arXiv subjects

D. Semikoz

Publications and source records attributed to D. Semikoz.

At least 37 records · Page 2Linked to original sources

arXiv2021

Possibility of measurement of cosmic ray electron spectrum up to 100 TeV with two-layer water Cherenkov detector array

Measurements of cosmic ray electron+positron spectrum above 10 TeV with ground-based experiments is challenging because of the difficulty of rejection of hadronic extensive air shower background. We study the efficiency of rejection of the hadronic background with water Cherenkov detector array supplemented by muon detection layer. We show that addition of a continuous muon detection layer to the experimental setup allows to achieve a ~ 1e-5 rejection factor for hadronic background at 10 TeV and enables measurement of electron spectrum in 10-100 TeV energy range. We show that measurements of electron spectrum in this energy range do not require a high-altitude experiment and can be done with a sea-level detector.

astro-ph.HE↗

arXiv2020

Sensitivity reach of gamma-ray measurements for strong cosmological magnetic fields

A primordial magnetic field with the strength in the 1-10 pG range can resolve the tension between different measurements of the Hubble constant and provide an explanation for the excess opacity in the 21 cm line at redshift $15<z<20$, if it is present during the recombination and reionization epochs. This field can also survive in the voids of the large-scale Structure in the present day universe. We study the sensitivity reach of the gamma-ray technique for measurement of such relatively strong cosmological magnetic field using deep exposure(s) of the nearest hard spectrum blazar(s) with CTA telescopes. We show that the gamma-ray measurement method can sense the primordial magnetic field with a strength of up to $10^{-11}$~G. Combination of the cosmic microwave background and gamma-ray constraints can thus sense the full range of possible cosmological magnetic fields to confirm or rule out their relevance to the problem of the origin of cosmic magnetic fields, as well as their influence on recombination and reionization epochs.

astro-ph.CO↗

arXiv2020

Viscous damping of chiral dynamos in the early universe

Chiral dynamo converting asymmetry between right and left-handed leptons in the early universe into helical magnetic field has been proposed as a possible cosmological magnetogenesis scenario. We show that this mechanism is strongly affected by viscous damping of primordial plasma motions excited by the dynamo. This effect modifies the expected range of strength and correlation length of the chiral dynamo field which could have survived till present epoch in the voids of the Large Scale Structure. We show the range of parameters of chiral dynamo field that may have survived in the voids is still consistent with existing lower bounds on intergalactic magnetic field from gamma-ray observations, but only if the right-left lepton asymmetry at the temperature T~80 TeV is very high, close to the maximal possible value.

hep-ph↗

arXiv2020

Neutrinos from the gamma-ray source eHWC J1825-134: predictions for Km$^3$ detectors

The eHWC J1825-134 source is located in the southern sky and has been recently detected by the HAWC observatory. It presents an hard spectral index and its gamma-ray flux extends up to energies close to 100 TeV without significant suppression. Amongst the HAWC sources, it is the most luminous in the multi-TeV domain and therefore is one of the first that should be searched for with a neutrino telescope in the northern hemisphere. Using an updated effective area for the forthcoming KM3NeT detector, we study the possibility to detect this source within ten years of its running time. We conclude that about a 4 to 5 sigma detection has to be expected after ten years of observations, depending on the details of the considered scenario.

astro-ph.HE↗

arXiv2020

Mapping large-scale diffuse gamma-ray emission in 10-100 TeV band with Cherenkov telescopes

Measurement of diffuse gamma-ray emission from the Milky Way with Imaging Atmospheric Cherenkov Telescopes (IACT) is difficult because of high level of charged cosmic ray background and small field-of-view. We show that such a measurement is nevertheless possible in the energy band 10-100 TeV. The minimal charged particle background for IACTs is achieved in event selections used for the analyses of the cosmic ray electrons. Tight cuts on the event quality in these event selections allow to achieve the background level sufficiently low to allow the measurements of the diffuse Galactic gamma-ray flux above 10 TeV. We calculate the sensitivities of different types of IACT arrays for the Galactic diffuse emission measurements and compare them with the diffuse gamma-ray flux from different parts of the sky measured by the Fermi Large Area Telescope below 3 TeV and with the astrophysical neutrino signal measured by IceCube telescope. We show that deep exposure of existing IACT systems is sufficient for detection of the diffuse flux from all the Galactic Plane up to Galactic latitude |b|~ degrees. Medium Size Telescope array of CTA will be able to detect the diffuse flux up 30 degree Galactic latitude. Its sensitivity will be sufficient for detection of the gamma-ray counterpart of the Galactic component of the IceCube astrophysical neutrino signal above 10 TeV. We also propose that a dedicated IACT system composed of small but wide field-of-view telescopes could be used for mapping the 10-100 TeV the diffuse gamma-ray emission from across all the sky.

