arXiv subjects

Dmitri Semikoz

Publications and source records attributed to Dmitri Semikoz.

18 recordsLinked to original sources

arXiv2025

Leptonic and hadronic models of high-energy nebula around V4641 Sgr

A prominent, 200-pc-scale high-energy nebula surrounding the microquasar V4641 Sgr is the brightest known gamma-ray source in the Southern sky at $E > 100\,\mathrm{TeV}$. In this paper, we develop self-consistent leptonic, hadronic, and leptohadronic models that reproduce both the observed spectrum and morphology of the source. Purely leptonic models are energetically more favorable yet they require rather specific morphological assumptions. The gamma-ray morphology of the source can be better explained within a hadronic scenario based on the identification of cold gas structures spatially correlated with the observed gamma-ray emission. However, a purely hadronic model for the source emission requires a substantial energy reservoir in protons and fails to reproduce the extended x-ray emission recently detected by XRISM. We show that emission including a combination of leptonic and hadronic components can reproduce both the spectral and morphological properties of the source. We provide predictions for the x-ray and neutrino spectra of the nebula that can discriminate the hadronic and leptonic contributions to the overall source signal.

astro-ph.HE↗

arXiv2025

A Self-Consistent Model of the Ultra High-Energy Gamma-Ray Emission of Pulsar Wind Nebulae: Insights from LHAASO and ATNF Catalogs

Pulsar wind nebulae (PWNe) are the dominant Ultra-high-energy (UHE) gamma-ray sources in the LHAASO catalog suggesting that they are the dominant leptonic PeVatrons in our Galaxy. Despite this, still very little is known about their UHE gamma-ray emission, their number in the Galaxy, or their contribution to the gamma-ray emission of our Galaxy. In this work, we propose a self-consistent data-driven model of the UHE gamma-ray emission of PWNe based on the ATNF and LHAASO catalogs. More specifically, we build a model of the UHE gamma-ray emission of PWNe that preserves the statistical relationships in the ATNF catalog and reproduces the number of PWNe detected in the LHAASO catalog. To cope with the limited data available in the LHAASO catalog when performing fits on gamma-ray data, we introduce the concept of censored regression that allows to also use the information provided by unresolved sources. Using our model, we find that reproducing the number of PWNe detected by LHAASO requires either fractions of misaligned pulsars smaller ($\lesssim60\%$) than usually found in the literature, or that some of the associations of PWNe to ATNF pulsars made by LHAASO may not be true. In both cases, we find that in order to reach self-consistency between radio and gamma-ray data, it is necessary that the majority of the unidentified sources in the LHAASO catalog are PWNe associated to an unseen pulsar. Moreover, using our model we also find that the contribution of unresolved PWNe to the total (diffuse) gamma-ray background measured by LHAASO in the $1-1000\,\rm{TeV}$ range is always smaller than $\lesssim10\%$ ($\lesssim30\%$). We conclude that PWNe mostly contribute to the source component of the UHE gamma-ray sky, while having almost no imprint on its diffuse component.

astro-ph.HE↗

arXiv2025

Bright Cosmic-Ray Source as a Solution to Auger-TA Tensions

The ultra-high-energy cosmic ray (UHECR) spectra measured by the Pierre Auger Observatory (Auger) and the Telescope Array (TA) agree very well below $10^{19.5}$ eV but differ significantly at higher energies. We show that these differences can be explained by a single nearby source superimposed on a nearly isotropic background. Taking into account deflections in Galactic and extragalactic magnetic fields, such a source can account for the excess in the TA spectrum without producing excessive anisotropy. The required hard spectrum of the source and intermediate-mass composition are consistent with previous fits of the Auger-only spectrum and composition. This scenario offers several additional advantages: (i) the source produces a broad excess partially overlapping the TA hotspots, suggesting their possible explanation; (ii) without additional tuning, it reproduces the $\sim90^\circ$ shift in dipole direction observed between the Auger-only and combined Auger-TA analyses; and (iii) the best-fit position of the source lies near M82, the brightest nearby starburst galaxy, making it a plausible source of the UHECR.

astro-ph.HE↗

arXiv2025

KM3-230213A: An Ultra-High Energy Neutrino from a Year-Long Astrophysical Transient

The Km3NET collaboration has recently reported the detection of a neutrino event with energy in excess of 100 PeV. This detection is in 2.5-3$\sigma$ tension with the upper limit on the neutrino flux at this energy imposed by IceCube and the Pierre Auger Observatory, if the event is considered part of the diffuse all-sky neutrino flux. We explore an alternative possibility that the event originates from a flare of an isolated source. We show that the data of Km3NET, IceCube and the Pierre Auger Observatory are consistent with the possibility of a source flare of duration $T \lesssim 2$ yr with muon neutrino flux $F \approx 3\times 10^{-10}(1\mbox{ yr }/ T)$ erg cm$^{-2}$ s$^{-1}$. Constraints on the neutrino spectrum indicate that the protons responsible for the neutrino emission have a very hard spectrum in the $E_p\gtrsim 10^{19}$ eV energy range, or otherwise that the neutrinos are produced by photohadronic interactions with infrared photons. The all-sky rate of similar neutrino flaring sources is constrained to be $R\lesssim 0.4/$ yr.

