Cyclotron lines: from magnetic field strength estimators to geometry tracers in neutron stars [HEAP]

With exactly forty years since the discovery of the first cyclotron line in Her X-1, there have been remarkable advancements in the field related to study of the physics of accreting neutron stars — cyclotron lines have been a major torchbearer in this regard, from being the only direct estimator of the magnetic field strength, a tracer of accretion geometry and an indicator of the emission beam in these systems. The main flurry of activities have centred around studying the harmonic separations, luminosity dependence, pulse phase dependence and more recently the shapes of the line and the trend for long term evolution in the line energy. This article visits the important results related to cyclotron lines since its discovery and reviews their significance. An emphasis is laid on pulse phase resolved spectroscopy and the important clues a joint timing and spectral study in this context can provide, to build a complete picture for the physics of accretion and hence X-ray emission in accreting neutron stars.

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C. Maitra
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

Dark Matter Annihilation into Four-Body Final States and Implications for the AMS Antiproton Excess [HEAP]

We consider dark matter annihilation into a general set of final states of Standard Model particles, including two-body and four-body final states that result from the decay of intermediate states. For dark matter masses ~10-10^5 GeV, we use updated data from Planck and from high gamma-ray experiments such as Fermi-LAT, MAGIC, and VERITAS to constrain the annihilation cross section for each final state. The Planck constraints are the most stringent over the entire mass range for annihilation into light leptons, and the Fermi-LAT constraints are the most stringent for four-body final states up to masses ~10^4 GeV. We consider these constraints in light of the recent AMS antiproton results, and show that for light mediators it is possible to explain the AMS data with dark matter, and remain consistent with Fermi-LAT Inner Galaxy measurements, for m_\chi ~ 60-100 GeV mass dark matter and mediator masses m_\phi / m_\chi ~< 1.

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S. Clark, B. Dutta and L. Strigari
Fri, 22 Sep 17

Comments: 12 pages, 6 figures

Revisiting field burial by accretion onto neutron stars [HEAP]

The surface magnetic field strength of millisecond pulsars (MSPs) is found to be about 4 orders of magnitude lower than that of garden variety radio pulsars (with a spin of $\sim 0.5-5$ s and $B\sim 10^{12}$G). The exact mechanism of the apparent reduction of field strength in MSPs is still a subject of debate. One of the proposed mechanisms is burial of the surface magnetic field under matter accreted from a companion. In this article we review the recent work on magnetic confinement of accreted matter on neutron stars poles. We present the solutions of the magneto-static equations with a more accurate equation of state of the magnetically confined plasma and discuss its implications for the field burial mechanism.

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D. Mukherjee
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

Radiation from rapidly rotating oblate neutron stars [HEAP]

A theoretical framework for emission originating from rapidly rotating oblate compact objects is described in detail. By using a Hamilton-Jacobi formalism, we show how the special relativistic rotational effects such as aberration of angles, Doppler boosting, and time dilatation naturally emerge from the general relativistic treatment of rotating compact objects. We use the Butterworth-Ipser metric expanded up to the second order in rotation and hence include effects of light bending, frame-dragging, and quadrupole deviations to our geodesic calculations. We also give detailed descriptions of the numerical algorithms used and provide an open source implementation of the numerical framework called Bender. As an application, we study spectral line profiles (i.e., smearing kernels) from rapidly rotating oblate neutron stars. We find that in this metric description the second order quadrupole effects are not strong enough to produce narrow observable features in the spectral energy distribution for almost any physically realistic parameter combination, and hence, actually detecting them is unlikely. The Full Width at Tenth Maximum and Full Width at Half Maximum of the smearing kernels are also reported for all of the possible viewing angles. These can be then used to quantitatively estimate the effects of rotational smearing on the observed spectra. We also calculate accurate pulse profiles and observer skymaps of emission from hot spots on rapidly rotating accreting millisecond pulsars. These allow us to quantify the strength of the pulse fractions one expects to observe from typical fast spinning millisecond pulsars.

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J. Nattila and P. Pihajoki
Fri, 22 Sep 17

Comments: 19 pages, 12 figures. Submitted to A&A. Comments very welcome!

The impact of vorticity waves on the shock dynamics in core-collapse supernovae [HEAP]

Convective perturbations arising from nuclear shell burning can play an important role in propelling neutrino-driven core-collapse supernova explosions. In this work, we analyze the impact of vorticity waves on the shock dynamics and the post-shock flow using the solution of the linear hydrodynamics equations. We show that the entropy perturbations generated by the interaction of the shock with vorticity waves may play a dominant role in generating buoyancy-driven turbulence in the gain region. We estimate that the resulting reduction in the critical luminosity is 17-24%, which approximately agrees with the results of three-dimensional neutrino-hydrodynamics simulations.

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C. Huete, E. Abdikamalov and D. Radice
Fri, 22 Sep 17

Comments: Submitted to MNRAS

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