Searcharxiv⌕ Search

arXiv subjects

Norman K. Glendenning

Publications and source records attributed to Norman K. Glendenning.

At least 19 recordsLinked to original sources

Likely formation of general relativistic radiation pressure supported stars or "eternally collapsing objects"

Hoyle and Folwler showed that there could be Radiation Pressure Supported Stars (RPSS) even in Newtonian gravity. Much later, Mitra found that one could also conceive of their General Relativistic (GR) version, "Relativistic Radiation Pressure Supported Stars" (RRPSSs). While RPSSs have $z\ll 1$, RRPSSs have $z \gg 1$, where $z$ is the surface gravitational redshift. Here we elaborate on the formation of RRPSSs during continued gravitational collapse by recalling that a contracting massive star must start trapping radiation as it would enter its {\em photon sphere}. It is found that, irrespective of the details of the contraction process, the trapped radiation flux should attain the corresponding Eddington value at sufficiently large $z\gg 1$. This means that continued GR collapse may generate an intermediate RRPSS with $z\gg 1$ before a true BH state with $z=\infty$ is formed asymptotically. An exciting consequence of this is that the stellar mass black hole candidates, at present epoch, should be hot balls of quark gluon plasma, as has been discussed by Royzen in a recent article entitled "{\it QCD against black holes?}".

astro-ph.HE↗

Spin Clustering of Accreting X-ray Neutron Stars as Possible Evidence of Quark Matter

A neutron star in binary orbit with a low-mass non-degenerate companion becomes a source of x-rays with millisecond variability when mass accretion spins it up. Centrifugally driven changes in density profile may initiate a phase transition in a growing region of the core parallel to what may take place in an isolated millisecond pulsar, but in reverse. Such a star will spend a longer time in the spin frequency range over which the transition occurs than elsewhere because the change of phase, paced by the spinup rate, is accompanied by a growth in the moment of inertia. The population of accreters will exhibit a clustering in the critical frequency range. A phase change triggered by changing spin and the accompanying adjustment of moment of inertia has its analogue in rotating nuclei.

astro-ph↗

Signal of Quark Deconfinement in Millisecond Pulsars and Reconfinement in Accreting X-ray Neutron Stars

Theoretically, the phase transition between the confined and deconfined phases of quarks can have a remarkable effect on the spin properties of millisecond pulsars and on the spin distribution of the population of x-ray neutron stars in low-mass binaries. In the latter class of stars, the effect has already been observed---a strong clustering in the population in a narrow band of spins. The observed clustering cannot presently be uniquely assigned to the phase transition as cause. However, there is another possible signal---not so far observed---in millisecond pulsars that we also discuss. It would have the same origin, and its discovery would tend to confirm the interpretation in terms of a phase transition in the stellar core and that the high density phase exists in the higer mass canonical neutron stars.

astro-ph↗

Evolution from Canonical to Millisecond Pulsar through the X-ray Accretion Stage

We model the evolution of canonical pulsars from the death line to millisecond pulsars through the X-ray neutron star stage of accretion from a low-mass companion. We trace this evolution in magnetic field strength starting at $B=10^{12}$ to $10^8$ G and in period of about 1 s to milliseconds. Important factors are accretion rate and the decay rate of the magnetic field. A broad swathe is traced in the $B-P$ plane according to the value of these factors, which represent different conditions of the binary pair. An important ingredient is the dependence of the stellar moment of inertia on rotation frequency (time).

astro-ph↗

On the uncertainty of hyperon couplings and the electrochemical potential in neutron star matter

Uncertainty of the hyperon couplings, in particular, that of the $Σ^-$, in dense matter raises the question of the behavior of the electrochemical potential in neutron star matter, which is crucial to the possible presence of the kaon condensed phase. We show that regardless of this uncertainty, the $Λ$ hyperon, whose coupling can be constrained by its binding in nuclear matter and other observations, also aided by the $Ξ^-$, introduce a saturation of the electrochemical potential just as the $Σ^-$ would otherwise do, which tends to mitigate against kaon condensation. The maximum possible mass of neutron stars appears to be $\sim 1.5 \msun$ independent of the uncertainties.

