1,011 research outputs found
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&
A variability analysis of low-latitude unidentified gamma-ray sources
We present a study of 40 low-latitude unidentified 3EG gamma-ray sources
which were found to be not positionally coincident with any known class of
potential gamma-ray emitters in the Galaxy (Romero, Benaglia & Torres, 1999).
We have performed a variability analysis which reveals that many of these 40
sources are variable. These sources have, in addition, a steep mean value of
the gamma-ray spectral index, , which, combined with
the high level of variability seems to rule out a pulsar origin. The positional
coincidences with uncatalogued candidates to supernova remnants were also
studied. Only 7 sources in the sample are spatially coincident with these
candidates, a result that is shown to be consistent with the expected level of
pure chance association. A complementary search for weak radio counterparts was
also conducted and the results are presented as an extensive table containing
all significant point-like radio sources within the 40 EGRET fields. We argue
that in order to produce the high variability, steep gamma-ray spectra, and
absence of strong radio counterparts observed in some of the gamma-ray sources
of our sample a new class of objects should be postulated, and we analyze a
viable candidate.Comment: Paper updated to match the accepted version to appear in Astronomy
and Astrophysics, 2001. Tables 5,6,7 and 8 are in ascii format and need to be
printed separately. they can also be obtained from
http://www.iar.unlp.edu.ar/garra Table 5 is 62 pages long. Download the
source to obtain the table
Long-Term Flux Monitoring of LSI +61 303 at 2.25 and 8.3 GHz
LSI +61 303 is an exotic binary system consisting of a ~10 Msun B star and a
compact object which is probably a neutron star. The system is associated with
the interesting radio source GT0236+610 that exhibits bright radio outbursts
with a period of 26.5 days. We report the results of continuous daily radio
interferometric observations of GT0236+610 at 2.25 and 8.3 GHz from 1994
January to 1996 February. The observations cover 25 complete (and 3 partial)
cycles with multiple observations each day. We detect substantial
cycle-to-cycle variability of the radio emission characterized by a rapid onset
of the radio flares followed by a more gradual decrease of the emission. We
detect a systematic change of the radio spectral index alpha which typically
becomes larger than zero at the onset of the radio outbursts. This behavior is
suggestive of expansion of material initially optically thick to radio
frequencies, indicating either that synchrotron or inverse Compton cooling are
important or that the free-free optical depth to the source is rapidly
changing. After two years of observations, we see only weak evidence for the
proposed 4-year periodic modulation in the peak flux of the outbursts. We
observe a secular trend in the outburst phases according the the best published
ephemeris. This trend indicates either orbital period evolution, or a drift in
outburst orbital phase in response to some other change in the system.Comment: 23 pages, LaTex, 7 figures, to appear in ApJ, v491, Dec 10th issue,
for associated info and preprints see
http://www.srl.caltech.edu/personnel/paulr/lsi.htm
Stochastic wake field particle acceleration in Gamma-Ray Bursts
Gamma-Ray Burst (GRB) prompt emission can, for specific conditions, be so
powerful and short-pulsed to strongly influence any surrounding plasma. In this
paper, we briefly discuss the possibility that a very intense initial burst of
radiation produced by GRBs satisfy the intensity and temporal conditions to
cause stochastic wake-field particle acceleration in a surrounding plasma of
moderate density. Recent laboratory experiments clearly indicate that powerful
laser beam pulses of tens of femtosecond duration hitting on target plasmas
cause efficient particle acceleration and betatron radiation up to tens of MeV.
We consider a simple but realistic GRB model for which particle wake-field
acceleration can first be excited by a very strong low-energy precursor, and
then be effective in producing the observed prompt X-ray and gamma-ray GRB
emission. We also briefly discuss some of the consequences of this novel GRB
emission mechanism.Comment: 5 pages, 1 figure, submitted to MNRA
Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts
We present the main results of a study of spectral and energetics properties
of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our
sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the
redshift estimates and the good-quality BeppoSAX time-integrated spectra we
deduce the main properties of GRBs in their cosmological rest frames. All
spectra in our sample are satisfactorily represented by the Band model with no
significant soft X-ray excesses or spectral absorptions. We find a positive
correlation between the estimated total (isotropic) energies in the 1-10000 keV
energy range (E_rad) and redshifts z. Interestingly, more luminous GRBs are
characterized also by larger peak energies E_p of their EF(E) spectra.
Furthermore, more distant GRBs appear to be systematically harder in the X-ray
band compared to GRBs with lower redshifts. We discuss how selection and data
truncation effects could bias our results and give possible explanations for
the correlations that we found.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
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