion measures (DMs) of a large sample of pulsars. Without the need of invoking multiple tracers, we obtain nonuniversal density spectra in the multiphase interstellar medium over different ranges of length scales. By comparing the analytical structure function of DMs with the observationally measured one in different areas of sky, we find a shallow density spectrum arising from the supersonic turbulence in cold interstellar phases, and a Kolmogorov-like density spectrum in the diffuse warm ionized medium (WIM). Both spectra extend up to hundreds of parsecs. On larger scales, we identify, for the first time, a steep density spectrum in the diffuse WIM extending up to several kiloparsecs. Our results show that the DMs of pulsars can provide unique new information on the interstellar turbulence.

The character of the warm ionised medium

$\textit{Herschel}$ observations of far infrared N$^+$ emission lines have demonstrated that dense plasma, with $n_e\sim30\,{\rm cm^{-3}}$, is ubiquitous in the inner Galactic plane. By combining the information from $\textit{Herschel}$ with other tracers of ionised gas, we build a picture of this dense plasma. We adopt a collisional ionisation model, so the analysis is not tied to a specific energisation mechanism. We find that the dense plasma is concentrated in a disk that is $130\;{\rm pc}$ thick, and makes a significant contribution to radio pulsar dispersion measures in the inner Galactic plane. The strength of the far infrared N$^+$ emission requires high temperatures in the plasma, with $T \simeq 19{,}000\,{\rm K}$ indicated both by the ratio of N$^+$ to C$^+$, and by the ratio of N$^+$ to microwave bremsstrahlung in the inner Galactic plane. This parallels the situation at high Galactic latitudes, where strong optical emission is observed from N$^+$ (and S$^+$), relative to both H$\alpha$ and microwave bremsstrahlung, and suggests a common origin. If so, the same gas provides a natural explanation for the extreme radio-wave scattering phenomena that are sometimes observed in pulsars and quasars. We therefore propose a new picture of the warm ionised medium as seen in emission, in which the plasma is dense, hot, and localised in numerous structures of size $\sim 10^2\,{\rm AU}$ that are clustered around stars.

The decomposition of temporal variations of pulsar dispersion measures

Pulsar dispersion measure (DM) accounts for the total electron content between a pulsar and us. High-precision observations for projects of pulsar timing arrays show temporal DM variations of millisecond pulsars. The aim of this paper is to decompose the DM variations of 30 millisecond pulsars by using Hilbert-Huang Transform (HHT) method, so that we can determine the general DM trends from interstellar clouds and the annual DM variation curves from solar wind, interplanetary medium and/or ionosphere. We find that the decomposed annual variation curves of 22 pulsars exhibit quasi-sinusoidal, one component and double components features of different origins. The amplitudes and phases of the curve peaks are related to ecliptic latitude and longitude of pulsars, respectively.

The distance to the pulsar B1818-04 and the distribution of turbulent interstellar plasma toward B0833-45, B1818-04, and B1933+16

The dependence of the scatter broadening of extragalactic sources on the dispersion measures of distant pulsars observed along nearby lines of sight and the dependence of broadening of pulsar pulses on the scatter broadening observed for the pulsars themselves and for extragalactic sources observed along nearby lines of sight are constructed and analyzed. These dependences can be used to study turbulent plasma in the Galaxy. The effective scattering layer in the direction toward the pulsar B1933+16 is located in the Sagittarius arm at a distance of ≈3.4 kpc from the observer, and has an extent of ≈0.55 kpc. The scatter broadening and pulse broadening of B0833-45 are due to the turbulent medium in the shell of the Gum Nebula. The distance from the pulsar to the center of the scattering layer is≈43 pc. Data on scattering of the radiation of the pulsar B1818-04 and of the extragalactic source J1821-0502, together with data on the distribution of OB stars in the direction toward this pulsar, are used to show that the distance to the pulsar is ≈0.6 kpc; an H II region around the O7V star HD 171198, located 0.42 kpc from the Sun, is responsible for the scattering of this pulsar’s radiation.

Tomography of the Galactic free electron density with the Square Kilometer Array

We present a new algorithm to reconstruct the Galactic free electron density from pulsar dispersion measures. The algorithm performs a nonparametric tomography for a density field with an arbitrary amount of degrees of freedom. It is based on approximating the Galactic free electron density as the product of a profile function with a statistically isotropic and homogeneous log-normal field. Under this approximation the algorithm generates a map of the free electron density as well as an uncertainty estimate without the need of information about the power spectrum. The uncertainties of the pulsar distances are treated consistently by an iterative procedure. We test the algorithm using the NE2001 model with modified fluctuations as a Galaxy model, pulsar populations generated from the Lorimer population model, and mock observations emulating the upcoming Square Kilometer Array. We show the quality of the reconstruction for mock data sets containing between 1000 and 10000 pulsars with distance uncertainties up to 25%. Our results show, that with the SKA nonparametric tomography of the Galactic free electron density becomes feasible, but the quality of the reconstruction is very sensitive to the distance uncertainties.

