Editorial: Neutron Stars and Quark Stars Inside Out

Chen Zhang^1*Yongfeng Huang^2,3*Renxin Xu⁴Hajime Togashi⁵Cheng-Jun Xia⁶Alexander Rodin⁷

• ¹Hong Kong University of Science and Technology, Kowloon, Hong Kong, SAR China
• ²School of Astronomy and Space Science, Nanjing University, Nanjing, Jiangsu Province, China
• ³Key Laboratory of Modern Astronomy and Astrophysics, Ministry of Eduction, School of Astronomy and Space Science, Nanjing University, Nanjing, Liaoning Province, China
• ⁴Department of Astronomy, School of Physics, Faculty of Science, Peking University, Beijing, Beijing Municipality, China
• ⁵Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka, Japan
• ⁶Center for Gravitation and Cosmology, College of Physical Science and Technology, Yangzhou University, Yangzhou, China
• ⁷P.N. Lebedev Physical Institute, Russian Academy of Sciences, Russia, Moscow, Russia

Since the theoretical proposal of neutron stars in the 1930s and the observational discovery of pulsars in the 1960s, neutron star physics has opened a new window into the extremes of the universe. With the simultaneous development of quantum chromodynamics in the 1970s, the hypothetical notion of strange quark matter and strange quark stars was born. In the 21st century, the physics of compact stars have been actively explored. In recent years, observational advances, especially the detection of gravitational waves, have come to the era of multi-messenger astrophysics, which enhance our ability to probe the nature of these compact stars. However, there are still many challenges in this flourishing field. The equations of state are not firmly determined. The existence of hybrid stars associated with hadron-quark phase transition is not conclusive. Distinguishing between neutron stars and quark stars remains a challenge.The research topic "Neutron Stars and Quark Stars Inside Out" brings together cutting-edge investigations that sheds light on the distinct characteristics of neutron stars and quark stars, especially on their internal compositions and observable manifestations. Spanning theoretical modeling, observational analysis, and computational approaches, the contributed articles collectively advance our understanding of compact stars across multiple fronts. This interdisciplinary field draws from nuclear physics, particle physics, general relativity, and astrophysics to address fundamental questions about the nature of matter under extreme conditions. Zhang, Huang, and Zou provide a valuable overview of recent developments in the field of strange quark stars in their review article "Recent progresses in strange quark stars." Starting with the hypothesis that strange quark matter may be the true ground state of matter at extremely high densities, they introduce three popular phenomenological models widely used to describe strange quark matter, with special attention to the corresponding equation of state in each model. By combining these equations of state with the Tolman-Oppenheimer-Volkov equations, they demonstrate how the inner structure and mass-radius relation can be determined for strange stars. The authors also explore tidal deformability and oscillations, which are sensitive to the composition and equations of state, and discuss hybrid stars as a special kind of quark star. Particularly noteworthy is their discussion of gravitational wave emissions that may be generated by strange stars through various mechanisms, potentially providing a means to identify these objects observationally. The authors also highlight how close-in strange quark planets could provide a unique test for the existence of strange quark objects, and how electromagnetic bursts, such as short gamma-ray bursts and fast radio bursts, might be generated by strange stars.Xia, Huang, Shao, and Xu address an intriguing puzzle in their article "Ultra-low-mass and small-radius white dwarfs made of heavy elements." They examine seven recently identified ultra-low-mass and small-radius white dwarfs with masses ranging from ∼0.02 to ∼0.08 solar masses and radii from ∼4,270 to 10,670 km. These measurements challenge traditional white dwarf models that assume composition primarily of light elements like carbon, oxygen, and helium. The authors propose that these anomalous white dwarfs might instead be composed of heavier elements such as iron, nickel, palladium, and lead. They demonstrate that the smaller charge-to-mass ratios in heavy elements effectively reduce electron number density in white dwarf matter, which reduces pressure with additional contributions from lattice energy and electron polarization corrections. This mechanism elegantly explains the observed smaller masses and radii. The authors also suggest that this composition could account for sub-Chandrasekhar progenitors in underluminous Type Ia supernovae, addressing another observational puzzle. They compare their model with alternative explanations involving strange quark matter or quark nuggets, and emphasize the need for further observations through spectroscopy and asteroseismology to determine the actual composition of these unusual white dwarfs.In "Hadron-quark phase transition in the neutron star with vector MIT bag model and Korea-IBS-Daegu-SKKU functional," Sen, Gil, and Hyun investigate the critical transition between hadronic and quark matter in neutron stars. Employing the Korea-IBS-Daegu-SKKU (KIDS) density functional for the hadron phase and the MIT bag model with vector (vBag) model for the quark phase, they obtain hadron-quark phase transitions considering Maxwell construction. The authors compute the structural properties of the resulting hybrid stars for three different values of bag constant in the range B 1/4 = (145-160 MeV). Their study reveals that symmetry energy has important influence not only on transition properties like transition mass, transition radius, and density jump due to phase transition, but also on the stability of hybrid stars. Furthermore, they demonstrate that vector repulsion in the quark phase has profound influence in obtaining reasonable hybrid star configurations consistent with recent astrophysical constraints. Their work effectively constrains the vector coupling constant to be 0.3 ≲ G V ≲ 0.4 for reasonable hybrid star configurations within their studied range of bag constants.In "Search for thermonuclear burst oscillations in the Swift/BAT data set," Li and colleagues conduct a comprehensive analysis of type I X-ray bursts observed by Swift/BAT from 2005 to April 2024. Their study focuses on X-ray burst oscillations (XBOs), which are periodic signals detected within these bursts that typically range from 11 to 620 Hz. Using the high-sensitivity and precise timing capabilities of Swift/BAT, the authors identified 50 type I X-ray bursts from 37 neutron star low-mass X-ray binaries. For sources with known burst oscillation frequencies, their findings largely corroborate previous studies, though many sources displayed low confidence levels in the oscillation signals. More significantly, for sources without known oscillation or spin frequencies, their FFT analysis revealed potential oscillation signals across a broad frequency range, with several bursts showing significance levels exceeding 3σ.Bernal and colleagues present a complementary model to the well-established canonical model for spin evolution of rotation-powered pulsars in "On the overall properties of young neutron stars: an application to the Crab pulsar." Their research report analytically explores the growth of the magnetic field during a pulsar's early history, a period shortly after supernova explosion when the neutron star forms. This encompasses the hypercritical phase and subsequent reemergence of the magnetic field, analyzing the impact of such growth on the early dynamics of the pulsar. The authors expand existing knowledge by examining evolutionary implications in a scenario governed by growth functions. Their proposed growth functions, calibrated with data from the Crab pulsar, exhibit satisfactory physical behaviors. This work provides valuable insights into how the periodicity of pulsars might be altered during early development, contrasting with the assumption of stable, unchanging periodicity in the canonical model, and helps explain observations of young neutron stars that don't strictly adhere to the rotating magnetic dipole model.Collectively, these articles highlight the rich interplay between theoretical modeling and observational constraints in advancing our understanding of neutron stars, quark stars, and related compact objects. They demonstrate how multi-messenger astronomy, combining electromagnetic observations with gravitational wave detections, is crucial for resolving outstanding questions about the internal composition and structure of these objects. The research presented here has significant implications for fundamental physics, potentially informing our understanding of quantum chromodynamics, the nuclear equation of state, and the behavior of matter under extreme conditions.