351 research outputs found
Christian Solutions to Modern Problems
https://digitalcommons.acu.edu/crs_books/1171/thumbnail.jp
A human homologue of the Drosophila sex determination factor transformer-2 has conserved splicing regulatory functions.
Evaluating the Maximum Likelihood Method for Detecting Short-Term Variability of AGILE gamma-ray Sources
The AGILE space mission (whose instrument is sensitive in the energy ranges
18-60 keV, and 30 MeV - 50 GeV) has been operating since 2007. Assessing the
statistical significance of time variability of gamma-ray sources above 100 MeV
is a primary task of the AGILE data analysis. In particular, it is important to
check the instrument sensitivity in terms of Poisson modeling of the data
background, and to determine the post-trial confidence of detections. The goals
of this work are: (i) evaluating the distributions of the likelihood ratio test
for "empty" fields, and for regions of the Galactic plane; (ii) calculating the
probability of false detection over multiple time intervals. In this paper we
describe in detail the techniques used to search for short-term variability in
the AGILE gamma-ray source database. We describe the binned maximum likelihood
method used for the analysis of AGILE data, and the numerical simulations that
support the characterization of the statistical analysis. We apply our method
to both Galactic and extra-galactic transients, and provide a few examples.
After having checked the reliability of the statistical description tested with
the real AGILE data, we obtain the distribution of p-values for blind and
specific source searches. We apply our results to the determination of the
post-trial statistical significance of detections of transient gamma-ray
sources in terms of pre-trial values. The results of our analysis allow a
precise determination of the post-trial significance of {\gamma}-ray sources
detected by AGILE.Comment: 12 pages, 13 figures, 8 tables, accepted to A&
The Identification of EGRET Sources with Flat-Spectrum Radio Sources
We present a method to assess the reliability of the identification of EGRET
sources with extragalactic radio sources. We verify that EGRET is detecting the
blazar class of AGN. However, many published identifications are found to be
questionable. We provide a table of 42 blazars which we expect to be robust
identifications of EGRET sources. This includes one previously unidentified
EGRET source, the lensed AGN PKS 1830-210 near the direction of the Galactic
center. We provide the best available positions for 16 more radio sources which
are also potential identifications for previously unidentified EGRET sources.
All high Galactic latitude EGRET sources (b>3 degrees) which demonstrate
significant variability can be identified with flat spectrum radio sources.
This suggests that EGRET is not detecting any type of AGN other than blazars.
This identification method has been used to establish with 99.998% confidence
that the peak gamma-ray flux of a blazar is correlated with its average 5 GHz
radio flux. An even better correlation is seen between gamma-ray flux and the
2.29 GHz flux density measured with VLBI at the base of the radio jet. Also,
using high confidence identifications, we find that the radio sources
identified with EGRET sources have larger correlated VLBI flux densities than
the parent population of flat radio spectrum sources.Comment: To be published in Ap J, May 20, 1997 issue, Vol. 481. The complete
paper is available at http://bu-ast.bu.edu/~mattox/src_id.htm
A High-Energy Study of the Geminga Pulsar
We present the results of deep X-ray and gamma-ray observations of the
Geminga pulsar obtained in the final years of the ASCA and CGRO missions, and
an upper limit from RXTE. A phase-connected ephemeris from the gamma-rays is
derived that spans the years 1973-2000, after allowing for a minor glitch in
frequency of Delta f/f = 6.2 x 10^-10 in late 1996. ASCA observations of the
hard X-ray pulse profile in 1994 and 1999 confirm this glitch. An improved
characterization of the hard X-ray pulse profile and spectrum from the long
ASCA observation of 1999 confirms that there is a non-thermal X-ray component
that is distinct from the gamma-ray spectrum as measured by EGRET. It can be
parameterized as a power-law of photon index Gamma = 1.72 +/- 0.10 with a flux
of 2.62 x 10^-13 ergs/cm^2/s in the 0.7-5 keV band and pulsed fraction 0.54 +/-
0.05, similar to, but more precise than values measured previously. An
extrapolation of this spectrum into the energy band observed by the RXTE PCA is
consistent with the non-detection of pulsed emission from Geminga with that
instrument. These results are interpreted in the context of outer-gap models,
and motivations for future X-ray observations of Geminga are given.Comment: 22 pages including 7 figure
A search for the radio counterpart of the unidentified gamma-ray source 3EG J1410-6147
We have made radio continuum, HI and X-ray observations in the direction of
the unidentified EGRET source 3EG J1410-6147, using the Australia Telescope
Compact Array and the Chandra X-ray Observatory. The observations encompass the
supernova remnant (SNR) G312.4-0.4 and the two young pulsars PSRs J1412-6145
and J1413-6141.
