arXiv subjects

A. Neronov

Publications and source records attributed to A. Neronov.

At least 127 records · Page 7Linked to original sources

arXiv2012

Superorbital modulation of X-ray emission from gamma-ray binary LSI +61 303

We report the discovery of a systematic constant time lag between the X-ray and radio flares of the gamma-ray binary LSI +61 303, persistent over long, multi-year, time scale. Using the data of monitoring of the system by RXTE we show that the orbital phase of X-ray flares from the source varies from $ϕ_X\simeq 0.35$ to $ϕ_X\simeq 0.75$ on the superorbital 4.6 yr time scale. Simultaneous radio observations show that periodic radio flares always lag the X-ray flare by $Δϕ_{X-R}\simeq 0.2$. We propose that the constant phase lag corresponds to the time of flight of the high-energy particle filled plasma blobs from inside the binary to the radio emission region at the distance ~10 times the binary separation distance. We put forward a hypothesis that the X-ray bursts correspond to the moments of formation of plasma blobs inside the binary system.

astro-ph.HE↗

arXiv2011

Low-energy break in the spectrum of Galactic cosmic rays

Measurements of the low energy spectrum of Galactic Cosmic Rays (GCR) by detectors at or near the Earth are affected by Solar modulation. To overcome this difficulty, we consider nearby molecular clouds as GCR detectors outside the Solar system. Using gamma-ray observations of the clouds by the Fermi telescope we derive the spectrum of GCRs in the clouds from the observed gamma-ray emission spectrum. We find that the GCR spectrum has a low energy break with the spectral slope hardening by 1.1+/-0.3 at an energy of 9+/-3 GeV. Detection of a low-energy break enables a measurement of GCR energy density in the interstellar space 0.9+/-0.3 eV/cm3.

astro-ph.HE↗

arXiv2011

Discovery of High-Energy Gamma-Ray Emission from the Binary System PSR B1259-63/LS 2883 Around Periastron with Fermi

We report on the discovery of \geq 100 MeV γ rays from the binary system PSR B1259-63/LS 2883 using the Large Area Telescope (LAT) on board Fermi. The system comprises a radio pulsar in orbit around a Be star. We report on LAT observations from near apastron to ~ 60 days after the time of periastron, tp, on 2010 December 15. No γ-ray emission was detected from this source when it was far from periastron. Faint γ-ray emission appeared as the pulsar approached periastron. At ~ tp + 30d, the \geq 100 MeV γ-ray flux increased over a period of a few days to a peak flux 20-30 times that seen during the pre-periastron period, but with a softer spectrum. For the following month, it was seen to be variable on daily time scales, but remained at ~ 1 - 4 \times 10^-6 cm^-2 s^-1 before starting to fade at ~ tp + 57d. The total γ-ray luminosity observed during this period is comparable to the spin-down power of the pulsar. Simultaneous radio and X-ray observations of the source showed no corresponding dramatic changes in radio and X-ray flux between the pre-periastron and post-periastron flares. We discuss possible explanations for the observed γ-ray-only flaring of the source.

astro-ph.HE↗

arXiv2011

EGMF Constraints from Simultaneous GeV-TeV Observations of Blazars

Attenuation of the TeV gamma-ray flux from distant blazars through pair production with extragalactic background light leads to the development of electromagnetic cascades and subsequent, lower energy, GeV secondary gamma-ray emission. Due to the deflection of VHE cascade electrons by extragalactic magnetic fields (EGMF), the spectral shape of this arriving cascade gamma-ray emission is dependent on the strength of the EGMF. Thus, the spectral shape of the GeV-TeV emission from blazars has the potential to probe the EGMF strength along the line of sight to the object. We investigate constraints on the EGMF derived from observations of blazars for which TeV observations simultaneous with those by the Fermi telescope were reported. We study the dependence of the EGMF bound on the hidden assumptions it rests upon. We select blazar objects for which simultaneous Fermi/LAT GeV and Veritas, MAGIC or HESS TeV emission have been published. We model the development of electromagnetic cascades along the gamma-ray beams from these sources using Monte Carlo simulations, including the calculation of the temporal delay incurred by cascade photons, relative to the light propagation time of direct gamma-rays from the source. Constraints on EGMF could be derived from the simultaneous GeV-TeV data on the blazars RGB J0710+591, 1ES 0229+200, and 1ES 1218+304. The measured source flux level in the GeV band is lower than the expected cascade component calculated under the assumption of zero EGMF. Assuming that the reason for the suppression of the cascade component is the extended nature of the cascade emission, we find that B>10^{-15} G (assuming EGMF correlation length of ~1 Mpc) is consistent with the data. Alternatively, the assumption that the suppression of the cascade emission is caused by the time delay of the cascade photons the data are consistent with B>10^{-17} G for the same correlation length.

