Searcharxiv⌕ Search

arXiv subjects

H. Grigorian

Publications and source records attributed to H. Grigorian.

At least 37 records · Page 2Linked to original sources
arXiv2007

Modern compact star observations and the quark matter equation of state

We present a hybrid equation of state (EoS) for dense matter that satisfies phenomenological constraints from modern compact star (CS) observations which indicate high maximum masses (M = 2 M_sun) and large radii (R> 12 km). The corresponding isospin symmetric EoS is consistent with flow data analyses of heavy-ion collisions and a deconfinement transition at approx. 0.55 fm^{-3}. The quark matter phase is described by a 3-flavor Nambu--Jona-Lasinio model that accounts for scalar diquark condensation and vector meson interactions while the nuclear matter phase is obtained within the Dirac-Brueckner-Hartree-Fock (DBHF) approach using the Bonn-A potential. We demonstrate that both pure neutron stars and neutron stars with quark matter cores (QCSs) are consistent with modern CS observations. Hybrid star configurations with a CFL quark core are unstable.

nucl-th↗
arXiv2006

Compact star constraints on the high-density EoS

A new scheme for testing the nuclear matter (NM) equation of state (EoS) at high densities using constraints from compact star (CS) phenomenology is applied to neutron stars with a core of deconfined quark matter (QM). An acceptable EoS shall not to be in conflict with the mass measurement of 2.1 +/- 0.2 solar mass (1 sigma level) for PSR J0751+1807 and the mass radius relation deduced from the thermal emission of RX J1856-3754. Further constraints for the state of matter in CS interiors come from temperature-age data for young, nearby objects. The CS cooling theory shall agree not only with these data, but also with the mass distribution inferred via population synthesis models as well as with LogN-LogS data. The scheme is applied to a set of hybrid EsoS with a phase transition to stiff, color superconducting QM which fulfills all above constraints and is constrained otherwise from NM saturation properties and flow data of heavy-ion collisions. We extrapolate our description to low temperatures and draw conclusions for the QCD phase diagram to be explored in heavy-ion collision experiments.

astro-ph↗
arXiv2006

Unmasking neutron star interiors using cooling simulations

We introduce a new tool for 'unmasking' the composition of neutron star (NS) interiors which is based on the fact that the state of matter at high densities determines the statistics of both NS observables, the temperature-age (TA) data as well as the mass distribution. We use modern cooling simulations to extract distributions of NS masses required to reproduce those of the yet sparse data in the TA plane. By comparing the results with a mass distribution for young, nearby NSs from population synthesis we can sharpen two NS cooling problems. The direct Urca (DU) problem consists in a narrowing of the NS population at the mass value for which the DU process as the most effective cooling mechanism in the hadronic layer of the star can occur. The Vela mass problem is a broadening of the population beyond the range of the typical mass window of 1.1 - 1.5 M_sun. Applying this tool to modern EoS we discuss examples for pure hadronic stars which are in conflict with these constraints while hybrid stars with a color superconducting quark matter core can predict a satisfactory mass distribution, provided the smallest diquark pairing gap has a properly defined density dependence.

astro-ph↗
arXiv2006

Nonlocality effects on Color Spin Locking condensates

We consider the color spin locking (CSL) phase of two-flavor quark matter at zero temperature for nonlocal instantaneous, separable interactions. We employ a Lorentzian-type form factor allowing a parametric interpolation between the sharp (Nambu-Jona-Lasinio (NJL) model) and very smooth (e.g. Gaussian) cut-off models for systematic studies of the influence on the CSL condensate the deviation from the NJL model entails. This smoothing of the NJL model form factor shows advantageous features for the phenomenology of compact stars: (i) a lowering of the critical chemical potential for the onset of the chiral phase transition as a prerequisite for stability of hybrid stars with extended quark matter cores and (ii) a reduction of the smallest pairing gap to the order of 100 keV, being in the range of values interesting for phenomenological studies of hybrid star cooling evolution.

