arXiv subjects

Sanjay Reddy

Publications and source records attributed to Sanjay Reddy.

At least 55 records · Page 3Linked to original sources

arXiv2017

Nuclear pasta in hot dense matter and its implications for neutrino scattering

We find that the abundance of large clusters of nucleons in neutron-rich matter at sub-nuclear density is greatly reduced by finite temperature effects when matter is close to beta-equilibrium. Large nuclei and exotic non-spherical nuclear configurations called pasta, favored in the vicinity of the transition to uniform matter at $T=0$, dissolve at relatively low temperature. For matter close to beta-equilibrium we find that the pasta melting temperature is $T_m^β\simeq 4\pm 1$~MeV for realistic equations of state. The mechanism for pasta dissolution is discussed, and in general $T_m^β$ is shown to be sensitive to the proton fraction. We find that coherent neutrino scattering from nuclei and pasta makes a modest contribution to the opacity under the conditions encountered in supernovae and neutron star mergers. Implications for neutrino signals from galactic supernovae are briefly discussed.

astro-ph.HE↗

arXiv2017

Energy Conservation and the Chiral Magnetic Effect

We analyze the chiral magnetic effect in a homogeneous neutral plasma from the point of view of energy conservation, and construct an effective potential for the growth of maximally helical perturbations of the electromagnetic field. We show that a negative curvature at the origin of the potential, indicating instability of the plasma, is induced by a chiral asymmetry in electron Fermi energy, as opposed to number density, while the potential grows at large field value. It follows that the ground state for a plasma has zero magnetic helicity; a nonzero electron mass will allow an excited state of a plasma with nonzero helicity to relax to that ground state quickly. We conclude that a chiral plasma instability triggered by weak interactions is not a viable mechanism for explaining magnetic fields in stars except possibly when dynamics drives the system far from equilibrium.

hep-ph↗

arXiv2017

Charged current neutrino interactions in hot and dense matter

We derive the charged current absorption rate of electron and anti-electron neutrinos in dense matter using a fully relativistic approach valid at arbitrary matter degeneracy. We include mean field energy shifts due to nuclear interactions and the corrections due to weak magnetism. The rates are derived both from the familiar Fermi's Golden Rule, and from the techniques of finite temperature field theory, and shown to be equivalent. In various limits, these results can also be used to calculate neutral current opacities. We find that some pieces of the response have been left out in previous derivations and their contribution at high density can be significant. Useful formulae and detailed derivations are presented and we provide a new open source implementation of these opacities for use in radiation hydrodynamic simulations of core-collapse supernovae and neutron star mergers.

astro-ph.HE↗

arXiv2016

Neutrino Signatures From Young Neutron Stars

After a successful core collapse supernova (CCSN) explosion, a hot dense proto-neutron star (PNS) is left as a remnant. Over a time of twenty or so seconds, this PNS emits the majority of the neutrinos that come from the CCSN, contracts, and loses most of its lepton number. This is the process by which all neutron stars in our galaxy are likely born. The emitted neutrinos were detected from SN 1987A and they will be detected in much greater numbers from any future galactic CCSN. These detections can provide a direct window into the properties of the dense matter encountered inside neutron stars, and they can affect nucleosynthesis in the mate- rial ejected during the CCSN. In this chapter, we review the basic physics of PNS cooling, including the basic equations of PNS structure and neutrino diffusion in dense matter. We then discuss how the nuclear equation of state, neutrino opacities in dense matter, and convection can shape the temporal behavior of the neutrino signal. We also discuss what was learned from the late time SN 1987A neutrinos, the prospects for detection of these neutrinos from future galactic CCSNe, and the effects these neutrinos can have on nucleosynthesis.

