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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

VERITAS detection of γ-ray flaring activity from the BL LAC object 1ES 1727+502 during bright moonlight observations

During moonlit nights, observations with ground-based Cherenkov telescopes at very high energies (VHEs, E>100 GeV) are constrained since the photomultiplier tubes (PMTs) in the telescope camera are extremely sensitive to the background moonlight. Observations with the VERITAS telescopes in the standard configuration are performed only with a moon illumination less than 35% of full moon. Since 2012, the VERITAS collaboration has implemented a new observing mode under bright moonlight, by either reducing the voltage applied to the PMTs (reduced-high-voltage; RHV configuration), or by utilizing UV-transparent filters. While these operating modes result in lower sensitivity and increased energy thresholds, the extension of the available observing time is useful for monitoring variable sources such as blazars and sources requiring spectral measurements at the highest energies. In this paper we report the detection of γ-ray flaring activity from the BL Lac object 1ES 1727+502 during RHV observations. This detection represents the first evidence of VHE variability from this blazar. The integral flux is (1.1±0.2)×10{sup −11} cm{sup −2} s{sup −1} above 250 GeV, which is about five times higher than the low-flux state. The detection triggered additional VERITAS observations during standard dark-time. Multiwavelength observations with the FLWO 48″ telescope, and the Swift and Fermi satellites are presented and used to produce the first spectral energy distribution (SED) of this object during γ-ray flaring activity. The SED is then fitted with a standard synchrotron-self-Compton model, placing constraints on the properties of the emitting region and of the acceleration mechanism at the origin of the relativistic particle population in the jet.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Near-equipartition jets with log-parabola electron energy distribution and the blazar spectral-index diagrams

Fermi-LAT analyses show that the γ-ray photon spectral indices Γ{sub γ} of a large sample of blazars correlate with the νF{sub ν} peak synchrotron frequency ν{sub s} according to the relation Γ{sub γ}=d−klog ν{sub s}. The same function, with different constants d and k, also describes the relationship between Γ{sub γ} and peak Compton frequency ν{sub C}. This behavior is derived analytically using an equipartition blazar model with a log-parabola description of the electron energy distribution (EED). In the Thomson regime, k=k{sub EC}=3b/4 for external Compton (EC) processes and k=k{sub SSC}=9b/16 for synchrotron self-Compton (SSC) processes, where b is the log-parabola width parameter of the EED. The BL Lac object Mrk 501 is fit with a synchrotron/SSC model given by the log-parabola EED, and is best fit away from equipartition. Corrections are made to the spectral-index diagrams for a low-energy power-law EED and departures from equipartition, as constrained by absolute jet power. Analytic expressions are compared with numerical values derived from self-Compton and EC scattered γ-ray spectra from Lyα broad-line region and IR target photons. The Γ{sub γ} versus ν{sub s} behavior in the model depends strongly on b, with progressively and predictably weaker dependences on γ-ray detection range, variability time, and isotropic γ-ray luminosity. Implications for blazar unification and blazars as ultra-high energy cosmic-ray sources are discussed. Arguments by Ghisellini et al. that the jet power exceeds the accretion luminosity depend on the doubtful assumption that we are viewing at the Doppler angle.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TIME-DEPENDENT STOCHASTIC ACCELERATION MODEL FOR FERMI BUBBLES

We study stochastic acceleration models for the Fermi bubbles. Turbulence is excited just behind the shock front via Kelvin–Helmholtz, Rayleigh–Taylor, or Richtmyer–Meshkov instabilities, and plasma particles are continuously accelerated by the interaction with the turbulence. The turbulence gradually decays as it goes away from the shock fronts. Adopting a phenomenological model for the stochastic acceleration, we explicitly solve the temporal evolution of the particle energy distribution in the turbulence. Our results show that the spatial distribution of high-energy particles is different from those for a steady solution. We also show that the contribution of electrons that escaped from the acceleration regions significantly softens the photon spectrum. The photon spectrum and surface brightness profile are reproduced by our models. If the escape efficiency is very high, the radio flux from the escaped low-energy electrons can be comparable to that of the WMAP haze. We also demonstrate hadronic models with the stochastic acceleration, but they are unlikely in the viewpoint of the energy budget.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Electronic structure and effective mass of pristine and Cl-doped CsPbBr 3

