arXiv subjects

Andrii Neronov

Publications and source records attributed to Andrii Neronov.

35 records · Page 2Linked to original sources

arXiv2017

Supernova origin of cosmic rays from a gamma-ray signal in the Constellation III region of the Large Magellanic Cloud

Cosmic rays could be produced via shock acceleration powered by supernovae. The supernova hypothesis implies that each supernova injects on average some 1e50 erg in cosmic rays, while the shock acceleration model predicts a powerlaw cosmic ray spectrum with the slope close to 2. Verification of these predictions requires measurements of spectrum and power of cosmic ray injection from supernova population(s). Here we obtain such measurements based on gamma-ray observation of Constellation III region of Large Magellanic Cloud. We show that gamma-ray emission from this young star formation region originates from cosmic rays injected by approximately two thousand supernovae, rather than by massive star wind powered superbubble pre-dating supernova activity. Cosmic ray injection power is found to be (1.1-0.2+0.5)x 1e50 erg/supernova (for the estimated interstellar medium density 0.3/cm3). The spectrum is a powerlaw with slope 2.09-0.07+0.06. This agrees with the model of particle acceleration at supernova shocks and provides a direct proof of the supernova origin of cosmic rays.

astro-ph.HE↗

arXiv2017

Cosmic ray spectrum in the local Galaxy

We study the spectral properties of the cosmic-ray spectrum in the interstellar medium within 1 kpc distance from the Sun. We used eight-year exposure data of molecular clouds of the Gould Belt obtained with the Fermi/LAT telescope to precisely measure the cosmic-ray spectrum at different locations in the local Galaxy. We compared this measurement with the direct measurements of the cosmic-ray flux in and around the solar system obtained by Voyager and AMS-02 or PAMELA. We find that the average cosmic-ray spectrum in the local Galaxy in the 1-100 GeV range is well described by a broken power-law in rigidity with a low-energy slope of $2.33^{+0.06}_{-0.08}$ and a break at $18^{+7}_{-4}$ GV, with a slope change by $0.59\pm 0.11$. This result is consistent with an earlier analysis of the gamma-ray signal from the Gould Belt clouds based on a shorter exposure of Fermi/LAT and with a different event selection. The break at 10-20 GV is also consistent with the combined Voyager + AMS-02 measurements in/around the solar system. The slope of the spectrum below the break agrees with the slope of the average cosmic-ray spectrum in the inner part of the disk of the Milky Way that was previously derived from the Fermi/LAT gamma-ray data. We conjecture that it is this slope of 2.33 and not the locally measured softer slope of 2.7-2.8 that is determined by the balance between a steady-state injection of cosmic rays with a power-law slope of 2-2.1 that is due to Fermi acceleration and the energy-dependent propagation of cosmic-ray particles through the turbulent interstellar magnetic field with a Kolmogorov turbulence spectrum. The approximation of a continuous-in-time injection of cosmic rays at a constant rate breaks down, which causes the softening of the spectrum at higher energies.

astro-ph.HE↗

arXiv2017

White paper on EUSO-SPB2

EUSO-SPB2 is a second generation Extreme Universe Space Observatory (EUSO) on a Super-Pressure Balloon (SPB). This document describes the physics capabilities, the proposed technical design of the instruments, and the simulation and analysis software.

astro-ph.HE↗

arXiv2016

First Study of Combined Blazar Light Curves with FACT and HAWC

For studying variable sources like blazars, it is crucial to achieve unbiased monitoring, either with dedicated telescopes in pointing mode or survey instruments. At TeV energies, the High Altitude Water Cherenkov (HAWC) observatory monitors approximately two thirds of the sky every day. It uses the water Cherenkov technique, which provides an excellent duty cycle independent of weather and season. The First G-APD Cherenkov Telescope (FACT) monitors a small sample of sources with better sensitivity, using the imaging air Cherenkov technique. Thanks to its camera with silicon-based photosensors, FACT features an excellent detector performance and stability and extends its observations to times with strong moonlight, increasing the duty cycle compared to other imaging air Cherenkov telescopes. As FACT and HAWC have overlapping energy ranges, a joint study can exploit the longer daily coverage given that the observatories' locations are offset by 5.3 hours. Furthermore, the better sensitivity of FACT adds a finer resolution of features on hour-long time scales, while the continuous duty cycle of HAWC ensures evenly sampled long-term coverage. Thus, the two instruments complement each other to provide a more complete picture of blazar variability. In this presentation, the first joint study of light curves from the two instruments will be shown, correlating long-term measurements with daily sampling between air and water Cherenkov telescopes. The presented results focus on the study of the variability of the bright blazars Mrk 421 and Mrk 501 during the last two years featuring various flaring activities.

