POLARIX: a pathfinder mission of X-ray polarimetry

Abstract

Since the birth of X-ray astronomy, spectral, spatial and timing observation improved dramatically, procuring a wealth of information on the majority of the classes of the celestial sources. Polarimetry, instead, remained basically unprobed. X-ray polarimetry promises to provide additional information procuring two new observable quantities, the degree and the angle of polarization. Polarization from celestial X-ray sources may derive from emission mechanisms themselves such as cyclotron, synchrotron and non-thermal bremsstrahlung, from scattering in aspheric accreting plasmas, such as disks, blobs and columns and from the presence of extreme magnetic field by means of vacuum polarization and birefringence. Matter in strong gravity fields and Quantum Gravity effects can be studied by X-ray polarimetry, too. POLARIX is a mission dedicated to X-ray polarimetry. It exploits the polarimetric response of a Gas Pixel Detector, combined with position sensitivity, that, at the focus of a telescope, results in a huge increase of sensitivity. The heart of the detector is an Application-Specific Integrated Circuit (ASIC) chip with 105,600 pixels each one containing a full complete electronic chain to image the track produced by the photoelectron. Three Gas Pixel Detectors are coupled with three X-ray optics which are the heritage of JET-X mission. A filter wheel hosting calibration sources unpolarized and polarized is dedicated to each detector for periodic on-ground and in-flight calibration. POLARIX will measure time resolved X-ray polarization with an angular resolution of about 20 arcsec in a field of view of 15 × 15 arcmin and with an energy resolution of 20% at 6 keV. The Minimum Detectable Polarization is 12% for a source having a flux of 1 mCrab and 105 s of observing time. The satellite will be placed in an equatorial orbit of 505 km of altitude by a Vega launcher. The telemetry down-link station will be Malindi. The pointing of POLARIX satellite will be gyroless and it will perform a double pointing during the earth occultation of one source, so maximizing the scientific return. POLARIX data are for 75% open to the community while 25% + SVP (Science Verification Phase, 1 month of operation) is dedicated to a core program activity open to the contribution of associated scientists. The planned duration of the mission is one year plus three months of commissioning and SVP, suitable to perform most of the basic science within the reach of this instrument. A nice to have idea is to use the same existing mandrels to build two additional telescopes of iridium with carbon coating plus two more detectors. The effective area in this case would be almost doubled

X-Ray Emission From Transient Jet Model in Black Hole Binaries

“…On the other hand, in RIAF models the polarization signal is expected to be weaker, due to averaging of the synchrotron signal from different parts of the disk, as well as contribution from free-free emission from the outer accretion flow. We thus expect that proposed future X-ray polarization missions, such as the POLARIX mission (Costa et al 2010) or the Gravity and Extreme Magnetism Small Explorer mission (Jahoda 2010), may play a significant role in determining the relative contributions from the disks and the jets in producing the X-ray spectra.…”

Measurement of the position resolution of the Gas Pixel Detector

“…Still the GPD was devised primarily to be exploited at the focus of conventional and multi-layer hard X-ray optics [22]. At this purpose the GPD has been considered for different proposed space missions [14,12,13,15]. While X-ray optics with the exquisite quality of those of Chandra will not be again available in the near future, for POLARIX (or XIPE, proposed in 2012 to ESA as a small mission for a launch on 2017) the JET-X optics (three complete mirrors, two Flight Models, FM, the heritage of the former Spectrum X-Gamma mission, and one Qualification Model), can still be used.…”

The background of the gas pixel detector and its impact on imaging X-ray polarimetry

“…The expected HEW of the instrument is 20 arcsec, the result of a simulation including both the contribution of the measured PSF of the optics and the contribution of the inclined penetration of the reflected photons absorbed within the GPD drift length. 15 Such simulation is derived by that already performed for POLARIX 16 An observation with XIPE of SgrB2 of 2 10 6 s has an MDP of 20.5 % constraining the position angle (1σ) at 3.5 • if the polarization is 40%. Such observation is foreseen in a XIPE class experiment, notwithstanding its difficulty, just because of the large amount of polarization expected.…”