astro-ph.HE↗

arXiv2010

No evidence for gamma-ray halos around active galactic nuclei resulting from intergalactic magnetic fields

We analyze the gamma-ray halo around stacked AGNs reported in Ap.J.Lett., 2010, 722, L39. First, we show that the angular distribution of gamma-rays around the stacked AGNs is consistent with the angular distribution of the gamma-rays around the Crab pulsar, which is a point source for Fermi/LAT. This makes it unlikely that the halo is caused by an electromagnetic cascade of TeV photons in the intergalactic space. We then compare the angular distribution of gamma-rays around the stacked AGNs with the point-spread function (PSF) of Fermi/LAT and confirm the existence of an excess above the PSF. However, we demonstrate that the magnitude and the angular size of this effect is different for photons converted in the front and back parts of the Fermi/LAT instrument, and thus is an instrumental effect.

astro-ph.HE↗

arXiv2010

Very high-energy gamma-ray emission from IC 310

We construct a systematic survey of extragalactic γ-ray sky at the energies above 100 GeV using the data of Fermi telescope. Such survey has not been previously done by the ground-based Cherenkov gamma-ray telescopes which have, contrary to Fermi, narrow field of view. We study a map of arrival directions of the highest energy photons detected by Fermi at Galactic latitudes |b| > 10 degrees and search for significant point source like excesses above the diffuse Galactic and extragalactic γ-ray backgrounds. We identify eight significant point source like excesses in this map. Seven of the eight sources are known TeV blazars. The previously unknown source is identified with a head-tail radio galaxy IC 310, situated in Perseus cluster of galaxies. The source is detected with significance 6 sigma above 30 GeV. We identify two possible scenaria for gamma-ray emission from this source. One possibility is that emission originates from the base of relativistic outflow from the active nucleus, as in the BL Lacs and FR I type radio galaxies. Otherwise gamma-ray photons could be produced at the bow shock formed in result of fast motion of the galaxy through the intracluster medium. The two models could be distinguished via the study of variability of the γ-ray signal.

astro-ph.HE↗

arXiv2010

Galactic sources of E>100 GeV gamma-rays seen by Fermi telescope

We perform a search for sources of gamma-rays with energies E>100 GeV at low Galactic latitudes |b|<10 deg using the data of Fermi telescope. To separate compact gamma-ray sources from the diffuse emission from the Galaxy, we use the Minimal Spanning Tree method with threshold of 5 events in inner Galaxy (Galactic longitude |l|<60 deg) and of 3 events in outer Galaxy. Using this method, we identify 22 clusters of very-high-energy (VHE) gamma-rays, which we consider as "source candidates". 3 out of 22 event clusters are expected to be produced in result of random coincidences of arrival directions of diffuse background photons. To distinguish clusters of VHE events produced by real sources from the background we perform likelihood analysis on each source candidate. We present a list of 19 higher significance sources for which the likelihood analysis in the energy band E>100 GeV gives Test Statistics (TS) values above 25. Only 10 out of the 19 high-significance sources can be readily identified with previously known VHE gamma-ray sources. 4 sources could be parts of extended emission from known VHE gamma-ray sources. Five sources are new detections in the VHE band. Among these new detections we tentatively identify one source as a possible extragalactic source PMN J1603-4904 (a blazar candidate), one as a pulsar wind nebula around PSR J1828-1007. High significance cluster of VHE events is also found at the position of a source coincident with the Eta Carinae nebula. In the Galactic Center region, strong VHE gamma-ray signal is detected from Sgr C molecular cloud, but not from the Galactic Center itself.

astro-ph.HE↗

arXiv2010

The catalog of variable sources detected by INTEGRAL I: Catalog and Techniques

Context: In 6 years of operation, INTEGRAL/ISGRI revealed more than 500 sources. Many of these sources are variable. Taking into account that nearly half of INTEGRAL/ISGRI sources are new and many of them are still unidentified, the variability properties of the sources can serve as additional parameters that may help to classify and identify the unknown sources. Aims: In order to study the variability properties of the sources detected by INTEGRAL/ISGRI we develop a method to quantify the variability of a source. We describe here our techniques and compile a catalog of the sources that fit our criteria of variability. Methods: We use the natural time binning of INTEGRAL observations called Science Window ($\approx 2000$ seconds) and test the hypothesis that the detected sources are constant using a $χ^2$ all-sky map in three energy bands (20-40, 40-100, 100-200 keV). We calculate an intrinsic variance of the flux in individual pixels and use it to define the fractional variability of a source. The method is sensitive to the source variability on time scales of one Science Window and higher. We concentrate only on the sources which were already reported to be detected by INTEGRAL. Results: We present a catalog of 202 sources which are found to be significantly variable. For the catalog sources we give the measure of variability and fluxes with corresponding errors in 20-40, 40-100, 100-200 keV energy bands, and we present some statistics about the population of variable sources. The description of the physical properties of the variable sources will be given in a forthcoming paper.

