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Search Results: 1 - 10 of 200 matches for " Andrii Neronov "
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Neutrinos from Extra-Large Hadron Collider in the Milky Way
Andrii Neronov,Dmitry Semikoz
Physics , 2014, DOI: 10.1016/j.astropartphys.2015.06.004
Abstract: Neutrino telescope IceCube has recently discovered astrophysical neutrinos with energies in the TeV-PeV range. We use the data of Fermi gamma-ray telescope to demonstrate that the neutrino signal has significant contribution from the Milky Way galaxy. Matching the gamma-ray and neutrino spectra we find that TeV-PeV Galactic cosmic rays form a powerlaw spectrum with the slope p~2.45. This spectral slope is consistent with the average cosmic ray spectrum in the disks of the Milky Way and Large Magellanic Cloud galaxies. It is also consistent with the theoretical model of cosmic ray injection by diffusive shock acceleration followed by escape through the Galactic magnetic field with Kolmogorov turbulence. The locally observed TeV-PeV cosmic ray proton spectrum is softer than the average Galactic cosmic ray spectrum. This could be readily explained by variability of injection of cosmic rays in the local interstellar medium over the past 1e7 yr and discreetness of the cosmic ray source distribution.
Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars
Andrii Neronov,Ievgen Vovk
Physics , 2010, DOI: 10.1126/science.1184192
Abstract: Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound $B\ge 3\times 10^{-16}$~gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera-electron volt gamma-ray in intergalactic medium. The bound improves as $\lambda_B^{-1/2}$ if magnetic field correlation length, $\lambda_B$, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.
Fast variability of gamma-ray emission from supermassive black hole binary OJ 287
Andrii Neronov,Ievgen Vovk
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17997.x
Abstract: We report the discovery of fast variability of gamma-ray flares from blazar OJ 287. This blazar is known to be powered by binary system of supermassive black holes. The observed variability time scale T_var < 3-10 hr is much shorter than the light crossing time of more massive (1.8x10^10 solar masses) black hole and is comparable to the light crossing time of the less massive (1.3x10^8 solar masses) black hole. This indicates that gamma-ray emission is produced by relativistic jet ejected by the black hole of smaller mass. Detection of gamma rays s with energies in excess of 10 GeV during the fast variable flares constrains the Doppler factor of the jet to be larger than 4. Possibility of the study of orbital modulation of emission from relativistic jet makes OJ 287 a unique laboratory for the study of the mechanism(s) of formation of jets by black holes, in particular, of the response of the jet parameters to the changes of the parameters of the medium from which the black hole accretes and into which the jet expands.
High energy gamma rays from the massive black hole in the Galactic Center
Felix Aharonian,Andrii Neronov
Physics , 2004, DOI: 10.1086/426426
Abstract: Accreting black holes are believed to be sites of possible particle acceleration with favorable conditions also for effective gamma-ray production. However, because of photon-photon pair production, only low energy (MeV) gamma-rays can escape these compact objects with typically very large compactness parameter, given that in most cases the accretion disks within 10 Schwarzschild radii radiate with a power exceeding 10 percent of the Eddington luminosity. Therefore, the high-energy gamma-ray emission of these objects (both stellar mass and supermassive BHs) is generally suppressed, and consequently the unique information on possible particle acceleration process near the event horizon of the BH is essentially lost. Fortunately, this is not the case for the super-massive BH located at the dynamical center of our Galaxy (Sgr A*) which, thanks to its extraordinary low bolometric luminosity is transparent fo gamma-rays up to very high energies, about 10 TeV. We discuss different scenarios of gamma-ray production in Sgr A* and show that for a reasonable set of parameters one can expect detectable gamma-ray fluxes of both hadronic and electronic origin. Some of these scenarios are applicable not only for the TeV gamma-ray emission recently reported from the direction of Galactic Center, but may have broader implications relevant to highly variable nonthermal emission of Sgr A* in radio, IR and X-ray bands.
Variability of gamma-ray emission from blazars on the black hole timescales
Ievgen Vovk,Andrii Neronov
Physics , 2013, DOI: 10.1088/0004-637X/767/2/103
Abstract: We investigate the variability properties of blazars in the GeV band using the data of the Fermi/LAT telescope. We find that blazars exhibit variability on the scales down to the minimal timescale resolvable by Fermi, which is a function of the peak photon count rate in the LAT. This implies that the real minimal variability timescales for the majority of blazars are typically shorter than those resolvable by the LAT. We find that for several blazars these minimal variability timescales reach those associated to the blazar central engine, the supermassive black hole. At the same time, none of the blazars exhibits variability on the timescale shorter than the black hole horizon light crossing time and/or the period of rotation around the last stable circular orbit. Based on this fact, we argue that the timing properties of the gamma-ray signal could be determined by the processes in the direct vicinity of the supermassive black hole.
Microlensing constraint on the size of the gamma-ray emission region in blazar B0218+357
Ievgen Vovk,Andrii Neronov
Physics , 2015,
Abstract: Context. Observations of the effect of microlensing in gravitationally lensed quasars could potentially be used to study the structure of the source on distance scales down to the size of the supermassive black hole powering the quasar activity. Aims. We search for the microlensing effect in the gamma-ray band using the signal from a gravitationally lensed blazar B0218+357. Methonds. We develop a method of deconvolution of contributions of two images of the source into the gamma-ray band flaring lightcurve. We use this method to study the evolution of the magnification factor ratio between the two images throughout the flaring episodes. We interpret the time variability of the ratio as a signature of the microlensing effect and derive constraints on the physical parameters of the gamma-ray source by comparing the observed variability properties of the magnification factor ratio with those derived from numerical simulations of the microlensing caustics networks. Results. We find that the magnification factor ratio has experienced a change characteristic for a microlensing caustic crossing event during a 100 d flaring period in 2012. It has further changed between 2012 and a recent flaring episode in 2014. We use the measurement of the maximal magnification and duration of the caustic crossing event to derive an estimate of the projected size of the gamma-ray emission region in B0218+357, $R_\gamma \sim 10^{14}$ cm. This estimate is compatible with a complementary estimate found from the minimal variability time scale. The microlensing / minimal variability time scale measurements of the source size suggest that the gamma-ray emission is produced at the base of the blazar jet, in the direct vicinity of the central supermassive black hole.
