Abstract Radio emission from pulsars is known to exhibit a diverse range of emission phenomena, among which nulling, where the emission becomes temporarily undetectable, is an intriguing example. Observations suggest nulling is prevalent in many long-period pulsars and thus must be understood to obtain a more comprehensive picture of pulsar emission and its evolution. However, one of the limitations in the observational characterization of nulling is the limited signal-to-noise ratio, which often makes individual pulses difficult to distinguish from noise or from any putative faint emission. Although some of the approaches in the published literature attempt to address this, they lose efficacy when individual pulses appear indistinguishable from noise, and as a result can lead to less accurate measurements. Here, we develop a new method (the N sum algorithm) that uses sums of pulses to improve distinguishability from noise, thus measuring the nulling fraction more robustly. The algorithm can be employed to measure nulling fractions in weaker pulsars and can be applied to observations with a limited number of pulses. We compare our method with the recently developed Gaussian mixture modeling approach, using both simulated and real data, and find that our approach yields consistent results for generic and weaker pulsars. We also explore quasiperiodicity in nulling and measure the related parameters for five pulsars, including PSRs J1453−6413, J0950+0755, and J0026−1955, for which these are also the first such measurements. We compare and contrast our analysis of quasiperiodic nulling with previously published work, and explore the use of spin-down energy loss ( E ̇ ) to distinguish between different types of modulation behaviour.

The mysterious Galactic center (GC) gamma-ray (γ-ray) excess (GCE) could be explained by a large population of millisecond pulsars (MSPs) hiding in the Galactic bulge, too faint to be detected as individual high-energy point sources by the Large Area Telescope and too fast and dispersed to be detected in shallow radio pulsation surveys. Fermi Motivated by an innovative candidate selection method, we aim to detect millisecond pulsars associated with the GCE by carrying out deep radio pulsation searches toward promising candidates detected in the inner Galaxy in X rays by and in radio or γ rays by the Very Large Array or . Chandra Fermi We conducted deep radio observation and follow-up campaigns with MeerKAT, as well as the Murriyang and Green Bank telescopes toward nine X-ray candidate sources. We report the detection of two new MSPs, including a black widow candidate, toward the Galactic bulge: PSRs J1740--2805 and J1740--28. These discoveries double the number of MSPs discovered within the innermost 2 from the GC.

Abstract This paper investigates how a self-bound equation of state (EOS), which describes strange quark stars, affects the rotational properties of compact stars, focusing on deviations from universal relations governing gravitational mass and radius changes due to rotation. The analysis reveals significant deviations in stars with higher surface-to-center total energy-density ratios, ϵ s ϵ c + c 2 P c , challenging the established universal relations. For Newtonian stars, hydrostatic equilibrium ensures that the difference between the gravitational potential at the center, Φ c , and at the poles, Φ p , remains constant within sequences of rotating neutron stars characterized by the same central and polar specific enthalpy (Φ c − Φ p = − h c + h p ). Combined with the scaling Φ ∝ R e 2 , where R e denotes the equatorial radius, this condition naturally leads to a quasi-universal behavior in the rotational change of radius within these sequences. Similarly, in general relativistic stars, the hydrostatic equilibrium maintains that Φ p GR − Φ c GR remains unchanged within these sequences, where Φ GR is one of the metric potentials. Inspired by this theoretical framework, a toy model has been developed to capture the dependence of gravitational mass and radius deviations on the surface-to-central total energy density ratio. Subsequently, an improved set of empirical universal relations has been proposed for accurately modeling rapidly rotating compact stars with self-bound EOSs.

Abstract Central compact objects (CCOs) are a subclass of neutron stars (NSs) with a dipole magnetic field strength considerably weaker than those of radio pulsars and magnetars. One possible explanation for such weak magnetic fields in the CCOs is the hidden magnetic field scenario, in which supernova fallback submerges the magnetosphere of a proto-NS (PNS) beneath a newly formed crust. However, the fallback mass and timescale required for this submergence process remain uncertain. We perform one-dimensional general relativistic magnetohydrodynamic simulations of the supernova fallback onto a magnetized PNS, while considering neutrino cooling. In our simulations, the infalling material compresses the magnetic field and drives a strong shock. The shock initially expands outward, but eventually stalls and recedes as neutrino cooling becomes significant. After the shock stalls, the gas density above the magnetosphere increases rapidly, potentially leading to the formation of a new crust. To understand the shock dynamics, we develop semianalytic models that describe the resulting magnetospheric and shock radii when the shock stalls. By comparing the fallback timescale with the shock stalling timescale, corresponding to the waiting time for the new crust formation, we derive a necessary condition for the submergence of the PNS’s magnetic field. Our results will provide guidance for investigating the diversity of young isolated NSs through multidimensional simulations.

