Abstracts of Interest
Selected by:
Roger Clay
Abstract: 1807.10153
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Title: Solar energetic particles and galactic cosmic rays over millions of years as inferred from data on cosmogenic $^{26}$Al in lunar samples
(Submitted on 26 Jul 2018)
Abstract: Aims. Lunar soil and rocks are not protected by a magnetic field or an atmosphere and are continuously irradiated by energetic particles that can produce cosmogenic radioisotopes directly inside rocks at different depths depending on the particle's energy. This allows the mean fluxes of solar and galactic cosmic rays to be assessed on the very long timescales of millions of years. Methods. Here we show that lunar rocks can serve as a very good particle integral spectrometer in the energy range 20-80 MeV. We have developed a new method based on precise modeling, that is applied to measurements of $^{26}$Al (half-life ~0.7 megayears) in lunar samples from the Apollo mission, and present the first direct reconstruction (i.e., without any a priori assumptions) of the mean energy spectrum of solar and galactic energetic particles over a million of years. Results. We show that the reconstructed spectrum of solar energetic particles is totally consistent with that over the last decades, despite the very different levels of solar modulation of galactic cosmic rays ($\phi=496\pm 40$ MV over a million years versus $\phi= 660\pm 20$ MV for the modern epoch). We also estimated the occurrence probability of extreme solar events and argue that no events with the F(>30 MeV) fluence exceeding $5*10^{10}$ and $10^{11}$ cm$^2$ are expected on timescales of a thousand and million years, respectively. Conclusions. We conclude that the mean flux of solar energetic particles hardly depends on the level of solar activity, in contrast to the solar modulation of galactic cosmic rays. This puts new observational constraints on solar physics and becomes important for assessing radiation hazards for the planned space missions.
Abstract: 1807.10034
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Full Text: [ PostScript, PDF]
Title: Is it Mixed dark matter or neutrino masses?
(Submitted on 26 Jul 2018)
Abstract: In this paper we explore a scenario where the dark matter is a mixture of interacting and non interacting species. We observe that the effect on the matter power spectrum can ressemble the suppression of power expected in the case of non-negligible neutrino masses, though the effect described here manifests itself at smaller scales. We conclude that the lack of precise determination of the bias parameter could hinder our ability to disentangle the effect of neutrino masses from a mixture of collisional and collisionless dark matter fluids. This analysis is the first to highlight such a potential complication in the determination of neutrino masses owing to dark matter particle properties.
Abstract: 1807.09409
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Title: Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole
(Submitted on 25 Jul 2018)
Abstract: The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +\- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.
Abstract: 1807.09750
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Full Text: [ PostScript, PDF]
Title: Bright gamma-ray flares powered by magnetic reconnection in QED-strength magnetic fields
(Submitted on 25 Jul 2018)
Abstract: Strong magnetic fields in magnetospheres of neutron stars (especially magnetars) and other astrophysical objects may release their energy in violent, intense episodes of magnetic reconnection. While reconnection has been studied extensively, the extreme field strength near neutron stars introduces new effects: synchrotron cooling and electron-positron pair production. Using massively parallel particle-in-cell simulations that self-consistently incorporate these new quantum-electrodynamic effects, we investigate relativistic magnetic reconnection in the strong-field regime. We show that reconnection in this regime can efficiently convert magnetic energy to X-ray and gamma-ray radiation and thus power bright high-energy astrophysical flares. Rapid radiative cooling causes strong plasma and magnetic field compression in compact plasmoids. In the most extreme cases, the field can approach the critical quantum limit, leading to copious pair production.
Abstract: 1807.08762
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Title: A wildly flickering jet in the black hole X-ray binary MAXI J1535-571
(Submitted on 23 Jul 2018)
Abstract: We report on the results of optical, near-infrared (NIR) and mid-infrared observations of the black hole X-ray binary candidate (BHB) MAXI J1535-571 during its 2017/2018 outburst. During the first part of the outburst (MJD 58004-58012), the source shows an optical-NIR spectrum that is consistent with an optically thin synchrotron power-law from a jet. After MJD 58015, however, the source faded considerably, the drop in flux being much more evident at lower frequencies. Before the fading, we measure a de-reddened flux density of $\gtrsim$100 mJy in the mid-infrared, making MAXI J1535-571 one of the brightest mid-infrared BHBs known so far. A significant softening of the X-ray spectrum is evident contemporaneous with the infrared fade. We interpret it as due to the suppression of the jet emission, similar to the accretion-ejection coupling seen in other BHBs. However, MAXI J1535-571 did not transition smoothly to the soft state, instead showing X-ray hardness deviations, associated with infrared flaring. We also present the first mid-IR variability study of a BHB on minute timescales, with a fractional rms variability of the light curves of $\sim 15-22 \%$, which is similar to that expected from the internal shock jet model, and much higher than the optical fractional rms ($\lesssim 7 \%$). These results represent an excellent case of multi-wavelength jet spectral-timing and demonstrate how rich, multi-wavelength time-resolved data of X-ray binaries over accretion state transitions can help refining models of the disk-jet connection and jet launching in these systems.