Modeling of the Vela complex including the Vela supernova remnant, the binary system gamma2 Velorum, and the Gum nebula
Iurii Sushch,Bohdan Hnatyk,Andrii Neronov
Physics , 2010, DOI: 10.1051/0004-6361/201015346
Abstract: We study the geometry and dynamics of the Vela complex including the Vela supernova remnant (SNR), the binary system gamma2 Velorum and the Gum nebula. We show that the Vela SNR belongs to a subclass of non-Sedov adiabatic remnants in a cloudy interstellar medium (ISM), the dynamics of which is determined by the heating and evaporation of ISM clouds. We explain observable characteristics of the Vela SNR with a SN explosion with energy 1.4 x 10^50 ergs near the step-like boundary of the ISM with low intercloud densities (~ 10^{-3} cm^{-3}) and with a volume-averaged density of clouds evaporated by shock in the north-east (NE) part about four times higher than the one in the south-west (SW) part. The observed asymmetry between the NE and SW parts of the Vela SNR could be explained by the presence of a stellar wind bubble (SWB) blown by the nearest-to-the Earth Wolf-Rayet (WR) star in the gamma2 Velorum system. We show that the size and kinematics of gamma2 Velorum SWB agree with predictions of numerical calculations for the evolution of the SWB of M_ini = 35M* star. The low initial mass of the WR star in gamma2 Velorum implies that the luminosity of the nuclear line of 26Al, produced by gamma2 Velorum, is below the sensitivity of existing gamma-ray telescopes.
Evolution of Primordial Magnetic Fields from Phase Transitions
Tina Kahniashvili,Alexander G. Tevzadze,Axel Brandenburg,Andrii Neronov
Physics , 2012, DOI: 10.1103/PhysRevD.87.083007
Abstract: We consider the evolution of primordial magnetic fields generated during cosmological, electroweak or QCD, phase transitions. We assume that the magnetic field generation can be described as an injection of magnetic energy to cosmological plasma at a given scale determined by the moment of magnetic field generation. A high Reynolds number ensures strong coupling between magnetic field and fluid motions. The subsequent evolution of the magnetic field is governed by decaying hydromagnetic turbulence. Both our numerical simulations and a phenomenological description allow us to recover "universal" laws for the decay of magnetic energy and the growth of magnetic correlation length in the turbulent (low viscosity) regime. In particular, we show that during the radiation dominated epoch, energy and correlation length of non-helical magnetic fields scale as conformal time to the powers -1/2 and +1/2, respectively. For helical magnetic fields, energy and correlation length scale as conformal time to the powers -1/3 and +2/3, respectively. The universal decay law of the magnetic field implies that the strength of magnetic field generated during the QCD phase transition could reach $\sim 10^{-9}$\,G with the present day correlation length $\sim 50$ kpc. The fields generated at the electroweak phase transition could be as strong as $\sim 10^{-10}$ G with correlation lengths reaching $\sim 0.3$\,kpc. These values of the magnetic fields are consistent with the lower bounds of the extragalactic magnetic fields.
Fermi/LAT observations of 1ES 0229+200: implications for extragalactic magnetic fields and background light
Ievgen Vovk,Andrew M. Taylor,Dmitri Semikoz,Andrii Neronov
Physics , 2011, DOI: 10.1088/2041-8205/747/1/L14
Abstract: 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.
Searching for decaying dark matter in deep XMM-Newton observation of the Draco dwarf spheroidal
Oleg Ruchayskiy,Alexey Boyarsky,Dmytro Iakubovskyi,Esra Bulbul,Dominique Eckert,Jeroen Franse,Denys Malyshev,Maxim Markevitch,Andrii Neronov
Physics , 2015,
Abstract: We present results of a search for the 3.5 keV emission line in our recent very long (~1.4 Ms) XMM-Newton observation of the Draco dwarf spheroidal galaxy. The astrophysical X-ray emission from such dark matter-dominated galaxies is faint, thus they provide a test for the dark matter origin of the 3.5 keV line previously detected in other massive, but X-ray bright objects, such as galaxies and galaxy clusters. We do not detect a statistically significant emission line from Draco; this constrains the lifetime of a decaying dark matter particle to tau > (7-9) x 10^27 s at 95% CL (combining all three XMM-Newton cameras; the interval corresponds to the uncertainty of the dark matter column density in the direction of Draco). The PN camera, which has the highest sensitivity of the three, does show a positive spectral residual (above the carefully modeled continuum) at E = 3.54 +/- 0.06 keV with a 2.3 sigma significance. The two MOS cameras show less-significant or no positive deviations, consistently within 1 sigma with PN. Our Draco limit on tau is consistent with previous detections in the stacked galaxy clusters, M31 and the Galactic Center within their 1-2 sigma uncertainties, but is inconsistent with the high signal from the core of the Perseus cluster (which has itself been inconsistent with the rest of the detections). We conclude that this Draco observation does not exclude the dark matter interpretation of the 3.5 keV line in those objects.
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