Organic–inorganic lead halide perovskites (LHPs) have attracted great interest owing to their outstanding optoelectronic properties. Typically, the underlying electronic structure would determinate the physical properties of materials. But as for now, limited studies have been done to reveal the underlying electronic structure of this material system, comparing to the huge amount of investigations on the material synthesis. The effective mass of the valance band is one of the most important physical parameters which plays a dominant role in charge transport and photovoltaic phenomena. In pristine CsPbBr 3 , the Fröhlich polarons associated with the Pb–Br stretching modes are proposed to be responsible for the effective mass renormalization. In this regard, it would be very interesting to explore the electronic structure in doped LHPs. Here, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) studies on both pristine and Cl-doped CsPbBr 3 . The experimental band dispersions are extracted from ARPES spectra along both $\overline{\Gamma}$–$\overline{M}$–$\overline{\Gamma}$ and $\overline{X}$–$\overline{M}$–$\overline{X}$ high symmetry directions. DFT calculations are performed and directly compared with the ARPES data. Our results have revealed the band structure of Cl-doped CsPbBr 3 for the first time, which have also unveiled the effective mass renormalization in the Cl-doped CsPbBr 3 compound. Doping dependent measurements indicate that the chlorine doping could moderately tune the renormalization strength. These results will help understand the physical properties of LHPs as a function of doping.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Interpreting Crab Nebula’s synchrotron spectrum: two acceleration mechanisms

ABSTRACT We outline a model of the Crab pulsar wind nebula with two different populations of synchrotron emitting particles, arising from two different acceleration mechanisms: (i) Component-I due to Fermi-I acceleration at the equatorial portion of the termination shock, with particle spectral index pI ≈ 2.2 above the injection break corresponding to γwindσwind ∼ 105, peaking in the ultraviolet (UV, γwind ∼ 102 is the bulk Lorentz factor of the wind, σwind ∼ 103 is wind magnetization); and (ii) Component-II due to acceleration at reconnection layers in the bulk of the turbulent Nebula, with particle index pII ≈ 1.6. The model requires relatively slow but highly magnetized wind. For both components, the overall cooling break is in the infrared at ∼0.01 eV, so that the Component-I is in the fast cooling regime (cooling frequency below the peak frequency). In the optical band, Component-I produces emission with the cooling spectral index of αo ≈ 0.5, softening towards the edges due to radiative losses. Above the cooling break, in the optical, UV, and X-rays, Component-I mostly overwhelms Component-II. We hypothesize that acceleration at large-scale current sheets in the turbulent nebula (Component-II) extends to the synchrotron burn-off limit of ϵs ∼ 100 MeV. Thus in our model acceleration in turbulent reconnection (Component-II) can produce both hard radio spectra and occasional gamma-ray flares. This model may be applicable to a broader class of high-energy astrophysical objects, like active galactic nuclei and gamma-ray burst jets, where often radio electrons form a different population from the high-energy electrons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Electron capture decay of 58-min U-229(92) and levels in Pa-229(91)

Electron capture decay of U-229 is investigated by measuring the gamma-ray and conversion electron spectra of mass-separated and unseparated U-229 sources with high-resolution germanium and silicon detectors, respectively. Gamma-gamma coincidence measurements are also performed using germanium detectors. These studies provide level energies and level ordering in Pa-229. Single-particle assignments are given to these levels which are in agreement with the systematics in this region and also with theory. In a previous study, we report the observation of a 5/2(+/-) parity doublet in the Pa-229 ground state, which is a signature of octupole deformation. The present analysis of the data still shows a splitting of 60 +/- 50 eV, but with this large uncertainty the existence of the doublet is not certain.

