Study of Magnetized accretion flow with cooling processes [HEAP]

We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with bremsstrahlung cooling and cyclotron cooling. All accretion solutions terminate with a shock close to the neutron star surface, but at some region of the parameter space, it also harbours a second shock away from the star surface. We have found that cyclotron cooling is necessary for correct accretion solutions which match the surface boundary conditions.

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K. Singh and I. Chattopadhyay
Mon, 19 Feb 18

Comments: 8 pages, 6 figures, published in a special issue of JApA

Efficiency of Centrifugal Mechanism in Producing PeV Neutrinos From Active Galactic Nuclei [HEAP]

A several-step theoretical model is constructed to trace the origin of ultra high energy (UHE) $[1-2]$PeV neutrinos detected, recently, by the IceCube collaboration. Protons in the AGN magnetosphere, experiencing different gravitational centrifugal force, provide free energy for the parametric excitation of Langmuir waves via a generalized two-stream instability. Landau damping of these waves, outside the AGN magnetosphere, can accelerate protons to ultra high energies. The ultimate source for this mechanism, the Langmuir-Landau-Centrifugal-Drive (LLCD), is the gravitational energy of the compact object. The LLCD generated UHE protons provide the essential ingredient in the creation of UHE neutrinos via appropriate hadronic reactions; protons of energy~ $10^{17}$eV can be generated in the plasmas surrounding AGN with bolometric luminosities of the order of $10^{43}$ergs s$^{-1}$. By estimating the diffusive energy flux of extragalactic neutrinos in the energy interval $[1-2]$PeV, we find that an acceptably small fraction $0.003\%$ of the total bolometric luminosity will suffice to create the observed fluxes of extragalactic ultra-high energy neutrinos.

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Z. Osmanov, S. Mahajan, G. Machabeli, et. al.
Fri, 16 Feb 18

Comments: 7 pages, 1 figure

On the timing behavior of PSR B1259-63 under the propeller torque from a transient accretion disc [HEAP]

The $\gamma$-ray pulsar binary system PSR B1259-63 flares in GeV after each periastron. The origin of those flares is still under debate. Recently, [2017ApJ…844..114Y] proposed a mechanism that might explain the GeV flares. In that model, a transient accretion disc is expected to be formed from the matter which was gravity-captured by the neutron star from the main sequence companion’s circumstellar disc. The transient accretion disc exerts a spin down torque on the neutron star (propeller effect), which might be traceable via pulsar timing observation of PSR B1259-63. In this paper, we phenomenologically consider the propeller effect with a parameter $\chi$, which describes the coupling between the disc matter and the neutron star. Comparing the expected timing residuals against the recent observation in [2014MNRAS.437.3255S], we conclude that the angular momentum transfer is very weak (with the coupling parameter $\chi\le10^{-4}$).

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S. Yi and K. Cheng
Fri, 16 Feb 18

Comments: 5 Pages, 7 Figures,accepted for publication in MNRAS

Hydrodynamical Neutron-star Kicks in Electron-capture Supernovae and Implications for the CRAB Supernova [HEAP]

Neutron stars (NSs) obtain kicks of typically several 100 km/s at birth. The gravitational tug-boat mechanism can explain these kicks as consequences of asymmetric mass ejection during the supernova (SN) explosion. Support for this hydrodynamic explanation is provided by observations of SN remnants with associated NSs, which confirm the prediction that the bulk of the explosion ejecta, in particular chemical elements between silicon and the iron group, are dominantly expelled in the hemisphere opposite to the direction of the NS kick. Here, we present a large set of two- and three-dimensional explosion simulations of electron-capture SNe, considering explosion energies between ~3×10^49 erg and ~1.6×10^50 erg. We find that the fast acceleration of the SN shock in the steep density gradient delimiting the O-Ne-Mg core of the progenitor enables such a rapid expansion of neutrino-heated matter that the growth of neutrino-driven convection freezes out quickly in a high-mode spherical harmonics pattern. Since the corresponding momentum asymmetry of the ejecta is very small and the gravitational acceleration by the fast-expanding ejecta abates rapidly, the NS kick velocities are at most a few km/s. The extremely low core compactness of O-Ne-Mg-core progenitors therefore favors hydrodynamic NS kicks much below the ~160 km/s measured for the Crab pulsar. This suggests either that the Crab Nebula is not the remnant of an electron-capture SN, but of a low-mass iron-core progenitor, or that the Crab pulsar was not accelerated by the gravitational tug-boat mechanism but received its kick by a non-hydrodynamic mechanism such as, e.g., anisotropic neutrino emission.

