Pulsar Observations Research Articles

Probing noncommutative gravity with gravitational wave and binary pulsar observations

Noncommutative gravity is a natural method of quantizing spacetime by promoting the spacetime coordinates themselves to operators which do not commute. This approach is motivated, for example, from a quantum gravity perspective, among others. Noncommutative gravity has been tested against the binary black hole merger event GW150914. Here, we extend and improve upon such a previous analysis by (i) relaxing an assumption made on the preferred direction due to noncommutativity, (ii) using posterior samples produced by the LIGO/Virgo Collaborations, (iii) consider other gravitational wave events, namely GW151226, GW170608, GW170814 and GW170817, and (iv) consider binary pulsar observations. Using Kepler's law that contains the noncommutative effect at second post-Newtonian order, we derive corrections to the gravitational waveform phase and the pericenter precession. Using the gravitational wave and double pulsar binary observations, we find bounds on a space-time noncommutative tensor $\theta^{0i}$ in terms of the preferred frame direction with respect to the orientation of each binary. We find that the gravitational wave bounds are stronger than the binary pulsar one by an order of magnitude and the noncommutative tensor normalized by the Planck length and time is constrained to be of order unity.

Quantum nucleation of up-down quark matter and astrophysical implications

Quark matter with only $u$ and $d$ quarks ($ud$QM) might be the ground state of baryonic matter at large baryon number $A>A_{\rm min}$. With $A_{\rm min}\gtrsim 300$, this has no direct conflict with the stability of ordinary nuclei. An intriguing test of this scenario is to look for quantum nucleation of $ud$QM inside neutron stars due to their large baryon densities. In this paper, we study the transition rate of cold neutron stars to $ud$ quark stars ($ud$QSs) and the astrophysical implications, considering the relevant theoretical uncertainties and observational constraints. It turns out that a large portion of parameter space predicts an instantaneous transition, and so the observed neutron stars are mostly $ud$QSs. We find this possibility still viable under the recent gravitational wave and pulsar observations, although there are debates on its compatibility with some observations that involve complicated structure of quark matter. The tension could be partially relieved in the two-families scenario, where the high-mass stars ($M\gtrsim2 M_{\odot}$) are all $ud$QSs and the low-mass ones ($M\sim1.4\, M_{\odot}$) are mostly hadronic stars. In this case, the slow transition of the low-mass hadronic stars points to a very specific class of hadronic models with moderately stiff EOSs, and $ud$QM properties are also strongly constrained.

Characterizing Astrophysical Binary Neutron Stars with Gravitational Waves

Abstract Merging binary neutron stars are thought to be formed predominantly via isolated binary evolution. In this standard formation scenario, the first-born neutron star goes through a recycling process and might be rapidly spinning during the final inspiral, whereas the second-born star is expected to have effectively zero spin at merger. Based on this feature, we propose a new framework for the astrophysical characterization of binary neutron stars observed from their gravitational wave emission. We further propose a prior for the dimensionless spins of recycled neutron stars, given by a gamma distribution with a shape parameter of 2 and a scale parameter of 0.012, extrapolated from radio pulsar observations of Galactic binary neutron stars. Interpreting GW170817 and GW190425 in the context of the standard formation scenario and adopting the gamma-distribution prior, we find positive support (with a Bayes factor of 6, over the nonspinning hypothesis) for a spinning recycled neutron star in GW190425, whereas the spin of the recycled neutron star in GW170817 is small and consistent with our prior. We measure the masses of the recycled (slow) neutron stars in GW170817 and GW190425 to be and , with 68% credibility, respectively. We discuss implications for the astrophysical origins of these two events and outline future prospects of studying binary neutron stars using our framework.

Strengthening the bounds on the r-mode amplitude with X-ray observations of millisecond pulsars

ABSTRACT R-mode oscillations have been shown to have a significant potential to constrain the composition of fast spinning neutron stars. Due to their high rotation rates, millisecond pulsars (MSPs) provide a unique platform to constrain the properties of such oscillations, if their surface temperatures can be inferred. We present the results of our investigations of archival X-ray data of a number of MSPs, as well as recent XMM–Newton observations of PSR J1810+1744 and PSR J2241−5236. Using the neutron star atmosphere model and taking into account various uncertainties, we present new bounds on the surface temperature of these sources. Thereby, we significantly strengthen previous bounds on the amplitude of the r-mode oscillations in MSPs and find rigorous values as low as α ≲ 3 × 10−9. This is by now about three orders of magnitude below what standard saturation mechanisms in neutron stars could provide, which requires very strong dissipation in the interior, strongly pointing towards a structurally complex or exotic composition of these sources. At such low temperatures, sources could even be outside of the instability region, and taking into account the various uncertainties, we obtain for an observed surface temperature a simple frequency bound below which r-modes are excluded in slower spinning pulsars.