Ahmad, I.↗
DOE OSTI2021

Effects of Shielding on Gamma Rays [Slides]

The interaction of gamma rays with matter results in an effect we call attenuation (i.e. ‘shielding’). Attenuation can dramatically alter the appearance of a spectrum. Attenuating materials may actually create features in a spectrum via x-ray fluorescence. Attenuation is a function of the incident gamma-ray, material properties, and the thickness of the material. Half Value Layers (HVL) are a useful concept for estimating how much a given thickness of material will impact transmission of gamma-rays. Shielding impacts spectra in predictable ways. We can use clues in the spectrum to infer information about the source’s local environment.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

BROADBAND OBSERVATIONS OF HIGH REDSHIFT BLAZARS

We present a multi-wavelength study of four high redshift blazars, S5 0014+81 ( z = 3.37), CGRaBS J0225+1846 ( z = 2.69), BZQ J1430+4205 ( z = 4.72), and 3FGL J1656.2−3303 ( z = 2.40) using quasi-simultaneous data from the Swift , Nuclear Spectroscopic Telescope Array ( NuSTAR ) and the Fermi -Large Area Telescope (LAT) and also archival XMM-Newton observations. Other than 3FGL J1656.2−3303, none of the sources were known as γ -ray emitters, and our analysis of ∼7.5 yr of LAT data reveals the first time detection of statistically significant γ -ray emission from CGRaBS J0225+1846. We generate the broadband spectral energy distributions (SED) of all the objects, centering at the epoch of NuSTAR observations and reproduce them using a one-zone leptonic emission model. The optical−UV emission in all the objects can be explained by radiation from the accretion disk, whereas the X-ray to γ -ray windows of the SEDs are found to be dominated by inverse Compton scattering off the broad line region photons. All of them host black holes that are billions of solar masses. Comparing the accretion disk luminosity and the jet power of these sources with a large sample of blazars, we find them to occupy a high disk luminosity–jet power regime. We also investigate the X-ray spectral properties of the sources in detail with a major focus on studying the causes of soft X-ray deficit, a feature generally seen in high redshift radio-loud quasars. We summarize that this feature could be explained based on the intrinsic curvature in the jet emission rather than being due to the external effects predicted in earlier studies, such as host galaxy and/or warm absorption.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Very bright prompt and reverse shock emission of GRB L40512A

We report our observations of very bright prompt optical and reverse shock (RS) optical emission of GRB 140512A and analyze its multi-wavelength data observed with the Swift and Fermi missions. It is found that the joint optical–X-ray–gamma-ray spectrum with our first optical detection (R = 13.09 mag) at T{sub 0}+136 s during the second episode of the prompt gamma-rays can be fit by a single power law with an index of −1.32 ± 0.01. Our empirical fit to the afterglow light curves indicates that the observed bright optical afterglow with R = 13.00 mag at the peak time is consistent with predictions of the RS and forward shock (FS) emission of external shock models. A joint optical–X-ray afterglow spectrum is well fit with an absorbed single power law, with an index evolving with time from −1.86 ± 0.01 at the peak time to −1.57 ± 0.01 at a late epoch, which could be due to the evolution of the ratio of the RS to FS emission fluxes. We fit the light curves with standard external models, and derive the physical properties of the outflow. It is found that the ratio R{sub B}≡ϵ{sub B,r}/ϵ{sub B,f} is 8187, indicating a high magnetization degree in the RS region. Measuring the relative radiation efficiency with R{sub e}≡ϵ{sub e,r}/ϵ{sub e,f}, we have R {sub e} = 0.02, implying that the radiation efficiency of the RS is much lower than that in FS. We also show that the R {sub B} of GRBs 990123, 090102, and 130427A are similar to that of GRB 140512A and their apparent difference may be mainly attributed to the difference of the jet kinetic energy, initial Lorentz factor, and medium density among them.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

An Interactive Multi-instrument Database of Solar Flares

Solar flares are complicated physical phenomena that are observable in a broad range of the electromagnetic spectrum, from radio waves to γ -rays. For a more comprehensive understanding of flares, it is necessary to perform a combined multi-wavelength analysis using observations from many satellites and ground-based observatories. For an efficient data search, integration of different flare lists, and representation of observational data, we have developed the Interactive Multi-Instrument Database of Solar Flares (IMIDSF, https://solarflare.njit.edu/). The web-accessible database is fully functional and allows the user to search for uniquely identified flare events based on their physical descriptors and the availability of observations by a particular set of instruments. Currently, the data from three primary flare lists ( Geostationary Operational Environmental Satellites , RHESSI , and HEK) and a variety of other event catalogs ( Hinode , Fermi GBM, Konus- W IND, the OVSA flare catalogs, the CACTus CME catalog, the Filament eruption catalog) and observing logs ( IRIS and Nobeyama coverage) are integrated, and an additional set of physical descriptors (temperature and emission measure) is provided along with an observing summary, data links, and multi-wavelength light curves for each flare event since 2002 January. We envision that this new tool will allow researchers to significantly speed up the search of events of interest for statistical and case studies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

