We take advantage of the wealth of rotation measures data contained in the NRAO VLA Sky Survey catalog to derive new, statistically robust, upper limits on the strength of extragalactic magnetic ...fields. We simulate the extragalactic magnetic field contribution to the rotation measures for a given field strength and correlation length, by assuming that the electron density follows the distribution of Lyman-α clouds. Based on the observation that rotation measures from distant radio sources do not exhibit any trend with redshift, while the extragalactic contribution instead grows with distance, we constrain fields with Jeans' length coherence length to be below 1.7 nG at the 2σ level, and fields coherent across the entire observable Universe below 0.65 nG. These limits do not depend on the particular origin of these cosmological fields.
Primordial black holes (PBH), produced through a variety of processes in the early universe, could fill galactic halos accounting for a fraction or the totality of the dark matter. In particular, PBH ...with substellar masses could be captured by stars, start to swallow their material, and eventually turn them into BH, hence originating a peculiar new type of stellar catastrophic event. Here we revisit this process in the most favorable case of PBH capture by neutron stars. We detail a number of novel features in the capture phase, during the settling within the star and mass growth of the PBH, and illustrate some phenomenological consequences. In particular, we point out that in the subsonic regime the PBH drag takes the form of a Bondi accretion. As a result, the onset of the final transmutation of the NS into a stellar sized BH is expected with the PBH seed in slight off-center position. We also compute the gravitational wave energy-loss and signals associated to different phases of the PBH-stellar interaction. In particular, the emission associated to the accretion phase is periodic with a few kHz frequency; in the rare case of a nearby Galactic event and for light PBH, it could constitute a warning of the forthcoming transmutation.
Abstract
We propose a new method for the estimation of ultra-high energy cosmic ray (UHECR) mass composition from a distribution of their arrival directions. The method employs a test statistic (TS) ...based on a characteristic deflection of UHECR events with respect to the distribution of luminous matter in the local Universe. Making realistic simulations of the mock UHECR sets, we show that this TS is robust to the presence of galactic and non-extreme extra-galactic magnetic fields and sensitive to the mass composition of events in a set. This allows one to constrain the UHECR mass composition by comparing the TS distribution of a composition model in question with the data TS, and to discriminate between different composition models. While the statistical power of the method depends somewhat on the MF parameters, this dependence decreases with the growth of statistics. The method shows good performance even at GZK energies where the estimation of UHCER mass composition with traditional methods is complicated by a low statistics.
We study the influence of the random part of the galactic magnetic field on the propagation of ultrahigh energy cosmic rays. Within very mild approximations about the properties of the electron ...density fluctuations in the Galaxy, we are able to derive a clear and direct relation between the observed variance of rotation measures and the predicted cosmic ray deflections. Remarkably, this is obtained bypassing entirely the detailed knowledge of the magnetic properties of the turbulent plasma. Depending on the parameters of the electron density spectrum, we can either directly estimate the expected deflection, or constrain it from above. Thanks to the latest observational data on rotation measures, we build a direction-dependent map of such deflections. We find that over most of the sky the random deflections of 40 EeV protons do not exceed 1°-2°, and can be as large as 5° close to the Galactic plane.
A signature of EeV protons of Galactic origin Tinyakov, P. G; Urban, F. R; Ivanov, D ...
Monthly notices of the Royal Astronomical Society,
08/2016, Letnik:
460, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We investigate signatures that would be produced in the spectrum and sky distribution of ultra-high energy cosmic rays (UHECR) by a population of the Galactic sources of high-energy protons in the ...energy range around 1 EeV, i.e. around the diffusive-to-ballistic transition. In this regime, the CR flux has to be calculated numerically. We employ the approach that consists in backtracking anti-protons from Earth through the Galaxy and integrating the source emissivity along the trajectory. This approach makes evident two generic features of the transition region: sharp increase of the total flux as the energy decreases across the transition region, and its strong anisotropy (appearance of a bright compact spot) all the way until the onset of the diffusive regime. We then discuss and compare several methods to experimentally detect or constrain these features. We find that a few per cent admixture of the Galactic protons can in principle be detected by the current UHECR experiments.
We reanalyse the subset of the Faraday rotation measures data from the NRAO VLA Sky Survey catalogue for which redshift and spectral index information is available, in order to better elucidate the ...relations between these observables. We split this subset in two based on their radio luminosity, and find that higher power sources have a systematically higher residual rotation measure, once the regular field of the Milky Way is subtracted. This rotation measure stands well above the variances due to the turbulent field of our Galaxy and measurement errors, contrarily to low-power sources. The effect is more pronounced as the energy threshold becomes more restrictive. If the two sets are merged, one observes an apparent evolution of rotation measure with redshift, but our analysis shows that this can be interpreted as an artefact of the different intrinsic properties of brighter sources that are typically observed at larger distances.
We made use of the two latest sets of rotation measures (RMs) of extragalactic radio sources, namely the NRAO VLA Sky Survey rotation measures catalog and a compilation by Kronberg and Newton-McGee, ...to infer the global structure of the Galactic magnetic field (GMF). We have checked that these two data sets are mutually consistent. Given the existence of clear patterns in the all-sky RM distributions we considered GMF models consisting of two components: disk (spiral or ring) and halo. The parameters of these components were determined by fitting different model field geometries to the observed RMs. We found that the model consisting of a symmetric (with respect to the Galactic plane) spiral disk and antisymmetric halo fits the data best and reproduces the observed distribution of RMs over the sky very well. We confirm that ring disk models are disfavored. Our results favor small pitch angles around ~ -- 5? and an increased vertical scale of electron distribution, in agreement with some recent studies. Based on our fits, we select two benchmark models suitable for studies of cosmic ray propagation, including cosmic rays at ultrahigh energies.
In the present-day cosmic ray data, the strongest indication of anisotropy of the ultrahigh energy cosmic rays is the 20-degree hotspot observed by the Telescope Array with the statistical ...significance of 3.4sigma. In this work, we study the possibility of detecting such a spot by space-based all-sky observatories. We show that if the detected luminosity of the hotspot is attributed to a physical effect and not a statistical fluctuation, the KLYPVE and JEM-EUSO experiments would need to collect ~300 events with E>57EeV in order to detect the hotspot at the 5sigma confidence level with the 68% probability. We also study the dependence of the detection prospects on the hotspot luminosity.
We analyze the gamma-ray halo around stacked AGNs reported by Ando & Kusenko (2010, ApJ, 722, L39). First, we show that the angular distribution of γ-rays around the stacked AGNs is consistent with ...the angular distribution of the γ-rays around the Crab pulsar, which is a point source for Fermi/LAT. This makes it unlikely that the halo is caused by an electromagnetic cascade of TeV photons in the intergalactic space. We then compare the angular distribution of γ-rays around the stacked AGNs with the point-spread function (PSF) of Fermi/LAT and confirm the existence of an excess above the PSF. However, we demonstrate that the magnitude and the angular size of this effect is different for photons converted in the front and back parts of the Fermi/LAT instrument, and thus is an instrumental effect.
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