nucl-th↗

Possible evidence of quark matter in neutron star X-ray binaries

We study the spin evolution of X-ray neutron stars in binary systems, which are being spun up by mass transfer from accretion disks. Our investigation reveals that a quark phase transition resulting from the changing central density induced by the changing spin, can lead to a pronounced peak in the frequency distribution of X-ray neutron stars. This finding provides one of several possible explanations available in the literature, or at least a contributor to part of the observed anomalous frequency distribution of neutron stars in low-mass X-ray binaries (LMXBs), which lie in a narrow band centered at about 300 Hz, as found by the Rossi Explorer (RXTE).

astro-ph↗

Structured Mixed Phase is Favored in Neutron Stars

We give a general thermodynamical argument showing that in neutron stars, the Coulomb structured mixed phase is always favored for any first order phase transition involving systems in equilibrium with baryon number and electric charge as the two independent components. This finding is likely to have important consequences for many neutron star properties, e.g., glitch phenomena, transport and superfluid properties, r-mode instabilities, and the braking index.

astro-ph↗

Surface Tension between Kaon Condensate and Normal Nuclear Matter Phase

We calculate for the first time the surface tension and curvature coefficient of a first order phase transition between two possible phases of cold nuclear matter, a normal nuclear matter phase in equilibrium with a kaon condensed phase, at densities a few times the saturation density. We find the surface tension is proportional to the difference in energy density between the two phases squared. Furthermore, we show the consequences for the geometrical structures of the mixed phase region in a neutron star.

nucl-th↗

Lower Limit on Radius as a Function of Mass for Neutron Stars

While model-independent limits are always interesting, a limit on neutron star radius as a function of mass attains special interest in light of recent interpretations of the periodic as well as quasi-periodic oscillations (QPOs) in brightness of X-rays emitted from neutron stars that are accreting matter from a low-mass companion. Here we derive such a limit based only on well accepted principles. We discuss our limit in connection with a recent interpretation of X-ray pulsations from SAX J1808.4-3658 as indicating a strange-star candidate, and show that this object can also be a normal neutron star, though one whose central core has very high density. The most plausible high-density phase of hadronic matter, which is also expected to be very compressible, is quark matter. So an alternative to the strange star interpretation of SAX J1808.4-3658 is that it is a hybrid neutron star.

astro-ph↗

Non-Identical Neutron Star Twins

The work of J. A. Wheeler in the mid 1960's showed that for smooth equations of state no stable stellar configurations with central densities above that corresponding to the limiting mass of ``neutron stars'' (in the generic sense) were stable against acoustical vibrational modes. A perturbation would cause any such star to collapse to a black hole or explode. Accordingly, there has been no reason to expect that a stable degenerate family of stars with higher density than the known white dwarfs and neutron stars might exist. We have found a class of exceptions corresponding to certain equations of state that describe a first order phase transition. We discuss how such a higher density family of stars could be formed in nature, and how the promising new exploration of oscillations in the X-ray brightness of accreting neutron stars might provide a means of identifying them. Our proof of the possible existence of a third family of degenerate stars is one of principle and rests on general principles like causality, microstability of matter and General Relativity.

astro-ph↗

First Order Kaon Condensate

First order Bose condensation in asymmetric nuclear matter and in neutron stars is studied, with particular reference to kaon condensation. We demonstrate explicitly why the Maxwell construction fails to assure equilibrium in multicomponent substances. Gibbs conditions and conservation laws require that for phase equilibrium, the charge density must have opposite sign in the two phases of isospin asymmetric nuclear matter. The mixed phase will therefore form a Coulomb lattice with the rare phase occupying lattice sites in the dominant phase. Moreover, the kaon condensed phase differs from the normal phase, not by the mere presence of kaons in the first, but also by a difference in the nucleon effective masses. The mixed phase region, which occupies a large radial extent amounting to some kilometers in our model neutron stars, is thus highly heterogeneous. It should be particularly interesting in connection with the pulsar glitch phenomenon as well as transport properties.

