Pulsar Astronomy

Pulsars are a type of fast-rotating magnetized neutron star that emits beamed multi-wavelength electromagnetic radiation [1]. These compact objects, which typically have a mass of 1.4–2$M_{\odot }$ within the radius about of 10–15 km [2,3], are generally thought to be produced during supernova explosions [4]. Since the first radio pulsar was discovered in 1967 [5], more than 4000 pulsars have been located with different telescopes, from radios to X-rays and gamma rays [6,7]. The known pulsars typically have a rotation period distributed from about 1.4 ms to 20 s [8,9]. They are characterized by a strong magnetic field on the surface, e.g., a strength of ∼${10}^{8–9}$ G for millisecond pulsars [10], around ${10}^{12–13}$ G for normal radio pulsars [1], and up to ${10}^{14–15}$ G for magnetars [11]. In addition, the inner structure and composition of neutron stars are still uncertain, which is connected to their micro-physics, e.g., their hyperon-dominated matter, deconfined quark matter, superfluidity, and superconductivity [12]. Binary neutron stars are promising gravitational wave sources [13] and can test gravitational theories, setting strong limits on deviations from general relativity [14,15]. Therefore, pulsars provide a natural lab for the strong magnetic fields, dense-matter physics, and strong gravity of the universe.

In recent years, studies on pulsars have also made significant progress in China with the development of new telescopes, especially the Shanghai Tianma 65 m telescope and the Five-hundred-meter Aperture Spherical radio Telescope (FAST). FAST, as the biggest single-dish radio telescope in the world, has the highest sensitivity on radio observations in the L-band (1.05–1.45 GHz) [16]. At present, FAST has reported the discovery of more than 1000 new radio pulsars in our Galaxy [7], including millisecond pulsars [17], special binary neutron star systems [18], and a large number of radio pulsars in globular clusters [19]. In the future, high-sensitivity FAST surveys will be expected to discover more than 5000 radio pulsars, possibly including extra-galactic pulsars. The notable increase in the pulsars and new populations would also provide challenges for pulsar physics, evolution, and emission mechanisms in the magnetosphere. Therefore, we proposed this Special Issue to evaluate the progress and status of pulsar observations, emission mechanisms, and neutron star physics, which would be helpful for young scientists devoted to pulsar astronomy and related realms of astrophysics.

This Special Issue of Universe, “Pulsar Astronomy”, published in 2024, collected reviews and research articles on observations and theories related to pulsars, focusing on the recent progress of multiple dimensions in pulsar astronomy, including the structure and internal composition of pulsars (neutron stars or strange stars), observational properties of pulsars in different bands, millisecond pulsars in binaries, X-ray pulsars, pulsar timing and gravitational waves, and radiation mechanisms of pulsars. It should be noted that this collection does not include X-ray pulsars in accretion binary systems, which have different energy power and radiation processes from rotation-powered pulsars [20]. Accreting X-ray pulsars can produce a much wider spin period of neutron stars (e.g., $P_{\mathrm{spin}}>1000$ s) [21], and cyclotron resonant scattering features such as their special X-ray spectral properties provide a unique tool to directly measure the surface magnetic field (B∼${10}^{12–13}$ G) of these accreting neutron stars [22,23]. This Special Issue collected a total of five papers, including three review papers and two research articles (see the list of contributions in the References). We summarize the main contents and implications of these contribution in the following.

Millisecond pulsars (MSPs) have a very stable rotation period, with spin periods from ∼1.4 to 30 ms and very small period derivatives of $\dot{P}\le {10}^{-18}$ s ${\mathrm{s}}^{-1}$. Thus, high-precision observations of these MSPs allow them to play important roles in positioning, navigation, and timing. Zheng et al. [24] performed an X-ray timing analysis of six MSPs based on the 6-year observations of the Neutron Star Interior Composition Explorer (NICER). Compared with radio bands, X-ray timing aboard spacecrafts offers unique advantages due to the effects of negligible dispersion measures (DMs), miniaturized detectors, and independence from ground-based systems, so it can be used for future space navigation. The authors obtained the timing precision on the order of ${10}^{-14}$ for six pulsars over 1000 days. Further investigation of the red noise and efforts to mitigate or minimize its impact on timing residuals will further enhance the utility of this approach. Therefore, their results show the feasibility of X-ray pulsar based timekeeping as a promising application for space missions with the long-term stability of MPSs.

The spin period detected in pulsars also shows some irregularities in timing properties, e.g., glitches and noises, which will provide more physical information on neutron stars’ internal structure and rotation variation. Specially, the origin of pulsar timing noise is uncertain. Yuan et al. [25] presented the timing noise of 85 normal pulsars observed with the Nanshan 26 m radio telescope, covering about 13 years. By fitting the pulsar ephemeris and characterizing the timing noise, the red noise is evident in normal pulsars. For the observed sample, about half of them have timing residuals attributed to rotational irregularities, and the red noise in normal pulsars may originate from a random walk in the spin frequency or spin-down rate. The Nanshan telescope also reported a different spin-down rate for the pulsar PSR J1320-351 from previous observations, which could be related to pulsar position and timing noise uncertainties.

The Shanghai Tianma Radio Telescope (TMRT), constructed in 2013, covers a very broad radio observational band from 1.3 to 50 GHz; it has been be a good instrument for pulsar studies, with particular advantages at high frequencies. Yan et al. [26] wrote a review of pulsar-related observations with TMRT, e.g., long-term pulsar timing, magnetar monitoring, high-frequency pulsar observations, pulsar interstellar scintillation, and very-long-baseline interferometry (VLBI) observations. In this review, the pulsar observation systems equipped in TMRT are also briefly introduced. Due to its high sensitivity and simultaneous dual-frequency observation capacity, in future TMRT will be expected to make more discoveries and contributions to pulsar astronomy.

The internal composition and structure of pulsars are still debated, but they are intriguing for both the physical and astronomical community. The central region of pulsars could contain normal baryon matter, e.g., neutrons, protons, and, possibly, exotic matter like condensed mesons, hyperons, or free quarks, which establishes strong connections between particle physics, nuclear physics, and astrophysics. Li et al. [27] reviewed the equation of state (EOS) of dense matter for pulsars, introducing three possible objects in nature: neutron stars, strange quark stars, and strangeon stars. These compact stars should have different global structures (mass–radius relations) and elastic properties, which may be resolved in observations. The review article concentrates on the asteroseismology of these compact stars, their oscillation modes, and related physics. This astronomical phenomena is a promising tool to probe the EOS and structure. The authors have suggested that the detection of quasi-periodic oscillations from giant flares to soft gamma-ray repeaters and corresponding gravitational waves will provide key information about the elastic properties and internal compositions of stars.

Pulsars are interesting to us because they contain mysteries in both internal physics and radiation processes from outside the magnetosphere. Cao et al. [28] presented a detailed review on the advances in gamma-ray pulsar observations and provided an introduction to the electrodynamics of pulsar magnetospheres and different radiation models, including the vacuum model and plasma-filled model. The present important progress comes from recent numerical simulations which support the pulsar magnetosphere scenario being a near force-free field structure with particle acceleration near the light cylinder. However, the emission models and emission mechanisms in pulsar magnetospheres are still difficult to distinguish based on the observations of light curves and spectra. Polarization will provide new information on the magnetosphere. Thus, the authors have suggested that, in the near future, multi-wavelength light curves, spectra, and polarization combined with realistic numerical simulations will place a stronger constraint on the location and geometry of the particle acceleration, emission region and mechanism.