Search and detection of northern pulsars in the side lobes of the murchison wide-field array

New pulsar searching methods can significantly enhance the discovery of new pulsars and observations of single pulsars. This paper searches for pulsars among the observations centered at 185 MHz collected by the Murchison Wide-field Array (MWA). The declinations of two high-latitude northern sky pulsars, identified as J0332+5434 and J2219+4754, exceeded the commonly considered detection limit of MWA (declination≤30°), and appeared in the side lobes of MWA’s incoherent beams. Although the sensitivity is substantially lower in the side lobes than in the main lobe, pulsars are generally very bright at low frequencies, so are observable in the side lobes of low-frequency, wide field of view telescopes. This article describes the search for pulsars with the MWA, and discusses the possibility of further searching and observing the northern sky pulsars in the MWA side lobes. In a simulation study, the MWA detection sensitivity was evaluated at several flux densities of known radio pulsars. We estimated that approximately 30 known northern sky pulsars can be detected in the MWA side lobes. This work promises future pulsar searching by the Square Kilometer Array at low frequencies.

Systematic study on the quark-hadron mixed phase in compact stars

We investigate systematically the quark-hadron mixed phase in dense stellar matter, and its influence on compact star structures. The properties of quark matter and hadronic matter are fixed based on various model predictions. Beside adopting constant values, the surface tension $\Sigma$ for the quark-hadron interface is estimated with the multiple reflection expansion method and equivparticle model. To fix the structures of quark-hadron pasta phases, a continuous dimensionality of the structure is adopted as proposed by Ravenhall, Pethick, and Wilson. The corresponding properties of hybrid stars are then obtained and confronted with pulsar observations. It is found that the correlation between radius and tidal deformability in traditional neutron stars preserves in hybrid stars. For those permitted by pulsar observations, in almost all cases the quark phase persists inside the most massive compact stars. The quark-hadron interface plays an important role on hybrid star structures once quark matter emerges. The surface tension $\Sigma$ estimated with various methods increases with density, which predicts stiffer EOSs for the quark-hadron mixed phase and increases the maximum mass of hybrid stars. The EOSs of hybrid star matter are well constrained at densities $n\lesssim 0.8$ fm${}^{-3}$, while larger uncertainty is expected at higher densities.

Constraining the dense matter equation-of-state with radio pulsars

ABSTRACT Radio pulsars provide some of the most important constraints for our understanding of matter at supranuclear densities. So far, these constraints are mostly given by precision mass measurements of neutron stars (NS). By combining single measurements of the two most massive pulsars, J0348+0432 and J0740+6620, the resulting lower limit of 1.98 M⊙ (99 per cent confidence) of the maximum NS mass, excludes a large number of equations of state (EOSs). Further EOS constraints, complementary to other methods, are likely to come from the measurement of the moment of inertia (MOI) of binary pulsars in relativistic orbits. The Double Pulsar, PSR J0737−3039A/B, is the most promising system for the first measurement of the MOI via pulsar timing. Reviewing this method, based in particular on the first MeerKAT observations of the Double Pulsar, we provide well-founded projections into the future by simulating timing observations with MeerKAT and the SKA. For the first time, we account for the spin-down mass-loss in the analysis. Our results suggest that an MOI measurement with 11 per cent accuracy (68 per cent confidence) is possible by 2030. If by 2030 the EOS is sufficiently well known, however, we find that the Double Pulsar will allow for a 7 per cent test of Lense–Thirring precession, or alternatively provide a ∼3σ-measurement of the next-to-leading order gravitational wave damping in GR. Finally, we demonstrate that potential new discoveries of double NS systems with orbital periods shorter than that of the Double Pulsar promise significant improvements in these measurements and the constraints on NS matter.

