196,873 research outputs found
Log-parabolic spectra and particle acceleration in the BL Lac object Mkn 421: Spectral analysis of the complete BeppoSAX wide band X-ray data set
No full textWe report the results of a new analysis of 13 wide band BeppoSAX observations
of the BL Lac object Mkn 421. The data from LECS, MECS and PDS,
covering an energy interval from 0.1 to over 100 keV, have been used to
study the spectral variability of this source. We show that the energy
distributions in different luminosity states can be fitted very well by a log-parabolic
law , which provides good estimates
of the energy and flux of the synchrotron peak in the SED.
In the first four short observations of 1997 Mkn 421 was characterized by a very
stable spectral shape, with average values and , independently
of the source brightness and of the fact that the source luminosity was increasing or decreasing.
In the observations of 1998 smaller values for both parameters, and 0.34, were found and the peak energy in the SED was in the range 0.5–0.8 keV.
The observations of May 1999 and April–May 2000 covered runs of
a duration of several days and provided a very high number of events for all the
instruments. The resulting spectral fits were then limited by
some instrumental systematics. Also in these cases, the log-parabolic model gave a satisfactory
description of the overall SED of Mkn 421. In particular, in the observations of
spring 2000 the source was brighter than the other observations and showed a
large change of the spectral curvature. Spectral parameters estimates gave
and 0.19 and the energy of the maximum in the SED moved to
the range 1–5.5 keV.
We give a possible interpretation of the log-parabolic spectral model in terms of
particle acceleration mechanisms. An energy distribution of emitting particles
with curvature close to the one observed can be explained by a simple model for statistical
acceleration with the assumption that the probability for a particle to increase
its energy is a decreasing function of the energy itself.
A consequence of this mechanism is the existence of a linear
relation between the spectral parameters a and b, well confirmed by the estimated
values of these two parameters for Mkn 421
Log-parabolic spectra and particle acceleration in blazars. II. The BeppoSAX wide band X-ray spectra of Mkn 501
No full textWe present the results of a spectral and temporal study of the complete set of
BeppoSAX NFI (11) and WFC (71) observations of the BL Lac object Mkn 501.
The WFC 2–28 keV data, reported here for the first time, were collected over
a period of about five years, from September 1996 to October 2001.
These observations, although not evenly distributed, show that Mkn 501,
after going through a very active phase from spring 1997 to early 1999,
remained in a low brightness state until late 2001.
The data from the LECS, MECS and PDS instruments, covering the wide energy
interval 0.1–150 keV, have been used to study in detail the spectral
variability of the source.
We show that the X-ray energy distribution of Mkn 501 is well described by a
log-parabolic law in all luminosity states.
This model allowed us to obtain good estimates of the SED synchrotron peak
energy and of its associated power.
The strong spectral variability observed, consisting of strictly correlated
changes between the synchrotron peak energy and bolometric flux
(), suggests that the main physical changes
are not only due to variations of the maximum Lorentz factor of the emitting
particles but that other quantities must be varying as well.
During the 1997 flare the high energy part of the spectrum of Mkn 501
shows evidence of an excess above the best fit log-parabolic law suggesting the
existence of a second emission component that may be responsible for most of the
observed variability
M-quantile small area estimation for panel data
No full textEconomic indicators need to be estimated at regional level. Small area estimation based on M-quantile regression has recently been introduced by Chambers and Tzavidis (Biometrika 93:255–268, 2006) and it has proved to provide a valid alternative to traditional methods. Thus far, this method has only been applied to cross-sectional data. However, it is well known that the use of panel data may provide significant gains in terms of efficiency of the estimators. This paper explores possible extensions of M-quantile-based small area estimators to the panel data context. A model-based simulation study is presented
Log-parabolic spectra and particle acceleration in blazars. III. SSC emission in the TeV band from Mkn501
No full textCurved broad-band spectral distributions of non-thermal sources like
blazars are described well by a log-parabolic law where the second
degree term measures the curvature.
