Pulsar Research Articles

Cyclical accretion regime change in the slow X-ray pulsar 4U 0114+65 observed with Chandra

4U 0114+65 is a high-mass X-ray binary system formed by the luminous supergiant B1Ia, known as V V662 Cas, and one of the slowest rotating neutron stars (NSs) with a spin period of about 2.6 hours. This provides a rare opportunity to study interesting details of the accretion within each individual pulse of the compact object. For this paper we analyzed 200 ks of Chandra grating data, divided into nine uninterrupted observations around the orbit. The changes in the circumstellar absorption column through the orbit suggest an orbital inclination of ∼ 40^∘ with respect to the observer and a companion mass-loss rate of ∼ 8.6 10^-7 M yr^-1. The peaks of the NS pulse exhibit a large pulse-to-pulse variability. Three of them show an evolution from a brighter regime to a weaker one. We propose that the efficiency of Compton cooling in this source fluctuates throughout an accumulation cycle. After significant depletion of matter within the magnetosphere, since the settling velocity is ∼ 2 lower than the free-fall velocity, the source gradually accumulates matter until the density exceeds a critical threshold. This increase in density triggers a transition to a more efficient Compton cooling regime, leading to a higher mass accretion rate and consequently to an increased brightness.

Deep Searches for Radio Pulsations and Bursts from Four Magnetar and a Magnetar-like Pulsar with FAST

Abstract We report on radio observations of four magnetars SGR 0501+4516, Swift 1834.9–0846, 1E 1841–045, SGR 1900+14, and a magnetar-like pulsar PSR J1846–0258 with the Five-hundred-meter Aperture Spherical radio Telescope at 1250 MHz. Notably, PSR J1846–0258 was observed 1 month after its 2020 X-ray outburst. The data from these observations were searched for periodic emissions and single pulses. No radio emission was detected for any of our targets. After accounting for the effect of red noise, the nondetections yield stringent upper limits on the radio flux density, with S 1250 ≤ 16.9 μJy for the four magnetars and the magnetar-like pulsar, along with constraints on single-pulse flux densities. Our deep radio observations suggest that these magnetars and the magnetar-like pulsar are indeed radio-quiet sources or unfavorably beamed. The resulting flux upper limits, along with previous findings, are discussed, highlighting the significance of further radio observations of radio-quiet magnetars and the high-B magnetar-like pulsar.

Giant Hall Waves Launched by Superconducting Phase Transition in Pulsars

The cores of pulsars are expected to become superconducting soon after birth. The transition to type-II superconductivity is associated with the bunching of magnetic field lines into discrete superconducting flux tubes which possess enormous tension. The coupling of the crust to the flux tubes implies the existence of huge tangential magnetic fields at the crust–core interface. We show that the transition to superconductivity triggers a highly nonlinear response in the Hall drift of the crustal magnetic field, an effect which was neglected in previous numerical modeling. We argue that at the time of the phase transition giant Hall waves are launched from the crust–core interface toward the surface. Our models show that if the crust contains a multipolar magnetic field ∼1013 G, the amplitude of the Hall waves is ∼1015 G. The elastic deformation of the lattice is included in our models, which allows us to track the time-dependent shear stresses everywhere in the crust. The simulations indicate that the Hall waves may be strong enough to break the crust, and could cause star quakes which trigger rotation glitches and changes in the radio pulse profile. The Hall waves also couple to slow magnetospheric changes, which cause anomalous braking indices. The emission of the giant Hall waves from the crust–core interface facilitates fast flux expulsion from the superconducting core, provided that the flux tubes in the core are themselves sufficiently mobile. For all of the flux tube mobility prescriptions implemented in this work, the core approaches the Meissner state with B = 0 at late times.

A Review of Miniature Radio Transmitters for Wildlife Tracking.

This article surveys the literature on miniature radio transmitters designed to track free-ranging wild animals using emitter-localization techniques. The articles covers the topics of power sources used in such transmitters, including miniature batteries and energy harvesting, techniques for generating the transmitted radio-frequency carrier, techniques for creating short radio pulses and more general on-off schedules, modulation in modern wildlife-tracking transmitters, construction, manufacturing, and tuning techniques, and recent trends in this area. The article also describes the recreation of the first successful wildlife-tracking transmitter, a nontrivial invention that had a profound impact on wildlife ecology, and explores its behavior.