Abstract Extended periods of radio pulsations have been observed for six magnetars, displaying characteristics different from those of ordinary pulsars. In this Letter, we argue that radio emission is generated in a closed, twisted magnetic flux bundle originating near the magnetic pole and extending beyond 100 km from the magnetar. The electron–positron flow in the twisted bundle has to carry electric current and, at the same time, experiences a strong drag from the radiation field of the magnetar. This combination forces the plasma into a “radiatively locked” state with a sustained two-stream instability, generating radio emission. We demonstrate this mechanism using novel first-principles simulations that follow the plasma behavior by solving the relativistic Vlasov equation with the discontinuous Galerkin method. First, using one-dimensional simulations, we demonstrate how radiative drag induces the two-stream instability, sustaining turbulent electric fields. When extended to two dimensions, the system produces electromagnetic waves, including superluminal modes capable of escaping the magnetosphere. We measure their frequency and emitted power and incorporate the local simulation results into a global magnetospheric model. The model explains key features of the observed radio emission from magnetars: its appearance after an X-ray outburst, wide pulse profiles, luminosities ∼10 30 erg s −1 , and a broad range of frequencies extending up to ∼100 GHz.

Nuclear magnetism is typically investigated by perturbing the spin system with radio frequency pulses, but low polarization and detection using induction coils limit direct access to the longitudinal magnetization. The hyperpolarization technique SABRE-SHEATH requires ultra-low magnetic fields for spin order transfer; consequently, SQUID sensors with a frequency-independent sensitivity are well-suited for unperturbed detection in this regime. We demonstrate direct observation of hyperpolarization build up (TB) and spin lattice relaxation (T1) in [1-13C]pyruvate, hyperpolarized with SABRE-SHEATH at 150 nT and 500 nT. The values for TB of 36 s and 26 s and T1 of 40 s and 43 s, respectively, suggests a shift in dominant polarization transfer efficacy or complexes, highlighting the method's merit in characterizing hyperpolarization pathways. Moreover, as demand for hyperpolarized probes in metabolic imaging continues to grow, the exceptional time resolution makes direct detection a valuable tool for understanding and optimizing polarization dynamics and reactor designs.

Binary (and trinary) radio pulsars are natural laboratories in space for understanding gravity in the strong field regime, with many unique and precise tests carried out so far, including the most precise tests of the strong equivalence principle and of the radiative properties of gravity. The Square Kilometre Array (SKA) telescope, with its high sensitivity in the Southern Hemisphere, will vastly improve the timing precision of recycled pulsars, allowing for a deeper search of potential deviations from general relativity (GR) in currently known systems. A Galactic census of pulsars will, in addition, will yield the discovery of dozens of relativistic pulsar systems, including potentially pulsar – black hole binaries, which can be used to test the cosmic censorship hypothesis and the “no-hair’’ theorem. Aspects of gravitation to be explored include tests of strong equivalence principles, gravitational dipole radiation, extra field components of gravitation, gravitomagnetism, and spacetime symmetries. In this chapter, we describe the kinds of gravity tests possible with binary pulsar and outline the features and abilities that SKA must possess to best contribute to this science.

Because of their extreme stellar densities, globular clusters are highly efficient factories of X-ray binaries and radio pulsars: per unit of stellar mass, they contain about 1000 times more of these exotic objects. Thus far, 345 radio pulsars have been found in globular clusters. These can be used as precision probes of the structure, gas content, magnetic field, and dynamic history of their host clusters; some of them are also highly interesting in their own right because they probe exotic stellar evolution scenarios as well as the physics of dense matter, accretion, and gravity; one of them (PSR J0514-4002E) might even be the first pulsar - black hole system known. Deep searches with SKA-MID and SKA-LOW will only require one to a few tied-array beams, and can be done during early commissioning of the telescope, before an all-sky pulsar survey using hundreds to thousands of tied-array beams is feasible. Even a conservative approach predicts new discoveries only with the core of SKA-MID AA , and the full AA and eventually AA4 is expected to increase the number of discoveries even more, leading to more than doubling the current known population. This offers a great opportunity for early SKAO pulsar science, even before all the collecting area is in place. On the other hand, a more optimistic prediction calls for a 4-5 times growth of the population, leading to a total of about 1700 pulsars to be detectable with SKA-MID AA4 configuration in all Galactic GCs visible by SKA telescopes. Thus, a dedicated search for pulsars in globular clusters will fully exploit the best possible natural laboratories to study many branches of physics and astrophysics, including properties of dense matter, stellar evolution, and the dynamical history of the Galactic globular cluster systems.

