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Register a new userPSR 0943+10: a bare strange star?
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R. X. Xu
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Recent work by Rankin & Deshpande strongly suggests that there exist strong ``micro-storms rotating around the magnetic axis of the 1.1s pulsar PSR 0943+10. Such a feature hints that most probably the large-voltage vacuum gap proposed by Ruderman & Sutherland (RS) does exist in the pulsar polar cap. However, there are severe arguments against the formation of the RS-type gap in pulsars, since the binding energies of both the Fe ions and the electrons in a neutron stars surface layer is too small to prevent thermionic ejection of the particles from the surface. Here we propose that PSR 0943+10 (probably also most of the other ``drifting pulsars) might be bare strange stars rather than normal neutron stars, in which the ``binding energy at the surface is merely infinity either for the case of ``pulsar or ``anti-pulsar. It is further proposed that identifying a drifting pulsar as an anti-pulsar is the key criterion to distinguish strange stars from neutron stars.
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The photon emissivity from the bremsstrahlung process ee-> eegamma occuring in the electrosphere at the bare surface of a strange quark star is calculated. For surface temperatures T<10^9K, the photon flux exceeds that of e+e- pairs that are produced via the Schwinger mechanism in the presence of a strong electric field that binds electrons to the surface of the quark star. The average energy of photons emitted from the bremsstrahlung process can be 0.5 MeV or more, which is larger than that in e+e- pair annihilation. The observation of this distinctive photon spectrum would constitute an unmistakable signature of a strange quark star and shed light on color superconductivity at stellar densities.
A recent X-ray observation has shown that the radio pulsar PSR B0943+10, with clear drifting subpulses, has a much smaller polar cap area than that of conventional pulsars with mass of $simmsun$ and radius of $sim10$ km. Zhang et al. (2005) addressed then that this new result conflicts with the standard vacuum gap model. Nonetheless, the discrepancy could be explained if PSR B0943+10 is actually a low-mass quark star. It is found that the potential drop in the open-field-line region of oblique pulsars (i.e., inclination angle $alpha eq 0$) might be $sim 10^2$ times that of aligned pulsars, and that PSR B0943+10 with $alpha = 12.4^{rm o}$ could be well above the deathline. We thus conclude that the Ruderman-Sutherland-type vacuum gap model still works well for this pulsar if it is a bare quark star with a mass of $sim 0.02M_odot$ and a radius of $sim 2.6$ km.
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.
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.
We have identified a new intermediate polar, HS 0943+1404, as part of our ongoing search for cataclysmic variables in the Hamburg Quasar Survey. The orbital and white dwarf spin periods determined from time-resolved photometry and spectroscopy are Porb ~= 250 min and Pspin = 69.171 +- 0.001 min, respectively. The combination of a large ratio Pspin/Porb ~= 0.3 and a long orbital period is very unusual compared to the other known intermediate polars. The magnetic moment of the white dwarf is estimated to be mu1 ~ 10^{34} G cm^3, which is in the typical range of polars. Our extensive photometry shows that HS 0943+1404 enters into deep (~ 3 mag) low states, which are also a characteristic feature of polars. We therefore suggest that the system is a true ``intermediate polar that will eventually synchronise, that is, a transitional object between intermediate polars and polars. The optical spectrum of HS 0943+1404 also exhibits a number of unusual emission lines, most noticeably NII 5680, which is likely to reflect enhanced nitrogen abundances in the envelope of the secondary.
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