Abstracts of Interest
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Abstract: 1710.09893
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Title: Supernova and prompt gravitational-wave precursors to LIGO gravitational-wave sources and short-GRBs
(Submitted on 24 Oct 2017)
Abstract: Binary black-holes (BHs) and binary neutron-stars (NSs) mergers had been recently detected through gravitational-wave (GW) emission, with the latter followed by post-merger electromagnetic counterparts, appearing seconds up to weeks after the merger. While post-merger electromagnetic counterparts had been anticipated theoretically, very little electromagnetic precursors to GW-sources had been proposed, and non observed yet. Here we show that a fraction of ${\rm a}\,{\rm few\times}10^{-4}-10^{-1}$ of LIGO GW-sources and short-GRBs, could be preceded by supernovae-explosions years up to decades before the merger. Each of the BH/NS-progenitors in GW-sources are thought to form following a supernova, likely accompanied by a natal velocity-kick to the newly born compact object. The evolution and natal-kicks determine the orbits of surviving binaries, and hence the delay-time between the birth of the compact-binary and its final merger through GW-emission. We use data from binary evolution population-synthesis models to show that the delay-time distribution has a non-negligible tail of ultra-short delay-times between 1-100 yrs, thereby giving rise to potentially observable supernovae precursors to GW-sources. Moreover, future LISA/DECIGO GW space-detectors will enable the detection of GW-inspirals in the pre-mergers stage weeks to decades before the final merger. These ultra-short delay-time sources could therefore produce a unique type of promptly appearing LISA/DECIGO-GW-sources accompanied by \emph{coincident} supernovae. The archival (and/or direct) detection of precursor (coincident) SNe with GW and/or short-GRBs will provide unprecedented characterizations of the merging-binaries, and their prior evolution through supernovae and natal kicks, otherwise inaccessible through other means.
Abstract: 1710.09894
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Full Text: [ PostScript, PDF]
Title: Modulations in Spectra of Galactic Gamma-ray sources as a result of photon-ALPs mixing
(Submitted on 24 Oct 2017)
Abstract: Axion like particles (ALPs) are fundamental pseudo scalar particles with properties similar to Axions which are a well-known extension of the standard model to solve the strong CP problem in Quantum Chromodynamics. ALPs can oscillate into photons and vice versa in the presence of an external tranversal magnetic field. This oscillation of photon and ALPs could have important implications for astronomical observations, i.e. a characteristic energy dependent attenuation in Gamma ray spectra for astrophysical sources. Here we have revisited the opportunity to search Photon-ALPs coupling in the disappearance channel. We use eight years of Fermi Pass 8 data of a selection of promising galactic Gamma-ray source candidates and study the modulation in the spectra in accordance with Photon-ALPs mixing and estimate best fit values of the parameters i.e. Photon-ALPs coupling constant$ (g_{\alpha\gamma\gamma} )$ and ALPs mass($m_{\alpha} $). For the magnetic field we use large scale galactic magnetic field models based on Faraday rotation measurements and we have also studied the survival probability of photons in the Galactic plane.
Abstract: 1710.09910
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Title: Hard X-ray properties of NuSTAR blazars
(Submitted on 26 Oct 2017)
Abstract: We present the results of our study on the hard X-ray properties of the NuSTAR blazars. We carried out timing, spectral and cross-correlation analysis of 31 NuSTAR observations of 13 blazars. We investigated the spectral shapes of the sources using single power-law, broken power-law and log-parabola models. We found that for most of the sources the hard X-ray emission can be well represented by single power-law model; and that the spectral slopes for different blazar sub-classes are consistent with so called "blazar sequence". We report a steepest spectra ($\Gamma\sim3$) in the BL Lacertae PKS 2155--304 and a hardest spectra ($\Gamma\sim1.4$) in the flat-spectrum radio quasar PKS 2149--306. In addition, we noted a close connection between the flux and spectral slope within the source sub-class in the sense that high flux and/or flux states tend to be harder in spectra. We also found the signatures of soft and hard lags of a few ours, and a complex relation between the flux and the hardness ratio. Using the minimum variability timescales, we infer the distribution of the emission region sizes. In BL Lacertae objects, assuming particle acceleration by diffusive shocks and synchrotron cooling as the dominant processes governing the observed flux variability, we constrain the magnetic field of the emission region to be a few gauss; whereas in flat-spectrum radio quasars, using external Compton models, we estimate the energy of the lower end of the injected electrons to be a few hundreds of Lorentz factors.
Abstract: 1710.06843
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Title: Neutron-star radius constraints from GW170817 and future detections
(Submitted on 18 Oct 2017 (v1), last revised 27 Oct 2017 (this version, v2))
Abstract: We introduce a new, powerful method to constrain properties of neutron stars (NSs). We show that the total mass of GW170817 provides a reliable constraint on the stellar radius if the merger did not result in a prompt collapse as suggested by the interpretation of associated electromagnetic emission. The radius R_1.6 of nonrotating NSs with a mass of 1.6 M_sun can be constrained to be larger than 10.68_{-0.04}^{+0.15} km, and the radius R_max of the nonrotating maximum mass configuration must be larger than 9.60_{-0.03}^{+0.14} km. We point out that detections of future events will further improve these constraints. Moreover, we show that a future event with a signature of a prompt collapse of the merger remnant will establish even stronger constraints on the NS radius from above and the maximum mass M_max of NSs from above. These constraints are particularly robust because they only require a measurement of the chirp mass and a distinction between prompt and delayed collapse of the merger remnant, which may be inferred from the electromagnetic signal or even from the presence/absence of a ringdown gravitational-wave (GW) signal. This prospect strengthens the case of our novel method of constraining NS properties, which is directly applicable to future GW events with accompanying electromagnetic counterpart observations. We emphasize that this procedure is a new way of constraining NS radii from GW detections independent of existing efforts to infer radius information from the late inspiral phase or postmerger oscillations, and it does not require particularly loud GW events.
Abstract: 1710.05784
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Title: Can an Axion be the Dark Energy particle ?
(Submitted on 16 Oct 2017)
Abstract: Following a phenomenological analysis done by the late Martin Perl for the detection of the dark energy, we show that an axion of energy $1.5\times 10^{-3}~eV/c^2$ can be a viable candidate for the dark energy particle. In particular, we obtain the characteristic length and frequency of the axion as a quantum particle. Then, employing a relation that connects the energy density with the frequency of a particle, i.e., $\rho\sim f^{4}$, we show that the energy density of axions, with the aforesaid value of mass, as obtained from our theoretical analysis is proportional to the dark energy density computed on observational data, i.e., $\rho_{a}/\rho_{DE}\sim \mathcal{O}(1)$.