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A. Gessner and H. Janka
Fri, 16 Feb 18

Comments: 21 pages, 9 figures; submitted to ApJ

On the Radar detection of high-energy neutrino-induced cascades in ice; From Radar scattering cross-section to sensitivity [HEAP]

In recent works we discussed the feasibility of the radar detection technique as a new method to probe high-energy cosmic-neutrino induced plasmas in ice. Using the different properties of the induced ionization plasma, an energy threshold of several PeV was derived for the over-dense scattering of a radio wave off the plasma. Next to this energy threshold the radar return power was determined for the different constituents of the plasma. It followed that the return signal should be detectable at a distance of several hundreds of meters to a few kilometers, depending on the plasma constituents and considered geometry. In this article we describe a more detailed modeling of the scattering process by expanding our model to include the full shower geometry, as well as the reflection off the under-dense plasma region. We include skin-effects, as well as the angular dependence of the scattered signal. As a first application of this more detailed modeling approach, we provide the effective area and sensitivity for a simplified detector setup. It follows that, depending on the detailed plasma properties, the radar detection technique provides a very promising method for the detection of neutrino induced particle cascades at energies above several PeV. Nevertheless, to determine the feasibility of the method more detailed information about the plasma properties, especially its lifetime and the free charge collision rate, are needed.

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K. Vries, P. Coppin, A. OMurchadha, et. al.
Fri, 16 Feb 18

Comments: Submitted to Astropart. Phys

Hadronic models of the Fermi bubbles: Future perspectives [HEAP]

The origin of sub-TeV gamma rays detected by Fermi-LAT from the Fermi bubbles at the Galactic center is still unknown. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce observed gamma rays. Such interactions naturally produce high-energy neutrinos, and detection of those can discriminate between a hadronic and a leptonic origin of gamma rays. Additional constraints on the Fermi bubbles gamma-ray flux in the TeV range from recent HAWC observations restrict hadronic model parameters, which in turn disfavor Fermi bubbles as the origin of a large fraction of neutrino events detected by IceCube along the bubble directions. We revisit our hadronic model and discuss future constraints on parameters from observations in very high-energy gamma rays and neutrinos.

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S. Razzaque and L. Yang
Fri, 16 Feb 18

Comments: 8 pages, 4 figures. Invited article submitted to MDPI journal Galaxies for its special issue, Searching for Connections among the Fermi Bubbles, the Galactic Center GeV Excess, and Loop I, edited by D. Malyshev

Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9$-$342058 [HEAP]

We report on two new quiescent {\it XMM-Newton} observations (in addition to the earlier {\it Swift}/XRT and {\it XMM-Newton} coverage) of the cooling neutron star crust in the low-mass X-ray binary 1RXS J180408.9$-$342058. Its crust was heated during the $\sim$4.5 month accretion outburst of the source. From our quiescent observations, fitting the spectra with a neutron star atmosphere model, we found that the crust had cooled from $\sim$ 100 eV to $\sim$73 eV from $\sim$8 days to $\sim$479 days after the end of its outburst. However, during the most recent observation, taken $\sim$860 days after the end of the outburst, we found that the crust appeared not to have cooled further. This suggested that the crust had returned to thermal equilibrium with the neutron star core. We model the quiescent thermal evolution with the theoretical crustal cooling code NSCool and find that the source requires a shallow heat source, in addition to the standard deep crustal heating processes, contributing $\sim$0.9 MeV per accreted nucleon during outburst to explain its observed temperature decay. Our high quality {\it XMM-Newton} data required an additional hard component to adequately fit the spectra. This slightly complicates our interpretation of the quiescent data of 1RXS J180408.9$-$342058. The origin of this component is not fully understood.

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A. Parikh, R. Wijnands, N. Degenaar, et. al.
Fri, 16 Feb 18

Comments: Accepted for publication by MNRAS