John W. Hewitt
A5024911603- Astrophysics and Cosmic Phenomena
- Gamma-ray bursts and supernovae
- Dark Matter and Cosmic Phenomena
- Neutrino Physics Research
- Pulsars and Gravitational Waves Research
- Radio Astronomy Observations and Technology
- Astrophysics and Star Formation Studies
- Solar and Space Plasma Dynamics
- Astrophysical Phenomena and Observations
- Particle Detector Development and Performance
- Particle Accelerators and Free-Electron Lasers
- Astro and Planetary Science
- Particle physics theoretical and experimental studies
- Galaxies: Formation, Evolution, Phenomena
- Ionosphere and magnetosphere dynamics
- Superconducting Materials and Applications
- Geophysics and Gravity Measurements
- Astronomy and Astrophysical Research
- Astronomical Observations and Instrumentation
- Atmospheric Ozone and Climate
- Atomic and Molecular Physics
- Radiation Therapy and Dosimetry
- Stellar, planetary, and galactic studies
- Molecular Spectroscopy and Structure
- Restraint-Related Deaths
The University of Texas at Arlington
2023
University of Alaska Anchorage
2021-2023
Providence College
2021-2023
Clark Atlanta University
2023
Lawrence Berkeley National Laboratory
2023
Southern University and Agricultural and Mechanical College
2023
University of North Florida
2015-2022
Goddard Space Flight Center
2009-2016
Max Planck Society
2016
University of Maryland, Baltimore County
2014-2016
We present the third Fermi Large Area Telescope (LAT) source catalog (3FGL) of sources in 100 MeV–300 GeV range. Based on first 4 yr science data from Gamma-ray Space mission, it is deepest yet this energy Relative to Second LAT catalog, 3FGL incorporates twice as much data, well a number analysis improvements, including improved calibrations at event reconstruction level, an updated model for Galactic diffuse γ-ray emission, refined procedure detection, and methods associating with...
This catalog summarizes 117 high-confidence ⩾0.1 GeV gamma-ray pulsar detections using three years of data acquired by the Large Area Telescope (LAT) on Fermi satellite. Half are neutron stars discovered LAT through periodicity searches in and radio around unassociated source positions. The pulsars evenly divided into groups: millisecond pulsars, young radio-loud radio-quiet pulsars. We characterize pulse profiles energy spectra derive luminosities when distance information exists. Spectral...
Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) the sources of galactic cosmic rays, unequivocal evidence for acceleration protons in these objects is still lacking. When encounter interstellar material, they produce neutral pions, which turn decay into gamma rays. This offers a compelling way detect sites protons. The identification pion-decay has been difficult because high-energy electrons also via...
The γ-ray sky can be decomposed into individually detected sources, diffuse emission attributed to the interactions of Galactic cosmic rays with gas and radiation fields, a residual all-sky component commonly called isotropic background (IGRB). IGRB comprises all extragalactic emissions too faint or resolved in given survey, as well any foregrounds that are approximately isotropic. first measurement Large Area Telescope (LAT) on board Fermi Gamma-ray Space (Fermi) used 10 months sky-survey...
The Fermi Large Area Telescope (LAT) has provided the most detailed view to date of emission towards Galactic centre (GC) in high-energy gamma-rays. This paper describes analysis data taken during first 62 months mission energy range 1-100 GeV from a $15^\circ \times 15^\circ$ region about direction GC, and implications for interstellar emissions produced by cosmic ray (CR) particles interacting with gas radiation fields inner Galaxy point sources detected. Specialised models (IEMs) are...
Abstract The region around the Galactic Center (GC) is now well established to be brighter at energies of a few GeV than what expected from conventional models diffuse gamma-ray emission and catalogs known sources. We study excess using 6.5 yr data Fermi Large Area Telescope. characterize uncertainty GC spectrum morphology due uncertainties in cosmic-ray source distributions propagation, distribution interstellar gas Milky Way, potential contribution bubbles. also evaluate properties...
