Fermi Spectra Research Articles

Very-high-energy Emission from Pulsars

Abstract Air-Cherenkov telescopes have detected pulsations at energies above 50 GeV from a growing number of Fermi pulsars. These include the Crab, Vela, PSR B1706−44, and Geminga, with the first two having pulsed detections above 1 TeV. In some cases, there appears to be very-high-energy (VHE) emission that is an extension of the Fermi spectra to high energies, while in other cases, additional higher-energy spectral components that require a separate emission mechanism may be present. We present results of broadband spectral modeling using global magnetospheric fields and multiple emission mechanisms that include synchro-curvature (SC) and inverse Compton scattered (ICS) radiation from accelerated particles (primaries) and synchrotron self-Compton (SSC) emission from lower-energy pairs. Our models predict three distinct VHE components: SC from primaries whose high-energy tail can extend to 100 GeV, SSC from pairs that can extend to several TeV, and ICS from primary particles accelerated in the current sheet that scatter pair synchrotron radiation, which appears beyond 10 TeV. Our models suggest that H.E.S.S.-II and MAGIC have detected the high-energy tail of the primary SC component that produces the Fermi spectrum in Vela, Geminga, and PSR B1706−44. We argue that the ICS component peaking above 10 TeV from Vela has been seen by H.E.S.S. Detection of this emission component from the Crab and other pulsars is possible with the High Altitude Water Cherenkov Observatory and Cherenkov Telescope Array, and will directly measure the maximum particle energy in pulsars.

Searching for TeV Candidates in 4LAC High-synchrotron- peaked Frequency BL Lac Objects

Abstract The next generation of TeV detectors is expected to have significantly enhanced performance. It is therefore constructive to search for new TeV candidates for observation. This paper focuses on TeV candidates among the high-synchrotron-peaked BL Lacertae objects (HBLs) reported in the fourth catalog of active galactic nuclei detected by Fermi’s Large Area Telescope, i.e., 4LAC. By cross-matching the Fermi data with radio and optical observations, we collected the multiwavelength features of 180 HBLs with known redshifts. The data set contains 39 confirmed TeV sources and 141 objects whose TeV detection has not yet been reported (either not yet observed, or observed but not detected). Using two kinds of supervised machine-learning (SML) methods, we searched for new possible TeV candidates (PTCs) among the nondetected objects by assessing the similarity of their multi-wavelength properties to existing TeV-detected objects. The classification results of the two SML classifiers were combined and the 24 highest-confidence PTCs were proposed as the best candidates. We calculate, here, the 12 year averaged Fermi spectra of these PTCs and estimate their detectability by extrapolating the Fermi spectra and including the extragalactic background light attenuation. Four candidates are suggested to have a high likelihood of being detected by the Large High Altitude Air Shower Observatory, and 24 are candidates for Cherenkov Telescope Array observations.

Prospective Annual Detection Rate of High-energy Gamma-Ray Bursts with LHAASO-WCDA

Abstract Gamma-ray bursts (GRBs) are considered one of the most violent, explosive events in the universe and serve as high-redshift probes for cosmological study due to their high-energy observations. Such observations, particularly in the GeV regime, have already proven fruitful for deriving useful scientific results, such as the determination of extragalactic background light (EBL) and the stringent constraint on the Lorentz invariance violation effect. Owing to the advantages of a very large effective area, a low threshold energy, a wide field of view, and high duty cycle, the upcoming Large High Altitude Air Shower Observatory–Water Cerenkov Detector Array (LHAASO-WCDA) will have potential sensitivity for discovering GRBs in the 100 GeV energy region. In this work, a sample of GRBs has been generated and examined based on existing observations reported by the Large Area Telescope on board the Fermi satellite. The Fermi spectra are extrapolated to high energy by taking into account the absorption due to the pair production processes occurring between γ rays and EBL. With an assumption that an ultrahigh-energy component accounts for 10% of the total luminosity, it is found that LHAASO-WCDA has a GRB detection rate of ∼one GRB per year.

