Binary Pulsar System Research Articles

Diagnosing pulsar winds in black-widow, redback, and other binary millisecond pulsar systems

ABSTRACTBinary systems composed of a recycled millisecond pulsar and a stellar companion in close orbit could be excellent sites to diagnose pulsar winds. In such systems, the pulsar outflow irradiates and heats up the companion atmosphere, which can lead to the observation of strong day/night modulations in temperature. We demonstrate with particle shower simulations that the particle energy of the wind affects the heating depth in the atmosphere: the wind heat can be deposited above or below the photosphere, leading to different signatures in the observed spectra. We apply our method to four specific systems: We find that systems with cool night-side companions showing strong temperature variations can give interesting lower limits on the particle energy in the winds. In particular, if the companion night side of PSR B1957+20 were to be suddenly irradiated, deep heating would only take place if particles with energy >100 TeV were present. Observational evidence of deep heating in this system thus suggests that (i) such particles exist in the pulsar wind and/or (ii) binary evolution non-trivially takes the companion to the observed temperature asymmetry. Besides, the observed temperature difference can be maintained only with particle energies of the order of 100 MeV.

J0453+1559: A Neutron Star–White Dwarf Binary from a Thermonuclear Electron-capture Supernova?

Abstract The compact binary radio pulsar system J0453+1559 consists of a recycled pulsar as primary component of 1.559(5) M ⊙ and an unseen companion star of 1.174(4) M ⊙. Because of the relatively large orbital eccentricity of e = 0.1125, it was argued that the companion is a neutron star (NS), making it the NS with the lowest accurately determined mass to date. However, a direct observational determination of the nature of the companion is currently not feasible. Moreover, state-of-the-art stellar evolution and supernova modeling are contradictory concerning the possibility of producing such a low-mass NS remnant. Here we challenge the NS interpretation by reasoning that the lower-mass component could instead be a white dwarf born in a thermonuclear electron-capture supernova (tECSN) event, in which oxygen–neon deflagration in the degenerate stellar core of an ultra-stripped progenitor ejects several 0.1 M ⊙ of matter and leaves a bound ONeFe white dwarf as the second-formed compact remnant. We determine the ejecta mass and remnant kick needed in this scenario to explain the properties of PSR J0453+1559 by a NS–white dwarf system. More work on tECSNe is needed to assess the viability of this scenario.

Light curve analysis of a transitional millisecond pulsar PSR J1023+0038 with Kepler K2 mission

PSR J1023+0038 has been known as a transitional pulsar-low-mass-X-ray binary (LMXB) system, which provides crucial information in understanding the evolution of pulsars in binary systems. We report analysis of 80-day continuous optical light curve of PSR J1023+0038 with the Kepler observations during K2 mission. The light curve was observed from May to August 2017, where the system appeared to be in its LMXB state. At ∼30 minutes time resolution, the data were fitted with a Gaussian window function, which generated a smoothed light curve with 100, 000 from 3545 raw data input. The time at the maxima and minima of the light curve was measured and compared with the orbit ephemeris predicted from a quadratic polynomial function published by previous work. We found that at MJD ∼57935 there appeared to be a constant delay of ∼1.06 hr added to the orbit, which then gradually recovered to the expected period at MJD ∼57965. Possible causes have been proposed to explain such delay includes a third body involvement or hot spots on the star.

Modelling the interaction between relativistic and non-relativistic winds in binary pulsar systems: strong magnetization of the pulsar wind

ABSTRACT We present a numerical study of the properties of the flow produced by the collision of a magnetized anisotropic pulsar wind with the circumbinary environment. We focus on studying the impact of the high wind magnetization on the geometrical structure of the shocked flow. This work is an extension of our earlier studies that focused on a purely hydrodynamic interaction and weak wind magnetization. We consider the collision in the axisymmetric approximation, i.e. the pulsar rotation axis is assumed to be oriented along the line between the pulsar and the optical star. The increase of the magnetization results in the expansion of the opening cone in which the shocked pulsar wind propagates. This effect is explained in the frameworks of the conventional theory of collimation of magnetized winds. This finding has a direct implication for scenarios that involve Doppler boosting as the primary mechanism behind the GeV flares detected with the Fermi Large Area Telescope from PSR B1259−63/LS 2883. The maximum enhancement of the apparent emission is determined by the ratio of 4$\pi$ to the solid in which the shocked pulsar wind propagates. Our simulations suggest that this enhancement factor is decreased by the impact of the magnetic field.

On the weak magnetic field of millisecond pulsars: does it decay before accretion?

