arXiv subjects

Grigory I. Rubtsov

Publications and source records attributed to Grigory I. Rubtsov.

8 recordsLinked to original sources

arXiv2026

Constraining Lorentz invariance violation from the depth of air-shower maximum

Hypothetical Lorentz invariance violation (LIV) remains strongly constrained but has not been excluded as a possible manifestation of new high-energy physics. In subluminal photon-sector LIV, suppressed Bethe$\unicode{x2013}$Heitler pair production modifies the electromagnetic component of extensive air showers. We present a simulation-based toy analysis of this effect using published Pierre Auger fluorescence-detector distributions of the reconstructed depth of air-shower maximum. The modified distributions of the depth of the shower maximum are folded with the Auger detector-response parametrizations and compared with the data using free composition fractions in each reconstructed-energy bin. Within this approach, the comparison gives a sensitivity to LIV compatible with that expected from ultra-high-energy photon-induced showers. We obtain $M_{\rm LIV}>1.5\times 10^{21}\,\text{GeV}$ ($95\%$ confidence level). This result should not be interpreted as a detector-level experimental limit because the detector response and reconstruction are simplified, and the result remains composition dependent. However, the approach presented here opens the way for constraining LIV in hadronic air showers with more detailed analyses.

astro-ph.HE↗

arXiv2026

Multi-TeV $γ$-ray candidates from GRB 221009A: a downturn in the intrinsic $γ$-ray spectrum, an echo of the prompt emission phase, and intergalactic electromagnetic cascades

The detection of $γ$-ray candidates up to the energy of $\approx$13 TeV from the exceptionally bright $γ$-ray burst GRB 221009A by the Large High Altitude Air-Shower Observatory (LHAASO) has raised considerable interest in the astrophysical community. The $γ$-ray dataset resulting from the LHAASO observations allows one to reconstruct the intrinsic spectrum of GRB 221009A with an unprecedented precision. This intrinsic spectrum reveals a downturn at the energy of several TeV (statistical significance $\gt 5 σ$), i.e. the reconstructed intensity is below the intensity expected for a power-law spectrum. We show that a significant TeV $γ$-ray component may be produced by neutrons from photohadronic interactions inside the fireball. These neutrons escape the fireball and interact with the surrounding matter, giving rise to a flux of electrons and positrons, eventually resulting in an observable flux of GeV--TeV synchrotron photons -- a high energy "echo" of the GRB prompt emission phase. Finally, we show that at multi-TeV energies the contribution of $γ$ rays from intergalactic electromagnetic cascades initiated by primary ultra high energy protons is severely limited during the early afterglow phase for the typical magnetic field strength in the intergalactic filaments above 1 nG.

astro-ph.HE↗

arXiv2024

Hypothetical Lorentz invariance violation and the muon content of extensive air showers

Extensive air showers (EAS), produced by cosmic rays in the atmosphere, serve as probes of particle interactions, providing access to energies and kinematical regimes beyond the reach of laboratory experiments. Measurements from multiple cosmic-ray detectors indicate a significant, yet unexplained, discrepancy between the observed muon content in EAS and that predicted by state-of-the-art interaction models, suggesting a need for refinements in our understanding of fundamental physics. Here we show that a tiny, experimentally allowed, violation of the Lorentz invariance (LIV) may result in the suppression of the number of electrons in EAS, leaving the muon number intact and explaining both the ''muon excess'' and its energy dependence. On the other hand, we use the lack of a much stronger discrepancy between EAS data and simulations to obtain strict constraints on the LIV scale. Future experimental tests of this LIV scenario are outlined.

astro-ph.HE↗

arXiv2023

First constraints on the strength of the extragalactic magnetic field from $γ$-ray observations of GRB 221009A

The extragalactic magnetic field (EGMF) could be probed with $γ$-ray observations of distant sources. Primary very high energy (VHE) $γ$-rays from these sources absorb on extragalactic background light photons, and secondary electrons/positrons from the pair production acts create cascade $γ$-rays. These cascade $γ$-rays could be detected with space $γ$-ray telescopes such as Fermi-LAT. The $γ$-ray burst GRB 221009A was an exceptionally bright transient well suited for intergalactic $γ$-ray propagation studies. Using publicly-available Fermi-LAT data, we obtain upper limits on the spectrum of delayed emission from GRB 221009A during the time windows of 10, 30, and 90 days after the burst, and compare these with model spectra calculated for various EGMF strengths $B$, obtaining constraints on $B$. We show that the values of $B$ between 10^{-20} G and 10^{-18} G are excluded.

astro-ph.HE↗

arXiv2018

Lower limit on the ultra-high-energy proton-to-helium ratio from the measurements of the tail of $X_{\mathrm{max}}$ distribution