THE MILKY WAY’S HOT GAS KINEMATICS: SIGNATURES IN CURRENT AND FUTURE O vii ABSORPTION LINE OBSERVATIONS

ABSTRACT Detections of z &approx; ?> 0 oxygen absorption and emission lines indicate the Milky Way hosts a hot ( &sim; 10 6 ?> K), low-density plasma extending &gsim; 50 kpc ?> into the Mily Way’s halo. Current X-ray telescopes cannot resolve the line profiles, but the variation of their strengths on the sky constrains the radial gas distribution. Interpreting the O vii Kα absorption line strengths has several complications, including optical depth and line of sight velocity effects. Here, we present model absorption line profiles accounting for both of these effects to show the lines can exhibit asymmetric structures and be broader than the intrinsic Doppler width. The line profiles encode the hot gas rotation curve, the net inflow or outflow of hot gas, and the hot gas angular momentum profile. We show how line of sight velocity effects impact the conversion between equivalent width and the column density, and provide modified curves of growth accounting for these effects. As an example, we analyze the LMC sight line pulsar dispersion measure and O vii equivalent width to show the average gas metallicity is &gsim; 0.6 Z &CircleDot; ?> and b &gsim; 100 ?> km s−1. Determining these properties offers valuable insights into the dynamical state of the Milky Way’s hot gas, and improves the line strength interpretation. We discuss future strategies to observe these effects with an instrument that has a spectral resolution of about 3000, a goal that is technically possible today.

CONSTRAINING THE MILKY WAY'S HOT GAS HALO WITH O VII AND O VIII EMISSION LINES

The Milky Way hosts a hot ($\approx 2 \times 10^6$ K), diffuse, gaseous halo based on detections of z = 0 OVII and OVIII absorption lines in quasar spectra and emission lines in blank-sky spectra. Here we improve constraints on the structure of the hot gas halo by fitting a radial model to a much larger sample of OVII and OVIII emission line measurements from XMM-Newton EPIC-MOS spectra compared to previous studies ($\approx$ 650 sightlines). We assume a modified $\beta$-model for the halo density distribution and a constant-density Local Bubble from which we calculate emission to compare with the observations. We find an acceptable fit to the OVIII emission line observations with $\chi^{2}_{red}$ (dof) = 1.08 (644) for best-fit parameters of $n_o r_c^{3\beta} = 1.35 \pm 0.24$ cm$^{-3}$ kpc$^{3\beta}$ and $\beta = 0.50 \pm 0.03$ for the hot gas halo and negligible Local Bubble contribution. The OVII observations yield an unacceptable $\chi^{2}_{red}$ (dof) = 4.69 (645) for similar best-fit parameters, which is likely due to temperature or density variations in the Local Bubble. The OVIII fitting results imply hot gas masses of $M$(< 50 kpc) = $3.8_{-0.3}^{+0.3} \times 10^{9} M_{\odot}$ and $M$(< 250 kpc) = $4.3_{-0.8}^{+0.9} \times 10^{10} M_{\odot}$, accounting for $\lesssim$ 50% of the Milky Way's missing baryons. We also explore our results in the context of optical depth effects in the halo gas, the halo gas cooling properties, temperature and entropy gradients in the halo gas, and the gas metallicity distribution. The combination of absorption and emission line analyses implies a sub-solar gas metallicity that decreases with radius, but that also must be $\geq 0.3 Z_{\odot}$ to be consistent with the pulsar dispersion measure toward the LMC.

Radio emission of RRAT pulsars at 111 MHz

Observations of the RRAT pulsars J0627+16, J0628+09, J1819−1458, J1826−1419, J1839−01, J1840−1419, J1846−0257, J1848−12, J1850+15, J1854+0306, J1919+06, J1913+1330, J1919+17, J1946+24, and J2033+00 observed earlier on the 64-m Parkes telescope (Australia) and the 300-m Arecibo radio telescope (Puerto Rico) at 1400 MHz were conducted at 111 MHz on the LSA radio telescope of the Pushchino Radio Astronomy observatory in 2010–2012. A characteristic feature of these pulsars is their sporadic radio emission during rare active epochs and the absence of radio emission during long time intervals. No appreciable flare activity of these pulsars was detected in the Pushchino observations. However, processing the observations using the Fast Folding Algorithm taking into account known information about the pulsar dispersion measures and periods shows that, even during quiescent intervals, the majority of the studied pulsars generate weak radio pulses with a period corresponding to that of the radio emission of the sporadic pulses observed at active epochs. The flux of this radio emission does not exceed 100 mJy at the pulse peak, even at the low frequency of 111 MHz. This considerably hinders detection of the radio emission of RRAT pulsars at high frequencies, since the radio fluxes of RRAT pulsars decreases with increasing frequency.

Radio properties of the magnetar near Sagittarius A* from observations with the Australia Telescope Compact Array

Abstract We have carried out observations of the newly discovered magnetar in the direction of Sagittarius A* using the Australia Telescope Compact Array (ATCA) in four frequency bands from 4.5 to 20 GHz. Radio pulsations are clearly detected at all frequencies. We measure the pulsar's dispersion measure to be 1650 ± 50 cm−3 pc, the highest of any of the known pulsars. Once Faraday rotation has been taken into account, the pulse profile is more than 80 per cent linearly polarized at all frequencies and has a small degree (5 per cent) of circular polarization. The rotation measure of −67000 ± 500 rad m−2 is the largest (in magnitude) ever measured for a pulsar but still a factor of 8 smaller than Sgr A* itself. The combination of the dispersion and rotation measures implies an integrated magnetic field strength of −50 μG along the line of sight. The flux density appears to have increased by about a factor of 2 between observations made 30 d apart. This object therefore joins the small class of radio emitting magnetars.