ABSTRACT Millisecond pulsars (MSPs) are old, fast spinning neutron stars (NSs) thought to have evolved from classical pulsars in binary systems, where the rapid rotation is caused by the accretion of matter and angular momentum from their companion. During this transition between classical and MSPs, there is a magnetic field reduction of ∼4 orders of magnitude, which is not well understood. According to the standard scenario, the magnetic field is reduced as a consequence of accretion, either through ohmic dissipation or through screening by the accreted matter. We explored an alternative hypothesis in which the magnetic field is reduced through ambipolar diffusion before the accretion. This is particularly effective during the long epoch in which the pulsar has cooled, but has not yet started accreting. This makes the final magnetic field dependent on the evolution time of the companion star and thus its initial mass. We use observed binary systems to constrain the time available for the magnetic field decay based on the current pulsar companion: a helium white dwarf, a carbon–oxygen white dwarf, or another NS. Based on a simplified model without baryon pairing, we show that the proposed process agrees with the general distribution of observed magnetic field strengths in binaries, but is not able to explain some mildly recycled pulsars where no significant decay appears to have occurred. We discuss the possibility of other formation channels for these systems and the conditions under which the magnetic field evolution would be set by the NS crust rather than the core.

Relativistic Shapiro delay measurements of an extremely massive millisecond pulsar

Despite its importance to our understanding of physics at supranuclear densities, the equation of state (EoS) of matter deep within neutron stars remains poorly understood. Millisecond pulsars (MSPs) are among the most useful astrophysical objects in the Universe for testing fundamental physics, and place some of the most stringent constraints on this high-density EoS. Pulsar timing - the process of accounting for every rotation of a pulsar over long time periods - can precisely measure a wide variety of physical phenomena, including those that allow the measurement of the masses of the components of a pulsar binary system (Lorimer & Kramer 2005). One of these, called relativistic Shapiro delay (Shapiro 1964), can yield precise masses for both an MSP and its companion; however, it is only easily observed in a small subset of high-precision, highly inclined (nearly edge-on) binary pulsar systems. By combining data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 12.5-year data set with recent orbital-phase-specific observations using the Green Bank Telescope, we have measured the mass of the MSP J0740+6620 to be $\mathbf{2.14^{+0.10}_{-0.09}}$ solar masses (68.3% credibility interval; 95.4% credibility interval is $\mathbf{2.14^{+0.20}_{-0.18}}$ solar masses). It is highly likely to be the most massive neutron star yet observed, and serves as a strong constraint on the neutron star interior EoS.

The Discovery of Six Recycled Pulsars from the Arecibo 327 MHz Drift-Scan Pulsar Survey

Abstract Recycled pulsars are old (≳108 yr) neutron stars that are descendants from close, interacting stellar systems. In order to understand their evolution and population, we must find and study the largest possible number of recycled pulsars in a way that is as unbiased as possible. In this work, we present the discovery and timing solutions of five recycled pulsars in binary systems (PSRs J0509+0856, J0709+0458, J0732+2314, J0824+0028, and J2204+2700) and one isolated millisecond pulsar (PSR J0154+1833). These were found in data from the Arecibo 327 MHz Drift-Scan Pulsar Survey (AO327). All these pulsars have a low dispersion measure (DM; ), and have a DM-determined distance of ≲3 kpc. Their timing solutions, with data spans ranging from 1 to ∼7 yr, include precise estimates of their spin and astrometric parameters, and for the binaries, precise estimates of their Keplerian binary parameters. Their orbital periods range from about 4 to 815 days and the minimum companion masses (assuming a pulsar mass of 1.4 M ⊙) range from ∼0.06 to 1.11 M ⊙. For two of the binaries we detect post-Keplerian parameters; in the case of PSR J0709+0458 we measure the component masses but with a low precision, in the not too distant future the measurement of the rate of advance of periastron and the Shapiro delay will allow very precise mass measurements for this system. Like several other systems found in the AO327 data, PSRs J0509+0854, J0709+0458, and J0732+2314 are now part of the NANOGrav timing array for gravitational wave detection.

