XPOL: a photoelectric polarimeter onboard XEUS
Enrico Costa, Ronaldo Bellazzini, Jean Bregeon, Alessandro Brez, Massimo Frutti, Sergio Di Cosimo, Luca Latronicio, Francesco Lazzarotto, Giorgio Matt, Massimo Minuti, Ennio Morelli, Fabio Muleri, Michele Pinchera, Massimiliano Razzano, Alda Rubini, Paolo Soffitta, Gloria Spandre
aa r X i v : . [ a s t r o - ph ] O c t XPOL: a photoelectric polarimeter onboard XEUS
Enrico Costa a , Ronaldo Bellazzini c , Jean Bregeon c , Alessandro Brez c , Massimo Frutti a ,Sergio Di Cosimo a , Luca Latronicio c , Francesco Lazzarotto a , Giorgio Matt d , Massimo Minuti c ,Ennio Morelli e , Fabio Muleri a,b , Michele Pinchera c , Massimiliano Razzano c , Alda Rubini a ,Paolo Soffitta a , Gloria Spandre ca Istituto di Astrofisica Spaziale e Fisica Cosmica, Via del Fosso del Cavaliere 100, I-00133Roma, Italy; b Universit`a di Roma Tor Vergata, Dipartimento di Fisica, via della Ricerca Scientifica 1,00133 Roma, Italy c Istituto Nazionale di Fisica Nucleare, Largo B. Pontecorvo 3, I-56127 Pisa, Italy d Universit`a di Roma Tre, Dipartimento di Fisica E.Amaldi, via della Vasca Navale 84, 00146Roma, Italy e Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Gobetti 101, I-40129 Bologna, Italy
ABSTRACT
The XEUS mission incorporates two satellites: the Mirror Spacecraft with 5 m of collecting area at 1 keV and2 m at 7 keV, and an imaging resolution of 5” HEW and the Payload Spacecraft which carries the focal planeinstrumentation. XEUS was submitted to ESA Cosmic Vision and was selected for an advanced study as alarge mission. The baseline design includes XPOL, a polarimeter based on the photoelectric effect, that takesadvantage of the large effective area which permits the study of the faint sources and of the long focal length,resulting in a very good spatial resolution, which allows the study of spatial features in extended sources. Weshow how, with XEUS, Polarimetry becomes an efficient tool at disposition of the Astronomical community. Keywords:
X-ray Astronomy, polarization
1. INTRODUCTION
XEUS is an ambitious mission planned to be flown ∼
55 years after the start of X-ray astronomy. XEUS focalplane instrumentation is extremely evolved, especially in the domain of imaging non-dispersive spectroscopyand of wide field imaging with a good spectral response. This follows an almost continuous development fromthe first rockets through very successful missions such as Einstein, ROSAT, ASCA, SAX, Chandra, XMM. Thedevelopment of polarimeters has not proceeded in parallel. In fact after the first attempts and the first successwith OSO-8, no polarimeter has been embarked aboard a mission, with the exception of SPECTRUM-X-Gamma,that never arrived to the launch. Polarimetry is therefore an all to dig field, and a relatively extended literature(at least compared with the shortage of data) suggests that the crop would be highly rewarding. Nowadays newpolarimeters based on the photoelectric effects are available. The INFN of Pisa has developed the Gas PixelDetector, in the frame of a collaboration with IASF.
These devices combine the capability to measure thepolarization with good imaging and can be employed as focal plane detectors, allowing for the same dramaticimprovement occurred for imaging with the arrival of Einstein mission. We remind that Einstein was a stepforward also from the point of view of satellite attitude. In the pioneering satellites, stabilized on one axis,the detectors had a slat collimator misaligned with respect to rotation. A source was detected as an excessof counts following the collimator profile. Einstein and all the following satellites were stabilized on three axisand a source was a cluster of events in the image consistent with the telescope psf. The diffraction polarimeteris non-dispersive and requires rotation (both of analyzer and of detector) to perform the measurement. Thescattering polarimeter is intrinsically non-dispersive but requires the rotation of the whole to compensate huge
Further author information: (Send correspondence to Enrico Costa)Enrico Costa: E-mail: [email protected], Telephone: +39-0649934004 ystematic effects. Since the rotation was no more provided by the satellite, the polarimeter introduced a seriouscomplication of the focal plane, to be added to the complication of swapping from one instrument to the otherin the focal plane. Since the scientific interest of polarimetry was out of question, these mismatching were thecause of the removal of polarimeters from the major X-ray missions (Einstein, Chandra), where it was foreseenin the beginning.Beside the advantage of being small and working at room temperature, there is the additional advantageof not requiring rotation: this removes a further area of mismatching with other instruments. The change ofinstruments in the focal plane is intrinsically and safely resolved by the formation flight technology itself.The first obvious answer to the question ”Why to include a polarimeter in the focal plane of XEUS?” couldbe that it is simple and does not require large resources.In fact we could object that XEUS will, in any case, devote a minor fraction of its time to polarimetry. It islikely that before XEUS a dedicated mission will be flown. Such a mission could perform very long pointing ofsome target of particular interest and partially compensate with observing time the smaller effective area.We will demonstrate in the following that a GPD polarimeter aboard XEUS can achieve scientific results ofhigh value, that there results will solve some hot topics which are within the scientific targets of XEUS and thatcan be achieved with XEUS only, and not with a pathfinder mission of lower performance.
