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تسجيل مستخدم جديدA Neutron Star is born
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Debora Peres Menezes
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A neutron star was first detected as a pulsar in 1967. It is one of the most mysterious compact objects in the universe, with a radius of the order of 10 km and masses that can reach two solar masses. In fact, neutron stars are star remnants, a kind of stellar zombies (they die, but do not disappear). In the last decades, astronomical observations yielded various contraints for the neutron star masses and finally, in 2017, a gravitational wave was detected (GW170817). Its source was identified as the merger of two neutron stars coming from NGC 4993, a galaxy 140 million light years away from us. The very same event was detected in $gamma$-ray, x-ray, UV, IR, radio frequency and even in the optical region of the electromagnetic spectrum, starting the new era of multi-messenger astronomy. To understand and describe neutron stars, an appropriate equation of state that satisfies bulk nuclear matter properties is necessary. GW170817 detection contributed with extra constraints to determine it. On the other hand, magnetars are the same sort of compact objects, but bearing much stronger magnetic fields that can reach up to 10$^{15}$ G on the surface as compared with the usual 10$^{12}$ G present in ordinary pulsars. While the description of ordinary pulsars is not completely established, describing magnetars poses extra challenges. In this paper, I give an overview on the history of neutron stars and on the development of nuclear models and show how the description of the tiny world of the nuclear physics can help the understanding of the cosmos, especially of the neutron stars.
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Context: The Mira variable LX Cyg showed a dramatic increase of its pulsation period in the recent decades and appears to undergo an important transition in its evolution. Aims: We aim at investigating the spectral type evolution of this star over th
e recent decades as well as during one pulsation cycle in more detail and discuss it in connection with the period evolution. Methods: We present optical, near- and mid-IR low-resolution as well as optical high-resolution spectra to determine the current spectral type. The optical spectrum of LX Cyg has been followed for more than one pulsation cycle. Recent spectra are compared to archival spectra to trace the spectral type evolution and a Spitzer mid-IR spectrum is analysed for the presence of molecular and dust features. Furthermore, the current period is derived from AAVSO data. Results: It is found that the spectral type of LX Cyg changed from S to C sometime between 1975 and 2008. Currently, the spectral type C is stable during a pulsation cycle. It is shown that spectral features typical of C-type stars are present in its spectrum from ~0.5 to 14 $mu{rm{m}}$. An emission feature at 10.7 $mu{rm{m}}$ is attributed to SiC grains. The period of LX Cyg has increased from ~460 d to ~580 d within only 20 years, and is stable now. Conclusions: We conclude that the change in spectral type and the increase in pulsation period happened simultaneously and are causally connected. Both a recent thermal pulse (TP) and a simple surface temperature decrease appear unlikely to explain the observations. We therefore suggest that the underlying mechanism is related to a recent third dredge-up mixing event that brought up carbon from the interior of the star, i.e. that a genuine abundance change happened. We propose that LX Cyg is a rare transition type object that is uniquely suited to study the transformation from O- to C-rich stars in detail.
We report on the results of a 4-year timing campaign of PSR~J2222$-0137$, a 2.44-day binary pulsar with a massive white dwarf (WD) companion, with the Nanc{c}ay, Effelsberg and Lovell radio telescopes. Using the Shapiro delay for this system, we find
a pulsar mass $m_{p}=1.76,pm,0.06,M_odot$ and a WD mass $m_{c},=,1.293,pm,0.025, M_odot$. We also measure the rate of advance of periastron for this system, which is marginally consistent with the GR prediction for these masses. The short lifetime of the massive WD progenitor star led to a rapid X-ray binary phase with little ($< , 10^{-2} , M_odot$) mass accretion onto the neutron star (NS); hence, the current pulsar mass is, within uncertainties, its birth mass; the largest measured to date. We discuss the discrepancy with previous mass measurements for this system; we conclude that the measurements presented here are likely to be more accurate. Finally, we highlight the usefulness of this system for testing alternative theories of gravity by tightly constraining the presence of dipolar radiation. This is of particular importance for certain aspects of strong-field gravity, like spontaneous scalarization, since the mass of PSR~J2222$-0137$ puts that system into a poorly tested parameter range.
We propose that the observed cooling of the neutron star in Cassiopeia A is due to enhanced neutrino emission from the recent onset of the breaking and formation of neutron Cooper pairs in the 3P2 channel. We find that the critical temperature for th
is superfluid transition is ~0.5x10^9 K. The observed rapidity of the cooling implies that protons were already in a superconducting state with a larger critical temperature. Our prediction that this cooling will continue for several decades at the present rate can be tested by continuous monitoring of this neutron star.
We report results from a convection dynamo simulation of proto-neutron star (PNS), with a nuclear equation of state (EOS) and the initial hydrodynamic profile taken from a neutrino radiation-hydrodynamics simulation of a massive stellar core-collapse
. A moderately-rotating PNS with the spin period of $170$ ms in the lepton-driven convection stage is focused. We find that large-scale flow and thermodynamic fields with north-south asymmetry develop in the turbulent flow, as a consequence of the convection in the central part of the PNS, which we call as a deep core convection. Intriguingly, even with such a moderate rotation, large-scale, $10^{15}$ G, magnetic field with dipole symmetry is spontaneously built up in the PNS. The turbulent electro-motive force arising from rotationally-constrained core convection is shown to play a key role in the large-scale dynamo. The large-scale structures organized in the PNS may impact the explosion dynamics of supernovae and subsequent evolution to the neutron stars.
It is now possible to model thermal relaxation of neutron stars after bouts of accretion during which the star is heated out of equilibrium by nuclear reactions in its crust. Major uncertainties in these models can be encapsulated in modest variation
s of a handful of fudge parameters that change the crustal thermal conductivity, specific heat, and heating rates. Observations of thermal relaxation constrain these fudge parameters and allow us to predict longer term variability in terms of the neutron star core temperature. We demonstrate this explicitly by modeling ongoing thermal relaxation in the neutron star XTE J1701-462. Its future cooling, over the next 5 to 30 years, is strongly constrained and depends mostly on its core temperature, uncertainties in crust physics having essentially been pinned down by fitting to the first three years of observations.
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