Gravitational waves from r-mode oscillations of stochastically accreting neutron stars

ABSTRACT r-mode oscillations in rotating neutron stars are a source of continuous gravitational radiation. We investigate the excitation of r-modes by the mechanical impact on the neutron star surface of stochastically accreted clumps of matter, assuming that the Chandrasekhar–Friedman–Schutz instability is not triggered. The star is idealized as a slowly rotating, unmagnetized, one-component fluid with a barotropic equation of state in Newtonian gravity. It is found that the r-mode amplitude depends weakly on the equation of state but sensitively on the rotation frequency $\nu _{\rm s}$. The gravitational wave strain implicitly depends on the equation of state through the damping time-scale. The root-mean-square strain is $h_{\rm rms} \approx 10^{-35} (\nu _{\rm s}/ 10\, {\rm Hz})^{2} (R_*/10\, {\rm km})^2 (\Delta t_{\rm acc}/1\, {\rm yr})^{1/2} (f_{\rm acc}/1\, {\rm kHz})^{-1/2} (\dot{M}/10^{-8}\text{M}_{\odot } \, \text{yr}^{-1}) (v/0.4c) (d/1\, {\rm kpc})^{-1}$, which is comparable to the strain from g-, p-, and f-modes excited by stochastic accretion, where $R_*$ is the radius of the star, $\Delta t_{\rm acc}$ is the uninterrupted duration of an accretion episode, $f_{\rm acc}$ is the mean clump impact frequency, $\dot{M}$ is the accretion rate, v is the impact speed, and d is the distance of the star from the Earth. An observational test is proposed, based on the temporal autocorrelation function of the gravitational wave signal, to discern whether the Chandrasekhar–Friedman–Schutz instability switches on and coexists with impact-excited r-modes before or during a gravitational wave observation.

The CRAFT coherent (CRACO) upgrade I: System description and results of the 110-ms radio transient pilot survey

Abstract We present the first results from a new backend on the Australian Square Kilometre Array Pathfinder, the Commensal Realtime ASKAP Fast Transient COherent (CRACO) upgrade. CRACO records millisecond time resolution visibility data, and searches for dispersed fast transient signals including fast radio bursts (FRB), pulsars, and ultra-long period objects (ULPO). With the visibility data, CRACO can localise the transient events to arcsecond-level precision after the detection. Here, we describe the CRACO system and report the result from a sky survey carried out by CRACO at 110-ms resolution during its commissioning phase. During the survey, CRACO detected two FRBs (including one discovered solely with CRACO, FRB 20231027A), reported more precise localisations for four pulsars, discovered two new RRATs, and detected one known ULPO, GPM J1839 $-$ 10, through its sub-pulse structure. We present a sensitivity calibration of CRACO, finding that it achieves the expected sensitivity of 11.6 Jy ms to bursts of 110 ms duration or less. CRACO is currently running at a 13.8 ms time resolution and aims at a 1.7 ms time resolution before the end of 2024. The planned CRACO has an expected sensitivity of 1.5 Jy ms to bursts of 1.7 ms duration or less and can detect $10\times$ more FRBs than the current CRAFT incoherent sum system (i.e. 0.5 $-$ 2 localised FRBs per day), enabling us to better constrain the models for FRBs and use them as cosmological probes.

Radio emission from atmosphere-skimming cosmic ray showers in high-altitude balloon-borne experiments

Atmosphere-skimming air showers are initiated by cosmic rays with incoming directions that allow the cascade to develop entirely within the atmosphere, without reaching the ground. Radio pulses induced by this type of showers have already been observed in balloon-borne experiments such as ANITA, but a detailed characterisation of their properties is lacking. The extreme range of densities in which these cascades can develop gives rise to a wide range of shower profiles, with radio emission characteristics that can differ significantly from those of regular downward-going showers. In this work, we have used the ZHAireS-RASPASS program to characterise the expected radio emission from atmosphere-skimming air showers and its properties. We have studied the interplay between the magnetic field and atmospheric density profile in the expected radio signal, focusing on its detection aboard balloon-borne experiments. The almost horizontal geometry of the events gives rise to a significant refractive asymmetry in the spatial distribution of the electric field, due to the propagation of the radio signals across a gradient of index of refraction. In addition, a unique coherence asymmetry appears in the intensity of the signals, as a consequence of the cumulative effect of the Earth's magnetic field over the very long distances that these particle cascades traverse. The implications of the peculiar characteristics of the emission are discussed regarding their impact both on the interpretation of collected data and in the exposure of balloon-borne experiments.