2. XPOL
The purpose of the XPOL is to provide, in the energy range 2 - 10keV, polarization measurements simultaneouslywith angular measurement (5 arcsec), spectral measurements (E/∆ E of ∼ µ s level.The FOV is of 1.5 × The heart of the polarimeter is the Gas Pixel Detector. It is a counter, with a beryllium window 50 µ m thick,filled with a mixture of low atomic number components (usually He 20% DME 80%). The photon is convertedin an absorption/drift gap 10 mm thick. The photoelectron interacts with atoms close to the impact point ad isslowed by ionization and scattered by the field of nuclei. The result is a track of electron-ion pairs. The electronsin the track are drifted by a constant electric field to a Gas Electron Multiplier, a polyimide film, metal coatedon both sides, with a matrix of holes on an hexagonal pattern, with a pitch of 50 µ m. Each hole multiplies ina proportional way the charge. Therefore the track is amplified, while preserving the information on the shapeand on the charge. Multiplied electrons are collected by a plane of metal pads, close to the GEM, also withhexagonal pattern and with the same pitch of 50 µ m. Each pad is the input of a complete electronic chain thatdetects the charge. Pads and front end electronics are part of a VLSI chip, based on 0.18 µ m CMOS technology.The chip has the capability to self trigger and fetches at the output only the content of a Region of Interest,including the pixels that triggered. Since the chip has a total of 105600 pixels, and a track typically produces acharge on 50-100 pixels (depending on the energy of the photon), this design prevents the divergence of dead-timethat would be needed to read the whole detector image. The analysis of the tracks allows to derive the impactpoint (with a precision one order of magnitude better than that of the centroid of the charge) and the ejectiondirection of the primary photoelectron. The latter carries the information of the polarization of the beam. Theprecision on the impact point is of the order of ∼ µ m FWHM, largely oversampling the PSF but this is notcompletely exploited because of the blurring due to the absorption of photons from an inclined beam at differentheights in the gas. This last effect is determining the actual resolution of XPOL.he GPD detector ad its polarimetric capabilities have been extensively described elsewhere. Below we givefor XPOL figures of sensitivity which are based on experimental data on existing prototypes, without includingany margin for the possible (and foreseen) improvements.
2, 4
The level of readiness of the detector is in a good shape. Sealed prototypes, built with low desorptionmaterials, have been tested for more than one year without any evidence of change. It should be consideredthat the technology for the manufacture of long duration gas cells for proportional counters and GSPCs to beemployed in the space, is very well established. The stability of the mixture has been tested. Further testing forthe robustness of the GEM to spark in presence of ions will be performed in a short time. Anyway it should beconsidered that the GEM is operated at a very low gain level ( ∼ Figure 1. A prototype Gas Pixel detector in the facility for vibration test
According to different operative modes a filter wheel will control and determine radiation impinging on thedetector. The filter wheel is a disc with different positions on it that can be rotated into the optical field of viewwith a motor. The Filter wheel position may be selected for observation, for calibration source deployment orfor safety. Safety operations may be required autonomously or via ground control to prevent excessive chargedparticle flux (solar flare or local magnetospheric storms).The filter wheel is foreseen to have 6 positions: • Position A) Closed (Operative mode: power-off, stand-by, electrical calibration, observation*) • Position B) Opened (Operative mode : Observation, normal rate ). • Position C) With transmission filter (Operative mode: observation, very high rate). • Position D) With diaphragm (Operative mode : Observation, small FOV). igure 2. The focal plane of XPOL including the detector, the back-end electronics, the filter wheel and the baffle (thedimensions of the baffle are not representative) • Position E) With calibration source I (Fe55). Operative mode : calibration. • Position F) With calibration source Ti/Cd-109 (TBC) . Operative mode : calibration.The presence of calibration sources aboard will be useful to monitor the stability of the gain and of theresponse to polarized photons. Therefore we foresee an source providing unpolarized photons and another oneemitting polarized ones (by bragg diffraction). They will be put periodically in the front of the detector window.A baffle of carbon fiber with thin metal plating will prevent the direct vision of the diffuse X-ray backgroundfrom the sky. The dimension of the baffle are still to be defined, on the basis of the extension of the skirt thatwill surround the optics satellite.The Detector, the back-end electronics and the filter wheel will be enclosed within a protective carter whichwill also support the baffle. In order to stabilize the gain the detector will have its own thermal control. In Fig. 2we show the focal plane. The dimensions of the baffle are not representative.
