Millisecond Pulsars Research Articles

Constraining the Orbital Inclination and Companion Properties of Three Black Widow Pulsars Detected by FAST

Black widows (BWs) are millisecond pulsars ablating their companion stars. The out-flowing material from the companion can block the radio emission of the pulsar, resulting in eclipses. In this paper, we construct a model for the radio eclipse by calculating the geometry of the bow shock between the winds of the pulsar and companion, where the shock shapes the eclipsing medium but had not been described in detail in previous works. The model is further used to explain the variations of the flux density and dispersion measure of three BW pulsars (i.e., PSR B1957+20, J2055+3829, and J2051−0827) detected by the Five-hundred-meter Aperture Spherical radio Telescope. Consequently, we constrained the parameters of the three BW systems such as the inclination angles and true anomalies of the observer as well as the mass-loss rates and wind velocity of the companion stars. With the help of these constraints, it is expected that magnetic fields of companion stars and even masses of pulsars could further be determined as some extra observation can be achieved in the future.

The Third Fermi Large Area Telescope Catalog of Gamma-Ray Pulsars

We present 294 pulsars found in GeV data from the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. Another 33 millisecond pulsars (MSPs) discovered in deep radio searches of LAT sources will likely reveal pulsations once phase-connected rotation ephemerides are achieved. A further dozen optical and/or X-ray binary systems colocated with LAT sources also likely harbor gamma-ray MSPs. This catalog thus reports roughly 340 gamma-ray pulsars and candidates, 10% of all known pulsars, compared to ≤11 known before Fermi. Half of the gamma-ray pulsars are young. Of these, the half that are undetected in radio have a broader Galactic latitude distribution than the young radio-loud pulsars. The others are MSPs, with six undetected in radio. Overall, ≥236 are bright enough above 50 MeV to fit the pulse profile, the energy spectrum, or both. For the common two-peaked profiles, the gamma-ray peak closest to the magnetic pole crossing generally has a softer spectrum. The spectral energy distributions tend to narrow as the spindown power Ė decreases to its observed minimum near 1033 erg s−1, approaching the shape for synchrotron radiation from monoenergetic electrons. We calculate gamma-ray luminosities when distances are available. Our all-sky gamma-ray sensitivity map is useful for population syntheses. The electronic catalog version provides gamma-ray pulsar ephemerides, properties, and fit results to guide and be compared with modeling results.

SAX J1810.8−2609: an outbursting neutron star X-ray binary with persistent spatially coincident radio emission

ABSTRACT Here, we report on joint X-ray and radio monitoring of the neutron star low-mass X-ray binary SAX J1810.8−2609. Our monitoring covered the entirety of its ${\sim }\, 5\,$ month outburst in 2021, revealing a temporal correlation between its radio and X-ray luminosity and X-ray spectral properties consistent with a ‘hard-only’ outburst. During the outburst, the best-fitting radio position shows significant variability, suggesting emission from multiple locations on the sky. Furthermore, our 2023 follow-up observations revealed a persistent, unresolved, steep spectrum radio source ${\sim }\, 2\,$ yr after SAX J1810.8−2609 returned to X-ray quiescence. We investigated potential origins of the persistent emission, which included an unrelated background source, long-lasting jet ejection(s), and SAX J1810 as a transitional millisecond pulsar. While the chance coincidence probability is low (${\lesssim }\, 0.16~{{\ \rm per\ cent}}$), an unrelated background source remains the most likely scenario. SAX J1810.8−2609 goes into outburst every ${\sim }\, 5\,$ yr, so monitoring of the source during its next outburst at higher sensitivities and improved spatial resolutions (e.g. with the Karl G. Jansky Very Large Array or Square Kilometre Array) should be able to identify two components (if the persistent emission originates from a background source). If only one source is observed, this would be strong evidence that the persistent emission is local SAX J1810.8−2609, and future monitoring campaigns should focus on understanding the underlying physical mechanisms, as no neutron star X-ray binary has shown a persistent radio signal absent any simultaneous X-ray emission.

