arXiv subjects

G. I. Rubtsov

Publications and source records attributed to G. I. Rubtsov.

64 records · Page 4Linked to original sources

arXiv2011

GRB observations by Fermi LAT revisited: new candidates found

We search the Fermi-LAT photon database for an extended gamma-ray emission which could be associated with any of the 581 previously detected gamma-ray bursts (GRBs) visible to the Fermi-LAT. For this purpose we compare the number of photons with energies E > 100 MeV and E > 1 GeV which arrived in the first 1500 seconds after the burst from the same region, to the expected background. We require that the expected number of false detections does not exceed 0.05 for the entire search and find the high-energy emission in 19 bursts, four of which (GRB 081009, GRB 090720B, GRB 100911 and GRB 100728A) were previously unreported. The first three are detected at energies above 100 MeV, while the last one shows a statistically significant signal only above 1 GeV.

astro-ph.HE↗

arXiv2010

Constraints on the flux of primary cosmic-ray photons at energies E > 10^18 eV from Yakutsk muon data

Comparing the signals measured by the surface and underground scintillator detectors of the Yakutsk Extensive Air Shower Array, we place upper limits on the integral flux and the fraction of primary cosmic-ray photons with energies E > 10^18 eV, E > 2*10^18 eV and E > 4*10^18 eV. The large collected statistics of the showers measured by large-area muon detectors provides a sensitivity to photon fractions < 10^-2, thus achieving precision previously unreachable at ultra-high energies.

astro-ph.HE↗

arXiv2009

Sensitivity of cosmic-ray experiments to ultra-high-energy photons: reconstruction of the spectrum and limits on the superheavy dark matter

We estimate the sensitivity of various experiments detecting ultra-high-energy cosmic rays to primary photons with energies above 10^19 eV. We demonstrate that the energy of a primary photon may be significantly (up to a factor of ~ 10) under- or overestimated for particular primary energies and arrival directions. We consider distortion of the reconstructed cosmic-ray spectrum for the photonic component. As an example, we use these results to constrain the parameter space of models of superheavy dark matter by means of both the observed spectra and available limits on the photon content. We find that a significant contribution of ultra-high-energy particles (photons and protons) from decays of superheavy dark matter is allowed by all these constraints.

astro-ph↗

arXiv2008

Muon content of ultra-high-energy air showers: Yakutsk data versus simulations

We analyse a sample of 33 extensive air showers (EAS) with estimated primary energies above 2\cdot 10^{19} eV and high-quality muon data recorded by the Yakutsk EAS array. We compare, event-by-event, the observed muon density to that expected from CORSIKA simulations for primary protons and iron, using SIBYLL and EPOS hadronic interaction models. The study suggests the presence of two distinct hadronic components, ``light'' and ``heavy''. Simulations with EPOS are in a good agreement with the expected composition in which the light component corresponds to protons and the heavy component to iron-like nuclei. With SYBILL, simulated muon densities for iron primaries are a factor of \sim 1.5 less than those observed for the heavy component, for the same electromagnetic signal. Assuming two-component proton-iron composition and the EPOS model, the fraction of protons with energies E>10^{19} eV is 0.52^{+0.19}_{-0.20} at 95% confidence level.

astro-ph↗

arXiv2007

Air-shower simulations with and without thinning: artificial fluctuations and their suppression

The most common way to simplify extensive Monte-Carlo simulations of air showers is to use the thinning approximation. We study its effect on the physical parameters reconstructed from simulated showers. To this end, we have created a library of showers simulated without thinning with energies from 10^17 eV to 10^18 eV, various zenith angles and primaries. This library is publicly available. Physically interesting applications of the showers simulated without thinning are discussed. Observables reconstructed from these showers are compared to those obtained with the thinning approximation. The amount of artificial fluctuations introduced by thinning is estimated. A simple method, multisampling, is suggested which results in a controllable suppression of artificial fluctuations and at the same time requires less demanding computational resources as compared to the usual thinning.

astro-ph↗

arXiv2007

Towards event-by-event studies of the ultrahigh-energy cosmic-ray composition

We suggest a method which improves the precision of studies of the primary composition of ultra-high-energy cosmic rays. Two principal ingredients of the method are (1) comparison of the observed and simulated parameters for individual showers, without averaging over arrival directions and (2) event-by-event selection of simulated showers by the physical observables and not by the reconstructed primary parameters. A detailed description of the algorithm is presented and illustrated by several examples.

astro-ph↗

arXiv2007

No-thinning simulations of extensive air showers and small scale fluctuations at the ground level

The particle density in extensive air showers fluctuates at the ground level. These fluctuations, at the scale of the scintillator detector size (several meters), lead to the diversity of the individual detector responses. Therefore, small scale fluctuations contribute to the error in the estimation of the primary energy by a ground array. This contribution is shown to be non-Gaussian. The impact on the primary energy spectrum measured by a ground array is estimated. Ir is argued that super-GZK events observed by AGASA experiment do not result from the energy overestimation, due to small scale fluctuations, of lower energy events.

astro-ph↗

arXiv2007

Constraining the fraction of primary gamma rays at ultra-high energies from the muon data of the Yakutsk extensive-air-shower array

By making use of the data on the total signal and on the muon component of the air showers detected by the Yakutsk array, we analyze, in the frameworks of the recently suggested event-by-event approach, how large the fraction of primary gamma-rays at ultra-high energies can be. We derive upper limits on the photon fraction in the integral flux of primary cosmic rays. At the 95% confidence level (CL), these limits are 22% for primary energies E_0>4\cdot 10^{19}eV and 12% for E_0>2\cdot 10^{19}eV. Despite the presence of muonless events, the data are consistent with complete absence of photons at least at 95% CL. The sensitivity of the results to systematic uncertainties, in particular to those of the energy determination for non-photon primaries, is discussed.

astro-ph↗

arXiv2006

Upper limit on the ultra-high-energy photon flux from AGASA and Yakutsk data

We present the interpretation of the muon and scintillation signals of ultra-high-energy air showers observed by AGASA and Yakutsk extensive air shower array experiments. We consider case-by-case ten highest energy events with known muon content and conclude that at the 95% confidence level (C.L.) none of them was induced by a primary photon. Taking into account statistical fluctuations and differences in the energy estimation of proton and photon primaries, we derive an upper limit of 36% at 95% C.L. on the fraction of primary photons in the cosmic-ray flux above 10^20 eV. This result disfavors the Z-burst and superheavy dark-matter solutions to the GZK-cutoff problem.

astro-ph↗

arXiv2003

Narrowing the window for millicharged particles by CMB anisotropy

We calculate the cosmic microwave background (CMB) anisotropy spectrum in models with millicharged particles of electric charge q\sim 10^{-6}-10^{-1} in units of electron charge. We find that a large region of the parameter space for the millicharged particles exists where their effect on the CMB spectrum is similar to the effect of baryons. Using WMAP data on the CMB anisotropy and assuming Big Bang nucleosynthesis value for the baryon abundance we find that only a small fraction of cold dark matter, Omega_{mcp}h_0^2 < 0.007 (at 95% CL), may consists of millicharged particles with the parameters (charge and mass) from this region. This bound significantly narrows the allowed range of the parameters of millicharged particles. In models without paraphoton millicharged particles are now excluded as a dark matter candidate. We also speculate that recent observation of 511 keV gamma-rays from the Galactic bulge may be an indication that a (small) fraction of CDM is comprised of the millicharged particles.

hep-ph↗