Predicting pulsar stars using a random tree boosting voting classifier (RTB-VC)

Abstract The recent exponential growth in the data volume and number of identified pulsar stars is due to pulsar candidate search experiments and surveys. In this study, we investigated the existing methods and techniques used for pulsar prediction, such as applying filters based on pulsar observations, which can adversely affect the success of accurate pulsar prediction. Some of the existing methods are not capable of dealing with large volumes of data and others fail to accurately select the best candidates from pulsar observations. Thus, we developed a new approach based on the traditional supervised machine learning algorithm, which yields faster and more accurate results. In this study, we present our hybrid machine learning classifier called the random trees boosting voting classifier (RTB-VC) for predicting pulsar stars. RTB-VC combines tree-based classifiers and it employs the High Time Resolution Universe 2 (HTRU2) data set comprising a set of eight features related to pulsars and non-pulsars. The HTRU2 data set is imbalanced and we solve this problem by using the synthetic minority oversampling technique to generate artificial data and obtain a balanced data set. A feature set is used to separate pulsar and non-pulsar candidates because the different distributions of variables in the data set are helpful for training models. In the proposed approach, the prediction stage of RTB-VC is based on a combination of soft voting, hard voting, and weighted voting to obtain highly accurate and relevant criteria for finally predicting pulsars or non-pulsars. The ensemble-based structure of RTB-VC yields accurate results based on pulsar observations with a high F 1 score for pulsars (98.3%). We evaluated the learning algorithm in terms of its accuracy, precision, recall, and F 1 score.

Pulsar timing observations with Haoping Radio Telescope

We report pulsar timing observations carried out in L-band with NTSC’s 40-meter Haoping Radio Telescope (HRT), which was constructed in 2014. These observations were carried out using the pulsar machine we developed. Timing observations toward millisecond pulsar J0437–4715 obtain a timing residual (r.m.s.) of 397 ns in the time span of 284 days. Our observations successfully detected Crab pulsar’s glitch that happened on 2019 July 23.

Masses of double neutron star mergers

Aims. I aim to explain the mass discrepancy between the observed double neutron-star binary population by radio pulsar observations and gravitational-wave observation. Methods. I performed binary population synthesis calculations and compared their results with the radio and the gravitational-wave observations simultaneously. Results. Simulations of binary evolution were used to link different observations of double neutron star binaries with each other. I investigated the progenitor of GW190425 in more detail. A distribution of masses and merger times of the possible progenitors is presented. Conclusions. A mass discrepancy between the radio pulsars in the Milky Way with another neutron star companion and the inferred masses from gravitational-wave observations of those kind of merging systems is naturally found in binary evolution.

Nonparametric constraints on neutron star matter with existing and upcoming gravitational wave and pulsar observations

Observations of neutron stars, whether in binaries or in isolation, provide information about the internal structure of the most extreme material objects in the Universe. In this work, we combine information from recent observations to place joint constraints on the properties of neutron star matter. We use (i) lower limits on the maximum mass of neutron stars obtained through radio observations of heavy pulsars, (ii) constraints on tidal properties inferred through the gravitational waves neutron star binaries emit as they coalesce, and (iii) information about neutron stars' masses and radii obtained through X-ray emission from surface hot spots. In order to combine information from such distinct messengers while avoiding the kind of modeling systematics intrinsic to parametric inference schemes, we employ a nonparametric representation of the neutron-star equation of state based on Gaussian processes conditioned on nuclear theory models. We find that existing astronomical observations imply $R_{1.4}=12.32^{+1.09}_{-1.47}\,\mathrm{km}$ for the radius of a $1.4\,M_{\odot}$ neutron star and $p(2\rho_\mathrm{nuc})=3.8^{+2.7}_{-2.9}\times10^{34}\,\mathrm{dyn}/\mathrm{cm}^2$ for the pressure at twice nuclear saturation density at the 90% credible level. The upper bounds are driven by the gravitational wave observations, while X-ray and heavy pulsar observations drive the lower bounds. Additionally, we compute expected constraints from potential future astronomical observations and find that they can jointly determine $R_{1.4}$ to ${\cal{O}}(1)\,\mathrm{km}$ and $p(2\rho_\mathrm{nuc})$ to 80% relative uncertainty in the next five years.

