72 research outputs found
Imaging and burst location with the EXIST high-energy telescope
The primary instrument of the proposed EXIST mission is a coded mask high
energy telescope (the HET), that must have a wide field of view and extremely
good sensitivity. It will be crucial to minimize systematic errors so that even
for very long total integration times the imaging performance is close to the
statistical photon limit. There is also a requirement to be able to reconstruct
images on-board in near real time in order to detect and localize gamma-ray
bursts. This must be done while the spacecraft is scanning the sky. The
scanning provides all-sky coverage and is key to reducing systematic errors.
The on-board computational problem is made even more challenging for EXIST by
the very large number of detector pixels. Numerous alternative designs for the
HET have been evaluated. The baseline concept adopted depends on a unique coded
mask with two spatial scales. Monte Carlo simulations and analytic analysis
techniques have been used to demonstrate the capabilities of the design and of
the proposed two-step burst localization procedure
Variability of EGRET Gamma-Ray Sources
The variability of the high-energy gamma ray sources in the Third EGRET
catalog is analyzed by a new method. We re-analyze the EGRET data to calculate
a likelihood function for the flux of each source in each observation, both for
detections and upper limits. These functions can be combined in a uniform
manner with a simple model of the flux distribution to characterize the flux
variation by a confidence interval for the relative standard deviation of the
flux. The main result is a table of these values for almost all the cataloged
sources. As expected, the identified pulsars are steady emitters and the
blazars are mostly highly variable. The unidentified sources are heterogeneous,
with greater variation at higher Galactic latitude. There is an indication that
pulsar wind nebulae are associated with variable sources. There is a population
of variable sources along the Galactic plane, concentrated in the inner spiral
arms.Comment: Accepted by ApJ. 13 pages, 3 figures, 2 tables. Machine-readable
copies of Table 1 are at http://giants.stanford.edu/~pln/vartbl1
Adjustment of the electric current in pulsar magnetospheres and origin of subpulse modulation
The subpulse modulation of pulsar radio emission goes to prove that the
plasma flow in the open field line tube breaks into isolated narrow streams. I
propose a model which attributes formation of streams to the process of the
electric current adjustment in the magnetosphere. A mismatch between the
magnetospheric current distribution and the current injected by the polar cap
accelerator gives rise to reverse plasma flows in the magnetosphere. The
reverse flow shields the electric field in the polar gap and thus shuts up the
plasma production process. I assume that a circulating system of streams is
formed such that the upward streams are produced in narrow gaps separated by
downward streams. The electric drift is small in this model because the
potential drop in narrow gaps is small. The gaps have to drift because by the
time a downward stream reaches the star surface and shields the electric field,
the corresponding gap has to shift. The transverse size of the streams is
determined by the condition that the potential drop in the gaps is sufficient
for the pair production. This yields the radius of the stream roughly 10% of
the polar cap radius, which makes it possible to fit in the observed
morphological features such as the "carousel" with 10-20 subbeams and the
system of the core - two nested cone beams.Comment: 8 pages, 1 figur
A Possible Site of Cosmic Ray Acceleration in the Supernova Remnant IC 443
We present evidence for shock acceleration of cosmic rays to high energies
(about 10 TeV) in the supernova remnant IC 443. X-ray imaging spectroscopy with
ASCA reveals two regions of particularly hard emission: an unresolved source
embedded in an extended emission region, and a ridge of emission coincident
with the southeastern rim. Both features are located on part of the radio shell
