Relativistic effects due to gravimagnetic moment of a rotating body [CL]

We compute exact Hamiltonian (and corresponding Dirac brackets) for spinning particle with gravimagnetic moment $\kappa$ in an arbitrary gravitational background. $\kappa=0$ corresponds to the Mathisson-Papapetrou-Tulczyjew-Dixon (MPTD) equations. $\kappa=1$ leads to modified MPTD equations with reasonable behavior in the ultrarelativistic limit. So we study the modified equations in the leading post-Newtonian approximation. Rotating body with unit gravimagnetic moment has qualitatively different behavior as compared with MPTD body: A) If a number of gyroscopes with various rotation axes are freely traveling together, the angles between the axes change with time. B) For specific binary systems, gravimagnetic moment gives a contribution to frame-dragging effect with the magnitude, that turns out to be comparable with that of Schiff frame dragging.

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W. Ramirez and A. Deriglazov
Fri, 22 Sep 17

Comments: 21 pages

Long-term Spectral Variability of the Ultra-luminous X-ray source Holmberg IX X–1 [HEAP]

We investigate the long-term spectral variability in the ultra-luminous X-ray source Holmberg IX X–1. By analyzing the data from eight {\it Suzaku} and 13 {\it XMM-Newton} observations conducted between 2001 and 2015, we perform a detailed spectral modeling for all spectra with simple models and complex physical models. We find that the spectra can be well explained by a disc plus thermal Comptonization model. Applying this model, we unveil correlations between the X-ray luminosity ($L_{\rm X}$) and the spectral parameters. Among the correlations, a particular one is the statistically significant positive correlation between $L_{\rm X}$ and the photon index ($\Gamma$), while at the high luminosities of $> 2\times10^{40}\,{\rm~erg\ s}^{-1}$, the source becomes marginally hard and that results a change in the slope of the $\Gamma – L_{\rm X}$ correlation. Similar variability behavior is observed in the optical depth of the source around $L_{\rm X} \sim 2\times10^{40}\,{\rm~erg\ s}^{-1}$ as the source becomes more optically thick. We consider the scenario that a corona covers the inner part of the disc, and the correlations can be explained as to be driven by the variability of seed photons from the disc input into the corona. On the basis of the disc-corona model, we discuss the physical processes that are possibly indicated by the variability of the spectral parameters. Our analysis reveals the complex variability behavior of Holmberg IX X–1 and the variability mechanism is likely related to the geometry of the X-ray emitting regions.

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V. Jithesh, R. Misra and Z. Wang
Fri, 22 Sep 17

Comments: Accepted for publication in ApJ, 12 Pages, 3 Tables, 3 Figures

Phase transition in compact stars: nucleation mechanism and $γ$-ray bursts revisited [HEAP]

We have revisited the nucleation process based on the Lifshitz-Kagan theory, which is the underlying mechanism of conversion of a pulsar constituted of hadronic matter to a quark star. We have selected appropriate models that have been tested against experimental and observational constraints to restrict the model arbitrariness present in previous investigations. The phase transition pressures and chemical potentials have been identified and afterwards, the tunneling probabilities and the nucleation time were computed. The critical pressures for which the half life of the metastable hadronic phase is one year were obtained. Even with the restrictions imposed to the selection of models, the results remained model dependent, but we found that the tunneling that makes possible the appearance of stable matter requires an overpressure that is practically independent of the quark matter bag constant. Finally, we have confirmed that the nucleation process can be one of the causes of gamma-ray bursts.

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K. Marquez and D. Menezes
Fri, 22 Sep 17

Comments: 19 pages, 7 figures

Whispers from the edge of physics [HEAP]

Neutron stars involve extreme physics which is difficult (perhaps impossible) to explore in laboratory experiments. We have to turn to astrophysical observations, and try to extract information from the entire range of the electromagnetic spectrum. In addition, neutron stars may radiate gravitational waves through a range of scenarios. In this brief summary I outline some of the main ideas, focussing on what we do and do not know, and describe the challenges involved in trying to catch these faint whispers from the very edge of physics.

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N. Andersson
Fri, 22 Sep 17

Comments: Has appeared in Journal of Astrophysics and Astronomy special issue on ‘Physics of Neutron Stars and related objects’, celebrating the 75th birth-year of G. Srinivasan

Non-standard neutrino self-interactions in a supernova and fast flavor conversions [CL]

We study the effects of non-standard self-interactions (NSSI) of neutrinos streaming out of a core-collapse supernova. We show that with NSSI, the standard linear stability analysis gives rise to linearly as well as exponentially growing solutions. We demonstrate analytically that flavor-preserving NSSI lead to suppression of collective oscillations, while flavor-violating NSSI allow an earlier onset with enhanced growth rate of oscillations. In the intersecting four-beam model, we show that flavor-violating NSSI can lead to fast oscillations even when the angle between the neutrino and antineutrino beams is obtuse, which is forbidden in the standard model. This leads to the new possibility of fast oscillations in a two-beam system with opposing neutrino-antineutrino fluxes, even in the absence of any spatial inhomogeneities. Finally, we solve the full non-linear equations of motion in the four-beam model numerically, and explore the interplay of fast and slow flavor conversions in the long-time behavior, in the presence of NSSI.

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A. Dighe and M. Sen
Fri, 22 Sep 17

Comments: 12 pages, Revtex style, 15 figures

Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above $8 \times 10^{18}$ eV [HEAP]

Cosmic rays are atomic nuclei arriving from outer space that reach the highest energies observed in nature. Clues to their origin come from studying the distribution of their arrival directions. Using $3 \times 10^4$ cosmic rays above $8 \times 10^{18}$ electron volts, recorded with the Pierre Auger Observatory from a total exposure of 76,800 square kilometers steradian year, we report an anisotropy in the arrival directions. The anisotropy, detected at more than the 5.2$\sigma$ level of significance, can be described by a dipole with an amplitude of $6.5_{-0.9}^{+1.3}$% towards right ascension $\alpha_{d} = 100 \pm 10$ degrees and declination $\delta_{d} = -24_{-13}^{+12}$ degrees. That direction indicates an extragalactic origin for these ultra-high energy particles.

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Pierre Auger Collaboration et. al.
Fri, 22 Sep 17

Comments: 19 pages (with supplementary material), 8 figures

Gravitational waves from single neutron stars: an advanced detector era survey [HEAP]

With the doors beginning to swing open on the new gravitational wave astronomy, this review provides an up-to-date survey of the most important physical mechanisms that could lead to emission of potentially detectable gravitational radiation from isolated and accreting neutron stars. In particular we discuss the gravitational wave-driven instability and asteroseismology formalism of the f- and r-modes, the different ways that a neutron star could form and sustain a non-axisymmetric quadrupolar “mountain” deformation, the excitation of oscillations during magnetar flares and the possible gravitational wave signature of pulsar glitches. We focus on progress made in the recent years in each topic, make a fresh assessment of the gravitational wave detectability of each mechanism and, finally, highlight key problems and desiderata for future work.

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K. Glampedakis and L. Gualtieri
Fri, 22 Sep 17

Comments: 39 pages, 12 figures, 2 tables. Chapter of the book “Physics and Astrophysics of Neutron Stars”, NewCompStar COST Action 1304