The He star donor channel towards the black widow PSR J1953+1844

ABSTRACT Black widows (BWs) are a type of eclipsing millisecond pulsars (MSPs) with low companion masses ($\lesssim 0.05\, \rm M_\odot$) and tight orbits (<1 d). PSR J1953+1844 (i.e. M71E) is a black window (BW) with the shortest orbital period (∼53 min) ever discovered, which was recently detected by Five-hundred-meter Aperture Spherical radio Telescope. Its companion mass is $\sim 0.01\, \rm M_\odot$ according to its mass function, indicating that the companion may be a hydrogen-deficient star. However, the origin of PSR J1953+1844 is highly unclear. In this paper, we explored the origin of PSR J1953+1844 through the neutron star+He star channel, in which the system can experience ultracompact X-ray binary phase. We found that the He star donor channel can reproduce the characteristics of PSR J1953+1844, indicating that this work provides an alternative formation channel for this source. Meanwhile, the minimum orbital period of BWs formed by this channel is ∼28 min, corresponding to the companion mass of $0.058\, \rm M_\odot$. In addition, we note that even though PSR J1953+1844 has a short orbital period, it cannot be detected by the gravitational wave (GW) observatories like Laser Interferometer Space Antenna, TaiJi, and TianQin. However, we still expect that the BWs with extremely tight orbit produced by this channel are the potential sources of future space-based GW observatories. Moreover, our simulations show that PSR J1953+1844 may eventually evolve into an isolated MSP.

Globular clusters contribute to the nuclear star clusters and galaxy centre γ-ray excess, moderated by galaxy assembly history

ABSTRACT Two unresolved questions at galaxy centres, namely the formation of the nuclear star cluster (NSC) and the origin of the γ-ray excess in the Milky Way (MW) and Andromeda (M31), are both related to the formation and evolution of globular clusters (GCs). They migrate towards the galaxy centre due to dynamical friction, and get tidally disrupted to release the stellar mass content including millisecond pulsars (MSPs), which contribute to the NSC and γ-ray excess. In this study, we propose a semi-analytical model of GC formation and evolution that utilizes the Illustris cosmological simulation to accurately capture the formation epochs of GCs and simulate their subsequent evolution. Our analysis confirms that our GC properties at z = 0 are consistent with observations, and our model naturally explains the formation of a massive NSC in a galaxy similar to the MW and M31. We also find a remarkable similarity in our model prediction with the γ-ray excess signal in the MW. However, our predictions fall short by approximately an order of magnitude in M31, indicating distinct origins for the two γ-ray excesses. Meanwhile, we utilize the catalogue of Illustris haloes to investigate the influence of galaxy assembly history. We find that the earlier a galaxy is assembled, the heavier and spatially more concentrated its GC system behaves at z = 0. This results in a larger NSC mass and brighter γ-ray emission from deposited MSPs.

The dominant mechanism(s) for populating the outskirts of star clusters with neutron star binaries

ABSTRACT It has been argued that heavy binaries composed of neutron stars (NSs) and millisecond pulsars (MSPs) can end up in the outskirts of star clusters via an interaction with a massive black hole (BH) binary expelling them from the core. We argue here, however, that this mechanism will rarely account for such observed objects. Only for primary masses ≲100 M⊙ and a narrow range of orbital separations should a BH–BH binary be both dynamically hard and produce a sufficiently low recoil velocity to retain the NS binary in the cluster. Hence, BH binaries are in general likely to eject NSs from clusters. We explore several alternative mechanisms that would cause NS/MSP binaries to be observed in the outskirts of their host clusters after a Hubble time. The most likely mechanism is a three-body interaction involving the NS/MSP binary and a normal star. We compare to Monte Carlo simulations of cluster evolution for the globular clusters NGC 6752 and 47 Tuc, and show that the models not only confirm that normal three-body interactions involving all stellar-mass objects are the dominant mechanism for putting NS/MSP binaries into the cluster outskirts, but also reproduce the observed NS/MSP binary radial distributions without needing to invoke the presence of a massive BH binary. Higher central densities and an episode of core collapse can broaden the radial distributions of NSs/MSPs and NS/MSP binaries due to three-body interactions, making these clusters more likely to host NSs in the cluster outskirts.

On the formation of eccentric millisecond pulsars by accretion-induced collapse of massive white dwarfs

ABSTRACT Millisecond pulsars (MSPs) are believed to be old neutron stars that have undergone spin-up by the accreting material from the donor. However, the discovery of eccentric MSPs (eMSPs) in the Galactic field challenges such a scenario of producing MSP–white dwarf systems only in a circular orbit. As the orbital periods and companion masses of these eMSPs are located in a narrow range, it is reasonable to postulate that they have the same origin. Although many models have been proposed to interpret the origin of eMSPs, the origin of the narrow range of the orbital period is still an open question. The accretion-induced collapse (AIC) of the oxygen–neon white dwarf is considered to be an important pathway to form MSPs, as it is expected to result in the formation of MSPs in a circular orbit due to tidal circularization. Here we revisited this scenario using binary population synthesis including the specific circularization calculation. Our results indicate that binaries with insufficient circularization in this scenario can evolve into eMSPs. The narrow initial binary parameters required by insufficient circularization can naturally account for the narrow range of the orbital period. Although the evolution of the white dwarf AIC process is not yet well understood, the characteristic of a narrow range in the orbital period of eMSPs can still set constraints on the physics of their evolution.