We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi-LAT catalog gamma-ray sources. Based on first twelve years science data in energy range from 50 MeV to 1 TeV, it contains 6658 The analysis improves that used 4FGL over eight data: more sources are fit with curved spectra, we introduce a robust spectral parameterization pulsars, and extend points TeV. parameters, distributions, associations updated all Light curves rebuilt yr intervals (not 2 month...
We present a catalog of sources detected above 10 GeV by the Fermi Large Area Telescope (LAT) in first 7 years data using Pass 8 event-level analysis. This is Third Catalog Hard Fermi-LAT Sources (3FHL), containing 1556 objects characterized - 2 TeV energy range. The sensitivity and angular resolution are improved factors 3 relative to previous LAT at same energies (1FHL). vast majority (79%) associated with extragalactic counterparts other wavelengths, including 16 located very high...
ABSTRACT Most of the celestial γ rays detected by Large Area Telescope (LAT) on board Fermi Gamma-ray Space originate from interstellar medium when energetic cosmic interact with nucleons and photons. Conventional point-source extended-source studies rely modeling this diffuse emission for accurate characterization. Here, we describe development Galactic Interstellar Emission Model (GIEM), which is standard adopted LAT Collaboration publicly available. This model based a linear combination...
Abstract The Large Area Telescope (LAT) aboard the Fermi spacecraft routinely observes high-energy emission from gamma-ray bursts (GRBs). Here we present second catalog of LAT-detected GRBs, covering first 10 yr operations, 2008 to 2018 August 4. A total 186 GRBs are found; these, 91 show in range 30–100 MeV (17 which seen only this band) and 169 detected above 100 MeV. Most these sources were discovered by other instruments ( /GBM, Swift /BAT, AGILE, INTEGRAL ) or reported Interplanetary...
We present a catalog of gamma-ray sources at energies above 10 GeV based on data from the Large Area Telescope (LAT) accumulated during first three years Fermi Gamma-ray Space mission. The Fermi-LAT >10GeV (1FHL) has 514 sources. For each source we location, spectrum, measure variability, and associations with cataloged other wavelengths. found that 449 (87%) could be associated known sources, which 393 (76% 1FHL sources) are active galactic nuclei. Of 27 pulsars, find 20 (12) to have...
Abstract We present a search for spatial extension in high-latitude ( ) sources recent Fermi point source catalogs. The result is the High-Latitude Extended Sources Catalog, which provides extensions (or upper limits thereof) and likelihood profiles suite of tested morphologies. find 24 extended sources, 19 were not previously characterized as extended. These include that are potentially associated with supernova remnants star-forming regions. also found γ -ray emission vicinity Cen A radio...
The central kpc of the Milky Way might be expected to differ significantly from rest Galaxy with regard gasdynamics and formation young stellar objects (YSOs). We probe this possibility mid-infrared observations obtained Infrared Array Camera Multiband Imaging Photometer on Spitzer Midcourse Space Experiment. use color–color diagrams spectral energy distribution (SED) fits explore nature YSO candidates (including 4.5 μm excesses possibly due molecular emission). There is an asymmetry in...
ABSTRACT To uniformly determine the properties of supernova remnants (SNRs) at high energies, we have developed first systematic survey energies from 1 to 100 GeV using data Fermi Large Area Telescope (LAT). Based on spatial overlap sources detected with SNRs known radio surveys, classify 30 as likely SNRs. We also report 14 marginal associations and 245 flux upper limits. A mock catalog in which positions are scrambled Galactic longitude allows us an limit 22% number candidates falsely...
We have developed a technique to provide short‐term warnings of solar energetic proton (SEP) events that meet or exceed the Space Weather Prediction Center threshold J (>10 MeV) = 10 pr cm −2 s −1 sr . The method is based on flare location, size, and evidence particle acceleration/escape as parameterized by longitude, time‐integrated soft X‐ray intensity, intensity type III radio emission at ∼1 MHz, respectively. In this technique, are issued min after maximum ≥M2 flares. For cycle 23...
The high energy activity in the inner few degrees of Galactic center is traced by diffuse radio, X-ray and gamma-ray emission. physical relationship between different components gas emitting at multiple wavelengths a focus this work. We first present radio continuum observations using Green Bank Telescope model nonthermal spectrum terms broken power-law distribution GeV electrons synchrotron radiation. show that emission detected Fermi primarily due to bremsstrahlung produced population...