astro-ph.HE↗

arXiv2025

UHECR deflections in the coherent Galactic magnetic field

We study the deflections of ultra-high-energy cosmic rays in several widely used models of the coherent Galactic magnetic field (GMF), including PT11 (Pshirkov et al. [1]), JF12 (Jansson and Farrar [2]), UF23 (Unger and Farrar [3]) and KST24 (Korochkin, Semikoz, and Tinyakov [4]). We propagate particles with rigidities of 5, 10, and 20 EV and analyze the differences in deflection predictions across these GMF models. We identify the GMF components responsible for deflections in various regions of the sky and discuss the uncertainties in modeling these components, as well as potential future improvements.

astro-ph.HE↗

arXiv2024

Multi-messenger signature of cosmic rays from the microquasar V4641 Sgr propagating along a Galactic Magnetic Field line

The recently detected extended, very-high-energy gamma-ray emission from the microquasar V4641 Sgr reveals a puzzling 200-parsec-long jet-like structure significantly misaligned with its radio jet. We propose that this gamma-ray structure is produced by high-energy cosmic-ray particles escaping from the microquasar along ordered field lines of the Galactic Magnetic Field and interacting with the interstellar medium. We show that if the gamma-ray emission is produced by interactions of high-energy cosmic ray nuclei, the system is detectable by future multi-km3 neutrino detectors. We argue that gamma-ray observations of jet-like features adjacent to high-energy sources in the Milky Way provide a new method to measure the regular and turbulent components of the Galactic magnetic field at different locations in the Milky Way.

astro-ph.HE↗

arXiv2024

On the Contribution of Unresolved Pulsars to the Ultra-high-energy Galactic Diffuse Gamma-Ray Emission

The ultra high-energy (UHE) Galactic diffuse gamma-ray emission holds important information on the propagation of cosmic rays in the Galaxy. However, its measurements suffer from a contamination from unresolved sources whose contribution remains unclear. In this Letter, we propose a novel data-driven estimate of the contribution of unresolved pulsar wind nebulae and TeV halos based on the information present in the ATNF and the LHAASO catalogs. We find that in the inner Galaxy, this contribution is limited to $\sim38\%\pm10\%$ of the diffuse flux measured by LHAASO at $\sim20\,\rm{TeV}$ in the case where all sources associated to pulsars contribute as unresolved sources, and this fraction drops with energy to less than $21\%\pm6\%$ above $100\,\rm{TeV}$. In the outer Galaxy, this contribution is always subdominant. In particular, it reaches at most $\sim18\%\pm2\%$ at $10\,\rm{TeV}$ and is less than $\sim7\%\pm1\%$ above $\sim25\,\rm{TeV}$. We conclude that the UHE Galactic diffuse gamma-ray emission cannot be dominated by unresolved pulsar sources above a few tens of $\rm{TeV}$.

astro-ph.HE↗

arXiv2024

The coherent magnetic field of the Milky Way halo, Local Bubble and Fan Region

Recent catalog of Faraday rotation measures (RM) of extragalactic sources together with the synchrotron polarization data from WMAP and Planck provides us with a wealth of information on magnetic fields of the Galaxy. However, the integral character of these observables together with our position inside the Galaxy makes the inference of the coherent Galactic magnetic field (GMF) complicated and ambiguous. We combine several phenomenological components of the GMF -- the spiral arms, the toroidal halo, the X-shaped field and the field of the Local Bubble -- to construct a new model of the regular GMF outside the thin disk. To have control over the relative contributions of the RM and polarization data to the fit we pay special attention to the estimation of errors in data bins. To this end we develop a systematic method which is uniformly applicable to different data sets. This method takes into account individual measurement errors, the variance in the bin as well as fluctuations in the data at angular scales larger than the bin size. This leads to decrease of the errors and, as a result, to better sensitivity of the data to the model content. We cross checked the stability of our method with the new LOFAR data. We found that the four components listed above are sufficient to fit both the RM and polarization data over the whole sky with only a small fraction masked out. Moreover, we have achieved several important improvements compared to previous approaches. Due to account of our location inside of the Local Bubble our model does not require introduction of striated fields. For the first time we showed that the Fan Region can be modeled as a Galactic-scale feature. The pitch angle of the magnetic field in our fit converged to the value around 20 degrees. Interestingly, with value is very close to the direction of the arms inferred recently from Gaia data on upper main sequence stars.