Radio Pulsars Resonantly Accelerating Electrons

Based on the recently demonstrated resonant wave–wave process, it is shown that electrons can be accelerated to ultra-relativistic energies in the magnetospheres of radio pulsars. The energization occurs via the resonant interaction of the electron wave (described by the Klein–Gordon (KG) equation) moving in unison with an intense electromagnetic (EM) wave; the KG wave/particle continuously draws energy from EM. In a brief recapitulation of the general theory, the high-energy (resonantly enhanced) electron states are investigated by solving the KG equation, minimally coupled to the EM field. The restricted class of solutions that propagate in phase with EM radiation (functions only of ζ=ωt−kz) are explored to serve as a possible basis for the proposed electron energization in the radio pulsars. We show that the wave–wave resonant energization mechanism could be operative in a broad class of radio pulsars with periods ranging from milliseconds to normal values (∼1 s); this could drive the magnetospheric electrons to acquire energies from 100 s of TeVs (millisecond pulsars) to 10 ZeVs (normal pulsars).

Timing Pulsars with CHIME and the NANOGrav 15yr Dataset

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) conducts long-term timing campaigns on large numbers of millisecond pulsars (MSPs). Wideband (WB) timing of the NANOGrav datasets was first done in the 12.5yr dataset, and proved to be extremely helpful for measuring the time-varying dispersion measure, which can significantly limit timing precision. However, we are limited by the fact that NANOGrav only observes most pulsars once per month. In contrast, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Telescope currently observes most NANOGrav MSPs at nearly daily cadence at 600 MHz with a 400 MHz bandwidth. Using WB timing techniques, we combined one year of data taken at CHIME with data over the same time period from the NANOGrav 15yr Dataset for three MSPs.

Multiband extension of the wideband timing technique

ABSTRACT The wideband timing technique enables the high-precision simultaneous estimation of pulsar times of arrival (ToAs) and dispersion measures (DMs) while effectively modelling frequency-dependent profile evolution. We present two novel independent methods that extend the standard wideband technique to handle simultaneous multiband pulsar data incorporating profile evolution over a larger frequency span to estimate DMs and ToAs with enhanced precision. We implement the wideband likelihood using the libstempo python interface to perform wideband timing in the tempo2 framework. We present the application of these techniques to the data set of 14 millisecond pulsars (MSPs) observed simultaneously in Band 3 (300–500 MHz) and Band 5 (1260–1460 MHz) of the upgraded Giant Metrewave Radio Telescope (uGMRT) with a large band gap of 760 MHz as a part of the Indian Pulsar Timing Array (InPTA) campaign. We achieve increased ToA and DM precision and sub-microsecond root mean square post-fit timing residuals by combining simultaneous multiband pulsar observations done in non-contiguous bands for the first time using our novel techniques.

Dataset of Classified Chandra Sources in Globular Clusters

We present a collection of classified X-ray sources in Globular Clusters (GCs) observed by the Chandra X-ray Observatory, including active binaries, cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries. We cross-match the most accurate published positions from multiwavelength observations of these sources to the Chandra Source Catalog Release 2.1, and the HST UV Globular Cluster Survey to extract their multiwavelength properties. The data set can be accessed via an interactive website (https://home.gwu.edu/~kargaltsev/XCLASS_GC) and used as a training data set for machine-learning classification of unidentified X-ray sources in GCs.

Defining Millisecond Pulsars

Millisecond pulsars (MSP) are an important subclass of rotation powered pulsars (RPP), traditionally defined as those with P s < 20–30 ms and B s ≲ 1010 G. We re-examine this definition by applying Gaussian mixture model analysis to identify distinct clusters within the RPP population and find that the MSPs appear to be better demarcated by the condition P s ≲ 16 ms.