Radio Sounding Measurements of the Solar Corona Using Giant Pulses of the Crab Pulsar in 2018

Observations of the Crab pulsar at 327 MHz were made at the Toyokawa Observatory of the Institute for Space-Earth Environmental Research, during the solar occultation in mid-June 2018 to investigate the coronal plasma density in the weak sunspot cycle, Cycle 24. The dispersion measurements (DMs) were determined using giant pulses detected from observations of the Crab pulsar. The systematic increase in DM over the background level, observed during the period of the closest approach of the Crab pulsar’s line-of-sight to the Sun, was ascribed to the effect of the coronal plasma. A coronal density model assuming spherical symmetry was determined by fitting it to the DM data, and was compared with those determined in past solar cycles. The best-fit model had large errors, and indicated a systematically higher value than those derived from past observations. The results obtained here are likely to be significantly affected by latitude/longitude variation in coronal plasma density, the time variation of the interstellar medium, mainly the Crab nebula, and increased measurement errors due to the reduced occurrence of giant pulses. Hence, further observations are needed to derive conclusions about a change of coronal density in the current cycle.

A broad-band perspective on circular polarization in radio pulsar observations

AbstractWe investigate the broad-band behaviour of circular polarization in radio pulsar profiles and show the relationship between polarization fraction and what proportion of that polarization is circular, both across frequency, and for a large number of pulsars viewed collectively. The behaviour observed may be explained by pulsar polarization originating from the partially-coherent combination of two linearly-polarized orthogonal modes with different flux spectral indices. (See also the poster in the “supplementary information”.)

An in-depth investigation of 11 pulsars discovered by FAST

ABSTRACT We present timing solutions and analyses of 11 pulsars discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These pulsars were discovered using an ultrawide bandwidth receiver in drift-scan observations made during the commissioning phase of FAST, and were then confirmed and timed using the 64-m Parkes Radio Telescope. Each pulsar has been observed over a span of at least one year. Highlighted discoveries include PSR J0344−0901, which displays mode-changing behaviour and may belong to the class of so-called swooshing pulsars (alongside PSRs B0919+06 and B1859+07); PSR J0803−0942, whose emission is almost completely linearly polarized; and PSRs J1900−0134 and J1945+1211, whose well-defined polarization angle curves place stringent constraints on their emission geometry. We further discuss the detectability of these pulsars by earlier surveys, and highlight lessons learned from our work in carrying out confirmation and monitoring observations of pulsars discovered by a highly sensitive telescope, many of which may be applicable to next-generation pulsar surveys. This paper marks one of the first major releases of FAST-discovered pulsars, and paves the way for future discoveries anticipated from the Commensal Radio Astronomy FAST Survey.

Improved binary pulsar constraints on the parametrized post-Einsteinian framework

The parameterized post-Einsteinian formalism was developed to search for generic deviations from general relativity with gravitational waves. We here present constraints on this framework using Bayesian analysis of a set of binary pulsar observations. In particular, we use measurements of the Keplerian and post-Keplerian parameters of six different binary pulsar systems, and Markov-Chain Monte-Carlo exploration to calculate posteriors on the parameterized post-Einsteinian parameters and derive robust constraints. We find improvements of 1-2 orders of magnitude in the strength of constraints when combining all six observations, relative to what one can achieve when using only the double binary pulsar. We also find that the constraints are robust to covariances with the binary's component masses. The bounds on the parameterized post-Einsteinian framework derived here could be used as a prior in future Bayesian tests of general relativity with gravitational wave observations.

The time and frequency standard system for FAST receivers

This paper reports on the time and frequency standard system for the Five-hundred meter Aperture Spherical radio Telescope (FAST), including the system design, stability measurements and pulsar timing observations. The stability and drift rate of the frequency standard are calculated using 1-year monitoring data. The UTC-NIM Disciplined Oscillator (NIMDO) system improves the system time accuracy and stability to the level of 5 ns. Pulsar timing observations were carried out for several months. The weighted RMS of timing residuals reaches the level of less than 3.0 μs.

