1,406 research outputs found
XIPE: the X-ray Imaging Polarimetry Explorer
X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and
temporal variability measurements and to imaging, allows a wealth of physical
phenomena in astrophysics to be studied. X-ray polarimetry investigates the
acceleration process, for example, including those typical of magnetic
reconnection in solar flares, but also emission in the strong magnetic fields
of neutron stars and white dwarfs. It detects scattering in asymmetric
structures such as accretion disks and columns, and in the so-called molecular
torus and ionization cones. In addition, it allows fundamental physics in
regimes of gravity and of magnetic field intensity not accessible to
experiments on the Earth to be probed. Finally, models that describe
fundamental interactions (e.g. quantum gravity and the extension of the
Standard Model) can be tested. We describe in this paper the X-ray Imaging
Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a
small mission with a launch in 2017 but not selected. XIPE is composed of two
out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD)
filled with a He-DME mixture at their focus and two additional GPDs filled with
pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 %
at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half
Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X
optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with
Malindi as down-link station and of a Mission Operation Center (MOC) at INPE
(Brazil).Comment: 49 pages, 14 figures, 6 tables. Paper published in Experimental
Astronomy http://link.springer.com/journal/1068
Hard X‐ray polarimetry of solar flares with BATSE
We describe a technique for measuring the polarization of hard X‐rays from solar flares based on the angular distribution of that portion of the flux which is scattered off the top of the Earth’s atmosphere. The scattering cross section depends not only on the scatter angle itself, but on the orientation of the scatter angle with respect to the incident polarization vector. Consequently, the distribution of the observed albedo flux will depend on the direction and the polarization properties (i.e., the level of polarization and polarization angle) of the source. Since the albedo component can represent a relatively large fraction (up to 40%) of the direct source flux, there will generally be sufficient signal for making such a measurement. The sensitivity of this approach is therefore dictated by the effective area and the ability of a detector system to ‘image’ the albedo flux. The 4π coverage of the BATSE detectors on the Compton Gamma‐RayObservatory provides an opportunity to measure both the direct and the albedo flux from a given solar flare event. Although the BATSE design (with its large field‐of‐view for each detector) is not optimized for albedo polarimetry, we have nonetheless investigated the feasibility of this technique using BATSE data
GRIPS and the Perspective of Next-generation Gamma-ray Surveys
GRIPS is one example of next generation telescopes proposed for astronomy the
energy range between hard X-ray mirror instruments such as NuStar and the Fermi
telescope. The Compton telescope principle is an advantageous concept in view
of background suppression, imaging sensitivity within a large field of view and
energy range, and capability to measure polarization. The diversity of
astrophysical sources at high energies (diffuse emission from cosmic-ray
interactions, nuclear lines from point-like and diffuse sources, accreting
binaries, cosmic-ray acceleration sites, novae and supernovae, GRBs) presents a
challenge, and in particular emphasizes the need for large fields of view and
surveys. We discuss the astrophysical challenges which are expected to remain
after the extended INTEGRAL mission, and how such a next-generation survey at
low-energy gamma-rays would impact on these. We argue that qualitatively new
and more direct insights could be obtained on cosmic high-energy phenomena and
their underlying physical processes.Comment: 7 pages, 2 figures. INTEGRAL Science Worlshop "The Restless Gamma-Ray
Universe", Dublin (IRL) Oct 201
Max '91: Flare research at the next solar maximum
To address the central scientific questions surrounding solar flares, coordinated observations of electromagnetic radiation and energetic particles must be made from spacecraft, balloons, rockets, and ground-based observatories. A program to enhance capabilities in these areas in preparation for the next solar maximum in 1991 is recommended. The major scientific issues are described, and required observations and coordination of observations and analyses are detailed. A program plan and conceptual budgets are provided
Tests of General Relativity in the Strong Gravity Regime Based on X-Ray Spectropolarimetric Observations of Black Holes in X-Ray Binaries
Although General Relativity (GR) has been tested extensively in the weak
gravity regime, similar tests in the strong gravity regime are still missing.
In this paper we explore the possibility to use X-ray spectropolarimetric
observations of black holes in X-ray binaries to distinguish between the Kerr
metric and the phenomenological metrics introduced by Johannsen and Psaltis
(2011) (which are not vacuum solutions of Einstein's equation) and thus to test
the no-hair theorem of GR. To this end, we have developed a numerical code that
calculates the radial brightness profiles of accretion disks and parallel
transports the wave vector and polarization vector of photons through the Kerr
and non-GR spacetimes. We used the code to predict the observational appearance
of GR and non-GR accreting black hole systems. We find that the predicted
energy spectra and energy dependent polarization degree and polarization
direction do depend strongly on the underlying spacetime. However, for large
regions of the parameter space, the GR and non-GR metrics lead to very similar
observational signatures, making it difficult to observationally distinguish
between the two types of models.Comment: 27 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Light bending scenario for accreting black holes in X-ray polarimetry
We discuss a model of an X-ray illuminating source above an accretion disk of
a rotating black hole. Within the so called lamp-post scheme we compute the
expected (observed) polarization properties of the radiation reaching an
observer. We explore the dependencies on model parameters, employing Monte
Carlo radiation transfer computations of the X-ray reflection on the accretion
disk and taking general relativity effects into account. In particular, we
discuss the role of the black hole spin, of the observer viewing angle, and of
the primary X-ray source distance from the black hole. We give several examples
of the resulting polarization degree for two types of exemplary objects -
active galactic nuclei and Galactic black holes. In order to assess potential
observability of the polarization features, we assume the sensitivity of the
proposed New Hard X-ray Mission (NHXM).
We examine the energy range from several keV to ~50 keV, so the iron-line
complex and the Compton hump are included in our model spectra. We find the
resultant polarization degree to increase at the higher end of the studied
energy band, i.e. at >~20 keV. Thus, the best results for polarimetry of
reflection spectra should be achieved at the Compton hump energy region. We
also obtain higher polarization degree for large spin values of the black hole,
small heights of the primary source, and low inclination angles of the
observer.Comment: 17 pages, 14 figures, accepted in Ap
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