The binary millisecond pulsar PSR J1023+0038 – II. Optical spectroscopy

ABSTRACT We present time-resolved optical spectroscopy of the ‘redback’ binary millisecond pulsar system PSR J1023+0038 during both its radio pulsar (2009) and accretion disc states (2014 and 2016). We provide observational evidence for the companion star being heated during the disc state. We observe a spectral type change along the orbit, from ∼G5 to ∼F6 at the secondary star’s superior and inferior conjunction, respectively, and find that the corresponding irradiating luminosity can be powered by the high-energy accretion luminosity or the spin-down luminosity of the neutron star. We determine the secondary star’s radial velocity semi-amplitude from the metallic (primarily Fe and Ca) and Hα absorption lines during these different states. The metallic and Hα radial velocity semi-amplitude determined from the 2009 pulsar-state observations allows us to constrain the secondary star’s true radial velocity K2 = 276.3 ± 5.6 km s−1 and the binary mass ratio q = 0.137 ± 0.003. By comparing the observed metallic and Hα absorption-line radial velocity semi-amplitudes with model predictions, we can explain the observed semi-amplitude changes during the pulsar state and during the pulsar/disc-state transition as being due to different amounts of heating and the presence of an accretion disc, respectively.

Pressure Balance and Intrabinary Shock Stability in Rotation-powered-state Redback and Transitional Millisecond Pulsar Binary Systems

Abstract A number of low-mass millisecond pulsar (MSP) binaries in their rotation-powered state exhibit double-peaked X-ray orbital modulation centered at inferior pulsar conjunction. This state, which has been known to persist for years, has recently been interpreted as emission from a shock that enshrouds the pulsar. However, the pressure balance for such a configuration is a crucial unresolved issue. We consider two scenarios for pressure balance: a companion magnetosphere and stellar mass loss with gas dominance. It is found that the magnetospheric scenario requires several kilogauss poloidal fields for isobaric surfaces to enshroud the MSP, as well as for the magnetosphere to remain stable if there is significant mass loss. For the gas-dominated scenario, it is necessary that the companion wind loses angular momentum prolifically as an advection- or heating-dominated flow. Thermal bremsstrahlung cooling in the flow may be observable as a UV to soft X-ray component independent of orbital phase if the mass rate is high. We formulate the general requirements for shock stability against gravitational influences in the pulsar rotation-powered state for the gas-dominated scenario. We explore stabilizing mechanisms, principally irradiation feedback, which anticipates correlated shock emission and companion variability and predicts F γ /F X ≲ 14 for the ratio of pulsar magnetospheric γ-ray to total shock soft-to-hard X-ray fluxes. This stability criterion implies an unbroken extension of X-ray power-law emission to hundreds of keV for some systems. We explore observational discriminants between the gas-dominated and magnetospheric scenarios, motivating contemporaneous radio through γ-ray monitoring of these systems.

The High Time Resolution Universe survey – XIV. Discovery of 23 pulsars through GPU-accelerated reprocessing

We have performed a new search for radio pulsars in archival data of the intermediate and high Galactic latitude parts of the Southern High Time Resolution Universe pulsar survey. This is the first time the entire dataset has been searched for binary pulsars, an achievement enabled by GPU-accelerated dedispersion and periodicity search codes nearly 50 times faster than the previously used pipeline. Candidate selection was handled entirely by a Machine Learning algorithm, allowing for the assessment of 17.6 million candidates in a few person-days. We have also introduced an outlier detection algorithm for efficient radio-frequency interference (RFI) mitigation on folded data, a new approach that enabled the discovery of pulsars previously masked by RFI. We discuss implications for future searches, particularly the importance of expanding work on RFI mitigation to improve survey completeness. In total we discovered 23 previously unknown sources, including 6 millisecond pulsars and at least 4 pulsars in binary systems. We also found an elusive but credible redback candidate that we have yet to confirm.

Testing velocity-dependent CPT -violating gravitational forces with radio pulsars

In the spirit of effective field theory, the Standard-Model Extension (SME) provides a comprehensive framework to systematically probe the possibility of Lorentz/CPT violation. In the pure gravity sector, operators with mass dimension larger than 4, while in general being advantageous to short-range experiments, are hard to investigate with systems of astronomical size. However, there is exception if the leading-order effects are CPT-violating and velocity-dependent. Here we study the lowest-order operators in the pure gravity sector that violate the CPT symmetry with carefully chosen relativistic binary pulsar systems. Applying the existing analytical results to the dynamics of a binary orbit, we put constraints on various coefficients for Lorentz/CPT violation with mass dimension 5. These constraints, being derived from the post-Newtonian dynamics for the first time, are complementary to those obtained from the kinematics in the propagation of gravitational waves.