astro-ph↗

Kaon Condensation and Dynamical Nucleons in Neutron Stars

We discuss the nature of the kaon condensation phase transition. We find several features which, if kaons condense in neutron stars, are not only remarkable, but must surely effect such properties as superfluidity and transport properties, which in turn are relevant to the glitch phenomenon and cooling rates of neutron stars. The mixed phase, because of the extensive pressure range that it spans, will occupy a broad radial extent in a neutron star. This region is permeated with microscopic drops (and other configurations) located at lattice sites of one phase immersed in the background of the other phase. The electric charge on drops is opposite to that of the background phase {\sl and} nucleons have a mass approximately a factor two different depending on whether they are in the drops or the background phase. A large part of the stellar interior has this highly non-homogeneous structure.

astro-ph↗

Neutron Star Constraints on the H Dibaryon

We study the influence of a possible H dibaryon condensate on the equation of state and the overall properties of neutron stars whose population otherwise contains nucleons and hyperons. In particular, we are interested in the question of whether neutron stars and their masses can be used to say anything about the existence and properties of the H dibaryon. We find that the equation of state is softened by the appearance of a dibaryon condensate and can result in a mass plateau for neutron stars. If the limiting neutron star mass is about that of the Hulse-Taylor pulsar a condensate of H dibaryons of vacuum mass 2.2 GeV and a moderately attractive potential in the medium could not be ruled out. On the other hand, if the medium potential were even moderately repulsive, the H, would not likely exist in neutron stars. If neutron stars of about 1.6 solar mass were known to exist, attractive medium effects for the H could be ruled out. Certain ranges of dibaryon mass and potential can be excluded by the mass of the Hulse-Taylor pulsar which we illustrate graphically.

astro-ph↗

Spin-up of Solitary Pulsars: Signal of Deconfinement

A phase transition in the nature of matter in the core of a neutron star, such as quark deconfinement or Bose condensation, can cause the spontaneous spin-up of a solitary millisecond pulsar. The spin-up epoch for our model lasts for $2\times 10^7$ years or 1/50 of the spin-down time.

astro-ph↗

Phase Transition in Pulsar Timing

A phase transition in the nature of matter in the core of a neutron star, such as quark deconfinement or Bose condensation, can cause the spontaneous spin-up of a solitary millisecond pulsar. The spin-up epoch for our model lasts for $2\times 10^7$ years or 1/50 of the spin-down time (Glendenning, Pei and Weber in Ref. \cite{glen97:a}). The possibility exists also for future measurements on X-ray neutron stars with low-mass companions for mapping out the tell-tale ``backbending'' behavior of the moment of inertia. Properties of phase transitions in substances such as neutron star matter, which have more than one conserved charge, are reviewed.

astro-ph↗

Pulsar Signal of Deconfinement

A solitary millisecond pulsar, if near the mass limit, and undergoing a phase transition, either first or second order, provided the transition is to a substantially more compressible phase, will emit a blatantly obvious signal---spontaneous spin-up. Normally a pulsar spins down by angular momentum loss to radiation. The signal is trivial to detect and is estimated to be ``on'' for 1/50 of the spin-down era of millisecond pulsars. Presently about 25 solitary millisecond pulsars are known. The phenomenon is analogous to ``backbending'' observed in high spin nuclei in the 1970's.

astro-ph↗

Strange Phases in Neutron Stars

The equation of state and the properties of neutron stars are studied for a phase transition to a charged kaon condensate. We study the mixed phase by using Gibbs condition with comparison to the hitherto applied Maxwell construction. Implications for kaon condensation and for the mass-radius relation of condensed neutron stars are examined.

nucl-th↗

Strangeness in Compact Stars and Signal of Deconfinement

Phase transitions in compact stars are discussed including hyperonization, deconfinement and crystalline phases. Reasons why kaon condensation is unlikely is reviewed. Particular emphasis is placed on the evolution of internal structure with spin-down of pulsars. A signature of a first order phase transition in the timing structure of pulsars which is strong and easy to measure, is identified.

astro-ph↗