126 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
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
Modelling the X-ray polarimetric signatures of complex geometry: the case study of the "changing look" AGN NGC 1365
"Changing look" Active Galactic Nuclei (AGN) are a subset of Seyfert galaxies
characterized by rapid transitions between Compton-thin and Compton-thick
regimes. In their Compton-thin state, the central engine is less obscured,
hence spectroscopy or timing observations can probe their innermost structures.
However, it is not clear if the observed emission features and the Compton hump
are associated with relativistic reflection onto the accretion disc, or complex
absorption by distant, absorbing gas clouds passing by the observer's
line-of-sight. Here, we investigate these two scenarios under the scope of
X-ray polarimetry, providing the first polarisation predictions for an
archetypal "changing look" AGN: NGC 1365. We explore the resulting polarisation
emerging from lamp-post emission and scattering off an accretion disc in the
immediate vicinity of a supermassive black hole. The computed polarisation
signatures are compared to the results of an absorption-dominated model, where
high column density gas partially covers the central source. While the shape of
the polarisation spectrum is similar, the two models differ in net polarisation
percentage, with the relativistic reflection scenario producing significantly
stronger polarisation. Additionally, the variation of the polarisation position
angle is distinctly different between both scenarios: the reflection-dominated
model produces smooth rotations of the polarisation angle with photon energy
whereas circumnuclear absorption causes an orthogonal switch of the
polarisation angle between the soft and the hard X-ray bands. By comparing the
predicted polarisation of NGC 1365 to the detectability levels of X-ray
polarimetry mission concepts proposed in the past, we demonstrate that with a
large, soft X-ray observatory or a medium-sized mission equipped with a hard (6
- 35 keV) polarimeter, the correct interpretation would be unambiguous.Comment: 6 pages, 4 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
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