Pulsar wind nebulae (PWNe) have been established as the most populous class of TeV gamma-ray emitters. Since launch, Fermi Large Area Telescope (LAT)identified five high-energy (100MeV <E< 100 GeV) sources PWNe, and detected a large number PWNe candidates, all powered by young energetic pulsars. The wealth multi-wavelength data available new results provided Fermi-LAT give us an opportunity to find explore radiative processes taking place in known ones. unidentifiedsources (UNIDs) are best...
We search for evidence of dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) measured with 50 months Fermi Large Area Telescope (LAT) observations. An improved theoretical description cosmological DM signal, based on two complementary techniques and assuming generic weakly interacting massive particle (WIMP) properties, renders more precise predictions compared to previous work. More specifically, we estimate cosmologically-induced intensity have an uncertainty a...
We present a sensitive $λ$20cm VLA continuum survey of the Galactic center region using new and archival data based on multi-configuration observations taken with relatively uniform {\it uv} coverage. The high dynamic range images cover regions within $-2^\circ < l 5^\circ$ $-40' b 40'$ spatial resolution $\approx30''$ 10$''$. wide field imaging technique is used to construct low-resolution mosaic 40 overlapping pointings. image includes Effelsburg filling low frequency data. also twenty...
We report on gamma-ray observations of the supernova remnant (SNR) RX J0852.0-4622 with Large Area Telescope (LAT) aboard Fermi Gamma-ray Space Telescope. In LAT data, we find a spatially extended source at location SNR. The extension is consistent SNR size seen in other wavelengths such as X-rays and TeV gamma rays, leading to identification spectrum well described power law photon index Gamma = 1.85 +/- 0.06 (stat) (+0.18,-0.19) (sys), which smoothly connects H.E.S.S. energy band. discuss...
Long-term monitoring of PSR J2021+4026 in the heart Cygnus region with Fermi Large Area Telescope unveiled a sudden decrease flux above 100 MeV over timescale shorter than week. The "jump" was near MJD 55850 (2011 October 16), decreasing from (8.33 ± 0.08) × 10−10 erg cm−2 s−1 to (6.86 0.13) s−1. Simultaneously, frequency spindown rate increased (7.8 0.1) 10−13 Hz (8.1 Significant (>5σ) changes pulse profile and marginal (<3σ) emission spectrum occurred at same time. There is also evidence...
Spitzer images of Tycho's supernova remnant in the mid-infrared reveal limb-brightened emission from entire periphery shell and faint filamentary structures interior. As with other young remnants, this is produced by dust grains, warmed to ∼100 K post-shock environment collisions energetic electrons ions. The ratio 70 24 μm fluxes a diagnostic temperature, which turn sensitive function plasma density. We find significant variations 70/24 flux around forward shock, implying order-of-magnitude...
ABSTRACT The Fermi Large Area Telescope (LAT) has an instantaneous field of view (FoV) covering <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>5</mml:mn> </mml:math> the sky and it completes a survey entire in high-energy gamma-rays every 3 hr. It enables searches for transient phenomena over timescales from milliseconds to years. Among these could be...
Galaxy clusters are one of the prime sites to search for dark matter (DM) annihilation signals. Depending on substructure DM halo a galaxy cluster and cross sections channels, these signals might be detectable by latest generation $\gamma$-ray telescopes. Here we use three years Fermi Large Area Telescope (LAT) data, which most suitable searching very extended emission in vicinity nearby Virgo cluster. Our analysis reveals statistically significant can well characterized uniformly emitting...
We present the first Fermi - Large Area Telescope (LAT) solar flare catalog covering 24 th cycle. This contains 45 -LAT flares (FLSFs) with emission in gamma-ray energy band (30 MeV 10 GeV) detected a significance greater than 5 sigma over years 2010-2018. A subsample containing 37 of these exhibit delayed beyond prompt-impulsive hard X-ray phase 21 showing lasting more two hours. No is four flares. also this observations GeV from 3 originating Active Regions located behind limb (BTL)...