astro-ph.HE↗

arXiv2016

LIC and LID considerations in the design and implementation of the MEMS laser pointing mechanism for the EUSO UV laser altimeter

The EUSO (Extreme Universe Space Observatory) project is developing a new mission concept for the scientific research of Ultra High Energy Cosmic Rays (UHECRs) from space. The EUSO wide-field telescope will look down from space onto the Earth night sky to detect UV photons emitted from air showers generated by UHECRs in our atmosphere. In this article we concentrate on the mitigation strategies agreed so far, and in particular on the implementation of a careful early selection and testing of subsystem materials (including optics), design and interfaces of the subsystem and an optimization of the instrument operational concept.

astro-ph.IM↗

arXiv2016

Decaying dark matter search with NuSTAR deep sky observations

We present the results of the search for decaying dark matter with particle mass in the 6-40 keV range with NuSTAR deep observations of COSMOS and ECDFS empty sky fields. We show that main contribution to the decaying dark matter signal from the Milky Way galaxy comes through the aperture of the NuSTAR detector, rather than through the focusing optics. High sensitivity of the NuSTAR detector, combined with the large aperture and large exposure times of the two observation fields allow us to improve previously existing constraints on the dark matter decay time by up to an order of magnitude in the mass range 10-30 keV. In the particular case of the nuMSM sterile neutrino dark matter, our constraints impose an upper bound m<20 keV on the dark matter particle mass. We report detection of four unidentified spectral lines in our data set. These line detections are either due to the systematic effects (uncertainties of calibrations of the NuSTAR detectors) or have an astrophysical origin. We discuss different possibilities for testing the nature of the detected lines.

astro-ph.HE↗

arXiv2016

An innovative silicon photomultiplier digitizing camera for gamma-ray astronomy

The single-mirror small-size telescope (SST-1M) is one of the three proposed designs for the small-size telescopes (SSTs) of the Cherenkov Telescope Array (CTA) project. The SST-1M will be equipped with a 4 m-diameter segmented mirror dish and an innovative fully digital camera based on silicon photo-multipliers (SiPMs). Since the SST sub-array will consist of up to 70 telescopes, the challenge is not only to build a telescope with excellent performance, but also to design it so that its components can be commissioned, assembled and tested by industry. In this paper we review the basic steps that led to the design concepts for the SST-1M camera and the ongoing realization of the first prototype, with focus on the innovative solutions adopted for the photodetector plane and the readout and trigger parts of the camera. In addition, we report on results of laboratory measurements on real scale elements that validate the camera design and show that it is capable of matching the CTA requirements of operating up to high-moon-light background conditions.

astro-ph.IM↗

arXiv2016

Searching for decaying dark matter in deep XMM-Newton observation of the Draco dwarf spheroidal

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.

astro-ph.HE↗

arXiv2015

Microlensing constraint on the size of the gamma-ray emission region in blazar B0218+357

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, $R_γ\sim 10^{14}$ 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.

astro-ph.HE↗

arXiv2015

Neutrinos from Extra-Large Hadron Collider in the Milky Way

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.

astro-ph.HE↗

arXiv2013

Evolution of Primordial Magnetic Fields from Phase Transitions

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.

astro-ph.CO↗

arXiv2013

Variability of gamma-ray emission from blazars on the black hole timescales

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.

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↗

arXiv2010

Fast variability of gamma-ray emission from supermassive black hole binary OJ 287

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.

astro-ph.HE↗

arXiv2010

Modeling of the Vela complex including the Vela supernova remnant, the binary system gamma2 Velorum, and the Gum nebula

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.

astro-ph.GA↗

arXiv2010

Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars

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 $λ_B^{-1/2}$ if magnetic field correlation length, $λ_B$, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.

astro-ph.HE↗

arXiv2004

High energy gamma rays from the massive black hole in the Galactic Center

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.

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