astro-ph.HE↗

arXiv2016

Late time cooling of neutron star transients and the physics of the inner crust

An accretion outburst onto a neutron star transient heats the neutron star's crust out of thermal equilibrium with the core. After the outburst the crust thermally relaxes toward equilibrium with the neutron star core and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established $\approx 1000 \, \mathrm{d}$ into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescale and continued to cool after $\approx 2500 \, \mathrm{d}$ into quiescence. Because the quiescent light curve reveals successively deeper layers of the crust, the observed late time cooling of MXB 1659-29 depends on the thermal transport in the inner crust. In particular, the observed late time cooling is consistent with a low thermal conductivity layer near the depth predicted for nuclear pasta that maintains a temperature gradient between the neutron star's inner crust and core for thousands of days into quiescence. As a result, the temperature near the crust-core boundary remains above the critical temperature for neutron superfluidity and a layer of normal neutrons forms in the inner crust. We find that the late time cooling of MXB 1659-29 is consistent with heat release from a normal neutron layer near the crust-core boundary with a long thermal time.

astro-ph.HE↗

arXiv2016

A lower limit on the heat capacity of the neutron star core

We show that observations of the core temperature of transiently-accreting neutron stars combined with observations of an accretion outburst give a lower limit to the neutron star core heat capacity. For the neutron stars in the low mass X-ray binaries KS 1731-260, MXB 1659-29, and XTE J1701-462, we show that the lower limit is a factor of a few below the core heat capacity expected if neutrons and protons in the core are paired, so that electrons provide the dominant contribution to the heat capacity. This limit rules out a core dominated by a quark color-flavor-locked (CFL) phase, which would have a much lower heat capacity. Future observations of or limits on cooling during quiescence will further constrain the core heat capacity.

astro-ph.HE↗

arXiv2016

Thermal conductivity and impurity scattering in the accreting neutron star crust

We calculate the thermal conductivity of electrons for the strongly correlated multi-component ion plasma expected in the outer layers of neutron star's crust employing a Path Integral Monte Carlo (PIMC) approach. This allows us to isolate the low energy response of the ions and use it to calculate the electron scattering rate and the electron thermal conductivity. We find that the scattering rate is enhanced by a factor 2-4 compared to earlier calculations based on the simpler electron-impurity scattering formalism. This findings directly impacts the interpretation of thermal relaxation observed in transiently accreting neutron stars and has implications for the composition and nuclear reactions in the crust that occur during accretion.

astro-ph.HE↗

arXiv2016

White Paper on Nuclear Astrophysics

This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21-23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9- 10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12-13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade.

astro-ph.SR↗

arXiv2016

Nucleon-nucleon bremsstrahlung of dark gauge bosons and revised supernova constraints

We calculate the rate of production of hypothetical light vector bosons (LVBs) from nucleon-nucleon bremsstrahlung reactions in the soft radiation limit directly in terms of the measured nucleon-nucleon elastic cross sections. We use these results and the observation of neutrinos from supernova SN1987a to deduce constraints on the couplings of vector bosons with masses $\lesssim 200$ MeV to either electric charge (dark photons) or to baryon number. We establish for the first time strong constraints on LVB that couple only to baryon number, and revise earlier constraints on the dark photon. For the latter, we find that the excluded region of parameter space is diminished by about a factor of 10.

nucl-th↗

arXiv2015

Quantum Monte Carlo calculations of the thermal conductivity of neutron star crusts

We use the quantum Monte Carlo (QMC) techniques to calculate the static structure function $S(q)$ of a one-component ion lattice and use it to calculate the thermal conductivity $κ$ of high-density solid matter expected in the neutron star crust. By making detailed comparisons with the results for the thermal conductivity obtained using standard techniques based on the one-phonon approximation (OPA) valid at low temperature, and the multi-phonon harmonic approximation expected to be valid over a wide range of temperatures, we asses the temperature regime where $S(q)$ from QMC can be used directly to calculate $κ$. We also compare the QMC results to those obtained using classical Monte Carlo to quantitatively asses the magnitude of the quantum corrections. We find that quantum effects became relevant for the calculation of $κ$ at temperature $T \lesssim 0.3 ~Ω_\mathrm{P}$, where $Ω_\mathrm{P}$ is the ion plasma frequency. At $T \simeq 0.1 ~Ω_\mathrm{P}$ the quantum effects suppress $κ$ by about $30\%$. The comparison with the results of the OPA indicates that dynamical information beyond the static structure is needed when $T \lesssim 0.1~ Ω_\mathrm{P}$. These quantitative comparisons help to establish QMC as a viable technique to calculate $κ$ at moderate temperatures in the range $T=0.1-1~Ω_\mathrm{P}$ of relevance to the study of accreting neutron stars. This finding is especially important because QMC is the only viable technique so far for calculating $κ$ in multi-component systems at low-temperatures.

