166 research outputs found
Measurement of the position resolution of the Gas Pixel Detector
The Gas Pixel Detector was designed and built as a focal plane instrument for
X-ray polarimetry of celestial sources, the last unexplored subtopics of X-ray
astronomy. It promises to perform detailed and sensitive measurements resolving
extended sources and detecting polarization in faint sources in crowded fields
at the focus of telescopes of good angular resolution. Its polarimetric and
spectral capability were already studied in earlier works. Here we investigate
for the first time, with both laboratory measurements and Monte Carlo
simulations, its imaging properties to confirm its unique capability to carry
out imaging spectral-polarimetry in future X-ray missions.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section A; 15 figures, 3 table
Probing magnetars magnetosphere through X-ray polarization measurements
The study of magnetars is of particular relevance since these objects are the
only laboratories where the physics in ultra-strong magnetic fields can be
directly tested. Until now, spectroscopic and timing measurements at X-ray
energies in soft gamma-repeaters (SGRs) and anomalous X-ray pulsar (AXPs) have
been the main source of information about the physical properties of a magnetar
and of its magnetosphere. Spectral fitting in the ~ 0.5-10 keV range allowed to
validate the "twisted magnetosphere" model, probing the structure of the
external field and estimating the density and velocity of the magnetospheric
currents. Spectroscopy alone, however, may fail in disambiguating the two key
parameters governing magnetospheric scattering (the charge velocity and the
twist angle) and is quite insensitive to the source geometry. X-ray
polarimetry, on the other hand, can provide a quantum leap in the field by
adding two extra observables, the linear polarization degree and the
polarization angle. Using the bright AXP 1RXS J170849.0-400910 as a template,
we show that phase-resolved polarimetric measurements can unambiguously
determine the model parameters, even with a small X-ray polarimetry mission
carrying modern photoelectric detectors and existing X-ray optics. We also show
that polarimetric measurements can pinpoint vacuum polarization effects and
thus provide an indirect evidence for ultra-strong magnetic fields.Comment: 12 pages, 8 figures, accepted for publication in MNRA
Correlation methods for the analysis of X-ray polarimetric signals
X-ray polarimetric measurements are based on studying the distribution of the
directions of scattered photons or photoelectrons and on the search of a
sinusoidal modulation with a period of {\pi}. We developed two tools for
investigating these angular distributions based on the correlations between
counts in phase bins separated by fixed phase distances. In one case we use the
correlation between data separated by half of the bin number (one period) which
is expected to give a linear pattern. In the other case, the scatter plot
obtained by shifting by 1/8 of the bin number (1/4 of period) transforms the
sinusoid in a circular pattern whose radius is equal to the amplitude of the
modulation. For unpolarized radiation these plots are reduced to a random point
distribution centred at the mean count level. This new methods provide direct
visual and simple statistical tools for evaluating the quality of polarization
measurements and for estimating the polarization parameters. Furthermore they
are useful for investigating distortions due to systematic effects
Determinazione delle correnti di lavoro degli XAA1.2 relative allo Stretcher_bias
Il presente rapporto tecnico riporta la curva caratteristica di polarizzazione dello stertcher bias degli XAA1.2 usati in SuperAGILE. Detta curva caratterisrtica è stata determinata in laboratori
a versatile facility for the calibration of x ray polarimeters
INAF/IASF BolognaVia Gobetti 101, I-40129 Bologna, ItalyE-mail:[email protected] presenta versatilefacility builtat IASF/INAF ofRomeforthecalibrationof X-rayinstrumentsin the energy range above 1.65 keV. Both unpolarized and polarized radiation can be generated.The former is produced by means of radioactive sources or X-ray tubes. Polarized photons areinstead obtainedbyBraggdiffractionat nearly45degrees. High fluxesat 2.29,2.69, 3.69and4.51keV are produced by diffracting the emission lines of X-ray tubes with molybdenum, rhodium,calcium and titanium anodes on suitable crystals. Diffraction of continuum emission is insteadexploited for the productionof polarized photons at 1.65 keV and 2.04 keV. Moreoverit is used togenerate polarized photons at higher energies corresponding to the different orders of diffraction.The size and the divergence of beam is controlled with diaphragms and capillary plates. Thedirection of polarization, the position and the inclination of the beam are accurately controlled bymeans of high precision motorized stages. This allows to map the response of imaging devicesto both polarized and unpolarized radiation. Moreover it is used to study the reponse to inclinedbeams up to ∼60 degrees.Polarimetry days in Rome: Crab status, theory and prospectsOctober 16-17, 2008Rome, Ital
A variable magnetic disc wind in the black hole X-ray binary GRS 1915+105?
GRS 1915+105 being one of the brightest transient black hole binary (BHB) in
the X-rays, offers a unique test-bed for the study of the connection between
accretion and ejection mechanisms in BHBs. In particular, this source can be
used to study the accretion disc wind and how it depends on the state changes
in BHBs. Our aim is to investigate the origin and geometry of the accretion
disc wind in GRS 1915+105. We analysed the spectra of GRS 1915+105 in the soft
and hard classes, using the high resolution spectroscopy offered
by Chandra HETGS. In the soft state, we find a series of wind absorption lines
that follow a non linear dependence of velocity width, velocity shift and
equivalent width with respect to ionisation, indicating a multiple component or
stratified outflow. In the hard state we find only a faint Fe XXVI absorption
line. We model the absorption lines in both the states using a dedicated MHD
wind model to investigate a magnetic origin of the wind and to probe the cause
of variability in the observed lines flux between the two states. The MHD disc
wind model provides a good fit for both states, indicating the possibility of a
magnetic origin of the wind. The multiple ionisation components of the wind are
well characterised as a stratification of the same magnetic outflow. We find
that the observed variability in the lines flux between soft and hard states
cannot be explained by photo-ionisation alone but it is most likely due to a
large (three orders of magnitude) increase in the wind density. We find the
mass outflow rate of the wind to be comparable to the accretion rate,
suggesting a intimate link between accretion and ejection processes that lead
to state changes in BHBs.Comment: 11 pages, 9 figures. Shortened abstract. Accepted for publication in
A&
A versatile facility for the calibration of X-ray polarimeters with polarized and unpolarized controlled beams
We devised and built a versatile facility for the calibration of the next
generation X-ray polarimeters with unpolarized and polarized radiation. The
former is produced at 5.9 keV by means of a Fe55 radioactive source or by X-ray
tubes, while the latter is obtained by Bragg diffraction at nearly 45 degrees.
Crystals tuned with the emission lines of X-ray tubes with molybdenum, rhodium,
calcium and titanium anodes are employed for the efficient production of highly
polarized photons at 2.29, 2.69, 3.69 and 4.51 keV respectively. Moreover the
continuum emission is exploited for the production of polarized photons at 1.65
keV and 2.04 keV and at energies corresponding to the higher orders of
diffraction. The photons are collimated by means of interchangeable capillary
plates and diaphragms, allowing a trade-off between collimation and high
fluxes. The direction of the beam is accurately arranged by means of high
precision motorized stages, controlled via computer so that long and automatic
measurements can be done. Selecting the direction of polarization and the
incidence point we can map the response of imaging devices to both polarized
and unpolarized radiation. Changing the inclination of the beam we can study
the systematic effects due to the focusing of grazing incidence optics and the
feasibility of instruments with large field of view.Comment: 12 pages, 11 figure
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