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

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

The radio and X-ray mode-switching pulsar PSR B0943+10 [HEAP]


Observations obtained in the last years challenged the widespread notion that rotation-powered neutron stars are steady X-ray emitters. Besides a few allegedly rotation-powered neutron stars that showed “magnetar-like” variability, a particularly interesting case is that of PSR B0943+10. Recent observations have shown that this pulsar, well studied in the radio band where it alternates between a bright and a quiescent mode, displays significant X-ray variations, anticorrelated in flux with the radio emission. The study of such synchronous radio/X-ray mode switching opens a new window to investigate the processes responsible for the pulsar radio and high-energy emission. Here we review the main X-ray properties of PSR B0943+10 derived from recent coordinated X-ray and radio observations.

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S. Mereghetti and M. Rigoselli
Fri, 22 Sep 17

Comments: Published in Journal of Astrophysics and Astronomy special issue on ‘Physics of Neutron Stars and Related Objects’, celebrating the 75th birth-year of G. Srinivasan

Spectral differences between the jets in `radio loud' and `radio quiet' hard state black hole binaries [HEAP]


We have compiled from the available literature a large set of radio measurements of black hole binaries in the hard X-ray state for which measurements of the gigahertz frequency radio spectral index are possible. We separate the sample into radio loud' andradio quiet’ subsets based upon their distribution in the radio — X-ray plane, and investigate the distribution of radio spectral indices within each subset. The distribution of spectral indices of the radio loud' subset is well described by a Gaussian distribution with mean spectral index $\alpha = +0.2$ and standard deviation $0.2$ (here spectral index is defined such that a positive spectral index means more flux at higher frequencies). The sparser sample for theradio quiet’ subset can be approximated, less well, by a Gaussian with mean $\alpha = -0.2$ and standard deviation $0.3$; alternatively the simple mean of the distribution of the radio quiet subset is $-0.3$. The two spectral index distributions are different at high statistical significance. Confirming previous work in the literature, we test to see if the differences in observed spectra could result from different distributions of jet viewing angles, but find no evidence for this. We conclude therefore that the jets in the two groups are physically different in some way, and briefly discuss possible origins and further possible diagnostics. Finally we note that extrapolating to lower frequencies the two subsets move closer together in the radio — X-ray plane, and approximately merge into a single distribution at around 400 MHz.

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M. Espinasse and R. Fender
Fri, 22 Sep 17

Comments: Accepted for publication in MNRAS

Relativistic disc line: a tool to constrain neutron star equation of state models [HEAP]


Relativistic iron K$\alpha$ spectral emission line from the inner disc of a neutron star low-mass X-ray binary (LMXB) was first detected in 2007. This discovery opened up new ways to probe strong gravity and dense matter. The past decade has seen detections of such a line from many neutron star LMXBs, and confirmation of this line from the same source with several X-ray satellites. These have firmly established the new field of relativistic disc line from neutron star systems in only ten years. Fitting the shape of such a line with an appropriate general relativistic model provides the accretion disc inner edge radius to the stellar mass ratio. In this review, we briefly discuss how an accurate measurement of this ratio with a future larger area X-ray instrument can be used to constrain neutron star equation of state models.

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S. Bhattacharyya
Fri, 22 Sep 17

Comments: 10 pages, 10 figures, 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

Cooling of Accretion-Heated Neutron Stars [HEAP]


We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.

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R. Wijnands, N. Degenaar and D. Page
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. In case of missing sources and/or references in the tables, please contact the first author and they will be included in updated versions of this review