PSR J1953+1844 Probably Being the Descendant of an Ultracompact X-Ray Binary

PSR J1953+1844 (i.e., M71E) is a millisecond pulsar in a 53 minute binary orbit discovered by the Five-hundred-meter Aperture Spherical radio Telescope. The mass function from pulsar timing is 2.3 × 10−7 M ⊙. The possible redback origin of this system has been discussed by Pan et al. We discuss here an alternative evolution track for this binary system, namely that PSR J1953+1844 is a descendant of an ultracompact X-ray binary (UCXB), which has a hydrogen-poor donor accreting onto a neutron star (NS) with an orbital period of ≤1 hr. We noticed that some UCXB systems hold an accreting millisecond X-ray pulsar (AMXP) and a donor with a mass of about 0.01 M ⊙. M71E has a very similar orbit to those of AMXPs, indicating that it might be evolved from a UCXB similar to PSR J1653–0158. The companion star of M71E should be significantly bloated and it most probably has a carbon and oxygen composition, otherwise a low inclination angle of the orbit is required for a helium companion. The discovery of this M71E binary system may shed light on when and how an NS in a UCXB turns into a radio pulsar.

The Effect of Irradiation on the Spin of Millisecond Pulsars

A millisecond pulsar (MSP) is an old neutron star (NS) that has accreted material from its companion star, causing it to spin up, which is known as the recycling scenario. During the mass transfer phase, the system manifests itself as an X-ray binary. PSR J1402+13 is an MSP with a spin period of 5.89 ms and a spin period derivative of . These properties make it a notable object within the pulsar population, as MSPs typically exhibit low spin period derivatives. In this paper, we aim to explain how an MSP can possess a high spin period derivative by binary evolution. By utilizing the stellar evolution code MESA, we examine the effects of irradiation on the companion star and the propeller effect on the NS during binary evolution. We demonstrate that irradiation can modify the spin period and mass of an MSP, resulting in a higher spin period derivative. These results suggest that the irradiation effect may serve as a key factor in explaining MSPs with high spin period derivatives.

Spin Equilibrium of Rapidly Spinning Neutron Stars via Transient Accretion

The concept of spin equilibrium due to an interaction between the stellar magnetosphere and a thin, Keplerian accretion disk, and a well-known formula of the corresponding equilibrium spin frequency, provide a key understanding of spin evolution and the distribution of rapidly spinning neutron stars, viz., millisecond pulsars. However, this concept and formula are for stable accretion, but the mass transfer to most accreting millisecond pulsars is transient and the accretion rate evolves by orders of magnitude during an outburst. In this short and focussed review, we briefly discuss a relatively new concept of the spin equilibrium condition and a new formula for the equilibrium spin frequency for transiently accreting millisecond pulsars. We also review a new method to estimate this equilibrium spin frequency for observed transiently accreting millisecond pulsars, even when a pulsar has not yet attained the spin equilibrium. These will be crucial to probe the spin evolution and distribution of millisecond pulsars, and should also be applicable to all magnetic stars transiently accreting via a thin, Keplerian accretion disk.

Polarized radiation from an accretion shock in accreting millisecond pulsars using exact Compton scattering formalism

Pulse profiles of accreting millisecond pulsars can be used to determine neutron star (NS) parameters, such as their masses and radii, and therefore provide constraints on the equation of state of cold dense matter. Information obtained by the Imaging X-ray Polarimetry Explorer (IXPE) can be used to decipher pulsar inclination and magnetic obliquity, providing ever tighter constraints on other parameters. In this paper, we develop a new emission model for accretion-powered millisecond pulsars based on thermal Comptonization in an accretion shock above the NS surface. The shock structure was approximated by an isothermal plane-parallel slab and the Stokes parameters of the emergent radiation were computed as a function of the zenith angle and energy for different values of the electron temperature, the Thomson optical depth of the slab, and the temperature of the seed blackbody photons. We show that our Compton scattering model leads to a significantly lower polarization degree of the emitted radiation compared to the previously used Thomson scattering model. We computed a large grid of shock models, which can be combined with pulse profile modeling techniques both with and without polarization included. In this work, we used the relativistic rotating vector model for the oblate NS in order to produce the observed Stokes parameters as a function of the pulsar phase. Furthermore, we simulated the data to be produced by IXPE and obtained constraints on model parameters using nested sampling. The developed methods can also be used in the analysis of the data from future satellites, such as the enhanced X-ray Timing and Polarimetry mission.

The second data release from the European Pulsar Timing Array

Pulsar timing arrays offer a probe of the low-frequency gravitational wave spectrum (1–100 nHz), which is intimately connected to a number of markers that can uniquely trace the formation and evolution of the Universe. We present the dataset and the results of the timing analysis from the second data release of the European Pulsar Timing Array (EPTA). The dataset contains high-precision pulsar timing data from 25 millisecond pulsars collected with the five largest radio telescopes in Europe, as well as the Large European Array for Pulsars. The dataset forms the foundation for the search for gravitational waves by the EPTA, presented in associated papers. We describe the dataset and present the results of the frequentist and Bayesian pulsar timing analysis for individual millisecond pulsars that have been observed over the last ~25 yr. We discuss the improvements to the individual pulsar parameter estimates, as well as new measurements of the physical properties of these pulsars and their companions. This data release extends the dataset from EPTA Data Release 1 up to the beginning of 2021, with individual pulsar datasets with timespans ranging from 14 to 25 yr. These lead to improved constraints on annual parallaxes, secular variation of the orbital period, and Shapiro delay for a number of sources. Based on these results, we derived astrophysical parameters that include distances, transverse velocities, binary pulsar masses, and annual orbital parallaxes.