Probing the holographic Fermi arc with scalar field: numerical and analytical study

Fermi arcs are disconnected contour of Fermi surface, which can be observed in pseudo-gap phase of high temperature superconductors. Aiming to understand this pseudo-gap phenomena, we study a holographic Fermionic system coupled with a massive scalar field in an AdS black hole background. Depending on the boundary condition on the scalar field mode, we discuss two possible scenarios. When the scalar condenses below a critical temperature Tc, Fermi surface undergoes a transition from normal phase to pseudo-gap phase. Hence Tc can be the reminiscent of well known cross over temperature T* in cuprate superconductor, below which pseudo-gap appears at constant doping. In the second scenario, the bulk scalar develops a non-normalizable profile at arbitrary temperature for non-zero source at the boundary. Therefore, we can tune the Fermi spectrum by tuning a dual source at the boundary. The dual source for this case can be the reminiscent of hole doping in the real cuprate superconductor. For both the cases we have studied Fermi spectrum and observed anisotropic gap in the spectral function depending on the model parameter and studied the properties of Fermi arcs across different phases.

Experimental Determination of the Effective Resonance Absorption Integrals of 238U and 158Gd in Urania–Gadolinia Rods

The experimental procedure and the results of determination of the effective resonance neutron absorption integrals of 238U and 158Gd for isolated UO2–Gd2O3 rods are presented. Measurements were performed using the activation method. UO2–Gd2O3 rods were irradiated in the neutron field with the Fermi spectrum formed at the center of the core of the F-1 research reactor at the National Research Centre Kurchatov Institute.

Simulation of decay processes and radiation transport times in radioactivity measurements

The Fortran subroutine package pennuc, which simulates random decay pathways of radioactive nuclides, is described. The decay scheme of the active nuclide is obtained from the nucleide database, whose web application has been complemented with the option of exporting nuclear decay data (possible nuclear transitions, branching ratios, type and energy of emitted particles) in a format that is readable by the simulation subroutines. In the case of beta emitters, the initial energy of the electron or positron is sampled from the theoretical Fermi spectrum. De-excitation of the atomic electron cloud following electron capture and internal conversion is described using transition probabilities from the LLNL Evaluated Atomic Data Library and empirical or calculated energies of released X rays and Auger electrons. The time evolution of radiation showers is determined by considering the lifetimes of nuclear and atomic levels, as well as radiation propagation times. Although pennuc is designed to operate independently, here it is used in conjunction with the electron-photon transport code penelope, and both together allow the simulation of experiments with radioactive sources in complex material structures consisting of homogeneous bodies limited by quadric surfaces. The reliability of these simulation tools is demonstrated through comparisons of simulated and measured energy spectra from radionuclides with complex multi-gamma spectra, nuclides with metastable levels in their decay pathways, nuclides with two daughters, and beta plus emitters.

CONTEMPORANEOUS BROADBAND OBSERVATIONS OF THREE HIGH-REDSHIFT BL LAC OBJECTS

ABSTRACT We have collected broadband spectral energy distributions (SEDs) of three BL Lac objects 3FGL J0022.1−1855 (z = 0.689), 3FGL J0630.9−2406 ( z ≳ ?> 1.239), and 3FGL J0811.2−7529 (z = 0.774), detected by Fermi with relatively flat gigaelectronvolt spectra. By observing simultaneously in the near-infrared to hard X-ray band, we can well characterize the high end of the synchrotron component of the SED. Thus, fitting the SEDs to synchro-Compton models of the dominant emission from the relativistic jet, we can constrain the underlying particle properties and predict the shape of the gigaelectronvolt Compton component. Standard extragalactic background light (EBL) models explain the high-energy absorption well, with poorer fits for high-ultraviolet models. The fits show clear evidence for EBL absorption in the Fermi spectrum of our highest-redshift source 3FGL J0630.9−2406. While synchrotron self-Compton models adequately describe the SEDs, the situation may be complicated by possible external Compton components. For 3FGL J0811.2−7529, we also discover a nearby serendipitous source in the X-ray data, which is almost certainly another lower synchrotron peak frequency ( &ngr; pk sy ?> ) BL Lac, that may contribute flux in the Fermi band. Since our sources are unusual high-luminosity, moderate &ngr; pk sy ?> BL Lacs, we compare these quantities and the Compton dominance, the ratio of peak inverse Compton to peak synchrotron luminosities ( L pk IC / L pk sy ?> ), with those of the full Fermi BL Lac population.