The NANOGrav 15 Yr Data Set: Removing Pulsars One by One from the Pulsar Timing Array

Evidence has emerged for a stochastic signal correlated among 67 pulsars within the 15 yr pulsar-timing data set compiled by the NANOGrav collaboration. Similar signals have been found in data from the European, Indian, Parkes, and Chinese pulsar timing arrays. This signal has been interpreted as indicative of the presence of a nanohertz stochastic gravitational-wave background (GWB). To explore the internal consistency of this result, we investigate how the recovered signal strength changes as we remove the pulsars one by one from the data set. We calculate the signal strength using the (noise-marginalized) optimal statistic, a frequentist metric designed to measure the correlated excess power in the residuals of the arrival times of the radio pulses. We identify several features emerging from this analysis that were initially unexpected. The significance of these features, however, can only be assessed by comparing the real data to synthetic data sets. After conducting identical analyses on simulated data sets, we do not find anything inconsistent with the presence of a stochastic GWB in the NANOGrav 15 yr data. The methodologies developed here can offer additional tools for application to future, more sensitive data sets. While this analysis provides an internal consistency check of the NANOGrav results, it does not eliminate the necessity for additional investigations that could identify potential systematics or uncover unmodeled physical phenomena in the data.

Multiple-quantum magic-angle spinning NMR spectra in the static limit: The I = 3/2 case.

A simplified theoretical description of multiple-quantum excitation and mixing for nuclear magnetic resonance of half-integer quadrupolar nuclei is presented. The approach recasts the multiple-quantum nutation behavior in terms of reduced excitation and mixing curves through a scaling of the first-order offset frequency by the quadrupolar coupling constant. The two-dimensional correlation of the static first-order anisotropic line shape to the second-order anisotropic magic-angle-spinning (MAS) line shape is utilized to transform the three-dimensional integral over the three Euler angles into a single integral over the dimensionless first-order offset parameter. These transformations lead to a highly efficient algorithm for simulating the multiple-quantum (MQ)-MAS spectrum for arbitrary excitation and mixing radio frequency (RF) field strengths, pulse durations, and MAS rates within the static limit approximation, which is defined in terms of the rotation period, pulse duration, RF field strength, and quadrupolar coupling parameters. This algorithm enables a more accurate determination of the relative site populations and quadrupolar coupling parameters in a least-squares analysis of MQ-MAS spectra. Furthermore, this article examines practical considerations for eliminating experimental artifacts and employing affine transformations to improve least-squares analyses of MQ-MAS spectra. The optimum ratio of RF field strength to the quadrupolar coupling constant and the corresponding pulse durations that maximize sensitivity within experimental constraints are also examined.

A Systematic Search for Redback and Black Widow Candidates Based on the 4FGL-DR3 Unassociated Sources and the Zwicky Transient Facility Data

Spider pulsars represent a unique subclass of radio millisecond pulsars in binaries, and are further categorized into black widows and redbacks according to the mass of the low-mass companion. These pulsars, observable across multiple wavelengths, exhibit periodic variability in optical. The discovery and study of additional spider-type pulsars are crucial for a fuller understanding the evolution of binary stars in close orbits and the recycling theory of millisecond pulsars. In this work, we systematically searched for spider pulsar binary systems using time-domain variability data from the Zwicky Transient Facility and unassociated gamma-ray sources from the Fermi 4FGL-DR3 catalog. We developed a time-domain data processing pipeline that employs the Lomb–Scargle periodogram algorithm. As a result, we identified a total of 194 ellipsoidal variables and irradiation-type binary stars. Further refinement using the Gaia Hertzsprung–Russell diagram resulted in a selection of 24 spider pulsar candidates. Incorporating the 4FGL 95% confidence error ellipse reduced the sample to 19 candidates. An additional filter using the Gaia color-reduced proper motion diagram yielded nine “gold sample” candidates. These newly identified spider pulsar candidates will guide future observational campaigns in radio, X-ray, and optical spectroscopy, aiding in the comprehensive validation of their nature.