astro-ph.HE↗

arXiv2020

LHAASO sensitivity for diffuse gamma-ray signals from the Galaxy

We estimate the sensitivity of LHAASO telescope for the large angular scale diffuse gamma-ray flux in multi-TeV - multi-PeV energy range. We discuss possible sources of the signal in this energy range including the guaranteed flux from cosmic ray interactions in the interstellar medium and possible flux from decaying dark matter. We show that LHAASO will be able to detect the diffuse cosmic ray induced gamma-ray flux up to high Galactic latitude regions thus providing firm identification of the Galactic cosmic ray component of the astrophysical neutrino signal detected by IceCube and clarification of the nature of the knee feature of the cosmic ray spectrum. Comparing the diffuse flux sensitivity with the diffuse gamma-ray flux expected from the dark matter decays, we show LHAASO will be able to detect the gamma-ray signal from dark matter particles of PeV-EeV mass decaying on the time scale up to 3e29 s.

astro-ph.HE↗

arXiv2019

Search for spectral features in extragalactic background light with gamma-ray telescopes

Cumulative optical and infrared emission from galaxies accumulated over cosmological time scales, the extragalactic background light (EBL), could be probed by complementary techniques of direct observations and source counting in the visible and infrared as well as via its imprint on the signal of distant active galactic nuclei in gamma rays. We compare the visible and infrared measurements with the gamma-ray constraints and study if the discrepancies of the measurements with different methods could be due to the presence of features in the EBL spectrum that are localised in the micron wavelength range. We combined data on time-averaged spectra of selected blazars that were obtained by Fermi and ground-based gamma-ray telescopes. We also modelled the effect of absorption on EBL while allowing for the existence of a previously unaccounted spectral feature. We show that a previously reported 'excess' in EBL flux in the $\sim 1$~micron wavelength range is consistent with gamma-ray measurements, that is, if the excess has the form of a narrow feature of the width $δλ< λ$ and an overall flux of up to 15 nW/(m$^2$ sr) above the 'minimal' EBL, which is estimated from the visible and infrared source counts. Such 'bump-like' spectral features could originate, for example, from decaying dark-matter particles, or either axions or peculiar astrophysical processes in the course of star-formation history. We discuss the possibilities for the search of spectral features in the EBL with the Cherenkov Telescope Array (CTA).

astro-ph.HE↗

arXiv2019

Search for decaying eV-mass axion-like particles using gamma-ray signal from blazars

Decaying axion-like particles (ALP) with masses in the eV range which might occupy dark matter halos of the Milky Way and other galaxies produce a characteristic "bump" feature in the spectrum of extragalactic background light (EBL). This feature leaves an imprint on the gamma-ray spectra of distant extragalactic sources. We derive constraints on the ALP coupling to photons based on analysis of spectra of very-high-energy gamma-ray loud blazars. We combine gamma-ray spectral measurements by Fermi/LAT and Cherenkov telescopes and fit a model in which the intrinsic source spectrum is modified by pair production on photons produced by ALP decays. We constrain the amplitude of gamma-ray flux suppression by this effect. We find that the combined Fermi/LAT and VERITAS data set for the source 1ES 1218+304 currently provides the tightest constraint on ALP-two-photon coupling which is complementary to the constraints imposed by non-observation of excess energy loss in Horizontal Branch stars, by the high-resolution spectroscopic observations of galaxy clusters with optical telescopes and by the searches of ALP signal with CERN Solar Axion Telescope. Our analysis favours existence of a bump in the EBL spectrum which could be produced by ALPs in the mass range 2-3 eV and axion-photon coupling $\sim 10^{-10}$ GeV$^{-1}$. We discuss possibilities for verification of this hint with deeper Cherenkov telescope observations of large number of blazars with current generation instruments and with the Cherenkov Telescope Array.