astro-ph.GA↗

arXiv2022

Upper limit on the diffuse extragalactic radio background from GZK photon observation

Here we point out that an observation of Ultra-High Energy Cosmic Ray (UHECR) photons, "GZK photons", could provide an upper limit on the level of the Extra-Galactic Radio Background, depending on the level of UHECR proton primaries (to be determined after a few years of data taking by the Pierre Auger Observatory upgrade AugerPrime). We also update our 2005 prediction of the range of GZK photon fluxes expected from proton primaries.

astro-ph.HE↗

arXiv2022

Gravitational wave signal from primordial magnetic fields in the Pulsar Timing Array frequency band

The NANOGrav, Parkes, European, and International Pulsar Timing Array (PTA) Collaborations have reported evidence for a common-spectrum process that can potentially correspond to a stochastic gravitational wave background (SGWB) in the 1--100 nHz frequency range. We consider the scenario in which this signal is produced by magnetohydrodynamic (MHD) turbulence in the early Universe, induced by a nonhelical primordial magnetic field at the energy scale corresponding to the quark confinement phase transition. We perform MHD simulations to study the dynamical evolution of the magnetic field and compute the resulting SGWB. We show that the SGWB output from the simulations can be very well approximated by assuming that the magnetic anisotropic stress is constant in time, over a time interval related to the eddy turnover time. The analytical spectrum that we derive under this assumption features a change of slope at a frequency corresponding to the GW source duration that we confirm with the numerical simulations. We compare the SGWB signal with the PTA data to constrain the temperature scale at which the SGWB is sourced, as well as the amplitude and characteristic scale of the initial magnetic field. We find that the generation temperature is constrained to be in the 1--200 MeV range, the magnetic field amplitude must be $>1$\% of the radiation energy density at that time, and the magnetic field characteristic scale is constrained to be $>10$\% of the horizon scale. We show that the turbulent decay of this magnetic field will lead to a field at recombination that can help to alleviate the Hubble tension and can be tested by measurements in the voids of the Large Scale Structure with gamma-ray telescopes like the Cherenkov Telescope Array.

astro-ph.CO↗

arXiv2022

Account of the baryonic feedback effect in gamma-ray measurements of intergalactic magnetic fields

Intergalactic magnetic fields in the voids of the large-scale structure can be probed via measurements of secondary gamma-ray emission from gamma-ray interactions with extragalactic background light. Lower bounds on the magnetic field in the voids were derived from the non-detection of this emission. It is not clear a priori what kind of magnetic field is responsible for the suppression of the secondary gamma-ray flux: a cosmological magnetic field that might be filling the voids or the field spread by galactic winds driven by star formation and active galactic nuclei. We used IllustrisTNG cosmological simulations to study the effect of magnetized galactic wind bubbles on the secondary gamma-ray flux. We show that within the IllustrisTNG model of baryonic feedback, the galactic wind bubbles typically provide energy-independent secondary flux suppression at the level of about 10%. The observed flux suppression effect has to be due to the cosmological magnetic field in the voids. This might not be the case for a special case when the primary gamma-ray source has a hard intrinsic gamma-ray spectrum that peaks in the energy range above 50 TeV. In this case, the observational data may be strongly affected by the magnetized bubble that is blown by the source host galaxy.

astro-ph.CO↗

arXiv2021

Limit on intergalactic magnetic field from ultra-high-energy cosmic ray hotspot in Perseus-Pisces region

Telescope Array collaboration has reported an evidence for existence of a source of ultra-high-energy cosmic ray (UHECR) events in Perseus-Pisces supercluster. We show that the mere existence of such a source imposes an upper bound on the strength of intergalactic magnetic field (IGMF) in the Taurus void lying between the Perseus-Pisces supercluster and the Milky Way galaxy. This limit is at the level of 10^{-10} G for a field with correlation length larger than the distance of the super-cluster ~ 70 Mpc. This bound is an order-of-magnitude stronger that the previously known bound on IGMF from radio Faraday rotation measurements and it is the first upper bound on magnetic field in the voids of the Large Scale Structure.

astro-ph.HE↗

arXiv2021

Detectability of large correlation length inflationary magnetic field with Cherenkov telescopes

Magnetic fields occupying the voids of the large scale structure may be a relic from the Early Universe originating from either Inflation or from cosmological phase transitions. We explore the possibility of identifying the inflationary origin of the void magnetic fields and measuring its parameters with gamma-ray astronomy methods. The large correlation length inflationary field is expected to impose a characteristic asymmetry of extended gamma-ray emission that is correlated between different sources on the sky. We show that a set of nearby blazars for which the extended emission is observable in the 0.1-1 TeV band with CTA can be used for the test of inflationary origin of the void magnetic fields.