Peculiarities of radio pulsars with long periods

The analysis of parameters of radio pulsars with periods P 5 sec has been carried out. It was found that there is not a clear dependence dP / dt on P on the diagram {dP / dt, P}. Such lack of any dependence can be explained in the frame of the disk model. It is shown that the pulse width decreases with increasing period for the sample considered. It is the opposite of the dependence in the generally accepted pulsar model. Such a behavior can be explained by the dependence of the level of radiation on the period, The alternative explanation is a non-dipole structure of magnetic field in the region of the generation of observed radiation. We consider the possibility of the description of extremely long intervals between successive pulses in pulsars J0901–4046 and J0250+5854 using the model of drift waves at the periphery of the magnetosphere. In the drift model rotation periods of these pulsars are several times shorter than the intervals between observed pulses.

Glitches and Glitching Clusters in Rotation-powered Pulsars

Abstract The study of pulsar glitch phenomena serves as a valuable probe into the dynamic properties of matter under extreme high-density conditions, offering insights into the physics within neutron stars. Providing theoretical explanations for the diverse manifestations observed in different pulsars has proven to be a formidable challenge. By analyzing the distribution of glitch sizes and waiting times, along with the evolution of cumulative glitch sizes over time, we have uncovered a long-term clustering phenomenon for pulsar glitches. This perspective allows us to approach the distinct glitch representations in various pulsars from a unified standpoint, connecting the same periodicity of observational data to the randomness. Without relying on specific physical models, we utilized the coefficient of variation to numerically determine optimal clustering numbers and clustering periods for sample pulsars. Our analysis involving 27 pulsars has revealed a clear linear relationship between the glitch cluster period and characteristic age. Of interest, the cumulative distribution of functions of cluster sizes and interval times have the same patterns, which can be synchronously fitted by Gaussian processes. These results may indicate novel understandings of glitches and the resulting processes.

UWB Chaotic Pulse-Based Ranging: Time-of-Flight Approach

Nowadays, indoor positioning using ultra-wideband (UWB) signals is actively being developed with the aim of implementing existing ideas and solutions, improving their performance, and searching for new measurement schemes. This paper proposes an approach to estimating the distance between wireless nodes by measuring radio signal propagation time using UWB chaotic radio pulses and UWB transceivers. This type of signal is a simple and practically interesting alternative to radio carriers of other types of UWB signals, which are based on packets of pulses (usually ultra-short pulses). The practical interest is caused by the noise-like nature of chaotic radio pulses, as well as their immunity to multipath fading and ease of generation. The aim of this work is to analyze such a system and identify the fundamental limitations inherent in the proposed approach. This paper describes a wireless system for measuring the signal propagation time based on the envelope of chaotic radio pulses using the SS-TWR (Single-Sided Two-Way Ranging) method. A difference scheme is used to determine the range. The characteristics of the proposed system are studied experimentally. The factors related to the threshold scheme for determining the time of arrival of a radio signal that introduce a systematic error into the measurement results are revealed, and approaches to correcting their influence are proposed.

Can Slow Pulsars in Milky Way Globular Clusters Form via Partial Recycling?

Abstract Alongside the population of several hundred radio millisecond pulsars currently known in Milky Way globular clusters, a subset of six slowly spinning pulsars (spin periods 0.3–4 s) are also observed. With inferred magnetic fields ​​​​​​≳1011 G and characteristic ages ≲​​​​​​108 yr, explaining the formation of these apparently young pulsars in old stellar populations poses a major challenge. One popular explanation is that these objects are not actually young but instead have been partially spun up via accretion from a binary companion. In this scenario, accretion in a typical low-mass X-ray binary (LMXB) is interrupted by a dynamical encounter with a neighboring object in the cluster. Instead of complete spin-up to millisecond spin periods, the accretion is halted prematurely, leaving behind a “partially recycled” neutron star. In this Letter, we use a combination of analytic arguments motivated by LMXB evolution and N-body simulations to show that this partial recycling mechanism is not viable. Realistic globular clusters are not sufficiently dense to interrupt mass transfer on the short timescales required to achieve such slow spin periods. We argue that collapse of massive white dwarfs and/or neutron star collisions are more promising ways to form slow pulsars in old globular clusters.

