1,433 research outputs found
Bragg concentrators for hard (> 10 keV) X-ray astronomy: Status report
The use of focusing telescopes in hard X-ray (E > 10 keV) astronomy will provide better flux sensitivity and imaging perfomances with respect to the directviewing detectors, utilized until now. We present recent results obtained from our Group regarding the possible use of Bragg-diffraction technique to design hard X-ray focusing telescopes
Probing phase coexistence and stabilization of the spin-ordered ferrimagnetic state by Calcium addition in the YBa_{1-x}Ca_{x}Co_{2}O_{5.5} layered cobaltites using neutron diffraction
In this article we study the effects of a partial substitution of Ba with the
smaller cation Ca in the layered cobaltites YBaCo_2O_{5+\delta} for \delta
\approx 0.5. Neutron thermodiffractograms are reported for the compounds
YBa_{0.95}Ca_{0.05}Co_2O_{5.5} (x_{Ca}=0.05) and YBa_{0.90}Ca_{0.10}Co_2O_{5.5}
(x_{Ca}=0.10) in the temperature range 20 K \leq T \leq 300 K, as well as high
resolution neutron diffraction experiments at selected temperatures for the
samples x_{Ca}=0.05, x_{Ca}=0.10 and the parent compound x_{Ca}=0. We have
found the magnetic properties to be strongly affected by the cationic
substitution. Although the "122" perovskite structure seems unaffected by Ca
addition, the magnetic arrangements of Co ions are drastically modified: the
antiferromagnetic (AFM) long-range order is destroyed, and a ferrimagnetic
phase with spin state order is stabilized below T \sim 290 K. For the sample
with x_{Ca}=0.05 a fraction of AFM phase coexists with the ferrimagnetic one
below T \sim 190 K, whereas for x_{Ca}=0.10 the AFM order is completely lost.
The systematic refinement of the whole series has allowed for a better
understanding of the observed low-temperature diffraction patterns of the
parent compound, YBaCo_2O_{5.5}, which had not yet been clarified. A two-phase
scenario is proposed for the x_{Ca}=0 compound which is compatible with the
phase coexistence observed in the x_{Ca}=0.05 sample
A Robust Filter for the BeppoSAX Gamma Ray Burst Monitor Triggers
The BeppoSAX Gamma Ray Burst Monitor (GRBM) is triggered any time a
statistically significant counting excess is simultaneously revealed by at
least two of its four independent detectors. Several spurious effects,
including highly ionizing particles crossing two detectors, are recorded as
onboard triggers. In fact, a large number of false triggers is detected, in the
order of 10/day. A software code, based on an heuristic algorithm, was written
to discriminate between real and false triggers. We present the results of the
analysis on an homogeneous sample of GRBM triggers, thus providing an estimate
of the efficiency of the GRB detection system consisting of the GRBM and the
software.Comment: Proc. 5th Huntsville GRB Symposiu
The hard X-ray tails in neutron star low mass X-ray binaries: BeppoSAX observations and possible theoretical explanation of the GX 17+2 case
We report results of a new spectral analysis of two BeppoSAX observations of
the Z source GX 17+2. In one of the two observations the source exhibits a
powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work
by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL
of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have
shown that bulk (dynamical) Comptonization of soft photons can be a possible
alternative mechanism for producing hard X-ray tails in such systems. We start
from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX
to better investigate the physical processes at work. We use GX 17+2 as a
representative case. Moreover, we suggest that weakening (or disappearance) of
the hard X-ray tail can be explained by increasing radiation pressure
originated at the surface of the neutron star (NS). As a result the high
radiation pressure stops the bulk inflow and consequently this radiation
feedback of the NS surface leads to quenching the bulk Comptonization.Comment: 6 pages, 3 figures, Accepted for publication in Ap
The 1998 outburst of the X-ray transient XTE J2012+381 as observed with BeppoSAX
We report on the results of a series of X-ray observations of the transient
black hole candidate XTE J2012+381 during the 1998 outburst performed with the
BeppoSAX satellite. The observed broad-band energy spectrum can be described
with the superposition of an absorbed disk black body, an iron line plus a high
energy component, modelled with either a power law or a Comptonisation tail.