astro-ph.HE↗

arXiv2021

Pion decay model of TIBET-AS$γ$ PeV gamma-ray signal

Tibet-AS$γ$ collaboration has recently reported a measurement of diffuse $γ$-ray flux from the outer Galactic disk in the energy range reaching PeV. We complement this measurement with the Fermi/LAT measurement of the diffuse flux from the same sky region and study the pion decay model of the combined Fermi/LAT+Tibet-AS$γ$ spectrum. We find that within such a model the average cosmic ray spectrum in the outer Galactic disk has the same characteristic features as the local cosmic ray spectrum. In particular, it experiences a hardening at several hundred GV rigidity and a knee feature in the PV rigidity range. The slope of the average cosmic ray spectrum above the break is close to the locally observed slope of the helium spectrum $γ\simeq 2.5$, but is harder than the slope of the local proton spectrum in the same rigidity range. Although the combination of Fermi/LAT and Tibet-AS$γ$ data points to the presence of the knee in the average cosmic ray spectrum, the quality of the data is not yet sufficient for the study of knee shape and cosmic ray composition.

astro-ph.HE↗

arXiv2017

Implications of Strong Intergalactic Magnetic Fields for Ultra-High-Energy Cosmic-Ray Astronomy

We study the propagation of ultra-high-energy cosmic rays in the magnetised cosmic web. We focus on the particular case of highly magnetised voids ($B \sim \text{nG}$), using the upper bounds from the Planck satellite. The cosmic web was obtained from purely magnetohydrodynamical cosmological simulations of structure formation considering different power spectra for the seed magnetic field in order to account for theoretical uncertainties. We investigate the impact of these uncertainties on the propagation of cosmic rays, showing that they can affect the measured spectrum and composition by up to $\simeq 80\%$ and $\simeq 5\%$, respectivelly. In our scenarios, even if magnetic fields in voids are strong, deflections of 50 EeV protons from sources closer than $\sim\;$50 Mpc are less than $15^\circ$ in approximately 10-50% of the sky, depending on the distribution of sources and magnetic power spectrum. Therefore, UHECR astronomy might be possible in a significant portion of the sky depending on the primordial magnetic power spectrum, provided that protons constitute a sizeable fraction of the observed UHECR flux.

astro-ph.HE↗

arXiv2014

Neutrinos in IceCube from AGN's

Recently IceCube collaboration has reported first evidence for the astrophysical neutrinos. Observation corresponds to the total astrophysical neutrino flux of the order of $3 \cdot 10^{-8}$ $GeV/cm^2/s/sr$ in a PeV energy range. Active Galactic Nuclei (AGN) are natural candidate sources for such neutrinos. To model the neutrino creation in AGNs we study photopion production processes on the radiation field of the Shakura-Sunyaev accretion disks in the black hole vicinity. We show that this model can explain detected neutrino flux and avoids, at the same time, existing constraints from the gamma-ray and cosmic ray observations.

astro-ph.HE↗

arXiv2012

Fermi/LAT observations of 1ES 0229+200: implications for extragalactic magnetic fields and background light

We report the observation in the GeV band of the blazar 1ES 0229+200, which over recent years has become one the primary sources used to put constraints on the Extragalactic Background Light (EBL) and Extragalactic Magnetic Field (EGMF). We derive constraints on both the EBL and EGMF from the combined Fermi-HESS data set taking into account the direct and cascade components of the source spectrum. We show that the limit on the EBL depends on the EGMF strength and vice versa. In particular, an EBL density twice as high as that derived by Franceschini et al. (2008) is allowed if the EGMF is strong enough. On the other hand, an EGMF strength as low as 6x10^-18 G is allowed if the EBL density is at the level of the lower bound from the direct source counts. We present the combined EBL and EGMF limits on as an exclusion plot in two dimensional parameter space: EGMF strength vs. EBL density.

astro-ph.CO↗

arXiv2011

Apparent superluminal neutrino propagation caused by nonlinear coherent interactions in matter

Quantum coherence can significantly increase the strength of the forward scattering of neutrinos propagating through the Earth and interacting with matter. The index of refraction of the neutrinos propagating in a medium and hence their phase velocity is determined by the forward scattering. So, depending on the nature of the interaction of neutrinos with matter, their phase velocity can be larger than the speed of light in vacuum. We show that such effects can explain the apparent superluminal propagation of muon neutrinos found recently by the OPERA experiment. Our proposal explains why the neutrino oscillations and the propagation of neutrinos from supernova 1987A are unaffected. It can be verified by changing the amount of neutrino coherence or by changing the composition of matter in which they propagate.

hep-ph↗