Discovery and Timing of Millisecond Pulsars with the Arecibo 327 MHz Drift-scan Survey

We present the discovery and timing solutions of four millisecond pulsars (MSPs) discovered in the Arecibo 327 MHz Drift-Scan Pulsar Survey. Three of these pulsars are in binary systems, consisting of a redback (PSR J2055+1545), a black widow (PSR J1630+3550), and a neutron star–white dwarf binary (PSR J2116+1345). The fourth MSP, PSR J2212+2450, is isolated. We present the multiyear timing solutions as well as polarization properties across a range of radio frequencies for each pulsar. We perform a multiwavelength search for emission from these systems and find an optical counterpart for PSR J2055+1545 in Gaia DR3, as well as a gamma-ray counterpart for PSR J2116+1345 with the Fermi-LAT telescope. Despite the close colocation of PSR J2055+1545 with a Fermi source, we are unable to detect gamma-ray pulsations, likely due to the large orbital variability of the system. This work presents the first two binaries found by this survey with orbital periods shorter than a day; we expect to find more in the 40% of the survey data that have yet to be searched.

Optical Identification of the Shortest-period Spider Pulsar System M71E

M71E is a spider pulsar (i.e., a millisecond pulsar with a tight binary companion) with the shortest known orbital period of P = 53.3 minutes, which was discovered by Pan et al. Their favored evolutionary model suggests that it bridges between two types of spider pulsars, namely, it descended from a “redback” and will become a “black widow.” Using Hubble Space Telescope (HST) archival imaging data, we report the first optical identification of its companion COM-M71E. The HST and pulsar timing coordinates are in excellent agreement (within ∼10 mas). If M71E is associated with the globular cluster M71, our measured brightness of COM-M71E (m F606W ≈ 25.3) is broadly consistent with the expectation from Pan et al.'s preferred binary evolutionary model of a stripped dwarf companion, while it is also compatible with an ultralow-mass degenerate companion. Future multiwavelength photometric and spectroscopic observations can characterize the companion and test the evolutionary scenarios.

The second data release from the European Pulsar Timing Array

We present the results of the search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies using the second data release of the European Pulsar Timing Array (EPTA) for 25 millisecond pulsars and a combination with the first data release of the Indian Pulsar Timing Array (InPTA). A robust GWB detection is conditioned upon resolving the Hellings-Downs angular pattern in the pairwise cross-correlation of the pulsar timing residuals. Additionally, the GWB is expected to yield the same (common) spectrum of temporal correlations across pulsars, which is used as a null hypothesis in the GWB search. Such a common-spectrum process has already been observed in pulsar timing data. We analysed (i) the full 24.7-year EPTA data set, (ii) its 10.3-year subset based on modern observing systems, (iii) the combination of the full data set with the first data release of the InPTA for ten commonly timed millisecond pulsars, and (iv) the combination of the 10.3-year subset with the InPTA data. These combinations allowed us to probe the contributions of instrumental noise and interstellar propagation effects. With the full data set, we find marginal evidence for a GWB, with a Bayes factor of four and a false alarm probability of 4%. With the 10.3-year subset, we report evidence for a GWB, with a Bayes factor of 60 and a false alarm probability of about 0.1% (≳3σ significance). The addition of the InPTA data yields results that are broadly consistent with the EPTA-only data sets, with the benefit of better noise modelling. Analyses were performed with different data processing pipelines to test the consistency of the results from independent software packages. The latest EPTA data from new generation observing systems show non-negligible evidence for the GWB. At the same time, the inferred spectrum is rather uncertain and in mild tension with the common signal measured in the full data set. However, if the spectral index is fixed at 13/3, the two data sets give a similar amplitude of (2.5 ± 0.7) × 10−15 at a reference frequency of 1 yr−1. Further investigation of these issues is required for reliable astrophysical interpretations of this signal. By continuing our detection efforts as part of the International Pulsar Timing Array (IPTA), we expect to be able to improve the measurement of spatial correlations and better characterise this signal in the coming years.