The source showed pronounced spectral variability between our five
observations. While the soft component in the spectrum remained almost
unchanged throughout our campaign, we detected a hard spectral tail which
extended to 200 keV in the first two observations, but became barely detectable
up to 50 keV in the following two. A further re-hardening is observed in the
final observation. The transition from a hard to a soft and then back to a hard
state occurred around an unabsorbed 0.1-200 keV luminosity of 10^38 erg/s (at
10 kpc). This indicates that state transitions in XTE 2012+281 are probably not
driven only by mass accretion rate, but additional physical parameters must
play a role in the evolution of the outburst.Comment: Paper accepted for publication on A&A (macro included, 9 pages, 5
figures
The GRB Variability/Peak Luminosity Correlation: new results
We report test results of the correlation between time variability and peak
luminosity of Gamma-Ray Bursts (GRBs), using a larger sample (32) of GRBs with
known redshift than that available to Reichart et al. (2001), and using as
variability measure that introduced by these authors. The results are puzzling.
Assuming an isotropic-equivalent peak luminosity, as done by Reichart et al.
(2001), a correlation is still found, but it is less relevant, and inconsistent
with a power law as previously reported. Assuming as peak luminosity that
corrected for GRB beaming for a subset of 16 GRBs with known beaming angle, the
correlation becomes little less significant.Comment: 11 pages, 10 figures, MNRAS, accepte
Disappearance of Hard X-ray Emission in the Last BeppoSAX Observation of the Z Source GX 349+2
We report on the results from two BeppoSAX observations of the Z source GX
349+2 performed in February 2001 and covering the broad energy range 0.12-200
keV. The light curve obtained from these observations shows a large flaring
activity, the count rate varying from ~130 to ~260 counts/s, indicating that
the source was in the flaring branch during these observations. The average
spectrum is well described by a soft blackbody and a Comptonized component. To
well fit the energy spectrum three gaussian lines are needed at 1.2 keV, 2.6
keV, and 6.7 keV with corresponding equivalent widths of 13 eV, 10 eV, and 39
eV, probably associated to L-shell emission of Fe XXIV, Ly-alpha S XVI, and Fe
XXV, respectively. These lines may be produced at different distances from the
neutron star, which increase when the count rate of the source increases. An
absorption edge is also needed at 9 keV with an optical depth of ~3 10^{-2}.
From the Color-Color Diagram (CD) we selected five zones from which we
extracted the corresponding energy spectra. The temperatures of the blackbody
and of the Comptonized component tend to increase when the intensity of the
source increases. We discuss our results comparing them to those obtained from
a previous BeppoSAX observation, performed in March 2000, during which the
source was a similar position of its Z-track. In particular we find that,
although the source showed similar spectral states in the 2000 and the 2001
observations, a hard tail, that was significantly detected in March 2000, is
not observed in these recent observations.Comment: Accepted for publication on Ap
Laue Lens Development for Hard X-rays (>60 keV)
Results of reflectivity measurements of mosaic crystal samples of Cu (111)
are reported. These tests were performed in the context of a feasibility study
of a hard X-ray focusing telescope for space astronomy with energy passband
from 60 to 600 keV. The technique envisaged is that of using mosaic crystals in
transmission configuration that diffract X-rays for Bragg diffraction (Laue
lens). The Laue lens assumed has a spherical shape with focal length . It is
made of flat mosaic crystal tiles suitably positioned in the lens. The samples
were grown and worked for this project at the Institute Laue-Langevin (ILL) in
Grenoble (France), while the reflectivity tests were performed at the X-ray
facility of the Physics Department of the University of Ferrara.Comment: 6 pages, 12 figures, accepted for publication in IEEE Transactions on
Nuclear Scienc
- …