astro-ph.HE↗

arXiv2015

Charged-current reactions in the supernova neutrino-sphere

We calculate neutrino absorption rates due to charged-current reactions $ν_e+n \rightarrow e^- + p $ and $\barν_e+p \rightarrow e^+ + n $ in the outer regions of a newly born neutron star called the neutrino-sphere. To improve on recent work which has shown that nuclear mean fields enhance the $ν_e$ cross-section and suppress the $\barν_e$ cross-section, we employ realistic nucleon-nucleon interactions that fit measured scattering phase shifts. Using these interactions we calculate the momentum-, density-, and temperature-dependent nucleon self-energies in the Hartree-Fock approximation. A potential derived from chiral effective field theory and a pseudo-potential constructed to reproduce nucleon-nucleon phase shifts at the mean-field level are used to study the equilibrium proton fraction and the charged-current rates are studied in detail. We compare our results to earlier calculations obtained using phenomenological mean-field models and to those obtained in the virial expansion valid at low density. We find that for typical ambient conditions in the neutrino-sphere, $T=5-10$ MeV and $ρ=10^{11}-10^{13}$ g/cm$^3$, the difference between the $ν_{e}$ and $\barν_e$ absorption rates is much larger than predicted earlier. Our results have important implications for heavy-element nucleosynthesis in supernovae and for supernova neutrino detection.

nucl-th↗

arXiv2014

Electron-neutron scattering and transport properties of neutron stars

We show that electrons can couple to the neutron excitations in neutron stars and find that this can limit their contribution to the transport properties of dense matter, especially the shear viscosity. The coupling between electrons and neutrons is induced by protons in the core, and by ions in the crust. We calculate the effective electron-neutron interaction for the kinematics of relevance to the scattering of degenerate electrons at high density. We use this interaction to calculate the electron thermal conductivity, electrical conductivity, and shear viscosity in the neutron star inner crust, and in the core where we consider both normal and superfluid phases of neutron-rich matter. In some cases, particularly when protons are superconducting and neutrons are in their normal phase, we find that electron-neutron scattering can be more important than the other scattering mechanisms considered previously.

nucl-th↗

arXiv2014

The Role of the Electron Mass in Damping Chiral Magnetic Instability in Supernova and Neutron Stars

We show that the nonzero electron mass plays a critical role in determining the magnetic properties of neutron stars, making it impossible to generate the chiral charge density needed to trigger a strong chiral magnetic instability during the core collapse of supernovae. This instability has been proposed as a plausible mechanism for generating extremely large helical magnetic fields in neutron stars at their birth; the mechanism relies on the generation of a large non-equilibrium chiral charge density via electron capture reactions that selectively deplete left-handed electrons during core-collapse and the early evolution of the protoneutron star. Our calculation shows that the electron chirality violation rate induced by Rutherford scattering, despite being suppressed by the smallness of the electron mass relative to the electron chemical potential, is still fast compared to the weak interaction electron capture rate. The resulting asymmetry between right and left-handed electron densities is therefore never able to attain an astrophysically relevant magnitude.