hep-ph↗
arXiv2006

Compact star constraints on the high-density EoS

A new scheme for testing the nuclear matter (NM) equation of state (EoS) at high densities using constraints from compact star (CS) phenomenology is applied to neutron stars with a core of deconfined quark matter (QM). An acceptable EoS shall not to be in conflict with the mass measurement of 2.1 +/- 0.2 M_sun (1 sigma level) for PSR J0751+1807 and the mass-radius relation deduced from the thermal emission of RX J1856-3754. Further constraints for the state of matter in CS interiors come from temperature-age data for young, nearby objects. The CS cooling theory shall agree not only with these data, but also with the mass distribution inferred via population synthesis models as well as with LogN-LogS data. The scheme is applied to a set of hybrid EsoS with a phase transition to stiff, color superconducting QM which fulfills all above constraints and is constrained otherwise from NM saturation properties and flow data of heavy-ion collisions. We extrapolate our description to low temperatures and draw conclusions for the QCD phase diagram to be explored in heavy-ion collision experiments.

astro-ph↗
arXiv2006

Astronomy meets QCD: cooling constraints for the theories of internal structure of compact objects

We discuss a set of tests which confront observations of cooling compact objects and theories of their thermal evolution. As an example we apply the recently developed $\mathrm{Log N}$-$\mathrm{Log S}$ test of compact star cooling theories to hybrid stars with a color superconducting quark matter core, we also apply and discuss other existing tests. While there is not yet a microscopically founded superconducting quark matter phase which would fulfill constraints from cooling phenomenology, we explore the hypothetical 2SC+X phase and show that the magnitude and density-dependence of the X-gap can be chosen to satisfy a set of tests: the temperature -- age ($\mathrm{T}$-$\mathrm{t}$) test, the $\mathrm{Log N}$-$\mathrm{Log S}$ test, the brightness constraint, and the mass spectrum constraint. Some recent modifications of the population synthesis model used to obtain the $\mathrm{Log N}$-$\mathrm{Log S}$ distribution are briefly discussed. In addition, we propose to use the age-distance diagram as a new tool to study the local population of young isolated neutron stars.

astro-ph↗
arXiv2006

Neutron star masses: dwarfs, giants and neighbors

We discuss three topics related to the neutron star (NS) mass spectrum. At first we discuss the possibility to form low-mass ($ M \stackrel{<}{\sim} 1 M_{\odot}$) and suggest this is possible only due to fragmentation of rapidly rotating proto-NSs. Such low-mass NSs should have very high spatial velocities which could allow identification. A critical assessment of this scenario is given. Secondly, we discuss mass growth due to accretion for NSs in close binary systems. With the help of numerical population synthesis calculations we derive the mass spectrum of massive ($M > 1.8 M_{\odot}$) NSs. Finally, we discuss the role of the mass spectrum in population studies of young cooling NSs. We formulate a kind of {\it mass constraint} which can be helpful, in our opinion, in discussing different competive models of the thermal evolution of NSs.

astro-ph↗
arXiv2006

Constraints on the high-density nuclear equation of state from the phenomenology of compact stars and heavy-ion collisions

A new scheme for testing nuclear matter equations of state (EsoS) at high densities using constraints from neutron star phenomenology and a flow data analysis of heavy-ion collisions is suggested. An acceptable EoS shall not allow the direct Urca process to occur in neutron stars with masses below $1.5~M_{\odot}$, and also shall not contradict flow and kaon production data of heavy-ion collisions. Compact star constraints include the mass measurements of 2.1 +/- 0.2 M_sun (1 sigma level) for PSR J0751+1807, of 2.0 +/- 0.1 M_sun from the innermost stable circular orbit for 4U 1636-536, the baryon mass - gravitational mass relationships from Pulsar B in J0737-3039 and the mass-radius relationships from quasiperiodic brightness oscillations in 4U 0614+09 and from the thermal emission of RX J1856-3754. This scheme is applied to a set of relativistic EsoS constrained otherwise from nuclear matter saturation properties with the result that no EoS can satisfy all constraints simultaneously, but those with density-dependent masses and coupling constants appear most promising.

