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What if normal baryonic matter is compressed so tightly that atomic nuclei come into close contact? This question has been asked since 1930s. The fist answer was presented by Lev Landau whose speculation has been developed, and the concept of neutron star is then popularized. However, another answer is related to strange star, which becomes worthy of attention especially after the establishment of the standard model of particle physics in 1960s. The basic ideas of this study are introduced pedagogically. We must point out emphatically that flavour symmetry of and strong coupling between quarks would be essential in seeking true answer to the question. The final answer is expected to appear in the era of multimessenger astronomy. It is emphasized too that, besides the differences of global properties (e.g., mass-radius relation, maximum mass, tidal deformability), the strong-bound surface of strange star (rather than the gravity-bound one for conventional neutron star) could play an important role in identifying a strange star by astronomical observations.
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There are strong indications that the process of conversion of a neutron star into a strange quark star proceeds as a strong deflagration implying that in a few milliseconds almost the whole star is converted. Starting from the three-dimensional hydrodynamic simulations of the combustion process which provide the temperature profiles inside the newly born strange star, we calculate for the first time the neutrino signal that is to be expected if such a conversion process takes place. The neutrino emission is characterized by a luminosity and a duration that is typical for the signal expected from protoneutron stars and represents therefore a powerful source of neutrinos which could be possibly directly detected in case of events occurring close to our Galaxy. We discuss moreover possible connections between the birth of strange stars and explosive phenomena such as supernovae and gamma-ray-bursts.
According to the recycling model, neutron stars in low-mass X-ray binaries were spun up to millisecond pulsars (MSPs), which indicates that all MSPs in the Galactic plane ought to be harbored in binaries. However, about $20%$ Galactic field MSPs are found to be solitary. To interpret this problem, we assume that the accreting neutron star in binaries may collapse and become a strange star when it reaches some critical mass limit. Mass loss and a weak kick induced by asymmetric collapse during the phase transition (PT) from neutron star to strange star can result in isolated MSPs. In this work, we use a population-synthesis code to examine the PT model. The simulated results show that a kick velocity of $sim60~{rm km~s}^{-1}$ can produce $sim6times10^3$ isolated MSPs and birth rate of $sim6.6times10^{-7} {rm ~yr}^{-1}$ in the Galaxy, which is approximately in agreement with predictions from observations. For the purpose of comparisons with future observation, we also give the mass distributions of radio and X-ray binary MSPs, along with the delay time distribution.
PSR J$1946+3417$ is a millisecond pulsar (MSP) with a spin period $Psimeq3.17rm~ms$. Harbored in a binary with an orbital period $P_{rm b}simeq27$ days, the MSP is accompanied by a white dwarf (WD). The masses of the MSP and the WD were determined to be $1.83rm~M_odot$ and $0.266rm~M_odot$, respectively. Specially, its orbital eccentricity is $esimeq0.134$, which is challenging the recycling model of MSPs. Assuming that the neutron star in a binary may collapse to a strange star when its mass reaches a critical limit, we propose a phase transition (PT) scenario to account for the origin of the system. The sudden mass loss and the kick induced by asymmetric collapse during the PT may result in the orbital eccentricity. If the PT event takes place after the mass transfer ceases, the eccentric orbit can not be re-circularized in the Hubble time. Aiming at the masses of both components, we simulate the evolution of the progenitor of PSR J$1946+3417$ via texttt{MESA}. The simulations show that a NS / main sequence star binary with initial masses of $1.4+1.6rm~M_odot$ in an initial orbit of 2.59 days will evolve into a binary consisting of a $2.0rm~M_odot$ MSP and a $0.27rm~M_odot$ WD in an orbit of $sim21.5$ days. Assuming that the gravitational mass loss fraction during PT is $10%$, we simulate the effect of PT via the kick program of texttt{BSE} with a velocity of $sigma_{rm PT}=60~{rm km~s}^{-1}$. The results show that the PT scenario can reproduce the observed orbital period and eccentricity with higher probability then other values.
The ultimate astronomical observatory would be a formation flying interferometer in space, immune to atmospheric turbulence and absorption, free from atmospheric and telescope thermal emission, and reconfigurable to adjust baselines according to the required angular resolution. Imagine the near/mid-infrared sensitivity of the JWST and the far-IR sensitivity of Herschel but with ALMA-level angular resolution, or imagine having the precision control to null host star light across 250m baselines and to detect molecules from the atmospheres of nearby exo-Earths. With no practical engineering limit to the formations size or number of telescopes in the array, formation flying interferometry will revolutionize astronomy and this White Paper makes the case that it is now time to accelerate investments in this technological area. Here we provide a brief overview of the required technologies needed to allow light to be collected and interfered using separate spacecrafts. We emphasize the emerging role of inexpensive smallSat projects and the excitement for the LISA Gravitational Wave Interferometer to push development of the required engineering building-blocks. We urge the Astro2020 Decadal Survey Committee to highlight the need for a small-scale formation flying space interferometer project to demonstrate end-to-end competency with a timeline for first stellar fringes by the end of the decade.
Hadronic supercriticalities are radiative instabilities that appear when large amounts of energy are stored in relativistic protons. When the proton energy density exceeds some critical value, a runaway process is initiated resulting in the explosive transfer of the proton energy into electron-positron pairs and radiation. The runaway also leads to an increase of the radiative efficiency, namely the ratio of the photon luminosity to the injected proton luminosity. We perform a comprehensive study of the parameter space by investigating the onset of hadronic supercriticalities for a wide range of source parameters (i.e., magnetic field strengths of 1 G$- 100$ kG and radii of $10^{11}$ cm$-10^{16}$ cm) and maximum proton Lorentz factors ($10^3-10^9$). We show that supercriticalities are possible for the whole range of source parameters related to compact astrophysical sources, like gamma-rays bursts, cores and jets of active galactic nuclei. We also provide an in-depth look at the physical mechanisms of hadronic supercriticalities and show that magnetized relativistic plasmas are excellent examples of non-linear dynamical systems in high-energy astrophysics.
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