Measuring the Spin of the Galactic Center Supermassive Black Hole with Two Pulsars.

As a key science project of the Square Kilometre Array (SKA), the discovery and timing observations of radio pulsars in the Galactic Center would provide high-precision measurements of the spacetime around the supermassive black hole, Sagittarius A* (Sgr A*), and initiate novel tests of general relativity. The spin of Sgr A* could be measured with a relative error of ≲1% by timing one pulsar with timing precision that is achievable for the SKA. However, the real measurements depend on the discovery of a pulsar in a very compact orbit, P_{b}≲0.5 yr. Here for the first time we propose and investigate the possibility of probing the spin of Sgr A* with two or more pulsars that are in orbits with larger orbital periods, P_{b}∼2-5 yr, which represents a more realistic situation from population estimates. We develop a novel method for directly determining the spin of Sgr A* from the timing observables of two pulsars, and it can be readily extended for combining more pulsars. With extensive mock data simulations, we show that combining a second pulsar improves the spin measurement by 2-3 orders of magnitude in some situations, which is comparable to timing a pulsar in a very tight orbit.

A Series of (Net) Spin-down Glitches in PSR J1522–5735: Insights from the Vortex Creep and Vortex Bending Models

Through a detailed timing analysis of Fermi-LAT data, the rotational behavior of the γ-ray pulsar PSR J1522−5735 was tracked from 2008 August (MJD 54692) to 2024 January (MJD 60320). During this 15.4 yr period, two overrecovery glitches and four antiglitches were identified, marking a rare occurrence in rotation-powered pulsars (RPPs). The magnitudes of these (net) spin-down glitches were determined to be ∣Δν g/ν∣ ∼ 10−8, well above the estimated detectability limit. For the two overrecovery glitches, the respective recovery fractions Q are 2.1(7) and 1.4(2). Further analysis showed no substantial variations in either the flux or pulse profile shape in any of these events, suggesting that small (net) spin-down glitches, unlike large events observed in magnetars and magnetar-like RPPs, may occur without leaving an impact on the magnetosphere. Within the framework of the vortex creep and vortex bending models, antiglitches and overrecoveries indicate the recoupling of vortex lines that moved inward as a result of a crustquake; meanwhile, the apparent fluctuations in the spin-down rate after the glitches occur as a result of the coupling of the oscillations of bent vortex lines to the magnetosphere.

Millisecond pulsars phenomenology under the light of graph theory

We compute and apply the minimum spanning tree (MST) of the binary millisecond pulsar population, and discuss aspects of the known phenomenology of these systems in this context. We find that the MST effectively separates different classes of spider pulsars – eclipsing radio pulsars in tight binary systems with a companion of either ~0.1–0.8 M⊙ (redbacks) or ≲0.06 M⊙ in mass (black widows) – into distinct branches. The MST also separates black widows (BWs) in globular clusters from those found in the field and groups other pulsar classes of interest, including transitional millisecond pulsars (tMSPs). Using the MST and a defined ranking for similarity, we identify possible candidates likely to belong to these pulsar classes. In particular, based on this approach, we propose the BW classification of J1300+1240, J1630+3550, J1317−0157, J1221−0633, J1627+3219, J1737−0314A, and J1701−3006F, discuss that of J1908+2105, and analyze J1723−2837, J1431−4715, and J1902−5105 as possible transitional systems. We introduce an algorithm that quickly locates where new pulsars fall within the MST and use this to examine the positions of the TMSP IGR J18245−2452 (PSR J1824−2452I), the tMSP candidate 3FGL J1544.6−1125, and the accreting millisecond X-ray pulsar SAX J1808.4−3658. Assessing the positions of these sources in the MST – assuming a range for their unknown variables (e.g., the spin period derivative of PSR J1824−2452I) –, we can effectively narrow down the parameter space necessary for searching for and determining key pulsar parameters through targeted observations.