astro-ph.HE↗

arXiv2010

Gamma ray emission from magnetized relativistic GRB outflows

We argue that small pitch angle synchrotron emission provides an important dissipation mechanism which has to be taken into account in the models of formation of relativistic magnetized gamma-ray burst (GRB) outflows from the newborn black holes and/or magnetars. We show that if the GRB outflow is proton loaded, spectral energy distribution of this emission is expected to sharply peak in 0.1-1~MeV energy band. If the small pitch angle synchrotron emission efficiently cools relativistic particles of the outflow, its spectrum below the peak energy is a powerlaw with spectral index alpha ~ -1, close to the typical spectral index of the time-resolved GRB spectra. Otherwise, the low energy spectral index can be as hard as alpha ~ 0, as observed at the beginning of the GRB pulses. We make a conjecture that small pitch angle synchrotron emission from proton-loaded magnetized GRB outflow could significantly contribute to the Band component of the prompt emission of GRBs while electromagnetic cascade initiated by the protons could be responsible for the GeV component.

astro-ph.HE↗

arXiv2010

INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission

We derive the spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range from the data of the IBIS instrument aboard the INTEGRAL satellite obtained during the four dedicated Earth-occultation observations of early 2006. We analyse the modulation of the IBIS/ISGRI detector counts induced by the passage of the Earth through the field of view of the instrument. Unlike previous studies, we do not fix the spectral shape of the various contributions, but model instead their spatial distribution and derive for each of them the expected modulation of the detector counts. The spectra of the diffuse emission components are obtained by fitting the normalizations of the model lightcurves to the observed modulation in different energy bins. The obtained CXB spectrum is consistent with the historic HEAO-1 results and falls slightly below the spectrum derived with Swift/BAT. A 10% higher normalization of the CXB cannot be completely excluded, but it would imply an unrealistically high albedo of the Earth. The derived spectrum of the GRXE confirms the presence of a minimum around 80 keV with improved statistics and yields an estimate of ~0.6 M_Sun for the average mass of white dwarfs in the Galaxy. The analysis also provides updated normalizations for the spectra of the Earth's albedo and the cosmic-ray induced atmospheric emission.

astro-ph.CO↗

arXiv2009

A compact pulsar wind nebula model of the gamma-ray loud binary LS I +61 303

We study a model of of LS I +61 303 in which its radio to TeV emission is due to interaction of a relativistic wind from a young pulsar with the wind from its companion Be star. We assume the fast polar wind is clumpy, which is typical for radiatively-driven winds. The clumpiness cause the two winds to mix. The relativistic electrons from the pulsar wind are retained in the moving clumps by inhomogeneities of the magnetic field, which explains the X-ray variability observed on time scales much shorter than the orbital period. We calculate detailed inhomogeneous spectral models reproducing the average broad-band spectrum from radio to TeVs. Given the uncertainties the form of the distribution of relativistic electrons, the X-ray spectrum could be dominated by either Compton or synchrotron emission. The recent Fermi observations constrain the high-energy cut-off in the electron distribution to be at the Lorentz factor of 2 10^4 or 10^8 in the former and latter model, respectively. We provide formulae comparing the losses of the relativistic electrons due to Compton, synchrotron and Coulomb processes vs. the distance from the Be star. We calculate the optical depth of the wind to free-free absorption, showing that it will suppress most of the radio emission within the orbit. We point out the importance of Compton and Coulomb heating of the stellar wind within and around the gamma-ray emitting region. Then, we find the most likely mechanism explaining the orbital modulation at TeV energies is anisotropy of emission.

astro-ph↗

arXiv2009

Study of gamma-ray loud binaries in the Fermi era

We discuss Fermi observations of gamma-ray loud binaries. We show that within hadronic model of activity of LSI +61 303, detection of cut-off in the GeV spectrum constrains maximal energy of the primary protons. In this way, the GeV gamma-ray data impose constraint on the expected neutrino signal (spectrum and lightcurve) from the source. We also briefly discuss perspectives of GeV band detection of PSR B1259-63 during the 2010 periastron passage.

astro-ph.HE↗

arXiv2009

Exceptional flaring activity of the anomalous X-ray pulsar 1E 1547.0-5408

(Abridged) We studied an exceptional period of activity of the anomalous X-ray pulsar 1E 1547.0-5408 in January 2009, during which about 200 bursts were detected by INTEGRAL. The major activity episode happened when the source was outside the field of view of all the INTEGRAL instruments. But we were still able to study the properties of 84 bursts detected simultaneously by the anti-coincidence shield of the spectrometer SPI and by the detector of the imager ISGRI. We find that the luminosity of the 22 January 2009 bursts of 1E 1547.0-5408 was > 1e42 erg/s. This luminosity is comparable to that of the bursts of soft gamma repeaters (SGR) and is at least two orders of magnitude larger than the luminosity of the previously reported bursts from AXPs. Similarly to the SGR bursts, the brightest bursts of 1E 1547.0-5408 consist of a short spike of ~100 ms duration with a hard spectrum, followed by a softer extended tail of 1-10 s duration, which occasionally exhibits pulsations with the source spin period of ~2 s. The observation of AXP bursts with luminosities comparable to the one of SGR bursts strengthens the conjecture that AXPs and SGRs are different representatives of one and the same source type.