A Fourier Domain “Jerk” Search for Binary Pulsars

Abstract While binary pulsar systems are fantastic laboratories for a wide array of astrophysics, they are particularly difficult to detect. The orbital motion of the pulsar changes its apparent spin frequency over the course of an observation, essentially “smearing” the response of the time series in the Fourier domain. We review the Fourier domain acceleration search (FDAS), which uses a matched filtering algorithm to correct for this smearing by assuming constant acceleration for a small enough portion of the orbit. We discuss the theory and implementation of a Fourier domain “jerk” search, developed as part of the PRESTO software package, which extends the FDAS to account for a linearly changing acceleration, or constant orbital jerk, of the pulsar. We test the performance of our algorithm on archival Green Bank Telescope observations of the globular cluster Terzan 5, and show that while the jerk search has a significantly longer runtime, it improves search sensitivity to binaries when the observation duration is 5%–15% of the orbital period. Finally, we present the jerk-search-enabled detection of Ter5am (PSR J1748−2446am), a new highly accelerated pulsar in a compact, eccentric, and relativistic orbit, with a likely pulsar mass of M ⊙.

On the timing behaviour of PSR B1259–63 under the propeller torque from a transient accretion disc

The $\gamma$-ray pulsar binary system PSR B1259-63 flares in GeV after each periastron. The origin of those flares is still under debate. Recently, [2017ApJ...844..114Y] proposed a mechanism that might explain the GeV flares. In that model, a transient accretion disc is expected to be formed from the matter which was gravity-captured by the neutron star from the main sequence companion's circumstellar disc. The transient accretion disc exerts a spin down torque on the neutron star (propeller effect), which might be traceable via pulsar timing observation of PSR B1259-63. In this paper, we phenomenologically consider the propeller effect with a parameter $\chi$, which describes the coupling between the disc matter and the neutron star. Comparing the expected timing residuals against the recent observation in [2014MNRAS.437.3255S], we conclude that the angular momentum transfer is very weak (with the coupling parameter $\chi\le10^{-4}$).

Constraining f(R) gravity in solar system, cosmology and binary pulsar systems

The f(R) gravity can be cast into the form of a scalar–tensor theory, and scalar degree of freedom can be suppressed in high-density regions by the chameleon mechanism. In this article, for the general f(R) gravity, using a scalar–tensor representation with the chameleon mechanism, we calculate the parametrized post-Newtonian parameters γ and β, the effective gravitational constant Geff, and the effective cosmological constant Λeff. In addition, for the general f(R) gravity, we also calculate the rate of orbital period decay of the binary system due to gravitational radiation. Then we apply these results to specific f(R) models (Hu–Sawicki model, Tsujikawa model and Starobinsky model) and derive the constraints on the model parameters by combining the observations in solar system, cosmological scales and the binary systems.

Radius of the neutron star magnetosphere during disk accretion

The dependence of the spin frequency derivative $\dot{\nu}$ of accreting neutron stars with a strong magnetic field (X-ray pulsars) on the mass accretion rate (bolometric luminosity, $L_{bol}$) has been investigated for eight transient pulsars in binary systems with Be stars. Using data from the Fermi/GBM and Swift/BAT telescopes, we have shown that for seven of the eight systems the dependence $\dot{\nu}$ can be fitted by the model of angular momentum transfer through an accretion disk, which predicts the relation $\dot{\nu}\sim L^{6/7}_{bol}$. Hysteresis in the dependence $\dot{\nu}(L_{bol})$ has been confirmed in the system V 0332+53 and has been detected for the first time in the systems KS 1947+300, GRO J1008-57, and 1A 0535+26. The radius of the neutron star magnetosphere in all of the investigated systems have been estimated. We show that this quantity varies from pulsar to pulsar and depends strongly on the analytical model and the estimates for the neutron star and binary system parameters.

Dissipation of the striped pulsar wind

Rapidly rotating neutron stars blow a relativistic, magnetized wind mainly composed of electron-positron pairs. The free expansion of the wind terminates far from the neutron star where a weakly magnetized pulsar wind nebula forms, implying efficient magnetic dissipation somewhere upstream. The wind current sheet that separates the two magnetic polarities is usually considered as the most natural place for magnetic dissipation via relativistic reconnection, but its efficiency remains an open question. Here, the goal of this work is to revisit this issue in light of the most recent progress in the understanding of reconnection and pulsar electrodynamics. We perform large two-dimensional particle-in-cell simulations of the oblique rotator to capture the multi-scale evolution of the wind. We find that the current sheet breaks up into a dynamical chain of magnetic islands separated by secondary thin current sheets. The sheet thickness increases linearly with radius while the Poynting flux decreases monotonically as reconnection proceeds. The radius of complete annihilation of the stripes is given by the plasma multiplicity parameter at the light cylinder. Current starvation within the sheets does not occur before complete dissipation as long as there is enough charges where the sheets form. Particles are efficiently heated up to a characteristic energy set by the magnetization parameter at the light cylinder. Energetic pulsed synchrotron emission peaks close to the light cylinder, and presents sub-pulse variability associated with the formation of plasmoids in the sheet. This study suggests that the striped component of the wind dissipates far before reaching the termination shock in isolated pulsars, even in very-high-multiplicity systems such as the Crab pulsar. Pulsars in binary systems may provide the best environments to study magnetic dissipation in the wind.