EVIDENCE FOR TWO LOGNORMAL STATES IN MULTI-WAVELENGTH FLUX VARIATION OF FSRQ PKS 1510-089

We present a systematic characterization of multi-wavelength emission from blazar PKS 1510-089 using well-sampled data at near-infrared (NIR), optical, X-ray, and γ -ray energies. The resulting flux distributions, except at X-rays, show two distinct lognormal profiles corresponding to a high and a low flux level. The dispersions exhibit energy-dependent behavior except in the LAT γ -ray and optical B-band. During the low level flux states, it is higher toward the peak of the spectral energy distribution, with γ -ray being intrinsically more variable followed by IR and then optical, consistent with mainly being a result of varying bulk Lorentz factor. On the other hand, the dispersions during the high state are similar in all bands except the optical B-band, where thermal emission still dominates. The centers of distributions are a factor of ∼4 apart, consistent with anticipation from studies of extragalactic γ -ray background with the high state showing a relatively harder mean spectral index compared to the low state.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Direct Observation of Two-step Magnetic Reconnection in a Solar Flare

We report observations of an eruptive X2.8 flare on 2013 May 13, which shows two distinct episodes of energy release in the impulsive phase. The first episode is characterized by the eruption of a magnetic flux rope, similar to the energy-release process in most standard eruptive flares. The second episode, which is stronger than the first normal one and shows enhanced high-energy X-ray and even γ-ray emissions, is closely associated with magnetic reconnection of a large-scale loop in the aftermath of the eruption. The reconnection inflow of the loop leg is observed in the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) 304 Å passband and accelerates toward the reconnection region to speeds as high as ∼130 km s{sup −1}. Simultaneously, the corresponding outflow jets are observed in the AIA hot passbands with speeds of ∼740 km s{sup −1} and a mean temperature of ∼14 MK. RHESSI observations show a strong burst of hard X-ray (HXR) and γ-ray emissions with hard electron spectra of δ≈3, exhibiting a soft–hard–harder behavior. A distinct altitude decrease of the HXR loop-top source coincides with the inward swing of the loop leg observed in the AIA 304 Å passband, which is suggested to be related to the coronal implosion. This fast inflow of magnetic flux contained in the loop leg greatly enhances the reconnection rate and results in very efficient particle acceleration in the second-step reconnection, which also helps to achieve a second higher temperature peak up to T ≈ 30 MK.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

The POPOP4 library and codes for preparing secondary gamma-ray production cross sections

The POPOP4 code for converting secondary gamma ray yield data to multigroup secondary gamma ray production cross sections and the POPOP4 library of secondary gamma ray yield data are described. Recent results of the testing of uranium and iron data sets from the POPOP4 library are given. The data sets were tested by comparing calculated secondary gamma ray pulse height spectra measured at the ORNL TSR-II reactor.

Ford, W. E., III↗
NASA NTRS1983

On the method of positron lifetime measurement

A fast-slow coincidence system was constructed for the measurement of positron lifetimes in material. The time resolution of this system was 270 ps for the (60)Co gamma rays. Positron lifetime spectra for 14 kinds of alkali halides were measured with this system. Two lifetime components and their intensities were derived from analyses of the lifetime spectra.

Nishiyama, F.↗
NASA NTRS1985

Nineteenth International Cosmic Ray Conference. HE Sessions, Volume 6

Papers contributed to the 19th International Cosmic Ray Conference which address high energy interactions and related phenomena are compiled. Particular topic areas include cross sections; particle production; nuclei and nuclear matter; nucleus-nucleus collisions; gamma ray and hadron spectra; C-jets, a-jets, and super families; and emulsion chamber simulations.

Jones, F. C.↗