Estimating the energy of stitching together the Maxwell and Fermi neutron spectra

The energy of stitching together the Maxwell and Fermi spectra is estimated on the basis of experimental data and the results of calculations carried out using the MCU-RFFI and MCU-5 codes.

THE γ-RAY SPECTRUM OF GEMINGA AND THE INVERSE COMPTON MODEL OF PULSAR HIGH-ENERGY EMISSION

We reanalyze the Fermi spectra of the Geminga and Vela pulsars. We find that the spectrum of Geminga above the break is exceptionally well approximated by a simple power law without the exponential cut-off, making Geminga's spectrum similar to that of Crab. Vela's broadband gamma-ray spectrum is equally well fit with both the exponential cut-off and the double power law shapes. In the broadband double power-law fits, for a typical Fermi spectrum of a bright \gamma-ray pulsar, most of the errors accumulate due to the arbitrary parametrization of the spectral roll-off. In addition, a power law with an exponential cut-off gives an acceptable fit for the underlying double power-law spectrum for a very broad range of parameters, making such fitting procedures insensitive to the underlying Fermi photon spectrum. Our results have important implications for the mechanism of pulsar high energy emission. A number of observed properties of \gamma-ray pulsars, i.e., the broken power law spectra without exponential cut-offs and stretching in case of Crab beyond the maximal curvature limit, spectral breaks close to or exceeding the maximal breaks due to curvature emission, a Crab patterns of relative intensities of the leading and trailing pulses repeated in the X-ray and \gamma-ray regions, all point to the inverse Compton origin of the high energy emission from majority of pulsars.

CALCULATION OF ISODOSE CURVES FOR CCA AND CCB CONCAVE EYE APPLICATORS

BEBIG Ruthenium-106 ophthalmic plaques have been used for treatment of uveal melanoma, retinoblastoma, melanoma of the iris and other special applications for more than 30 years. The plaques consist of a thin film of Ru-106, a beta emitter encapsulated in pure silver. Simulations of small concave applicators CCA and CCB, manufactured by Bebig, were performed using Monte Carlo MCNP4C code which allows for description of the applicator (geometry and materials) in detail. Electrons are emitted from the 106 Ru nuclei isotropically with initial energy randomly sampled from the corresponding Fermi spectra and with initial positions uniformly distributed on the radioactive layer. In this work, relative doses were calculated in soft tissue phantom near the active layer. Isodose curves for CCA and CCB eye applicators were determined. Our calculated data agrees well with the measured phantom data reported in literature.

Variation of the γ opacity by the He II Lyman continuum constrains the location of the γ-ray emission region in the blazar 3C 454.3

Abstract We study spectral properties of the brightest γ-ray blazar 3C 454.3 using 138 weeks of observations by the Fermi Gamma-ray Space Telescope (Fermi). We probe the behaviour of the source as a function of time at different brightness levels. The Fermi spectra in the GeV range can be well described by a wide underlying lognormal distribution with the photon—photon absorption breaks produced by the He ii and H i Lyman recombination continua (LyC). We find a power-law dependence of the peak energy on flux and discover anticorrelation between the column density of the He ii LyC and flux. This implies that the γ-ray emission zone lies close to the boundary of the high-ionization part of the broad-line region and moves away from the black hole when the flux increases. Identification of the γ-ray production with the relativistic jet implies that the jet is already accelerated at sub-parsec distances from the central black hole, which favours the Blandford—Znajek process as the jet launching mechanism.