The First Direct Detection of Gravitational Waves Opens a Vast New Frontier in Astronomy

The first direct detection of gravitational waves (GWs), announced on 11 February 2016, has opened a vast new frontier in astronomy. Albert Einstein predicted the existence of these waves about a century ago as a consequence of his general theory of relativity. Radio astronomy observations of the binary pulsar system PSR 1913 + 16 over a 20 year period beginning in 1975 provided strong observational evidence that gravitational waves carried energy away from the orbits of neutron stars at precisely the level predicted by general relativity (GR). This relentless conversion of orbital energy into gravitational wave energy causes binary orbits to decay until the objects eventually collide and merge. The frontier of precision measurement science, using laser interferometers, was pushed for more than four decades to achieve this first direct detection, marking a milestone in experimental physics and engineering. Even more significantly, this milestone also opens a new window onto our universe and a completely new kind of astronomy to explore.

The nature of the bimodal luminosity distribution of ultraluminous X-ray pulsars

The mechanism that can be responsible for the bimodal luminosity distribution of super-Eddington X-ray pulsars in binary systems is pointed out. The transition from the high to low state of these objects is explained by accretion flow spherization due to the radiation pressure at certain (high) accretion rates. The transition between the states can be associated with a gradual change in the accretion rate. The complex behavior of the recently discovered ultraluminous X-ray pulsars M 82 X-2, NGC 5907 ULX-1, and NGC 7793 P13 is explained by the proposed mechanism. The proposed model also naturally explains the measured spinup of the neutron star in these pulsars, which is slower than the expected one by several times.

Novel directed search strategy to detect continuous gravitational waves from neutron stars in low- and high-eccentricity binary systems

We describe a novel, very fast and robust, directed search incoherent method for periodic gravitational waves (GWs) from neutron stars in binary systems. As directed search, we assume the source sky position to be known with enough accuracy, but all other parameters are supposed to be unknown. We exploit the frequency-modulation due to source orbital motion to unveil the signal signature by commencing from a collection of time and frequency peaks. We validate our pipeline adding 131 artificial continuous GW signals from pulsars in binary systems to simulated detector Gaussian noise, characterized by a power spectral density Sh = 4x10^-24 Hz^-1/2 in the frequency interval [70, 200] Hz, which is overall commensurate with the advanced detector design sensitivities. The pipeline detected 128 signals, and the weakest signal injected and detected has a GW strain amplitude of ~10^-24, assuming one month of gapless data collected by a single advanced detector. We also provide sensitivity estimations, which show that, for a single- detector data covering one month of observation time, depending on the source orbital Doppler modulation, we can detect signals with an amplitude of ~7x10^-25. By using three detectors, and one year of data, we would easily gain more than a factor 3 in sensitivity, translating into being able to detect weaker signals. We also discuss the parameter estimate proficiency of our method, as well as computational budget, which is extremely cheap. In fact, sifting one month of single-detector data and 131 Hz-wide frequency range takes roughly 2.4 CPU hours. Due to the high computational speed, the current procedure can be readily applied in ally-sky schemes, sieving in parallel as many sky positions as permitted by the available computational power.

The relaxed Einstein equations in the context of a mixed UV-IR modified theory of gravity

In this article we will modify the Einstein field equations by promoting Newton’s constant G to a covariant differential operator composed of two terms which operate in different energy regimes (IR and UV). The IR term inside the covariant differential operator acts like a high-pass filter with a macroscopic distance filter scale and effectively degravitates energy sources characterized by wavelengths larger than the filter scale. While this term is predominant for cosmological energy processes it is almost inessential on astrophysical scales where the UV contribution inside leads to much stronger deviations compared to GR. In the context of this particular theory of gravity we work out the effective relaxed Einstein equations, the effective 1.5 post-Newtonian near zone mass for n-body systems as well as the IR and UV modified Schwarzschild metrics. We use these results in the context of the Double Pulsar binary system and observe that we recover, in the limit of vanishing UV-IR modification parameters, the corresponding general relativistic results.