The known population of non-accreting neutron stars is ever growing and currently consists of more than 3500 sources. Pulsar surveys with the SKAO telescopes will greatly increase the known population, adding radio pulsars to every subgroup in the radio-loud neutron star family. These discoveries will not only add to the current understanding of neutron star physics by increasing the sample of sources that can be studied, but will undoubtedly also uncover previously unknown types of sources that will challenge our theories of a wide range of physical phenomena. A broad variety of scientific studies will be made possible by a significantly increased known population of neutron stars, unravelling questions such as: How do isolated pulsars evolve with time; What is the connection between magnetars, high B-field pulsars, and the newly discovered long-period pulsars; How is a pulsar’s spin-down related to its radio emission; What is the nuclear equation of state? Increasing the known numbers of pulsars in binary or triple systems may enable both larger numbers and higher precision tests of gravitational theories and general relativity, as well as probing the neutron star mass distribution. The excellent sensitivity of the SKAO telescopes combined with the wide field of view, large numbers of simultaneous tied-array beams that will be searched in real time, wide range of observing frequencies, and the ability to form multiple sub-arrays will make the SKAO an excellent facility to undertake a wide range of neutron star research. In this paper, we give an overview of different types of neutron stars and discuss how the SKAO telescopes will aid in our understanding of the neutron star population.

The magnetic field structure of the Milky Way can offer critical insights into the origin of galactic magnetic fields. Measurements of magnetic structures of the Milky Way are still sparse in far regions of the Galactic disk and halo. Pulsars are the best probes for the three-dimensional structure of the Galactic magnetic field, primarily owing to their highly polarized short-duration radio pulses, negligible intrinsic Faraday rotation compared to the contribution from the medium in front, and their widespread distribution throughout the Galaxy across the thin disk, spiral arms, and extended halo. In this article, we give an overview of Galactic magnetic field investigation using pulsars. The sensitive SKA1 design baseline (AA4) will increase the number of known pulsars by a factor of around three, and the initial staged delivery array (AA ) will probably double the total number of the current pulsar population. Polarization observations of pulsars with the AA telescopes will give rotation measures along several thousand lines of sight, enabling detailed exploration of the magnetic structure of both the Galactic disk and the Galactic halo.

We explore the symmetry energy effect on the bulk properties of static neutron stars and rotating millisecond pulsars (MSPs). The unified equations of state (EOSs) are constructed self-consistently within the relativistic mean-field framework from the inner crust to the outer core. To investigate the impact of a unified EOS, which uses the same nuclear interaction for both the crust and core, we compared the results of MSPs with those obtained using non-unified EOSs. These non-unified EOSs match the crust and core EOSs, which have different slopes of symmetry energy. For a given rotational period, we examined how symmetry energy influences the maximum mass, equatorial radius, and deformation from sphericity in MSPs. Our findings indicate that a softer EOS is favored by a higher Keplerian frequency, which corresponds to a smaller L for unified EOSs, but a larger L for the crust in matching EOSs. However, under the slow rotation approximation, there is no significant effect from the symmetry energy slope on the bulk properties of 2 M_⊙ MSPs. In contrast, clear differences are observed for those around and below 1.4 M_⊙.

Here we report on a new survey for pulsars and transients in the 10,pc region around using the Effelsberg radio telescope at frequencies between 4 to 8 GHz. Our calibrated full-Stokes data were searched for pulsars and transients using , , and . Polarisation information is used in the scoring of the candidates. Our periodicity acceleration and jerk searches allowed us to maintain good sensitivity towards binary pulsars in ≳ 10-hr orbits. In addition we performed a dedicated search in linear polarisation for slow transients. While our searches yielded no new discovery beyond the redetection of the magnetar SGR J1745-2900, we report on a faint single pulse candidate in addition to several weak periodicity search candidates. After thoroughly assessing our survey's sensitivity, we determined that it is still not sensitive to a population of millisecond pulsars. Next generation radio interferometers can overcome the limitations of traditional single-dish pulsar searches of the Galactic Centre. PulsarX TransientX PRESTO