hep-ph↗

arXiv2013

Neutrino scattering from hydrodynamic modes in hot and dense neutron matter

We calculate the scattering rate of low energy neutrinos in hot and dense neutron matter encountered in neutrons stars and supernova in the hydrodynamic regime. We find that the Brillouin peak, associated with the sound mode, and the Rayleigh peak, associated with the thermal diffusion mode, dominate the dynamic structure factor. Although the total scattering cross section is constrained by the compressibility sum rule, the differential cross-section calculated using the hydrodynamic response function differs from results obtained in approximate treatments often used in astrophysics such as random phase approximations (RPA). We identified these differences and discuss its implications for neutrino transport in supernova.

nucl-th↗

arXiv2013

Dark Matter Thermalization in Neutron Stars

We study how many-body effects alter the dark matter (DM) thermalization time inside neutron stars. We find that Pauli blocking, kinematic constraints, and superfluidity and superconductivity in the neutron star significantly affect the DM thermalization time, in general lengthening it. This could change the final DM mass and DM-nucleon cross section constraints by considering black hole formation in neutron stars due to DM accretion. We consider the class of models in which DM is an asymmetric, complex scalar particle with a mass between 1 keV and 5 GeV which couples to regular matter via some heavy vector boson. Interestingly, we find that the discovery of asymmetric, bosonic DM could motivate the existence of exotic neutron star cores. We apply our results to the case of mixed sneutrino DM.

hep-ph↗

arXiv2013

Goldstone modes in the neutron star core

We formulate a theory of Goldstone bosons and their interactions in the superfluid and superconducting phase of dense nucleonic matter at densities of relevance to the neutron star core. For typical neutron star temperatures in the range T = 10^6 to 10^9 K, the Goldstone mode associated with rotational symmetry, called angulons, couple weakly to each other and to electrons. Consequently, these modes have anomalously large mean free paths and can contribute to both diffusive and ballistic transport of heat and momentum. In contrast, the two Goldstone bosons associated with density oscillations of the neutron and electron + proton fluids, called superfluid phonons, mix and couple strongly to electrons. They have shorter mean free paths, and their contribution to transport is negligible. Long-wavelength superfluid phonons and angulons can play a role in neutron star seismology, and lead to interesting phenomenology as angulons couple to magnetic fields and have anisotropic dispersion relations.

nucl-th↗

arXiv2013

Forecasting neutron star temperatures: predictability and variability

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 variations 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.

astro-ph.HE↗

arXiv2012

Medium modification of the charged current neutrino opacity and its implications

Previous work on neutrino emission from proto-neutron stars which employed full solutions of the Boltzmann equation showed that the average energies of emitted electron neutrinos and antineutrinos are closer to one another than predicted by older, more approximate work. This in turn implied that the neutrino driven wind is proton rich during its entire life, precluding $r$-process nucleosynthesis and the synthesis of Sr, Y, and Zr. This work relied on charged current neutrino interaction rates that are appropriate for a free nucleon gas. Here, it is shown in detail that the inclusion of the nucleon potential energies and collisional broadening of the response significantly alters this conclusion. Iso-vector interactions, which give rise to the nuclear symmetry energy, produce a difference between the neutron and proton single-particle energies $ΔU=U_n-U_p$ and alter the kinematics of the charged current reactions. In neutron-rich matter, and for a given neutrino/antineutrino energy, the rate for $ν_e+n\rightarrow e^-+p$ is enhanced while $ \barν_e+p\rightarrow n+e^+$ is suppressed because the $Q$ value for these reactions is altered by $\pmΔU$, respectively. In the neutrino decoupling region, collisional broadening acts to enhance both $ν_e$ and $\barν_e$ cross-sections and RPA corrections decrease the $ν_e$ cross-section and increase the $\bar ν_e$ cross-section, but mean field shifts have a larger effect. Therefore, electron neutrinos decouple at lower temperature than when the nucleons are assumed to be free and have lower average energies. The change is large enough to allow for a reasonable period of time when the neutrino driven wind is predicted to be neutron rich. It is also shown that the electron fraction in the wind is influenced by the nuclear symmetry energy.

astro-ph.HE↗