Pulsed fraction of super-critical column accretion flows on to neutron stars: Modeling of ultraluminous X-ray pulsars

Abstract We calculate the pulsed fraction (PF) of the super-critical column accretion flows on to magnetized neutron stars (NSs), of which the magnetic axis is misaligned with the rotation axis, based on the simulation results by Kawashima et al. (2016, PASJ, 68, 83). Here, we solve the geodesic equation for light in the Schwarzschild spacetime in order to take into account the light-bending effect. The gravitational redshift and the relativistic Doppler effect from gas motions of the accretion columns are also incorporated. The pulsed emission appears since the observed luminosity, which exceeds the Eddington luminosity for the stellar-mass black holes, periodically changes via precession of the column caused by the rotation of the NS. The PF tends to increase as $\theta _{\rm obs}$ approaches to $\theta _{\rm B}$, where $\theta _{\rm obs}$ and $\theta _{\rm B}$ are the observer’s viewing angle and the polar angle of the magnetic axis measured from the rotation axis, respectively. The maximum PF is around 50%. Also, we find that the PF becomes less than 5% for $\theta _{\rm obs} \lesssim 5^\circ$ or for $\theta _{\rm B} \lesssim 5^\circ$. Our results are consistent with observations of ultraluminous X-ray pulsars (ULXPs) with few exceptions, since the ULXPs mostly exhibit the PF of $\lesssim 50$%. Our present study supports the hypothesis that the ULXPs are powered by the super-critical column accretion on to NSs.

Constraints on the magnetic field in the Galactic halo from globular cluster pulsars

The Galactic magnetic field plays an important role in the evolution of the Galaxy, but its small-scale behaviour is still poorly known. It is also unknown whether it permeates the halo of the Galaxy or not. By using observations of pulsars in the halo globular cluster 47 Tucanae, we probed the Galactic magnetic field at arcsecond scales for the first time and discovered an unexpected large gradient in the component of the magnetic field parallel to the line of sight. This gradient is aligned with a direction perpendicular to the Galactic disk and could be explained by magnetic fields amplified to some 60 {\mu}G within the globular cluster. This scenario supports the existence of a magnetized outflow that extends from the Galactic disk to the halo and interacts with the studied globular cluster.

A method of ground target positioning by observing radio pulsars

We use radio millisecond pulsars (MSPs) to determine the position of the Parkes telescope to explore the feasibility of pulsar navigation for terrestrial application, as well to investigate the relations between pulsar observations and the positional errors for potential X-ray pulsar navigation (XNAV). Different from the analytical algorithm (Han et al. Astrophys. Space Sci. 364, 3, 2018) which derives the relations between observatory coordinates and pulsar timing residuals, a grid search method is used in our work. The method divides the space adjacent to the radio telescope into small grids, from which we form the rms timing residuals and fit pulsar parameters to find the position with minimal χ2 value. Six pulsars from the Parkes Pulsar Timing Array (PPTA) are selected to perform position determinations. The positional errors are analyzed together with the pulsar reference phase, timing residuals, and observation numbers. Using a hybrid database of pulsar timing and cartographic data, we obtained best positional accuracy of around 10 m by PSR J0437 − 4715. To investigate the feasibility of pulsar terrestrial navigation, only few observations from PSR J0437 − 4715 are used for position determinations. The results show that the precision of about 100 m is achievable with only 3 or 4 observations. In this paper, we introduce our method in detail and discuss the positioning performance in different scenarios.

Heating of the real polar cap of radio pulsars

ABSTRACT The heating of the real polar cap surface of radio pulsars by the bombardment of ultra-relativistic charges is studied. The real polar cap is a significantly smaller area within or close by the conventional polar cap, which is encircled by the last open field lines of the dipolar field $\vec{B}_\mathrm{ d}$. It is surrounded by those field lines of the small-scale local surface field $\vec{B}_\mathrm{ s}$ that join the last open field lines of $\vec{B}_\mathrm{ d}$ in a height of ∼105 cm above the cap. As the ratio of radii of the conventional and real polar cap Rdip/Rpc ∼ 10, flux conservation requires Bs/Bd ∼ 100. For rotational periods P ∼ 0.5 s, Bs ∼ 1014 G creates a strong electric potential gap that forms the inner accelerating region (IAR) in which charges gain kinetic energies ∼3 × 1014 eV. This sets an upper limit for the energy that backflowing charges can release as heat in the surface layers of the real polar cap. Within the IAR, which is flown through with a dense stream of extremely energetic charges, no stable atmosphere of hydrogen can survive. Therefore, we consider the polar cap as a solidified ‘naked’ surface consisting of fully ionized iron ions. We discuss the physical situation at the real polar cap, calculate its surface temperatures Ts as functions of Bs and P, and compare the results with X-ray observations of radio pulsars.