S. Basso

XIPE: the X-ray imaging polarimetry explorer

Abstract 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. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. 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. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). 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). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

POLARIX: a pathfinder mission of X-ray polarimetry

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.

Development of a Prototype Nickel Optic for the Constellation-X Hard-X-Ray Telescope

The Constellation-X mission planned for launch in 2015-2020 timeframe, will feature an array of Hard X-ray telescopes (HXT) with a total collecting area greater than 1500 cm at 40 keV. Two technologies are being investigated for the optics of these telescopes, one of which is multilayer-coated Electroformed-Nickel-Replicated (ENR) shells. The attraction of the ENR process is that the resulting full-shell optics are inherently stable and offer the prospect of better angular resolution which results in lower background and higher instrument sensitivity. We are building a prototype HXT mirror module using an ENR process to fabricate the individual shells.This prototype consists of 5 shells with diameters ranging from 15 cm to 28 cm with a length of 42.6 cm. The innermost of these will be coated with iridium, while the remainder will be coated with graded d-spaced W/Si multilayers. The assembly structure has been completed and last year we reported on full beam illumination results from the first test shell mounted in this structure. We have now fabricated and coated two (15 cm and 23 cm diameter) 100 micron thick shells which have been aligned and mounted. This paper presents the results of full beam illumination X-ray tests, taken at MPE-Panter. The HEW of the individual shells will be discussed, in addition to results from the full two shell optic test.

Accurate integration of segmented x-ray optics using interfacing ribs

Abstract. Future lightweight and long-focal-length x-ray telescopes must guarantee a good angular resolution (e.g., 5 arc sec HEW) and reach an unprecedented large effective area. This goal can be reached with the slumping of borosilicate glass sheets that allow the fabrication of lightweight and low-cost x-ray optical units (XOU). These XOUs, based on mirror segments, have to be assembled together to form complete multishell Wolter-I optics. The technology for the fabrication and the integration of these XOUs is under development in Europe, funded by European Space Agency, and led by the Brera Observatory (INAF-OAB). While the achievement of the required surface accuracy on the glass segments by means of a hot slumping technique is a challenging aspect, adequate attention must be given to the correct integration and coalignment of the mirror segments into the XOUs. To this aim, an innovative assembly concept has been investigated, based on glass reinforcing ribs. The ribs connect pairs of consecutive foils, stacked into a XOU, with both structural and functional roles, providing robust monolithic stacks of mirror plates. Moreover, this integration concept allows the correction of residual low-frequency errors still present on the mirror foil profile after slumping. We present the integration concept, the related error budget, and the results achieved so far with a semi-robotic integration machine especially designed and realized to assemble slumped glass foils into XOUs.

Glass mirrors by cold slumping to cover 100 m2 of the MAGIC II Cherenkov telescope reflecting surface

We report on the production and implementation of 100 square panels 1 m x 1 m, based on the innovative approach of cold slumping of thin glass sheets. The more than 100 segments will cover around one half of the 240 m-square reflecting surface of the MAGIC II, a clone of the atmospheric Cherenkov telescope MAGIC I (with a single-dish 17 m diameter mirror) which is already operating since late 2003 at La Palma. The MAGIC II telescope will be completed by the end of 2008 and will operate in stereoscopic mode with MAGIC I. While the central part of the of the reflector is composed of by diamond milled Aluminum of 1m2 area panels (following a design similar to that already used for MAGIC I), the outer coronas will be made of sandwiched glass segments. The glass panel production foresees the following steps: a) a thin glass sheet (1-2mm) is elastically deformed so as to retain the shape imparted by a master with convex profile - the radius of curvature is large, the sheet can be pressed against the master using vacuum suction -; b) on the deformed glass sheet a honeycomb structure that provides the needed rigidity is glued ; c) then a second glass sheet is glued on the top in order to obtain a sandwich; d) after on the concave side a reflecting coating (Aluminum) and a thin protective coating (Quartz) are deposited. The typical weight of each panel is about 12 kg and its resolution is better than 1 mrad at a level of diameter that contains the 90% of the energy reflected by the mirror; the areal cost of glass panels is ~2 k per 1m2. The technology based on cold slumping is a good candidate for the production of the primary mirrors of the telescopes forming the Cherenkov Telescope Array (CTA), the future large TeV observatory currently being studied in Europe. Details on the realization of MAGIC II new mirrors based on cold slumping glass will be presented.

THE IMAGING PROPERTIES OF THE GAS PIXEL DETECTOR AS A FOCAL PLANE POLARIMETER

ABSTRACTX-rays are particularly suited to probe the physics of extreme objects. However, despite the enormousimprovements of X-ray Astronomy in imaging, spectroscopy and timing, polarimetry remains largelyunexplored. We propose the photoelectric polarimeter Gas Pixel Detector (GPD) as an instrumentcandidate to fill the gap of more than thirty years of lack of measurements. The GPD, in the focusof a telescope, will increase the sensitivity of orders of magnitude. Moreover, since it can measurethe energy, the position, the arrival time and the polarization angle of every single photon, allows toperform polarimetry of subsets of data singled out from the spectrum, the light curve or the imageof source. The GPD has an intrinsic very fine imaging capability and in this work we report on thecalibrationcampaign carriedout in 2012at the PANTER X-raytest facility of the Max-Planck-Institutfu¨r extraterrestrische Physik of Garching (Germany) in which, for the first time, we coupled it to aJET-X optics module with a focal length of 3.5 m and an angular resolution of 18 arcsec at 4.5 keV.This configuration was proposed in 2012 aboard the X-ray Imaging Polarimetry Explorer (XIPE) inresponse to the ESA call for a small mission. We derived the imaging and polarimetric performancefor extended sources like Pulsar Wind Nebulae and Supernova Remnants as case studies for the XIPEconfiguration, discussing also possible improvements by coupling the detector with advanced optics,having finer angular resolution and larger effective area, to study with more details extended objects.Keywords: X-ray polarimetry, X-ray telescope, angular resolution