Abstract: 1807.09509
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Full Text: [ PostScript, PDF]
Title: VHE observations of binary systems performed with the MAGIC telescopes
(Submitted on 25 Jul 2018)
Abstract: The improvement on the Imaging Air Cherenkov Technique (IACT) led to the discovery of a new type of sources that can emit at very high energies: the gamma-ray binaries. Only six systems are part of this exclusive class. We summarize the latest results from the observations performed with the MAGIC telescopes on different systems as the gamma-ray binary LS I +61$^{\circ}$ 303 and the microquasars SS433, V404 Cygni and Cygnus X-1, which are considered potential VHE gamma-ray emitters. The binary system LS I +61$^{\circ}$ 303 has been observed by MAGIC in a long-term monitoring campaign. We show the newest results of our search for super-orbital variability also in context of contemporaneous optical observations. Besides, we will present the results of the only super-critical accretor known in our galaxy: SS 433. We will introduce the VHE results achieved with MAGIC after 100 hours of observations on the microquasar Cygnus X-1 and report on the microquasar V404 Cyg, which has been observed with MAGIC after it went through a series of exceptional X-ray outbursts in June 2015.
Abstract: 1807.09489
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Full Text: [ PostScript, PDF]
Title: Does slow and steady win the race? Investigating feedback processes in giant molecular clouds
(Submitted on 25 Jul 2018)
Abstract: We investigate the effects of gradual heating on the evolution of turbulent molecular clouds of mass $2\times 10^6$ M$_\odot$ and virial parameters ranging between $0.7-1.2$. This gradual heating represents the energy output from processes such as winds from massive stars or feedback from High Mass X-ray binaries (HMXBs), contrasting the impulsive energy injection from supernovae (SNe). For stars with a mass high enough that their lifetime is shorter than the life of the cloud, we include a SN feedback prescription. Including both effects, we investigate the interplay between slow and fast forms of feedback and their effectiveness at triggering/suppressing star formation. We find that SN feedback can carve low density chimneys in the gas, offering a path of least resistance for the energy to escape. Once this occurs the more stable, but less energetic, gradual feedback is able to keep the chimneys open. By funneling the hot destructive gas away from the centre of the cloud, chimneys can have a positive effect on both the efficiency and duration of star formation. Moreover, the critical factor is the number of high mass stars and SNe (and any subsequent HMXBs) active within the free-fall time of each cloud. This can vary from cloud to cloud due to the stochasticity of SN delay times and in HMXB formation. However, the defining factor in our simulations is the efficiency of the cooling, which can alter the Jeans mass required for sink particle formation, along with the number of massive stars in the cloud.
Abstract: 1807.09471
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Full Text: [ PostScript, PDF]
Title: Parameterisation of lateral density and arrival time distributions of Cherenkov photons in EASs as functions of independent shower parameters for different primaries
(Submitted on 25 Jul 2018)
Abstract: The simulation of Cherenkov photon's lateral density and arrival time distributions in Extensive Air Showers (EASs) was performed with the CORSIKA code in the energy range: 100 GeV to 100 TeV. On the basis of this simulation we obtained a set of approximating functions for the primary $\gamma$-ray photons, protons and iron nuclei incident at zenith angles from 0$^\circ$ to 40$^\circ$ over different altitudes of observation. Such a parameterisation is important for the primary particle identification, for the reconstruction of the shower observables and hence for a more efficient disentanglement of the $\gamma$-ray showers from the hadronic showers. From our parameterisation analysis, we have found that even though the geometry of the lateral density ($\rho_{ch}$) and the arrival time ($t_{ch}$) distributions is different for different primaries at a particular energy ($E$), at a particular incident angle ($\theta$) and at a particular altitude of observation ($H$) up to a given distance from the showe core ($R$), the distributions follow the same mathematical functions $\rho(E,R,\theta,H) = a E^{b}\exp[-\{c R + (\theta /d)^{2}-f H\}]$ and $t(E,R,\theta,H) = l E^{-m}\exp(n/R^{p})({\theta}^q+s)(u {H}^2+v)$ respectively but with different values of function parameters.