The back-end electronics is a small box connected with the detector with flexi cables at a distance of around 20cm. It includes the logics to program the ASIC chip, the logic to read the signal from the chip, to A/D convertthem, to flag with time and to transfer to the control electronics. All the logic functions are performed by anFPGA. Near to the detector there will be also High Voltage Power Supplies.The control electronics is a box that can be placed also at a certain distance from the focal plane consists of: • DC/DC converters to provide stabilized low voltages • A DSP processor • The Mass Memory • the housekeeping conditionerhe DPU of the control electronics programs the back-end electronics, receives the packets of events and organizesfor telemetry or, alternatively, for storage in the mass memory. Since the amount of data produced by thepolarimeter is high we are selecting the processor with the requirement that it is capable to perform on-boardthe analysis of the tracks.Operating modes foreseen are: • Electric calibration mode of pedestals (filter A = door closed) • Electric calibration mode by test pulse (filter A = door closed) • Calibration with radioactive source Fe55 (filter E) • Calibration with radioactive source mixed (filter F)The science modes are: • Normal (Filter B = all open ). No post-processing Diaphragm • Diaphragm (Filter D = f.o.v partially covered). No post-processing • High rate (Filter B = all open ). Post-processing • Extremely high rate (Filter C = all field attenuated) Post-processingAll these science operative modes are the same from the point of view of detectors and FEE configuration,time tagging, A/D conversion and zero suppression. They differ for the strategy to avoid overwhelming the massmemory in case bright sources are observed. In the normal mode data after zero suppression, the track image,are stored to the Mass Memory to be further forwarded to telemetry. In high rate, when the XPOL observation isover and the telescope is allocated to another instrument, data are recovered from the Mass Memory, compressedby DPU with onboard analysis of polarization (position, time and angle) and stored again in Mass Memory. Incase the target source is faint and another stronger source is present in the field of view, the latter can be removedby the use of a diaphragm: this is the diaphragm mode. In case of an extremely bright source, that could exceedthe capability of data handling, all the field will be attenuated with a filter C.The XPOL MM is dimensioned (16 GByte) to store data from a 5000 s observation of a 1 Crab source.The same function could be performed on the P/L Mass Memory provided that it is made available for thetime needed for post-processing. The processing time will normally be ¡20 times the acquisition time. Thedata flow for a very bright source could arrive to ∼
28 Mbit/s for a total memory occupation of 16GB.Theprocessing to compress data could take ∼ ∼
3. SCIENTIFIC PERFORMANCES
The science rationale of XEUS is built on three major topics: • Co-evolution of galaxies and their supermassive black holes • Evolution of large scale structure and nucleosynthesis • Matter under extreme conditions igure 3. The X-ray image of the M87 jet by Chandra. XPOL can measure the polarization of the brightest knot downto 5%
Moreover XEUS will be open to the community to face many other scientific items as an observatory. Polarimetrywill mainly contribute to the study of matter in extreme conditions. The main contribution of XEUS will be inthe study of matter under extreme conditions.One of the most interesting target for XEUS is the study of the effects of strong gravitation fields on theradiation, as predicted by General Relativity. The matter accreting on a compact object (Neutron Star or BlackHole) is organized in an accretion disk. Due to the high asymmetry of such a system the radiation emitted orscattered by the disk will have a certain degree of polarization. In 1960 Chandrasekhar had computed that thethomson scattering in an infinitely flat cloud will produce a polarization parallel to the major axis of the projectionof the diskin the sky. It will never exceed the limit of 11.7 %. Later Sunyaev and Titarchuk demonstrated thatif the X-ray emission from an accretion disk is produced by the Comptonization of low frequency radiation, avery high degree of polarization can be reached for the hard radiation. Polarization can be negative (parallel tothe disk axis) or positive (perpendicular to the disk axis). In any case, for reasons of symmetry the photons willbe polarized perpendicular or parallel to the disk. But in the path to the observer the radiation will experiencethe strong gravitational field from the central object. If this is a Black Hole the effect in the observer frame willbe observed as a rotation of the polarization angle. Stark Connors and Piran