There are multiple techniques to determine the chemical composition of the ultra-high-energy cosmic rays. While most of the methods are primarily sensitive to the average atomic mass, it is challenging to discriminate between the two lightest elements: proton and helium. In this paper, the proton-to-helium ratio in the energy range $10^{18.0} \mbox{eV}$ to $10^{19.3} \mbox{eV}$ is estimated using the tail of the distribution of the depth of the shower maximum $X_{\mathrm{max}}$. Using the exponential decay scale $Λ$ measured by the Pierre Auger Observatory and the Telescope Array experiment we derive the 68\%\,CL constraints on the proton-to-helium ratio $\mathrm{p}/\mathrm{He} > 7.3 $ and $\mathrm{p}/\mathrm{He} > 0.43 $ for $10^{18.0} < E < 10^{18.5}$ eV and $10^{18.3} < E < 10^{19.3}$ eV correspondingly. It is shown that the result is conservative with respect to the admixture of heavier elements. We evaluate the impact of the hadronic interaction model uncertainty. The implications for the astrophysical models of the origin of cosmic rays and the safety of the future colliders are discussed.

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arXiv2016

Constraining the production of cosmic rays by pulsars

One of the possible sources of hadronic cosmic rays (CRs) are newborn pulsars. If this is indeed the case, they should feature diffusive gamma-ray halos produced by interactions of CRs with interstellar gas. In this paper we try to identify extended gamma-ray emission around young pulsars, making use of the 7-year Fermi-LAT data. For this purpose we select and analyze a set of eight pulsars that are most likely to possess detectable gamma-ray halos. We find extended emission that might be interpreted as a gamma-ray halo only in the case of PSR J0007+7303. Its luminosity accords with the total energy of injected cosmic rays $\sim 10^{50}$ erg, although other interpretations of this source are possible. Irrespectively of the nature of this source we put bounds on the luminosity of gamma-ray halos which suggest that pulsars' contribution to the overall energy budget of galactic CRs is subdominant in the GeV-TeV range.

astro-ph.HE↗

arXiv2016

Probing Milky Way's hot gas halo density distribution using the dispersion measure of pulsars

A number of recent studies indicates a significant amount of ionized gas in a form of the hot gas halo around the Milky Way. The halo extends over the region of 100 kpc and may be acountable for the missing baryon mass. In this paper we calculate the contribution of the proposed halo to the dispersion measure (DM) of the pulsars. The Navarro, Frenk & White (NFW), Maller & Bullock (MB) and Feldmann, Hooper & Gnedin (FHG) density distibutions are considered for the gas halo. The data set includes pulsars with the distance known independently from the DM, e.g. pulsars in globular clusters, LMC, SMC and pulsars with known parallax. The results exclude the NFW distribution for the hot gas, while the more realistic MB and FHG models are compatible with the observed dispersion measure.

astro-ph.GA↗

arXiv2014

Time stretching of the GeV emission of GRBs: Fermi-LAT data vs geometrical model

It is known that the high energy $(> 100\,\text{MeV})$ emission of gamma-ray bursts is delayed with respect to the low energy emission. However, the dependence of light curves on energy has not been studied for the high energy bands. In this paper we consider the bursts observed by Fermi LAT from 2008 August 4 to 2011 August 1, for which at least $10$ photons were observed with the energy greater than $1\,\text{GeV}$. These include $4$ bursts: GRB 080916C, GRB 090510, GRB 090902B, and GRB 090926A. We use the Kolmogorov-Smirnov test to compare the light curves in the two bands, $100\,\text{MeV} < E < 1\,\text{GeV}$ and $E > 1\,\text{GeV}$. For GRB 080916C and GRB 090510 the light curves in the two bands are statistically compatible. However, for GRB 090926A, the higher-energy light curve is stretched compared to the lower-energy one with a statistical significance of $3.3 σ$ and, for GRB 090902B, on the contrary, the lower-energy curve is stretched with $2.3 σ$ significance. We argue that the observed diversity of stretching factors may be explained in a simple geometrical model. The model assumes that the jet opening angle depends on the emission energy in a way that the most energetic photons are radiated near the axis of the jet. All the bursts are considered equivalent in their rest frames and the observed light curves differ only due to different redshifts and view directions. The model conforms to the total burst energy constraint and matches the Fermi LAT observations of the fraction of GRBs visible in $100\,\text{MeV} < E < 1\,\text{GeV}$ band, which may be observed at higher energies. The model predicts the distribution of observable stretching factors, which may be tested in the future data. Finally, we propose a method to estimate observer's off-axis angle based on the stretching factor and the fraction of the high-energy photons.

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