Log-parabolic energy spectra can be obtained for relativistic electrons
by means of a statistical acceleration mechanism whose probability of
acceleration depends on energy.
In this paper we compute the spectra radiated by an electron population via synchrotron
and Synchro-Self Compton processes to derive the relations between
the log-parabolic parameters.
These spectra were obtained by means of an accurate numerical code
that takes the proper spectral distributions for single particle
emission into account.
We found that the ratio between the curvature parameters of the synchrotron spectrum
to that of the electrons is equal to ~0.2 instead of 0.25, the value foreseen
in the δ-approximation.
Inverse Compton spectra are also intrinsically curved and can be approximated
by a log-parabola only in limited ranges.
The curvature parameter, estimated around the SED peak, may vary from a lower value than
that of the synchrotron spectrum up to that of emitting electrons depending on
whether the scattering is in the Thomson or in the Klein-Nishina regime.
We applied this analysis to computing the synchro-self Compton emission from the BL Lac
object Mkn 501 during the large flare of April 1997. We fit simultaneous BeppoSAX
and CAT data and reproduced intensities and spectral curvatures of both components
with good accuracy.
The large curvature observed in the TeV range was found to be mainly
intrinsic, and therefore did not require a large pair production absorption against the
extragalactic background. We regard this finding as an indication that the Universe is
more transparent at these energies than previously assumed by several models found
in the literature.
This conclusion is supported by recent detection of two relatively
high redshift blazars with HESS.
Log Parabolic SSC spectra in HBL sources. A new analysis of the April 1997 large outburst of Mkn 501
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Lacuna e integrazione nel restauro dei metalli archeologici: oltre la ricomposizione, verso la restituzione dell’opera. Riflessioni e proposte, tra teoria e prassi
No full textThe task of the conservator, in cooperation with the archaeologist, is to analyse archaeological material from an historical and technical standpoint in such a way that the act of conserving prior to presentation of an artefact is designed and executed for the specific artefact. This intervention must be carried out in a competent manner, i.e. the form and recognition must not be disrupted. Unfortunately, due to the corrosion of metal objects, artefacts change, which make the work of the conservator difficult. During instruction at the ISCR (Istituto Superiore per la Conservazione ed il Restauro), the possibilities for reconstructing complex metal artefacts in a collection, which shows signs of formal and aesthetic integrity, were analysed as examples. It involved filling the caesurae and defects in morphology and size. This work was aimed at restoring the integrity of the work on the formal and aesthetic side. The study also recapitulates the contribution in terms of the historical utility function of the studied objects and their technical properties
Mapping landslides in lunar impact craters using Chebyshev polynomials and DEM's
No full textGeological slope failure processes have been observed on the Moon surface for decades, nevertheless a detailed and exhaustive lunar landslide inventory has not been produced yet. For a preliminary survey, WAC images and DEM maps from LROC at 100 m/pixels have been exploited in combination with the criteria applied by Brunetti et al. (2015) to detect the landslides. These criteria are based on the visual analysis of optical images to recognize mass wasting features. In the literature, Chebyshev polynomials have been applied to interpolate crater cross-sections in order to obtain a parametric characterization useful for classification into different morphological shapes. Here a new implementation of Chebyshev polynomial approximation is proposed, taking into account some statistical testing of the results obtained during Least-squares estimation. The presence of landslides in lunar craters is then investigated by analyzing the absolute values off odd coefficients of estimated Chebyshev polynomials. A case study on the Cassini A crater has demonstrated the key-points of the proposed methodology and outlined the required future development to carry out
Dr. Duane M. Jackson, Morehouse College, July 2011
No full textThis video is a conversation with Dr. Duane M. Jackson. Dr. Jackson talks about his paper, "Recall and the Serial Position Effect: The Role of Primacy and Recency on Accounting Students' Performance." Jackie Daniel, AUC Woodruff Library, is the interviewer
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