nucl-th↗
arXiv2006

Asymmetric neutrino propagation in newly born magnetized strange stars: GRB and kicks

We consider the early cooling evolution of strongly magnetized strange stars in a CFL phase with high gap Delta > 100 MeV. We demonstrate how this model may explain main features of the gamma-ray burst phenomena and also yield a strong star kick. The mechanism is based on beaming of neutrino emission along the magnetic vortex lines. We show that for sufficiently high initial temperatures T_0 ~ 30 - 60 MeV and surface magnetic fields B_s ~ 10^{15} - 10^{17} G, the energy release within the narrow beam is up to 10^{52} erg with a magnetic field dependent time scale between 10^{-2} s (for a smaller magnetic field) to 10 s. The above mechanism together with the parity violation of the neutrino-producing weak interaction processes in a magnetic field allow for the strange star kick. The higher the magnetic field the larger is the stars kick velocity. These velocities may cover the same range as observed pulsar kick velocities.

astro-ph↗
arXiv2005

Neutron star cooling constraints for color superconductivity in hybrid stars

We apply the recently developed Log N - Log S test of compact star cooling theories for the first time to hybrid stars with a color superconducting quark matter core. While there is not yet a microscopically founded superconducting quark matter phase which would fulfill constraints from cooling phenomenology, we explore the hypothetical 2SC+X phase and show that the magnitude and density-dependence of the X-gap can be chosen to satisfy a set of tests: temperature - age (T - t), the brightness constraint, Log N - Log S, and the mass spectrum constraint. The latter test appears as a new conjecture from the present investigation.

nucl-th↗
arXiv2005

Population synthesis as a probe of neutron star thermal evolution

The study of thermal emission from isolated, cooling neutron stars plays a key role in probing the physical conditions of both the star crust and the core. The comparison of theoretical models for the star thermal evolution with the surface temperature derived from X-ray observations of sources of different age is one of the main tools to investigate the properties of the interior and constrain the equation of state. Here we propose to use population synthesis studies as an independent approach to test the physics governing the star cooling. Theoretical Log N - Log S distributions depend on the assumed neutron star thermal evolution. We have computed distributions for several different cooling scenarios and found that comparison with the observed Log N - Log S of isolated neutron stars is effective in discriminating among cooling models. Among the eleven cooling models considered in this paper, all of which may reproduce the observed temperature vs. age diagram, only at most three can explain the Log N - Log S distribution of close-by cooling neutron stars. The Log N - Log S test, being a ``global'' one and despite some limitations, appears capable of complementing the standard temperature vs. age test used up to now.

astro-ph↗
arXiv2005

The phase diagram of three-flavor quark matter under compact star constraints

The phase diagram of three-flavor quark matter under compact star constraints is investigated within a Nambu--Jona-Lasinio model. Local color and electric charge neutrality is imposed for beta-equilibrated superconducting quark matter. The constituent quark masses and the diquark condensates are determined selfconsistently in the plane of temperature and quark chemical potential. Both strong and intermediate diquark coupling strengths are considered. We show that in both cases, gapless superconducting phases do not occur at temperatures relevant for compact star evolution, i.e., below T ~ 50 MeV. The stability and stucture of isothermal quark star configurations are evaluated. For intermediate coupling, quark stars are composed of a mixed phase of normal (NQ) and two-flavor superconducting (2SC) quark matter up to a maximum mass of 1.21 M_sun. At higher central densities, a phase transition to the three-flavor color flavor locked (CFL) phase occurs and the configurations become unstable. For the strong diquark coupling we find stable stars in the 2SC phase, with masses up to 1.326 M_sun. A second family of more compact configurations (twins) with a CFL quark matter core and a 2SC shell is also found to be stable. The twins have masses in the range 1.301 ... 1.326 M_sun. We consider also hot isothermal configurations at temperature T=40 MeV. When the hot maximum mass configuration cools down, due to emission of photons and neutrinos, a mass defect of 0.1 M_sun occurs and two final state configurations are possible.

hep-ph↗
arXiv2005

How robust is a 2SC quark matter phase under compact star constraints?

