The LIGO/VIRGO detection of the gravitational waves from a binary merger system, GW170817, has put a clean and strong constraint on the tidal deformability of the merging objects. From this constraint, deep insights can be obtained in compact star equation of states, which has been one of the most puzzling problems for nuclear physicists and astrophysicists. Employing one of the most widely used quark star EOS models, we characterize the star properties by the strange quark mass ($m_s$), an effective bag constant ($B_{\text{eff}}$), the perturbative QCD correction ($a_4$), as well as the gap parameter ($\mathrm{\Delta }$) when considering quark pairing, and investigate the dependences of the tidal deformablity on them. We find that the tidal deformability is dominated by $B_{\text{eff}}$ and insensitive to $m_s$, $a_4$. We discuss the correlation between the tidal deformability and the maximum mass ($M_{\mathrm{TOV}}$) of a static quark star, which allows the model possibility to rule out the existence of quark stars with future gravitational wave observations and mass measurements. The current tidal deformability measurement implies $M_{\mathrm{TOV}}\le 2.18M_{\odot }$ ($2.32M_{\odot }$ when pairing is considered) for quark stars. Combining with two-solar-mass pulsar observations, we also make constraints on the poorly known gap parameter $\mathrm{\Delta }$ for color-flavor-locked quark matter.

The nature of GW190814's secondary component m ₂ of mass 2.50–2.67 M _⊙ in the mass gap between the currently known maximum mass of neutron stars and the minimum mass of black holes is currently under hot debate. Among the many possibilities proposed in the literature, m ₂ was suggested to be a superfast pulsar, while its r-mode stability against runaway gravitational radiation through the Chandrasekhar–Friedman–Schutz mechanism is still unknown. Previously, Fortin et al. constructed a sample of 33 unified equations of state using the same nuclear interactions from the crust to the core consistently; from that sample we use those equations that fulfill all currently known astrophysical and nuclear physics constraints to compare the minimum frequency required for m ₂ to rotationally sustain a mass greater than 2.50 M _⊙ with the critical frequency above which the r-mode instability occurs. We use two extreme damping models assuming that the crust is either perfectly rigid or elastic. Using the stability of 19 observed low-mass X-ray binaries as an indication that the rigid crust damping of the r-mode dominates within the models studied, we find that m ₂ is r-mode-stable while rotating with a frequency higher than 870.2 Hz (0.744 times its Kepler frequency of 1169.6 Hz) as long as its temperature is lower than about 3.9 × 10⁷ K, further supporting the proposal that GW190814's secondary component is a supermassive and superfast pulsar.

Pulsar death line can be defined in a |$P\text{--}\dot{P}$| diagram. Traditionally, radio-loud pulsars are supposed to locate above the death line where there is a radio-loud region. With the development of observational equipment, the observational properties of the neutron star are remarkably diverse. In a |$P\text{--}\dot{P}$| diagram, some of the special sources are radio-quiet but lie above the death line. From the definition of the pulsar death line, different equations of state for a neutron star or strange star result in different death lines. We discuss the influence of the equation of state on the pulsar death line and the possible link between different neutron star groups. The results show that central compact objects would be the small mass of self-bound strange stars, and rotating radio transients might be old pulsars on the verge of death. We suggest that PSR J2144−3933 is likely to be a large mass pulsar, which would be larger than 2.0 M_⊙. Multiple observational facts would help us to reveal the nature of pulsars.

PSR J2150+3427 is a 0.654 s pulsar discovered by the Commensal Radio Astronomy FAST Survey. From the follow-up observations, we find that the pulsar is in a highly eccentric orbit (e = 0.601) with an orbital period of 10.592 days and a projected semimajor axis of 25.488 lt-s. Using 2.7 yr of timing data, we also measured the rate of periastron advance ω ̇ = 0.0115(4) deg yr⁻¹. An estimate for the total mass of the system using the ω ̇ gives M _tot = 2.59(13)M _⊙, which is consistent with most of the known double neutron star (DNS) systems and one neutron star (NS)–white dwarf (WD) system named B2303+46. Combining ω ̇ with the mass function of the system gives the masses of M _p < 1.67 and M _c > 0.98 M _⊙ for the pulsar and the companion star, respectively. This constraint, along with the spin period and orbital parameters, suggests that it is possibly a DNS system, and we cannot entirely rule out the possibility of an NS–WD system. Future timing observations will vastly improve the uncertainty in ω ̇ , and are likely to allow the detection of additional relativistic effects, which can be used to modify the values of M _p and M _c . With a spin-down luminosity of E ̇ = 5.07(6) × 10²⁹ erg s⁻¹, PSR J2150+3427 is a very low-luminosity pulsar, with only the binary pulsar J2208+4610 having a smaller E ̇ .

We report the evolution of the X-ray pulsations of EXO 2030+375 during its 2021 outburst using the observations from Insight-HXMT. Based on the accretion torque model, we study the correlation between the spin frequency derivatives and the luminosity. Pulsations can be detected in the energy band of 1–160 keV. The pulse profile evolves significantly with luminosity during the outburst, leading to that the whole outburst can be divided into several parts with different characteristics. The evolution of the pulse profile reveals the transition between the supercritical (fan-beam dominated) and the subcritical accretion (pencil-beam dominated) mode. From the accretion torque model and the critical luminosity model, based on a distance of 7.1 kpc, the inferred magnetic fields are (0.41 − 0.74) × 10¹² and (3.48 − 3.96) × 10¹² G, respectively, or based on a distance of 3.6 kpc, the estimated magnetic fields are (2.4 − 4.3) × 10¹³ and (0.98 − 1.11) × 10¹² G, respectively. Two different sets of magnetic fields both support the presence of multipole magnetic fields of the neutron star.

We report the timing analysis of PSR J1846−0513, a pulsar discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Commensal Radio Astronomy FAST Survey. The pulsar possesses a spin period of 23.36 ms and a spin-down rate ( P ̇ ) of 1.0106(3) × 10⁻¹⁸ s s⁻¹, and it is located in an eccentric orbit (e ∼0.208) with an orbital period of 0.61 days. The characteristic age and surface magnetic field of the pulsar are found to be 366.62 Myr and 4.9178 × 10⁹ G, respectively, indicating that it is a recycled pulsar. Using over two years of timing data, we measure the periastron advance ω ̇ = 0.8956(8) deg yr⁻¹. By assuming that this effect is purely relativistic, we have estimated the total mass M = 2.6287(35)M _⊙ and obtained an upper limit for the pulsar mass and a lower limit for the companion’s mass. Our results indicate that this is a double neutron star system.