Recent results from the AMS-02 data have confirmed that the cosmic ray positron fraction increases with energy between 10 and 200 GeV. This quantity should not exceed 50%, and it is hence expected ...that it will either converge towards 50% or fall. We study the possibility that future data may show the positron fraction dropping down abruptly to the level expected with only secondary production, and forecast the implications of such a feature in term of possible injection mechanisms that include both Dark Matter and pulsars.
Newly born pulsars offer favorable sites for the injection of heavy nuclei, and for their further acceleration to ultrahigh energies. Once accelerated in the pulsar wind, nuclei have to escape from ...the surrounding supernova envelope. We examine this escape analytically and numerically and discuss the pulsar source scenario in light of the latest ultrahigh energy cosmic ray (UHECR) data. Our calculations show that, at early times, when protons can be accelerated to energies E > 10 super(20) eV, the young supernova shell tends to prevent their escape. In contrast, because of their higher charge, iron-peaked nuclei are still accelerated to the highest observed energies at later times, when the envelope has become thin enough to allow their escape. Ultrahigh energy iron nuclei escape newly born pulsars with millisecond periods and dipole magnetic fields of ~10 super(12)-10 super(13) G, embedded in core-collapse supernovae. Due to the production of secondary nucleons, the envelope crossing leads to a transition of composition from light to heavy elements at a few EeV, as observed by the Auger Observatory. The escape also results in a softer spectral slope than that initially injected via unipolar induction, which allows for a good fit to the observed UHECR spectrum. We conclude that the acceleration of iron-peaked elements in a reasonably small fraction (<, ~0.01%) of extragalactic rotation-powered young pulsars would reproduce satisfactorily the current UHECR data. Possible signatures of this scenario are also discussed.
Recent results from the AMS-02 data have confirmed that the cosmic-ray positron fraction increases with energy between 10 and 200 GeV. This quantity should not exceed 50%, and it is hence expected ...that it will either converge toward 50% or fall. We study the possibility that future data may show the positron fraction dropping down abruptly to the level expected with only secondary production, and forecast the implications of such a feature in term of possible injection mechanisms that include both dark matter and pulsars. Were a sharp steepening to be found, rather surprisingly, we conclude that pulsar models would do at least as well as dark matter scenarios in terms of accounting for any spectral cut-off.
Aims. We investigate the evaporation of close-by pulsar companions, such as planets, asteroids, and white dwarfs, by induction heating. Methods. Assuming that the outflow energy is dominated by a ...Poynting flux (or pulsar wave) at the location of the companions, we calculate their evaporation timescales, by applying the Mie theory. Results. Depending on the size of the companion compared to the incident electromagnetic wavelength, the heating regime varies and can lead to a total evaporation of the companion. In particular, we find that inductive heating is mostly inefficient for small pulsar companions, although it is generally considered the dominant process. Conclusions. Small objects like asteroids can survive induction heating for 104 yr at distances as small as 1 R⊙ from the neutron star. For degenerate companions, induction heating cannot lead to evaporation and another source of heating (likely by kinetic energy of the pulsar wind) has to be considered. It was recently proposed that bodies orbiting pulsars are the cause of fast radio bursts; the present results explain how those bodies can survive in the pulsar’s highly energetic environment.
ABSTRACT
Binary systems composed of a recycled millisecond pulsar and a stellar companion in close orbit could be excellent sites to diagnose pulsar winds. In such systems, the pulsar outflow ...irradiates and heats up the companion atmosphere, which can lead to the observation of strong day/night modulations in temperature. We demonstrate with particle shower simulations that the particle energy of the wind affects the heating depth in the atmosphere: the wind heat can be deposited above or below the photosphere, leading to different signatures in the observed spectra. We apply our method to four specific systems: We find that systems with cool night-side companions showing strong temperature variations can give interesting lower limits on the particle energy in the winds. In particular, if the companion night side of PSR B1957+20 were to be suddenly irradiated, deep heating would only take place if particles with energy >100 TeV were present. Observational evidence of deep heating in this system thus suggests that (i) such particles exist in the pulsar wind and/or (ii) binary evolution non-trivially takes the companion to the observed temperature asymmetry. Besides, the observed temperature difference can be maintained only with particle energies of the order of 100 MeV.
Next-generation radio experiments such as the radio detector of the upgraded Pierre Auger Observatory and the planned GRAND and BEACON arrays target the detection of ultra-high-energy particle air ...showers arriving at low elevation angles. These inclined cosmic-ray air showers develop higher in the atmosphere than vertical ones, enhancing magnetic deflections of electrons and positrons inside the cascade. We evidence two novel features in their radio emission: a new polarization pattern, consistent with a geosynchrotron emission model and a coherence loss of the radio emission, both for showers with zenith angle θ≳65° and strong enough magnetic field amplitude (typical strength of B∼50 μT). Our model is compared with both ZHAireS and CoREAS Monte Carlo simulations. Our results break the canonical description of a radio signal made of Askaryan and transverse current emission only, and provide guidelines for the detection and reconstruction strategies of next-generation experiments, including cosmic-ray or neutrino discrimination.Next-generation radio experiments such as the radio detector of the upgraded Pierre Auger Observatory and the planned GRAND and BEACON arrays target the detection of ultra-high-energy particle air showers arriving at low elevation angles. These inclined cosmic-ray air showers develop higher in the atmosphere than vertical ones, enhancing magnetic deflections of electrons and positrons inside the cascade. We evidence two novel features in their radio emission: a new polarization pattern, consistent with a geosynchrotron emission model and a coherence loss of the radio emission, both for showers with zenith angle θ≳65° and strong enough magnetic field amplitude (typical strength of B∼50 μT). Our model is compared with both ZHAireS and CoREAS Monte Carlo simulations. Our results break the canonical description of a radio signal made of Askaryan and transverse current emission only, and provide guidelines for the detection and reconstruction strategies of next-generation experiments, including cosmic-ray or neutrino discrimination.
Autonomous radio-detection, i.e., detection of air-showers with standalone radio arrays, is one of the major technical challenges to overcome for the next generation astroparticle detectors. In this ...context, we study polarisation signatures of simulated radio signals to perform an identification of the associated air-showers initiated by cosmic-rays and neutrinos. We compare the two sources of radio emission (the charge excess and geomagnetic) and show that the former is almost negligible for inclined (zenith angle >65°) cosmic-ray air-showers. This provides an efficient background rejection criterion at the DAQ level, based on the projection of the total electric field along the direction of the local magnetic field. This relevant quantity can be computed, — even in an online treatment — for antennas measuring three orthogonal polarisations. Independently of the experimental antenna layout, we estimate that assuming a random polarisation of noise events, a rejection from ≈72% (for a non favourable detector location) to ≈93% (for a favourable one) of the noise induced events and a trigger efficiency of 86% (93%) with a 3σ (5σ) trigger threshold level should be achievable. We also show that neutrino-induced showers present a charge excess to geomagnetic signal ratio up to ∼10 times higher than for cosmic ray showers. Although this characteristic makes the identification of neutrino-induced showers challenging via the method developed here, it provides an efficient criterion to perform an offline discrimination between cosmic-ray and neutrino primaries. The stronger charge excess emission will also help the reconstruction of air-shower parameters, such as the core position.
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