X-Ray Hardening Preceding the Onset of SGR 1935+2154's Radio Pulsar Phase

Magnetars are neutron stars with extremely strong magnetic fields, frequently powering high-energy activity in X-rays. Pulsed radio emission following some X-ray outbursts has been detected, albeit its physical origin is unclear. It has long been speculated that the origin of magnetars’ radio signals is different from those from canonical pulsars, although convincing evidence is still lacking. Five months after magnetar SGR 1935+2154's X-ray outburst and its associated fast radio burst 20200428, a radio pulsar phase was discovered. Here we report the discovery of X-ray spectral hardening associated with the emergence of periodic radio pulsations from SGR 1935+2154 and a detailed analysis of the properties of the radio pulses. The observations suggest that radio emission originates from the outer magnetosphere of the magnetar, and the surface heating due to the bombardment of inward-going particles from the radio emission region is responsible for the observed X-ray spectral hardening.

Detection of Hidden Emissions in Two Rotating Radio Transients with High Surface Magnetic Fields

Abstract Rotating radio transients (RRATs) are neutron stars emitting sporadic radio pulses. The unique emission of RRATs has been proposed to resemble those of known pulsar types, such as extreme nulling pulsars or pulsars with giant pulses. However, the presence of additional radiation beyond these sporadic pulses remains unclear. Through high-sensitivity observations and extended tracking, we detected the sequential weak emissions in two RRATs with relatively high surface magnetic fields (B s ∼ 1013 G): J1846-0257 and J1854+0306. These emissions show peak flux densities of 0.15 and 0.41 mJy, up to 687 and 512 times weaker than our detected RRAT single pulses, respectively. The weak emissions contribute small fractions (~16% and 5%) to the total radio pulse energy releases, contrasting significantly with giant-pulse pulsars where normal pulses dominate. Polarization analysis of J1854+0306 suggests that its sporadic RRAT pulses may originate from intermittent enhanced sparking processes due to magnetospheric evolution. Our findings indicate that some RRATs may represent a novel class of pulsars, distinct from any previously known subclass. Further observations of sources with similar rotational properties using high-sensitivity instruments could validate the generality of these hidden emissions.

Three-dimensional magnetohydrodynamic simulations of core-collapse supernovae – I. Hydrodynamic evolution and protoneutron star properties

ABSTRACT We present results from three-dimensional, magnetohydrodynamic, core-collapse simulations of 16 progenitors following until 0.5 s after bounce. We use non-rotating solar-metallicity progenitor models with zero-age main-sequence mass between 9 and 24 ${\rm M}_{\odot }$. The examined progenitors cover a wide range of the compactness parameter including a peak around $23 \, {\rm M}_{\odot }$. We find that neutrino-driven explosions occur for all models within 0.3 s after bounce. We also find that the properties of the explosions and the central remnants are well correlated with the compactness. Early shock evolution is sensitive to the mass accretion rate on to the central core, reflecting the density profile of the progenitor stars. The most powerful explosions with diagnostic explosion energy $E_{\rm dia} \sim 0.75 \times 10^{51}$ erg are obtained by 23 and 24 ${\rm M}_{\odot }$ models, which have the highest compactness among the examined models. These two models exhibit spiral standing-accretion-shock-instability motions during 150–230 ms after bounce preceding a runaway shock expansion and leave a rapidly rotating neutron star with spin periods $\sim 50$ ms. Our models predict the gravitational masses of the neutron star ranging between $1.22$ and $1.67 {\rm M}_{\odot }$ and their spin periods 0.04 – 4 s. The number distribution of these values roughly matches observation. On the other hand, our models predict small hydrodynamic kick velocity (15–260 ${\rm km \, s}^{-1}$), although they are still growing at the end of our simulations. Further systematic studies, including rotation and binary effects, as well as long-term simulations up to several seconds, will enable us to explore the origin of various core-collapse supernova explosions.