214 research outputs found
Instrumentation and Future Missions in the Upcoming Era of X-Ray Polarimetry
The maturity of current detectors based on technologies that range from solid
state to gases renewed the interest for X-ray polarimetry, raising the
enthusiasm of a wide scientific community to improve the performance of
polarimeters as well as to produce more detailed theoretical predictions. We
will introduce the basic concepts about measuring the polarization of photons,
especially in the X-rays, and we will review the current state of the art of
polarimeters in a wide energy range from soft~to hard X-rays, from solar flares
to distant astrophysical sources. We will introduce relevant examples of
polarimeters developed from the recent past up to the panorama of upcoming
space missions to show how the recent development of the technology is allowing
reopening the observational window of X-ray polarimetry
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
A Study of background for IXPE
Focal plane X-ray polarimetry is intended for relatively bright sources with
a negligible impact of background. However this might not be always possible
for IXPE (Imaging X-ray Polarimetry Explorer) when observing faint extended
sources like supernova remnants. We present for the first time the expected
background of IXPE by Monte Carlo simulation and its impact on real
observations of point and extended X-ray sources. The simulation of background
has been performed by Monte Carlo based on GEANT4 framework. The spacecraft and
the detector units have been modeled, and the expected background components in
IXPE orbital environment have been evaluated. We studied different background
rejection techniques based on the analysis of the tracks collected by the Gas
Pixel Detectors on board IXPE. The estimated background is about 2.9 times
larger than the requirement, yet it is still negligible when observing point
like sources. Albeit small, the impact on supernova remnants indicates the need
for a background subtraction for the observation of the extended sources.Comment: 16 pages, 16 figure
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