The Tolman–Oppenheimer–Volkoff (TOV) limit represents the maximum mass a non-rotating, cold neutron star can sustain against gravitational collapse. However, in realistic astrophysical environments, neutron stars often possess intense magnetic fields (up to 10^15 G) and rapid rotation (up to millisecond periods), both of which significantly modifies the equilibrium structure and stability conditions. This paper is presented with the TOV limit by incorporating magnetic pressure, rotational effect with viscosity and collisional effect, and general relativistic corrections, aiming to estimate the upper mass threshold of a magnetized rotating neutron star (MRS). We derive the modified hydrostatic equilibrium equations; discuss numerical models based on General Relativity Magneto Hydro Dynamics (GRMHD), Navier-Stoke equation (for stress tensor consisting of bulk viscosity (collision effect) and shear viscosity) and present scaling relations between rotation rate, magnetic field strength, and the effective TOV mass. Recently GW170817 gravitational wave observed on 17 August 2017,the merger of two neutrons resulting into increased mass of 2.82 M☉[15] as reported is in complimenting results of Bhatia Hazarika limit (2.82 M☉ ) .Which definitely occurs due to collision of two neutron stars and also some effect shear viscosity is there .So our new model shows results in compliment with the observed data and Bhatia Hazarika Limit .The results suggest that the combined effects of rotation and magnetic field along with viscosity and collision support can increase the canonical TOV limit (~2.1 M☉) by up to 20–30% as Bhatia-Hazarika limit (~2.82 M☉) as observed [14] depending on field topology and equation of state (EOS).

Abstract We report the discovery of 12 new radio pulsars in the Large Magellanic Cloud (LMC) as part of the TRAPUM (TRAnsients and PUlsars with MeerKAT) Large Survey Project, using the MeerKAT L-band receivers (856–1712 MHz). These pulsars, discovered in 18 new pointings with 2 hour integration times, bring the total number of pulsars identified by this ongoing survey to 19 (yielding a total of 44 LMC radio pulsars now known), representing an 80 per cent increase in the LMC radio pulsar population to date. These include PSR J0454−6927, the slowest extragalactic radio pulsar discovered to date, with a spin period of 2238 ms, and PSR J0452−6921, which exhibits the highest dispersion measure (DM) among extragalactic radio pulsars, at 326 pc cm−3. The fastest spin period among the new discoveries is 245 ms, and the lowest DM is 62 pc cm−3. We also present timing results for our first pulsar discoveries with MeerKAT and the Murriyang radio telescope, obtaining phase-connected solutions for seven pulsars in the LMC. These results indicate that the pulsars are isolated, canonical radio pulsars with characteristic ages up to 8.8 Myr.

Assessing universal relations for rapidly rotating neutron stars: Insights from an interpretable deep learning perspective

The subject of this article is the process of estimating the parameters of a pulse signal with linear frequency modulation (LFM) used in airborne radar systems, particularly in synthetic aperture radars (SAR). The goal of this study is to synthesize algorithms for the optimal estimation of the key parameters of an LFM signal (i.e., carrier frequency, modulation frequency change rate, pulse length, and radio pulse envelope) and to develop a block diagram of a radar receiver that implements these algorithms. The tasks to be solved are as follows: build a mathematical model of a signal with linear frequency modulation emitted by a radar, an observation equation, and a likelihood functional; synthesize algorithms for estimating the parameters of an LFM signal using the maximum likelihood method; and develop a block diagram of a receiver based on the synthesized algorithms. The solutions to these tasks are based on the statistical theory of radio engineering systems and computer simulation. The following results were obtained: 1) algorithms for estimating the carrier frequency, frequency change rate, pulse length, and radio pulse envelope were synthesized; 2) simulations showed high noise robustness of the algorithms (up to a signal-to-noise ratio of –30 dB); 3) a block diagram of the radar was designed, which implements the synthesized algorithms and refines the estimated parameters in feedback. Conclusions. The scientific novelty of the obtained results is as follows: algorithms for estimating the parameters of both the point (carrier frequency, modulation frequency change rate, pulse length) and time characteristics (radio pulse envelope) of pulse LFM signals have been obtained using the maximum likelihood method. For the first time, it has been shown that estimating the pulse width requires solving a transcendental equation, and estimating the envelope requires smoothing in a sliding window. The obtained results expand the application of the maximum likelihood method in signal parameter estimation theory. The theory of phantomization of radio images has been further developed in terms of designing the receiving paths of phantomization radars.