We study the phase structure and equation of state for two-flavor quark matter at low temperature under compact star constraints within a nonlocal chiral quark model. We find that the occurence of a two-flavor color superconducting (2SC) phase is sensitive to variations of both the formfactor of the interaction and the ratio ηbetween the coupling constants in the diquark and the scalar meson channels. Our study suggests that for standard values of the coupling ratio 0.5<η<0.75 either the 2SC phase does not occur (Gaussian formfactor) or it exists only in a mixed phase with normal quark matter (NQ-2SC) with a volume fraction less than 20 - 40 %, occuring at high baryon chemical potentials μ_B >1200 MeV and most likely not relevant for compact stars. We also present the relevant region of the phase diagram for compact star applications and obtain that no gapless 2SC occurs at low temperatures.

hep-ph↗
arXiv2005

Cooling of Neutron Stars and 3P_2 neutron gap

We study the dependence of the cooling of isolated neutron stars on the magnitude of the $3P_2$ neutron gap. It is demonstrated that our ``Nuclear medium cooling scenario'' is in favor of a suppressed value of the $3P_2$ neutron gap.

astro-ph↗
arXiv2005

Cooling Evolution of Hybrid Stars

The cooling of compact isolated objects for different values of the gravitational mass has been simulated for two alternative assumptions. One is that the interior of the star is purely hadronic and second that the star can have a rather large quark core. It has been shown that within a nonlocal chiral quark model the critical density for a phase transition to color superconducting quark matter under neutron star conditions can be low enough for these phases to occur in compact star configurations with masses below 1.3 M_sun. For a realistic choice of parameters the equation of state (EoS) allows for 2SC quark matter with a large quark gap ~ 100 MeV for u and d quarks of two colors that coexists with normal quark matter within a mixed phase in the hybrid star interior. We argue that, if in the hadronic phase the neutron pairing gap in 3P_2 channel is larger than few keV and the phases with unpaired quarks are allowed, the corresponding hybrid stars would cool too fast. Even in the case of the essentially suppressed 3P_2 neutron gap if free quarks occur for M < 1.3 M_sun, as it follows from our EoS, one could not appropriately describe the neutron star cooling data existing by today. It is suggested to discuss a "2SC+X" phase, as a possibility to have all quarks paired in two-flavor quark matter under neutron star constraints, where the X-gap is of the order of 10 keV - 1 MeV. Density independent gaps do not allow to fit the cooling data. Only the presence of an X-gap that decreases with increase of the density could allow to appropriately fit the data in a similar compact star mass interval to that following from a purely hadronic model.

astro-ph↗
arXiv2004

Exploring the QCD phase diagram with compact stars

We investigate a nonlocal chiral quark model with separable 4-fermion interaction for the case of U(3) flavor symmetry and show that strange quark matter is unlikely to occur in a large enough volume of a compact star to entail remarkable observational consequences. The phase diagram in the two-flavor sector of such model has a critical end point of the line of first order chiral/deconfinement phase transitions on which a triple point marks the junction with the critical line for second order phase transitions to two-flavor color superconductivity (2SC) below T~80 MeV. Stable hybrid star configurations with large quark matter core in a color superconducting phase can exist. A consistent cooling phenomenology requires that all quark species be gapped, the minimal pairing gap of the order of 10 - 100 keV.

hep-ph↗
arXiv2004

Cooling delay for protoquark stars due to neutrino trapping

The influence of neutrino trapping (NT) on the early cooling evolution of hot proto quark stars (PQS) with initial temperatures in the range T ~ 40 MeV is studied. Within a simplified model for the neutrino transport it is shown that the time for reaching neutrino opacity temperature of T_{opac} ~ 1 MeV is about 10 sec. This is an order of magnitude larger than without NT and of the same order as the duration long gamma ray bursts.

astro-ph↗