Time and energy dependent characteristics of Thermonuclear burst oscillations [HEAP]


We have investigated temporal and spectral properties of a large sample of thermonuclear bursts with oscillations from eight different sources with spin frequencies varying from 270 to 620 Hz. For our sample we chose those bursts, for which the oscillation is sufficiently strong and of relatively long duration. The emission from the hot-spot that is formed during a thermonuclear burst is modulated by several physical processes and the burst oscillation profiles unavoidably carry signatures of these. In order to probe these mechanisms, we examined the amplitude and phase lags of the burst oscillations with energy. We also studied the frequency variation of oscillations during these thermonuclear bursts. We observed that the frequency drifts are more frequent in the cases where the spin frequency is lower. We found that the phase lag of the burst oscillations shows no systematic evolution with energy between the bursts, and also in between different sources. In 7 cases, we do indeed observe lag of soft energy photons, while there are a significant number of cases for which hard lag or no lag is observed.

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M. Chakraborty, Y. Bahar and E. Gogus
Wed, 22 Nov 17

Comments: 15 pages, 9 figures, accepted for publication in ApJ

Magnetic neutron star cooling and microphysics [HEAP]


We study the relative importance of several recent updates of microphysics input to the neutron star cooling theory and the effects brought about by superstrong magnetic fields of magnetars, including the effects of the Landau quantization in their crusts. We use a finite-difference code for simulation of neutron-star thermal evolution on timescales from hours to megayears with an updated microphysics input. The consideration of short timescales ($\lesssim1$ yr) is made possible by a treatment of the heat-blanketing envelope without the quasistationary approximation inherent to its treatment in traditional neutron-star cooling codes. For the strongly magnetized neutron stars, we take into account the effects of Landau quantization on thermodynamic functions and thermal conductivities. We simulate cooling of ordinary neutron stars and magnetars with non-accreted and accreted crusts and compare the results with observations. Suppression of radiative and conductive opacities in strongly quantizing magnetic fields and formation of a condensed radiating surface substantially enhance the photon luminosity at early ages, making the life of magnetars brighter but shorter. These effects together with the effect of strong proton superfluidity, which slows down the cooling of kiloyear-aged neutron stars, can explain thermal luminosities of about a half of magnetars without invoking heating mechanisms. Observed thermal luminosities of other magnetars are still higher than theoretical predictions, which implies heating, but the effects of quantizing magnetic fields and baryon superfluidity help to reduce the discrepancy.

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A. Potekhin and G. Chabrier
Wed, 22 Nov 17

Comments: 18 pages, 14 figures, accepted for publication in A&A

Lense-Thirring precession in ULXs as a possible means to constrain the neutron star equation-of-state [HEAP]


The presence of neutron stars in at least three ultraluminous X-ray sources is now firmly established and offers an unambiguous view of super-critical accretion. All three systems show long-timescale periods (60-80 days) in the X-rays and/or optical, two of which are known to be super-orbital in nature. Should the flow be classically super critical, i.e. the Eddington limit is reached locally in the disc (implying surface dipole fields that are sub-magnetar in strength), then the large scale-height flow can precess through the Lense-Thirring effect which could provide an explanation for the observed super-orbital periods. By connecting the details of the Lense-Thirring effect with the observed pulsar spin period, we are able to infer the moment-of-inertia and therefore equation-of-state of the neutron star without relying on the inclination of, or distance to the system. We apply our technique to the case of NGC 7793 P13 and demonstrate that stronger magnetic fields imply stiffer equations of state. We discuss the caveats and uncertainties, many of which can be addressed through forthcoming radiative magnetohydrodynamic (RMHD) simulations and their connection to observation.

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M. Middleton, P. Fragile, M. Bachetti, et. al.
Wed, 22 Nov 17

Comments: 14 pages, 9 figures, accepted to MNRAS

Contributions of the LOFAR Cosmic Ray Key Science Project to the 35th International Cosmic Ray Conference (ICRC 2017) [HEAP]


Contributions of the LOFAR Cosmic Ray Key Science Project to the 35th International Cosmic Ray Conference (ICRC 2017)

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A. Bonardi, S. Buitink, A. Corstanje, et. al.
Wed, 22 Nov 17

Comments: compendium of 8 proceedings for the ICRC 2017

Uncovering the identities of compact objects in high-mass X-ray binaries and gamma-ray binaries by astrometric measurements [HEAP]