We derive a lower distance limit of 6 kpc to the SNR, although interpretation
of positive velocity features in the HI spectrum may imply the SNR is more
distant than 14 kpc. PSR J1412-6145, with an age of 50 kyr, is the pulsar most
likely associated with SNR G312.4-0.4. X-rays are not detected from either
pulsar and diffuse X-ray emission near the bright western edge of the SNR is
weak. Although there is circumstantial evidence that this western region is a
pulsar wind nebula (PWN), the embedded pulsar PSR J1412-6145 is apparently not
sufficiently powerful to explain the radio enhancement. The origin of the
electron acceleration in this region and of the gamma-rays remain unidentified,
unless the distance to PSR J1413-6141 is at least a factor of 3 lower than its
dispersion measure distance.Comment: 10 pages, plus 5 jpeg figures, MNRAS, in press. Full postscript or
pdf including all figures is available at http://www.ast.cam.ac.uk/~m
AGILE detection of GeV gamma-ray emission from the SNR W28
Supernova remnants (SNRs) are believed to be the main sources of Galactic
cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray
emission through the interaction of accelerated particles with the concentrated
gas. The middle aged SNR W28, for its associated system of dense molecular
clouds, provides an excellent opportunity to test this hypothesis. We present
the AGILE/GRID observations of SNR W28, and compare them with observations at
other wavelengths (TeV and 12CO J=1-->0 molecular line emission). The gamma-ray
flux detected by AGILE from the dominant source associated with W28 is (14 +-
5) 10^-8 ph cm^-2 s^-1 for E > 400 MeV. This source is positionally well
correlated with the TeV emission observed by the HESS telescope. The local
variations of the GeV to TeV flux ratio suggest a difference between the CR
spectra of the north-west and south molecular cloud complexes. A model based on
a hadronic-induced interaction and diffusion with two molecular clouds at
different distances from the W28 shell can explain both the morphological and
spectral features observed by AGILE in the MeV-GeV energy range and by the HESS
telescope in the TeV energy range. The combined set of AGILE and H.E.S.S. data
strongly support a hadronic model for the gamma-ray production in W28.Comment: Accepted for publication in Astronomy & Astrophysics Letter
AGILE observation of a gamma-ray flare from the blazar 3C 279
Context. We report the detection by the AGILE satellite of an intense
gamma-ray flare from the gamma-ray source 3EG J1255-0549, associated to the
Flat Spectrum Radio Quasar 3C 279, during the AGILE pointings towards the Virgo
Region on 2007 July 9-13.
Aims. The simultaneous optical, X-ray and gamma-ray covering allows us to
study the spectral energy distribution (SED) and the theoretical models
relative to the flaring episode of mid-July.
Methods. AGILE observed the source during its Science Performance
Verification Phase with its two co-aligned imagers: the Gamma- Ray Imaging
Detector (GRID) and the hard X-ray imager (Super-AGILE) sensitive in the 30 MeV
- 50 GeV and 18 - 60 keV respectively. During the AGILE observation the source
was monitored simultaneously in optical band by the REM telescope and in the
X-ray band by the Swift satellite through 4 ToO observations.
Results. During 2007 July 9-13 July 2007, AGILE-GRID detected gamma-ray
emission from 3C 279, with the source at ~2 deg from the center of the Field of
View, with an average flux of (210+-38) 10^-8 ph cm^-2 s^-1 for energy above
100 MeV. No emission was detected by Super-AGILE, with a 3-sigma upper limit of
10 mCrab. During the observation lasted about 4 days no significative gamma-ray
flux variation was observed.
Conclusions. The Spectral Energy Distribution is modelled with a homogeneous
one-zone Synchrotron Self Compton emission plus the contributions by external
Compton scattering of direct disk radiation and, to a lesser extent, by
external Compton scattering of photons from the Broad Line Region.Comment: Accepted for publication in Astronomy and Astrophysic
PSRs J0248+6021 and J2240+5832: Young Pulsars in the Northern Galactic Plane. Discovery, Timing, and Gamma-ray observations
Pulsars PSR J0248+6021 (rotation period P=217 ms and spin-down power Edot =
2.13E35 erg/s) and PSR J2240+5832 (P=140 ms, Edot = 2.12E35 erg/s) were
discovered in 1997 with the Nancay radio telescope during a northern Galactic
plane survey, using the Navy-Berkeley Pulsar Processor (NBPP) filter bank. GeV
gamma-ray pulsations from both were discovered using the Fermi Large Area
Telescope. Twelve years of radio and polarization data allow detailed
investigations. The two pulsars resemble each other both in radio and in
gamma-ray data. Both are rare in having a single gamma-ray pulse offset far
from the radio peak. The high dispersion measure for PSR J0248+6021 (DM = 370
pc cm^-3) is most likely due to its being within the dense, giant HII region W5
in the Perseus arm at a distance of 2 kpc, not beyond the edge of the Galaxy as
obtained from models of average electron distributions. Its high transverse
velocity and the low magnetic field along the line-of-sight favor this small
distance. Neither gamma-ray, X-ray, nor optical data yield evidence for a
pulsar wind nebula surrounding PSR J0248+6021. The gamma-ray luminosity for PSR
J0248+6021 is L_ gamma = (1.4 \pm 0.3)\times 10^34 erg/s. For PSR J2240+5832,
we find either L_gamma = (7.9 \pm 5.2) \times 10^34 erg/s if the pulsar is in
the Outer arm, or L_gamma = (2.2 \pm 1.7) \times 10^34 erg/s for the Perseus
arm. These luminosities are consistent with an L_gamma ~ sqrt(Edot) rule.
Comparison of the gamma-ray pulse profiles with model predictions, including
the constraints obtained from radio polarization data, favor emission in the
far magnetosphere. These two pulsars differ mainly in their inclination angles
and acceleration gap widths, which in turn explains the observed differences in
the gamma-ray peak widths.Comment: 13 pages, Accepted to Astronomy & Astrophysic
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