hep-ph↗

arXiv2018

Improved limit on axion-like particles from gamma-ray data on Perseus cluster

Coupling of axion-like particles (ALPs) to photons in the presence of background magnetic field affects propagation of gamma-rays through magnetized environments. This results in modification in the gamma-ray spectra of sources in or behind galaxy clusters. We search for the ALP induced effects in the Fermi/LAT and MAGIC telescope spectra of the radio galaxy NGC 1275 embedded in Perseus galaxy cluster. We report an order-of-magnitude improved upper limit on the ALP-photon coupling constant in the 0.1-10 neV mass range from non-detection of the ALP imprints on the gamma-ray spectra. The improved upper limit extends into the coupling range in which the ALP particles could form the dark matter. We estimate the sensitivity improvements for the ALP search achievable with extension of the measurements to lower and higher energies with e-ASTROGAM and CTA and show that the gamma-ray probe of ALPs with masses in $10^{-11}-10^{-7}$ eV range will be have order-of-magnitude better sensitivity compared to ground-based experiment IAXO.

astro-ph.HE↗

arXiv2016

Detection prospects of the Telescope Array hotspot by space observatories

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 3.4σ. 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>57 EeV in order to detect the hotspot at the 5σ confidence level with the 68% probability. We also study the dependence of the detection prospects on the hotspot luminosity.

astro-ph.HE↗

arXiv2013

Magnetic Fields in the Large Scale Structure from Faraday Rotation measurements

We search for magnetic fields outside galaxies and galaxy clusters by investigating redshift evolution of Faraday rotation measures (RM) of extragalactic radio sources. Our analysis reveals a strong evidence for the redshift dependence of the mean of the absolute value of the RM. The evidence is further strengthened if the Galactic contribution to the RM is subtracted. The hypothesis of the absence of the redshift evolution of residual RM is ruled out at 5 sigma level. The observed redshift dependence of RM is consistent with the possibility of the presence of nano-Gauss strength magnetic fields with correlation length shorter than 0.1 Mpc in the weakly overdense elements of the Large Scale Structure traced by the Ly-alpha clouds.

astro-ph.CO↗

arXiv2012

Ultra-High Energy Cosmic Ray production in the polar cap regions of black hole magnetospheres

We develop a model of ultra-high energy cosmic ray (UHECR) production via acceleration in a rotation-induced electric field in vacuum gaps in the magnetospheres of supermassive black holes (BH). We show that if the poloidal magnetic field near the BH horizon is misaligned with the BH rotation axis, charged particles, which initially spiral into the BH hole along the equatorial plane, penetrate into the regions above the BH "polar caps" and are ejected with high energies to infinity. We show that in such a model acceleration of protons near a BH of typical mass 3e8 solar masses is possible only if the magnetic field is almost aligned with the BH rotation axis. We find that the power of anisotropic electromagnetic emission from an UHECR source near a supermassive BH should be at least 10-100 times larger then UHECR power of the source. This implies that if the number of UHECR sources within the 100 Mpc sphere is ~100, the power of electromagnetic emission which accompanies proton acceleration in each source, $10^{42-43}$ erg/s, is comparable to the typical luminosities of active galactic nuclei (AGN) in the local Universe. We also explore the acceleration of heavy nuclei, for which the constraints on the electromagnetic luminosity and on the alignment of magnetic field in the gap are relaxed.

astro-ph↗

arXiv2012

Degree-scale GeV "jets" from active and dead TeV blazars

We show that images of TeV blazars in the GeV energy band should contain, along with point-like sources, degree-scale jet-like extensions. These GeV extensions are the result of electromagnetic cascades initiated by TeV gamma-rays interacting with extragalactic background light and the deflection of the cascade electrons/positrons in extragalactic magnetic fields (EGMF). Using Monte-Carlo simulations, we study the spectral and timing properties of the degree-scale extensions in simulated GeV band images of TeV blazars. We show that the brightness profile of such degree-scale extensions can be used to infer the lightcurve of the primary TeV gamma-ray source over the past 1e7 yr, i.e. over a time scale comparable to the life-time of the parent active galactic nucleus. This implies that the degree-scale jet-like GeV emission could be detected not only near known active TeV blazars, but also from "TeV blazar remnants", whose central engines were switched off up to ten million years ago. Since the brightness profile of the GeV "jets" depends on the strength and the structure of the EGMF, their observation provides additionally information about the EGMF.