On the origin of correlated X-ray/VHE emission from LS I +61 303

The MAGIC collaboration has recently reported correlated X-ray and very high-energy gamma-ray emission from the gamma-ray binary LS I +61 303 during ~60% of one orbit. These observations suggest that the emission in these two bands has its origin in a single particle population. We aim at improving our understanding of the source behaviour by explaining the simultaneous X-ray and VHE data through a radiation model. We use a model based on a one zone population of relativistic leptonic particles assuming dominant adiabatic losses located at the position of the compact object. The adiabatic cooling timescale is inferred from the X-ray fluxes. The model can reproduce the spectra and lightcurves in the X-ray and VHE bands. Adiabatic losses could be the key ingredient to explain the X-ray and partially the VHE lightcurves. From the best fit result, we obtain a magnetic field of B=0.2 G, a minimum luminosity budget of ~2x10^35 erg/s and a relatively high acceleration efficiency. In addition, our results seem to confirm that the GeV emission detected by Fermi does not come from the same parent particle population as the X-ray and VHE emission and the Fermi spectrum poses a constraint on the hardness of the particle spectrum at lower energies. In the context of our scenario, more sensitive observations would allow to constrain the inclination angle, which could determine the nature of the compact object.

Redshifts of Distant Blazars Limited by Fermi and VHE γ-Ray Observations

Abstract Our goal is to limit the redshifts of three blazars, PG 1553$+$113, 3C 66A, and PKS 1424$+$240, through an investigation of their Fermi and VHE (very high energy) $\gamma$-ray observations. We assume that the intrinsic spectra of PG 1553$+$113, 3C 66A, and PKS 1424$+$240 have no cutoff across the Fermi and VHE $\gamma$-ray energy ranges. The intrinsic spectra of VHE $\gamma$-rays are obtained through extrapolation of the Fermi spectra. Comparing the measured and intrinsic VHE spectra due to extragalactic background light (EBL) absorption, we give the redshift upper limits of three blazars, while assuming a specific EBL model. The redshift upper limits of PG 1553$+$113, 3C 66A, and PKS 1424$+$240 are 0.78, 0.58, and 1.19, respectively. Near to the TeV energy the optical depth of VHE $\gamma$ photons might be overestimated by the Franceschini EBL model (Franceschini et al. 2008, A&A, 487, 837), or the second emission component might be present in the VHE spectra and lead the intrinsic photon index to be harder than the Fermi ones.

FERMIOBSERVATIONS OF TeV-SELECTED ACTIVE GALACTIC NUCLEI

We report on observations of TeV-selected AGN made during the first 5.5 months of observations with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope (Fermi). In total, 96 AGN were selected for study, each being either (i) a source detected at TeV energies (28 sources) or (ii) an object that has been studied with TeV instruments and for which an upper-limit has been reported (68 objects). The Fermi observations show clear detections of 38 of these TeV-selected objects, of which 21 are joint GeV-TeV sources and 29 were not in the third EGRET catalog. For each of the 38 Fermi-detected sources, spectra and light curves are presented. Most can be described with a power law of spectral index harder than 2.0, with a spectral break generally required to accommodate the TeV measurements. Based on an extrapolation of the Fermi spectrum, we identify sources, not previously detected at TeV energies, which are promising targets for TeV instruments. Evidence for systematic evolution of the $\gamma$-ray spectrum with redshift is presented and discussed in the context of interaction with the EBL.

On discrepancy between ATIC and Fermi data

Either ATIC or Fermi-LAT data can be fitted together with the PAMELA data by three components: primary background ∼ E−3.3, secondary background ∼ E−3.6, and an additional source of electrons ∼ E−γaExp(−E/Ecut). We find that the best fits for ATIC + PAMELA and for Fermi + PAMELA are approximately the same, γa ≈ 2 and Ecut ∼ 500 GeV. However, the ATIC data have a narrow bump between 300 GeV and 600 GeV which contradicts the smooth Fermi spectrum. An interpretation of the ATIC bump as well as the featureless Fermi spectrum in terms of dark matter models and pulsars is discussed.

Fermi Spectra and their Gauge Invariance in Hot and Dense Abelian and Non-Abelian theories

The one-loop Fermi spectra (one-particle and collective ones) are found for all momenta in the T2-approximation and their gauge invariance in hot and dense Abelian and non-Abelian theories is studied. It is shown that the one-particle spectrum, if the calculation accuracy is kept strictly, is gauge invariant for all momenta and has two branches as the bare one. The collective spectrum always has four branches which are gauge dependent including also their |q| = 0 limit. The exception is the case m, µ = 0 for which this spectrum is gauge invariant for a momenta as well.