Manufacturing of Wolter-I mirror segments with slumped glass

In our ongoing studies of high precision glass slumping we have successfully formed the first Wolter-I X-ray mirror segments with parabola and hyperbola in one piece. It could be demonstrated that the excellent surface roughness of the 0.55 mm thick display glass chosen is conserved during the slumping process. The influence of several parameters of the process, such as maximum temperature, heating and cooling rates etc. have to be measured and controlled with adequate metrology. Currently, we are optimizing the process to reduce the figure errors down to 1 micrometer what will be the starting point for further, final figure error corrections. We point out that metrology plays an important role in achieving a high precision optics, i.e. an angular resolution of a few arcsec. In this paper we report on the results of our studies and discuss them in the context of the requirements for future X-ray telescopes with large apertures.

X-ray optics developments at ESA

Future high energy astrophysics missions will require high performance novel X-ray optics to explore the Universe beyond the limits of the currently operating Chandra and Newton observatories. Innovative optics technologies are therefore being developed and matured by the European Space Agency (ESA) in collaboration with research institutions and industry, enabling leading-edge future science missions. Silicon Pore Optics (SPO) [1 to 21] and Slumped Glass Optics (SGO) [22 to 29] are lightweight high performance X-ray optics technologies being developed in Europe, driven by applications in observatory class high energy astrophysics missions, aiming at angular resolutions of 5” and providing effective areas of one or more square meters at a few keV. This paper reports on the development activities led by ESA, and the status of the SPO and SGO technologies, including progress on high performance multilayer reflective coatings [30 to 35]. In addition, the progress with the X-ray test facilities and associated beam-lines is discussed [36].

Design and development of the optics system for the NHXM Hard X-ray and Polarimetric Mission

The New Hard X-ray Mission (NHXM) Italian project will be operated by 2016. It is based on 4 hard X-ray optics modules, each formed by 60 evenly spaced multilayer coated Wolter I mirror shells. For the achievement of a long focal length (10 m) an extensible bench is used. The pseudo-cylindrical Wolter I monolithic substrates where the multilayer coating is applied will be produced using the Ni electroforming replica approach. For three of the four mirror modules the focal plane will host a hybrid a detector system, consisting in the combination of a Si-based low energy detector (efficient from 0.5 up to ~ 15 keV) , on top of a high energy CdTe pixellated detector (efficient from 10 keV up to ~ 80 keV); the two cameras will be surrounded by both a passive shield and an anticoincidence shield. The total on axis effective area of the three telescopes at 1 keV and at 30 kev is of 1500 cm2 and 350 cm2 respectively. The angular resolution requirement is better than 20 arcsec HEW at 30 keV, while the Field of View at 50% vignetting is 12 arcmin (diameter). The payload is finally completed with the fourth telescope module, that will have as a focal plane detector a high sensitivity imaging photoelectric polarimetric system, operating from 2 up to 35 keV. In this paper, after an overview of the mission configuration and its scientific goals, we report on the design and development of the multilayer optics of the mission, based on thin replicated Ni mirror shells.

Analytical computation of the off-axis effective area of grazing incidence X-ray mirrors

Aims. Focusing mirrors for X-ray telescopes in grazing incidence, introduced in the 70s, are characterized in terms of their p erformance by their imaging quality and effective area, which in turn determines their sensitivity. Ev en though the on-axis effective area is assumed in general to characterize the collecting power of an X-ray optic, the telescope capability of imaging extended X-ray sources is also determined by the variation in its effective area with the off-axis angle. The effective area, in general, decreases as the X-ray source moves off-axis, causing a loss of sensitivity in the peripheral regio ns of the telescope’s field of view. Methods. The complex task of designing optics for future X-ray telescopes entails detailed computations of both imaging quality and effective area on- and off-axis. Because of their apparent complexity, both aspects have been, so far, treated by using ray-tracing routines aimed at simulating the interaction of X-ray photons with the reflecting surfaces of a given focusing system. Al though this approach has been widely exploited and proven to be effective, it would also be attractive to regard the same proble m from an analytical viewpoint, to assess an optical design of an X-ray optical module with a simpler calculation than a ray-tracing r outine. This would also improve the effi ciency of optimization tasks when designing the X-ray optical modules. In this paper, we thereby focused on developing analytical solutions to compute the off-axis effective area of double-reflection X-ray mirrors. Results. We have developed useful analytical formulae for the off-axis effective area of a double-reflection mirror in the double cone approximation, requiring only an integration and the standard routines to calculate the X-ray coating reflectivity for a given incidence angle. The computation is easily applicable also to Wolter-I mirrors (such as those of NeXT, NuSTAR, HEXIT-SAT, IXO) and the approximation improves as the f-number of the mirror increases. Algebraic expressions are provided for the mirror geometric area, as a function of the off-axis angle. Finally, the results of the analytical computa tions presented here are validated by comparison with the corresponding predictions of a ray-tracing code.