7, 8 computed the effect for the caseof a galactic black hole in a binary system. Since the photons of higher energy are emitted close to the BH, therotation will be more effective at higher energies. This effect of rotation of polarization angle with energy is aunique signature of the presence of a Black Hole. Moreover the dependence of polarization amount and angleon energy will be different for Kerr and Schwartschild black holes. This is one of the most powerful probes ofgravitation near the BH horizon. The capabilities of XPOL to perform such a test on Cyg X-1 have been shownby Bellazzini et al. Another hot topics of high energy astrophysics is the structure and physics of jets. These are mainly observedby radio telescopes that provide both high resolution imaging and polarimetry. But in order to study the structureof regions of freshly accelerated electrons and improve the insight on the acceleration mechanisms themselves theX-ray imaging are a fundamental tool. Likely X-ray polarimetry will single out the formation of plasmoids bytime resolved polarimetry of the central object, but XEUS, with its large collecting area and with its excellentangular resolution will give the opportunity to perform angular resolved polarimetry of knots of brighter jets.In Fig. 3 we show the X-ray structure of M87 as detected by Chandra. With an observation of 10 s XPOLcan measure the polarization of knot A down to the level of 5%. It is also apparent that the angular resolutionof 5 arcseconds is essential for such a measurement. Also the very faint knot of the galactic micro-quasar XTEJ1550-564 can be observed by XPOL with a Minimum Detectable Polarization of 14%.Last but not least we want to mention the capability of XPOL to test theories of Quantum Gravity. The socalled Loop Quantum Gravity predicts that at very long timescale a violation of Lorentz invariance occurs. Thetwo states of circular polarization have a different velocity and this difference increases with energy. Since also thewave-number is proportional to the energy of the radiation, the total effect is a rotation of the polarization planewith the distance and with the square of the energy. The amount of this effect of birefringence is unknown igure 4. The very faint jet of the micro-quasar J1550-564. The Minimum Detectable Polarization for one day observationwith XPOL is 14%igure 5. The minimum variation of the polarization angle of blazar 1ES1101-232 (z=0.186) detectable by XPOL in oneday at 3 σ level. nd only upper limits are there. But since this is one of the few ways to derive experimental information aboutQG theories, the continuous search for more sensitive measurements (even of upper limits) is a worthwhile task.X-ray is the highest energy band where sensitive polarimetry of sources at cosmologic distances can be performed.We assume that Blazars are good candidates to have a high degree of polarization with angle independent onthe energy (at least within a decade). If this hypothesis is verified on nearby blazars (in the synchrotron regime)we move to far-away blazars and search for a rotation of polarization angle with the energy proportional tothe distance. In Fig. 5 we show that XPOL is capable to reject at 3 σ the hypothesis of constant angle with a100ks observation of blazar 1ES1101-232 if the coupling constant is 1 × − . This would improve of 5 orders ofmagnitude the previous upper limit. Figure 6. The Minimum Detectable Polarization as a function of observing time for a few representative sources
4. CONCLUSIONS
XPOL aboard XEUS is capable to perform some measurements which are a significant step forward in HighEnergy Astrophysics and that cannot be performed by none of the various lower profile proposed missions. InFig. 6 we show the time needed to achieve a certain level of Minimum Detectable Polarization with XPOL.XEUS is unique mainly under two respects: • The collecting area two orders of magnitude larger than any dedicated mission • The angular resolution of few arcseconds that derives from the long focal length. With such a length alsothe blurring due to the finite thickness of the detector is not very effective.How would it compare with pathfinder missions? XEUS could reasonably dedicate to polarimetry only a fractionof its time (let us say 1/10) while a pathfinder could perform full time polarimetry. The step in surface to havea drastic improvement with respect to pathfinders is of two orders of magnitude.(namely the area should be of ≈ m ). Both these parameters are subject to a potential reduction in the frame of design trade-off in ordero decrease costs or weights or, simply, to cope the performance of the actual optics technology. A decrease ofcollecting surface R s results in a proportional increase of the observing time: t −→ t/ R s . Or In a reduction ofMDP as R / s for the same observing time e.g. this would result in: • a reduced sample of AGN, with a poorer coverage of parameter space • a significant loss of sensitivity to variability of polarization angle with time (namely on testing stronggravity in extragalactic BHs).On the other side, since the source will still exceed the background a relaxation of the angular resolutionwould not impact on polarimetric sensitivity. but Would miss a few topical targets that only XEUS can do. Themost important: polarimetry of all details of the Crab and of other Pulsar Wind Nebulae, polarimetry of jets(galactic and extragalactic), polarimetry of bright knots of shell-like SNR, fast variability of polarization angle,due to General Relativity effects, in AGN. Acknowledgments
The authors acknowledges financial support from Agenzia Spaziale Italiana (ASI).
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