astro-ph.HE↗

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

IceCube Sensitivity for Neutrino Flux from Fermi Blazars in Quiescent States

We investigate the IceCube detection potential of very high energy neutrinos from blazars, for different classes of "hadronic" models, taking into account the limits imposed on the neutrino flux by the recent Fermi telescope observations. Assuming the observed gamma-ray emission is produced by the decay of neutral pions from proton-proton interactions, the measurement of the time-averaged spectral characteristics of blazars in the GeV energy band imposes upper limits on the time-averaged neutrino flux. Comparing these upper limits to the 5 sigma discovery threshold of IceCube for different neutrino spectra and different source locations in the sky, we find that several BL Lacs with hard spectra in the GeV band are within the detection potential of IceCube. If the gamma-ray emission is dominated by the neutral pion decay flux, none of the flat-spectrum radio quasars are detectable with IceCube. If the primary high energy proton spectrum is very hard and/or neutrinos are produced in proton-photon, rather than proton-proton reactions, the upper limit on the neutrino flux imposed by the measured gamma-ray spectra is relaxed and gamma-ray observations impose only lower bounds on the neutrino flux. We investigate whether these lower bounds guarantee the detection of blazars with very hard neutrino spectra, expected in the latter type model. We show that all the "hadronic" models of activity of blazars are falsifiable with IceCube. Furthermore, we show that models with gamma-ray emission produced by the decay of neutral pions from proton-proton interactions can be readily distinguished from the models based on proton-gamma interactions and/or models predicting very hard high energy proton spectra via a study of the distribution of spectral indices of gamma-ray spectra of sources detected with IceCube.

astro-ph.HE↗

arXiv2009

The search for decaying Dark Matter

We propose an X-ray mission called Xenia to search for decaying superweakly interacting Dark Matter particles (super-WIMP) with a mass in the keV range. The mission and its observation plan are capable of providing a major break through in our understanding of the nature of Dark Matter (DM). It will confirm, or reject, predictions of a number of particle physics models by increasing the sensitivity of the search for decaying DM by about two orders of magnitude through a wide-field imaging X-ray spectrometer in combination with a dedicated observation program. The proposed mission will provide unique limits on the mixing angle and mass of neutral leptons, right handed partners of neutrinos, which are important Dark Matter candidates. The existence of these particles is strongly motivated by observed neutrino flavor oscillations and the problem of baryon asymmetry of the Universe. In super-WIMP models, the details of the formation of the cosmic web are different from those of LambdaCDM. The proposed mission will, in addition to the search for decaying Dark Matter, provide crucial insight into the nature of DM by studying the structure of the "cosmic web". This will be done by searching for missing baryons in emission, and by using gamma-ray bursts as backlight to observe the warm-hot intergalactic media in absorption.

astro-ph.CO↗

arXiv2009

X-ray observations of PSR B1259-63 near the 2007 periastron passage

PSR B1259-63 is a 48 ms radio pulsar in a highly eccentric 3.4 year orbit with a Be star SS 2883. Unpulsed gamma-ray, X-ray and radio emission components are observed from the binary system. It is likely that the collision of the pulsar wind with the anisotropic wind of the Be star plays a crucial role in the generation of the observed non-thermal emission. The 2007 periastron passage was observed in unprecedented details with Suzaku, Swift, XMM-Newton and Chandra missions. We present here the results of this campaign and compare them with previous observations. With these data we are able, for the first time, to study the details of the spectral evolution of the source over a 2 months period of the passage of the pulsar close to the Be star. New data confirm the pre-periastron spectral hardening, with the photon index reaching a value smaller than 1.5, observed during a local flux minimum. If the observed X-ray emission is due to the inverse Compton (IC) losses of the 10 MeV electrons, then such a hard spectrum can be a result of Coulomb losses, or can be related to the existence of the low-energy cut-off in the electron spectrum. Alternatively, if the X-ray emission is a synchrotron emission of very high energy electrons, the observed hard spectrum can be explained if the high energy electrons are cooled by IC emission in Klein-Nishina regime. Unfortunately the lack of simultaneous data in the TeV energy band prevents us from making a definite conclusion on the nature of the observed spectral hardening and, therefore, on the origin of the X-ray emission.

astro-ph.GA↗

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↗