Monte Carlo calculation of the dose distributions of two 106Ru eye applicators

Background and purpose: Beta emitting 106Ru applicators are widely used to treat choroidal melanoma. In view of the importance of clinical applications of this radioisotope and the relative lack of knowledge of the dose distributions, three-dimensional dose maps of two concave applicators were calculated by means of Monte Carlo simulation. Materials and methods: Simulations of small CCA and CCB concave applicators manufactured by Bebig were performed using the Monte Carlo code PENELOPE, which allows the description of the structure (geometry and materials) of the applicator in detail. Electrons are emitted from the 106Ru nuclei isotropically, with initial energy randomly sampled from the corresponding Fermi spectra and with initial positions uniformly distributed on the radioactive layer. Primary electrons, as well as the produced δ-rays, are assumed to be absorbed in the medium when they slow down to an energy of 70 keV. Bremsstrahlung photons with energies larger than 7 keV are also simulated. The simulation code has been run on a 166 MHz PENTIUM PC. Results: Three-dimensional dose distributions produced by the CCA and CCB applicators in a water sphere, concentric with the applicator, were evaluated. To minimize the magnitude of statistical uncertainties, advantage has been taken of the cylindrical symmetry of the problem. The relative depth-dose (along the symmetry axis of the applicator) was also evaluated from the applicator surface up to distances larger than 1 cm, with statistical uncertainties of a few percent. Results compare well with data supplied by the manufacturer. Conclusions: We have performed accurate Monte Carlo calculations of three-dimensional dose distributions from CCA and CCB 106Ru applicators. The results, presented in the form of two-dimensional maps, depth-dose distributions along the symmetry axis and lateral dose profiles, provide a detailed description of the dose delivered in treatments of choroidal melanoma.

Spin excitons — a new mechanism of superconducting pairing in copper oxides

The Fermi and Bose quasiparticle spectrum in copper oxides is studied in a many-band p-d model taking account of the strong electronic correlations. It is shown that hole-doped systems possess a Bose mode — a spin exciton — which is associated with the singlet-triplet excitation of the two-hole ground-state term of CuO4 clusters. Intercluster hopping leads to fermion-boson interaction with a spin exciton as the intermediate boson. Such a mechanism does not exist for n-type systems.

Evaluation of beta-particle emitter spectra in liquid scintillation counting systems

A semi-analytical algorithm to evaluate the spectrum of beta-particle emitters in liquid scintillator counting systems is proposed. The spectrum of energy deposited into the scintillator is evaluated from the theoretical Fermi spectrum. The overall efficiency of the system to convert the deposited energy into useful photoelectrons which reach the first dynode of the photomultiplier tubes is determined from the ionization quenching function and the figure of merit of the scintillator-photocathode system. The statistical fluctuations in the photomultiplier amplification are described through a gamma distribution with its mean and variance expressed in terms of three parameters that completely characterize the photomultiplier. A closed formula to compute the collected spectrum for a counting system with two photomultiplier tubes working in coincidence is derived. Numerical results are compared with experimental measurements for two different beta-particle emitters.

Charge dependence of the solar modulation of multiply charged cosmic ray nuclei

The intensity and energy spectra of multiply charged cosmic ray nuclei, in the energy interval 250–1500 MeV/n, were studied at three different levels of solar activity, viz. in 1963, 1964 and 1967. The same detectors, nuclear emulsion stacks flown from Fort Churchill, Canada, were used to determine simultaneouslty the energy spectra of helium, C, N, O as well as H (Z=10–28) nuclei. An analysis of the measured spectra indicates that these can be interpreted in terms of: (a) the source spectrum as a Fermi spectrum with a spectral index of 2.65; (b) the interstellar propagation as in a Gaussian distribution of path lengths with a mean path length of 4 g cm−2 and (c) the interplanetary propagation as given by the numerical solution of the Fokker-Planck equation incorporating diffusion, convection and adiabatic deceleration. On comparing the measured ratios of He to H-nuclei (mean Z≃14) with the theoretically calculated values for the three levels of solar activity, it is found that within experimental uncertainties, the solar modulation is essentially the same for nuclei of same mass to charge ratio and is not dependent on the charge of the nuclei.