We develop a method for identifying a compact object in binary systems with astrometric measurements and apply it to some binaries. Compact objects in some high-mass X-ray binaries and gamma-ray binaries are unknown, which is responsible for the fact that emission mechanisms in such systems have not yet confirmed. The accurate estimate of the mass of the compact object allows us to identify the compact object in such systems. Astrometric measurements are expected to enable us to estimate the masses of the compact objects in the binary systems via a determination of a binary orbit. We aim to evaluate the possibility of the identification of the compact objects for some binary systems. We then calculate probabilities that the compact object is correctly identified with astrometric observation (= confidence level) by taking into account a dependence of the orbital shape on orbital parameters and distributions of masses of white dwarfs, neutron stars, and black holes. We find that the astrometric measurements with the precision of 70 $\mu$as for $\gamma$~Cas allows us to identify the compact object at 99% confidence level if the compact object is a white dwarf with 0.6 M$_{\odot}$. In addition, we can identify the compact object with the precision of 10 $\mu$as at 97% or larger confidence level for LS~I~+61$^{\circ}$~303 and 99% or larger for HESS~J0632+057. These results imply that the astrometric measurements with the 10-$\mu$as precision level can realize the identification of compact objects for $\gamma$~Cas, LS~I~+61$^{\circ}$~303, and HESS~J0632+057.

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M. Yamaguchi, T. Yano and N. Gouda
Wed, 22 Nov 17

Comments: 11 pages, 4 figures, 4 tables. Accepted for publication in MNRAS

State Transitions in Ultracompact Neutron Star LMXBs: towards the Low Luminosity Limit [HEAP]


Luminosity of X-ray spectral state transitions in black hole and neutron star X-ray binaries can put constraint on the critical mass accretion rate between accretion regimes. Previous studies indicate that the hard-to-soft spectral state transitions in some ultracompact neutron star LMXBs have the lowest luminosity. With X-ray monitoring observations in the past decade, we were able to identify state transitions towards the lowest luminosity limit in 4U 0614+091, 2S 0918-549 and 4U 1246-588. By analysing corresponding X-ray pointed observations with the Swift/XRT and the RXTE/PCA, we found no hysteresis of state transitions in these sources, and determined the critical mass accretion rate in the range of 0.002 – 0.04 $\dot{\rm M}{\rm Edd}$ and 0.003 – 0.05 $\dot{\rm M}{\rm Edd}$ for the hard-to-soft and the soft-to-hard transition, respectively, by assuming a neutron star mass of 1.4 solar masses. This range is comparable to the lowest transition luminosity measured in black hole X-ray binaries, indicating the critical mass accretion rate is not affected by the nature of the surface of the compact stars. Our result does not support the Advection-Dominated Accretion Flow (ADAF) model which predicts that the critical mass accretion rate in neutron star systems is an order of magnitude lower if same viscosity parameters are taken. The low transition luminosity and insignificant hysteresis in these ultracompact X-ray binaries provide further evidence that the transition luminosity is likely related to the mass in the disc.

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J. Lin and W. Yu
Wed, 22 Nov 17

Comments: 12 pages, 4 figures, to appear in MNRAS

Glimpse of the highly obscured HMXB IGR J16318-4848 with Hitomi [HEAP]


We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extremely strong absorption of N_H~10^{24} cm^{-2}. Previous X-ray studies revealed that its spectrum is dominated by strong fluorescence lines of Fe as well as continuum emission. For physical and geometrical insight into the nature of the reprocessing material, we utilize the high spectroscopic resolving power of the X-ray microcalorimeter (the soft X-ray spectrometer; SXS) and the wide-band sensitivity by the soft and hard X-ray imager (SXI and HXI) aboard Hitomi. Even though photon counts are limited due to unintended off-axis pointing, the SXS spectrum resolves Fe K{\alpha_1} and K{\alpha_2} lines and puts strong constraints on the line centroid and width. The line width corresponds to the velocity of 160^{+300}_{-70} km s^{-1}. This represents the most accurate, and smallest, width measurement of this line made so far from any X-ray binary, much less than the Doppler broadening and shift expected from speeds which are characteristic of similar systems. Combined with the K-shell edge energy measured by the SXI and HXI spectra, the ionization state of Fe is estimated to be in the range of Fe I–IV. Considering the estimated ionization parameter and the distance between the X-ray source and the absorber, the density and thickness of the materials are estimated. The extraordinarily strong absorption and the absence of a Compton shoulder component is confirmed. These characteristics suggest reprocessing materials which are distributed in a narrow solid angle or scattering primarily with warm free electrons or neutral hydrogen.

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Hitomi. Collaboration, F. Aharonian, H. Akamatsu, et. al.
Wed, 22 Nov 17

Comments: 17 pages, 9 figures, 2 tables, accepted for publication in PASJ