where the shock wave is interacting with molecular gas, and together they
account for a majority of the emission at 7 keV. Though we would not have
noticed it a priori, the unresolved feature is coincident with one resolved by
the ROSAT HRI. Because this feature overlaps a unique region of flat radio
spectral index (alpha < 0.24), has about equal light-crossing and synchrotron
loss times, and a power law spectrum with a spectral index of 2, we conclude
that the hard X-ray feature is synchrotron radiation from a site of enhanced
particle acceleration. Evidence against a plerion includes a lack of observed
periodicity (the pulsed fraction upper limit is 33%), the spectral similarity
with the more extended hard region, the location of the source outside the 95%
error circle of the nearby EGRET source, the fact that it is nestled in a bend
in the molecular cloud ring with which IC 443 is interacting, and the
requirement of an extremely high transverse velocity (>5,000 km/s). We conclude
that the anomalous feature is most likely tracing enhanced particle
acceleration by shocks that are formed as the supernova blast wave impacts the
ring of molecular clouds.Comment: 10 pages with embedded figures; accepted by the Ap.J; author's web
pages at http://lheawww.gsfc.nasa.gov/users/jonathan/ ,
http://lheawww.gsfc.nasa.gov/users/rob/Vita/petre.html ,
http://lheawww.gsfc.nasa.gov/users/evg/evg.htm
ASCA View of the Supernova Remnant Gamma Cygni (G78.2+2.1): Bremsstrahlung X-ray Spectrum from Loss-flattened Electron Distribution
We perform X-ray studies of the shell-type supernova remnant (SNR)
gamma-Cygni associated with the brightest EGRET unidentified source 3EG
J2020+4017. In addition to the thermal emissions with characteristic
temperature of kT = 0.5-0.9 keV, we found an extremely hard X-ray component
from several clumps localized in the northern part of the remnant. This
component is described by a power-law with a photon index of 0.8-1.5. Both the
absolute flux and the spectral shape of the nonthermal X-rays cannot be
explained by the synchrotron or inverse-Compton mechanisms. We argue that the
unusually hard X-ray spectrum can be naturally interpreted in terms of
nonthermal bremsstrahlung from Coulomb-loss-flattened electron distribution in
dense environs with the gas density about 10 to 100 cm^-3 . For given spectrum
of the electron population, the ratio of the bremsstrahlung X- and gamma-ray
fluxes depends on the position of the ``Coulomb break'' in the electron
spectrum. The bulk of gamma-rays detected by EGRET would come from the
radio-bright and X-ray dim cloud at southeast, where very dense gas and strong
magnetic field would illuminate the cloud in the radio and gamma-ray bands, but
suppress the bremsstrahlung X-ray emission due to the shift of the ``Coulomb
break'' in the electron spectrum towards higher energies.Comment: 14 pages, 8 figures, emulateapj5, accepted for publication in the
Astrophysical Journa
Polarization Observations of 1720 MHz OH Masers toward the Three Supernova Remnants W28, W44, and IC443
(abridged) - We present arcsecond resolution observations from the VLA of the
satellite line of the hydroxyl molecule (OH) at 1720.53 MHz toward three
Galactic supernova remnants: W28, W44 and IC443. All of our observations are
consistent with a model in which the OH(1720 MHz) is collisionally excited by
H2 molecules in the postshock gas heated by a non-dissociative shock. Supernova
remnants with OH(1720 MHz) maser emission may be promising candidates to
conduct high energy searches for the sites of cosmic ray acceleration.Comment: ApJ Let (accepted). Hardcopies available from [email protected]
CANGAROO-III observation of TeV gamma rays from the unidentified gamma-ray source HESS J1614-518
We report the detection, with the CANGAROO-III imaging atmospheric Cherenkov
telescope array, of a very high energy gamma-ray signal from the unidentified
gamma-ray source HESS J1614-518, which was discovered in the H.E.S.S. Galactic
plane survey. Diffuse gamma-ray emission was detected above 760 GeV at the 8.9