Abstract: 1807.10001
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Full Text: [ PostScript, PDF]
Title: Ongoing magnetic monopole searches with IceCube
(Submitted on 26 Jul 2018)
Abstract: The IceCube collaboration has instrumented a cubic kilometer of ice with $5160$ photo-multipliers. While mainly developed to detect Cherenkov light, any visible light can be used to detect particles within the ice. Magnetic monopoles are hypothetical particles predicted by many theories that extend the Standard model of Particle Physics. They are carriers of a single elementary magnetic charge. For this particle, different light production mechanisms dominate from direct Cherenkov light at highly relativistic velocities $\left(>0.76\,c\right)$, indirect Cherenkov light at mildly relativistic velocities $\left(\approx 0.5\,c \textrm{ to } 0.76\,c\right)$, luminescence light at low relativistic velocities $\left( \gtrsim 0.1\,c \textrm{ to } 0.5\,c\right)$, as well as catalysis of proton decay at non relativistic velocities $\left(\lesssim 0.1\,c\right)$. For each of this speed ranges, searches for magnetic monopoles at the IceCube experiment are either in progress or they have already set the world's best limits on the flux of magnetic monopoles. A summary of these searches will be presented, outlining already existing results as well as methods used by the currently conducted searches.
Abstract: 1807.08734
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Full Text: [ PostScript, PDF]
Title: Origins of Extragalactic Cosmic Ray Nuclei by Contracting Alignment Patterns induced in the Galactic Magnetic Field
(Submitted on 23 Jul 2018)
Abstract: We present a novel approach to search for origins of ultra-high energy cosmic rays. These particles are likely nuclei that initiate extensive air showers in the Earth's atmosphere. In large-area observatories, the particle arrival directions are measured together with their energies and the atmospheric depth at which their showers maximize. The depths provide rough measures of the nuclear charges. In a simultaneous fit to all observed cosmic rays we use the galactic magnetic field as a mass spectrometer and adapt the nuclear charges such that their extragalactic arrival directions are concentrated in as few directions as possible. Using different simulated examples we show that, with the measurements on Earth, reconstruction of extragalactic source directions is possible. In particular, we show in an astrophysical scenario that source directions can be reconstructed even within a substantial isotropic background.
Abstract: 1807.09024
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Full Text: [ PostScript, PDF]
Title: Deep learning techniques applied to the physics of extensive air showers
(Submitted on 24 Jul 2018)
Abstract: Deep neural networks are a powerful technique that have found ample applications in several branches of Physics. In this work, we apply machine learning algorithms to a specific problem of Cosmic Ray Physics: the estimation of the muon content of extensive air showers when measured at the ground. As a working case, we explore the performance of a deep neural network applied to the signals recorded by the water-Cherenkov detectors of the Surface Detector Array of the Pierre Auger Observatory. We apply deep learning architectures to large sets of simulated data. The inner structure of the neural network is optimized through the use of genetic algorithms. To obtain a prediction of the recorded muon signal in each individual detector, we train neural networks with a mixed sample of light, intermediate and heavy nuclei. When true and predicted signals are compared at detector level, the primary values of the Pearson correlation coefficients are above 95\%. The relative errors of the predicted muon signals are below 10\% and do not depend on the event energy, zenith angle, total signal size, distance range or the hadronic model used to generate the events.
Abstract: 1807.09273
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Full Text: [ PostScript, PDF]
Title: Statistical challenges in the search for dark matter
(Submitted on 24 Jul 2018)
Abstract: The search for the particle nature of dark matter has given rise to a number of experimental, theoretical and statistical challenges. Here, we report on a number of these statistical challenges and new techniques to address them, as discussed in the DMStat workshop held Feb 26 - Mar 3 2018 at the Banff International Research Station for Mathematical Innovation and Discovery (BIRS) in Banff, Alberta.
Abstract: 1807.09726
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Full Text: [ PostScript, PDF]
Title: Cosmic rays from supernova remnants and superbubbles
(Submitted on 25 Jul 2018)
Abstract: Recent high energy gamma-ray observations of both single supernova remnants and superbubbles, together with observations of supernovae, star formation regions, and local cosmic ray composition, now provide an integrated framework tying together the sources, injection, acceleration and propagation of the cosmic rays, so that it is possible to determine their relative contributions to cosmic ray acceleration for all but the very highest energies.
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