astro-ph.HE↗

arXiv2010

High-energy astroparticle physics

In these three lectures I discuss the present status of high-energy astroparticle physics including Ultra-High-Energy Cosmic Rays (UHECR), high-energy gamma rays, and neutrinos. The first lecture is devoted to ultra-high-energy cosmic rays. After a brief introduction to UHECR I discuss the acceleration of charged particles to highest energies in the astrophysical objects, their propagation in the intergalactic space, recent observational results by the Auger and HiRes experiments, anisotropies of UHECR arrival directions, and secondary gamma rays produced by UHECR. In the second lecture I review recent results on TeV gamma rays. After a short introduction to detection techniques, I discuss recent exciting results of the H.E.S.S., MAGIC, and Milagro experiments on the point-like and diffuse sources of TeV gamma rays. A special section is devoted to the detection of extragalactic magnetic fields with TeV gamma-ray measurements. Finally, in the third lecture I discuss Ultra-High-Energy (UHE) neutrinos. I review three different UHE neutrino detection techniques and show the present status of searches for diffuse neutrino flux and point sources of neutrinos.

hep-ph↗

arXiv2009

Sensitivity of gamma-ray telescopes for detection of magnetic fields in intergalactic medium

We explore potential of current and next-generation gamma-ray telescopes for the detection of weak magnetic fields in the intergalactic medium. We demonstrate that using two complementary techniques, observation of extended emission around point sources and observation of time delays in gamma-ray flares, one would be able to probe most of the cosmologically and astrophysically interesting part of the "magnetic field strength" vs. "correlation length" parameter space. This implies that gamma-ray observations with Fermi and ground-based Cherenkov telescopes will allow to (a) strongly constrain theories of the origin of magnetic fields in galaxies and galaxy clusters and (b) discover, constrain or rule out the existence of weak primordial magnetic field generated at different stages of evolution of the Early Universe.

astro-ph.CO↗

arXiv2009

Gamma-ray induced cascades and magnetic fields in intergalactic medium

We present the results of Monte-Carlo simulations of three-dimensional electromagnetic cascade initiated by interactions of the multi-TeV gamma-rays with the cosmological infrared/optical photon background in the intergalactic medium. Secondary electrons in the cascade are deflected by the intergalactic magnetic fields before they scatter on CMB photons. This leads to extended 0.1-10 degree scale emission at multi-GeV and TeV energies around extragalactic sources of very-high-energy gamma-rays. The morphology of the extended emission depends, in general, on the properties of magnetic fields in the intergalactic medium. Using Monte-Carlo simulated data sets, we demonstrate that the decrease of the size of extended source with the increase of energy allows to measure weak magnetic fields with magnitudes in the range from < 1e-16 G to 1e-12 G if they exist in the voids of the Large Scale Structure.

astro-ph.CO↗

arXiv2008

Measuring parameters of AGN central engines with very high energy gamma-ray flares

We discuss a "compact source" model of very high energy (VHE) emission from blazars in which the variability time is determined by the blazar central engine. In this model electron or proton acceleration close to the supermassive black hole is followed by the development of electromagnetic cascade in a radiatively inefficient accretion flow. Assuming such a model for the TeV blazar PKS 2155-304, we show that the variability properties of the TeV gamma-ray signal observed during a bright flare from this source, such as the minimal variability time scale and the recurrence period of the sub-flares, constrain the mass and the angular momentum of the supermassive black hole.

astro-ph↗

arXiv2007

Constraints on secondary 10-100 EeV gamma ray flux in the minimal bottom-up model of Ultra High Energy Cosmic Rays

In a recently proposed model the cosmic rays spectrum at energies above EeV can be fitted with a minimal number of unknown parameters assuming that the extragalactic cosmic rays are only protons with a power law source spectrum. Within this minimal model, after fitting the observed HiRes spectrum with four parameters (proton injection spectrum power law index, maximum energy, minimum distance to sources and evolution parameter) we compute the flux of ultra-high energy photons due to photon-pion production and e+e- pair production by protons for several radio background models and a range of average extragalactic magnetic fields.

astro-ph↗