اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدAn efficient photoelectric X-ray Polarimeter for the study of Black Holes and Neutron Stars
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Enrico Costa
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In astronomy there are basically four kinds of observations to extract the information carried by electromagnetic radiation: photometry, imaging, spectroscopy and polarimetry. By optimal exploitation of the first three techniques, X-ray astronomy has been able to unveil the violent world of compact high energy sources. Here we report on a new instrument that brings high efficiency also to X-ray polarimetry, the last unexplored field of X-ray astronomy. It will then be possible to resolve the internal structures of compact sources which otherwise would remain inaccessible, even to X-ray interferometry1. Polarimetry could provide a direct, visual picture of the state of matter under extreme magnetic and gravitational fields by measuring the radiation polarized through interaction with the highly asymmetric matter distribution (accretion disk) and with the magnetic field. The new instrument derives the polarization information from the track of the photoelectrons imaged by a finely subdivided gas detector. Its great improvement of sensitivity (at least two orders of magnitude) will allow direct exploration of the most dramatic objects of the X-ray sky.
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inese Space Agency aimed to survey the Hard X-ray Sky with Phoswich detectors, by exploitation of the direct demodulation technique. Since a fraction of the HXMT time will be spent on dedicated pointing of particular sources, it could host, with moderate additional resources a pair of X-ray telescopes, each with a photoelectric X-ray polarimeter in the focal plane. We present the design of the telescopes and the focal plane instrumentation and discuss the performance of this instrument to detect the degree and angle of linear polarization of some representative sources. Notwithstanding the limited resources the proposed instrument can represent a breakthrough in X-ray Polarimetry.
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It is found that there exists an empirical linear relation between the high frequency $ high$ and low frequency $ low$ of quasi-periodic oscillations (QPOs) for black hole candidate (BHC), neutron star (NS) and white dwarf (WD) in the binary systems,
which spans five orders of magnitude in frequency. For the NS Z (Atoll) sources, $ u_{high}$ and $ u_{low}$ are identified as the lower kHz QPO frequency and horizontal branch oscillations (HBOs) $ h$ (broad noise components); for the black hole candidates and low-luminosity neutron stars, they are the QPOs and broad noise components at frequencies between 1 and 10 Hz; for WDs, they are the ``dwarf nova oscillations (DNOs) and QPOs of cataclysmic variables (CVs). To interpret this relation, our model ascribes $ u_{high}$ to the Alfven wave oscillation frequency at a preferred radius and $ u_{low}$ to the same mechanism at another radius. Then, we can obtain $ low = 0.08 high$ and the relation between the upper kHz QPO frequency $ t$ and HBO to be $ h simeq 56 ({rm Hz}) ( t/{rm kHz})^{2}$, which are in accordance with the observed empirical relations. Furthermore, some implications of model are discussed, including why QPO frequencies of white dwarfs and neutron stars span five orders of magnitude in frequency.
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