Abstract Four neutron star radius measurements have already been obtained by modeling the X-ray pulses of rotation-powered millisecond pulsars observed by the Neutron Star Interior Composition ExploreR (NICER). We report here the radius measurement of PSR J0614−3329 employing the same method with NICER and XMM-Newton data using Bayesian inference. For all different models tested, including one with unrestricted inclination prior, we retrieve very similar nonantipodal hot region geometries and radii. For the preferred model, we infer an equatorial radius of R eq = 10.2 9 − 0.86 + 1.01 km for a mass of M = 1.4 4 − 0.07 + 0.06 M ⊙ (median values with equal-tailed 68% credible interval), the latter being essentially constrained from radio timing priors obtained by MeerKAT. A more complex model, fitting the data equally well, resulted in a consistent inferred radius. We find that, for all different models, the pulse emission originates from two hot regions, one at the pole and the other at the equator. The resulting radius constraint is consistent with previous X-ray and gravitational wave measurements of neutron stars in the same mass range. Equation of state inferences, including previous NICER and gravitational wave results, slightly soften the equation of state with PSR J0614−3329 included and shift the allowed mass–radius region toward lower radii by ∼300 m, which is compatible with previous analyses to within less than one standard deviation.

Abstract Fast Radio Bursts (FRBs) are bright millisecond radio pulses. Their origin is still unknown in the field of astronomy. A notable distinction among FRBs is that some sources repeat, while others appear to be non-repeating events. Interestingly, repeating FRBs tend to exhibit broader temporal widths and narrower spectral bandwidths compared to non-repeat events, suggesting they may arise from different physical mechanisms. However, current radio telescopes have limited coverage and sensitivity, which hinders a complete survey with continuous long-term monitoring. This issue makes it difficult to confirm repeat activity and potentially leads to misclassification of repeaters as non-repeaters; these are referred to as repeater candidates. To address this, machine learning techniques have emerged as a useful tool for classifying distinct FRB types in previous studies. In this study, we utilize the Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB baseband catalog with three orders of magnitude better time resolution than the intensity catalog. Measured fluences are available in the baseband catalog, while only upper limits are reported in the intensity catalog. We apply machine learning to the baseband catalog to evaluate classification outcomes. We identify 15 repeater candidates among 122 non-repeating FRBs in the baseband catalog. Additionally, our classification identifies 31 sources previously categorized as repeater candidates as non-repeaters, highlighting a significant difference from the prior work. Of these repeater candidates, 14 overlap with previous findings, while 1 is newly identified in this work. Notably, one of our candidates was confirmed as a repeater by CHIME/FRB. Follow-up observations for the 14 candidates are highly encouraged.

The radio continuum spectra of pulsars (PSRs) exhibit a wide variety of shapes, which are interpreted as pure and broken power laws, power laws with turnovers or cutoffs, and logarithmic-parabolic profiles. A notable fraction of these have well-defined power laws with ν^-2.1 exponential turnovers, indicative of free-free thermal absorption along the line of sight. We analysed a sample of 63 PSRs with such spectral shapes, compiled from four previously published studies, to investigate their statistical properties. We normalised each spectrum to a characteristic frequency and flux density of its own, facilitating a consistent treatment across the four subsamples. We show that these two fitted parameters are correlated by a power law, with its slope reflecting the median spectral index (∼ -2.0) of PSR emission. We find that the turnover frequencies in our sample are typically high, clustering around 558 MHz, which implies notably high emission measures (∼ 10^ pc cm^-6) for an inferred thermal absorbing medium with an electron temperature of $8000$ K. Moreover, by combining these emission measures with dispersion measures derived from pulse time delays, we break the degeneracy between the electron density and the path length of the absorbers. This reveals a discrete near-in population of absorbers characterised by small sizes ((∼ 0.1 pc )) and high electron densities ((∼ 10^ cm )), which exhibit a clear size-density anti-correlation reminiscent of that observed in Galactic and extragalactic H ii regions.

While broadband short-duration radio pulses from airplanes are commonly detected and used for calibration or as background in astrophysical observations, the precise locations of the emission regions cannot be determined in these studies. We show that it is possible to locate the few places on the body of an airplane, while it is flying through high clouds, from which broad-band, pulsed, radiation is emitted at very high frequency radio frequencies. This serendipitous discovery was made whilst imaging a lightning flash using the Low-Frequency Array (LOFAR). This observation provides insights into the way the airplane sheds the electrical charge it acquires when flying through clouds. Furthermore, this observation allowed us to test and improve the precision and accuracy for our lightning observation techniques. Our results indicate that with the improved procedure the location precision for strong pulses is better than 50 cm, with the orientation of linear polarization being accurate to within 25°. For the present case of a Boeing 777-300ER, very high frequency radio pulses were observed exclusively associated with the two engines, as well as a specific spot on the tail. Despite the aircraft flying through clouds at an altitude of 8 km, we did not detect any emissions from electrostatic wicks.