sigma level during an effective exposure of 54 hr from 2008 May to August. The
spectrum can be represented by a power-law:
8.2+-2.2_{stat}+-2.5_{sys}x10^{-12}x (E/1TeV)^{-Gamma} cm^{-2} s^{-1} TeV^{-1}
with a photon index Gamma of 2.4+-0.3_{stat}+-0.2_{sys}, which is compatible
with that of the H.E.S.S. observations. By combining our result with
multi-wavelength data, we discuss the possible counterparts for HESS J1614-518
and consider radiation mechanisms based on hadronic and leptonic processes for
a supernova remnant, stellar winds from massive stars, and a pulsar wind
nebula. Although a leptonic origin from a pulsar wind nebula driven by an
unknown pulsar remains possible, hadronic-origin emission from an unknown
supernova remnant is preferred.Comment: 9 pages, 7 figures, accepted for publication in Ap
Multiwavelength Examination of the COS-B Field 2CG 075+00 Yields a Blazar Identification for 3EG J2016+3657
We present a high-energy study of the intriguing COS-B gamma-ray field, 2CG
075+00, in order to search for possible counterparts. New EGRET data show that
the COS-B emission probably corresponds to two localized gamma-ray sources, 3EG
J2016+3657 and 3EG J2021+3716. Spectral fits to these EGRET sources, assuming a
power-law model, yield photon indices of ~ 2 for each object. We examine
archival ROSAT and ASCA X-ray data which overlap both EGRET error boxes, and
find several point sources in the region to a flux limit of approximately 6.5 x
10^{-13} erg cm^{-2} s^{-1}. We conclude that the most probable candidate for
3EG J2016+3657 is the compact, variable, flat-spectrum radio and millimeter
source B2013+370 (G74.87+1.22) which has blazar-like properties. The other
source, 3EG J2021+3716, remains unidentified.Comment: 32 pages, 9 figures, uses aasms4 and epsfig style files. Accepted for
publication in The Astrophysical Journa
GLAST: Understanding the High Energy Gamma-Ray Sky
We discuss the ability of the GLAST Large Area Telescope (LAT) to identify,
resolve, and study the high energy gamma-ray sky. Compared to previous
instruments the telescope will have greatly improved sensitivity and ability to
localize gamma-ray point sources. The ability to resolve the location and
identity of EGRET unidentified sources is described. We summarize the current
knowledge of the high energy gamma-ray sky and discuss the astrophysics of
known and some prospective classes of gamma-ray emitters. In addition, we also
describe the potential of GLAST to resolve old puzzles and to discover new
classes of sources.Comment: To appear in Cosmic Gamma Ray Sources, Kluwer ASSL Series, Edited by
K.S. Cheng and G.E. Romer
Is the Supernova Remnant RX J1713.7-3946 a Hadronic Cosmic Ray Accelerator ?
The non-thermal supernova remnant RX J1713.7-3946 (G347.3-0.5) has recently
been shown to be a site of cosmic ray (CR) electron acceleration to TeV
energies (Muraishi et al. 2000). Here we present evidence that this remnant is
also accelerating CR nuclei. Such nuclei can interact with ambient interstellar
gas to produce high energy gamma-rays via the decay of neutral pions. We
associate the unidentified EGRET GeV gamma- ray source 3EG J1714-3857 with a
massive (~3*10 5 Mo) and dense (~500 nucleons cm -3) molecular cloud
interacting with RX J1713.7-3946. Direct evidence for such interaction is
provided by observations of the lowest two rotational transitions of CO in the
cloud; as in other clear cases of interaction, the CO(J=2-1)/CO(J=1-0) ratio is
significantly enhanced. Since the cloud is of low radio and X-ray brightness,
CR electrons cannot be responsible for the bulk of its GeV emission there. A
picture thus emerges where both electrons and nuclei are being accelerated by
the SNR: whereas the CR electrons dominate the local non-thermal radio, X-ray
and TeV emission, the shock accelerated CR protons and ions (hadrons) are
revealed through their interactions in the adjacent massive cloud. Such a
scenario had been anticipated by Aharonian, Drury and Volk (1994).Comment: 15 pages, 2 low-resolution figures, submitted to ApJL Aug 10, 2001;
this version includes referee's suggestion
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