1. X-ray Polarimeter - POLIX: Design and development status
Rishin P.V., Gopala Krishna M.R., Biswajit Paul,
Varun B., Duraichelvan R., Mohamed Ibrahim, Rajagopala G., Sandhya P., Mamatha T.S., Ezhilarasi M.S., Nagaraja H.N.
Raman Research Institute, Bangalore, India
Abstract
POLIX is an X-ray polarimeter being developed at Raman Research Institute (RRI) for a small satellite mission of ISRO. The instrument is based on Thomson scattering of X-ray photons from a low atomic mass scatterer and subsequent detection in X-ray proportional counters. It works in energy band of 5-30 keV. It will be suitable for X-ray polarisation measurement in about 50 brightest X-ray sources, with a Minimum Detectable Polarisation (MDP) of 2-3%. The accretion powered X-ray pulsars, black hole X-ray binaries, rotation powered pulsars, non-thermal SNR and AGNs will be the prime targets for this mission. A laboratory model of the instrument has been made and tested successfully, an engineering model has been designed and fabrication of the same is in the final stages. Tests towards engineering model qualification are in progress. We present different aspects of the instrument design, sensitivity for polarisation measurement, current status of development and qualification test results.
Satellite requirements
The Engineering model
The fabrication of the four engineering model detectors is completed. Two of the detectors have been wired, assembled and tested. Long term stability tests is in progress. The fabrication of the collimator is complete. The unit has been machined out of a single block of Al 6061-T6 using EDM Wire-cutting technology and sand-blasted for surface cleaning. The complete assembly, including four detectors and the collimator is shown below.
Description
Value
Orbit
Low Inclination equatorial orbit
of altitude km
Pointing requirements
Pointing accuracy of 0.1 deg
Pointed along the viewing axis for long duration varying from a few days to a few weeks
Capability to point in any direction towards the sky with sun avoidance angle of 30°
Spin
Payload spin of rpm along roll axis
Instrument Configuration
The mechanical configuration of the polarimeter consists of X-ray detectors, placed on all sides of a scattering element. X-rays from cosmic sources are allowed to fall on this scatterer through a collimator which restricts the field of view of the instrument. The total configuration will be rotated about the viewing axis.
Introduction
X-ray polarimetry is an unexplored area in high energy astrophysics. In the whole history of X-ray astronomy, there was only one experiment for X-ray polarisation measurement and Crab nebula is the only X-ray source for which a definite polarisation measurement exists1. Some key scientific issues in high energy astrophysics require input from polarisation measurement2,3,4. Hence there is a strong need for X-ray polarisation measurement.  
A Thomson polarimeter working in the energy band 5-30keV is being developed at Raman Research Institute (RRI)5. A laboratory model has been made and tested successfully, an engineering model has been designed and fabrication of the same is in the final stages. This instrument will complement the soft X-ray polarisation measurements with photo-electron and Bragg reflection polarimeters and the hard X-ray polarisation measurements to be obtained with Compton polarimeters and allow study of polarisation characteristics of X-ray sources over a wide energy band.
Calibration and Space Qualification
Vibration tests on the detector at ISRO qualification levels is planned and a jig for holding the detector on the vibration table has been fabricated (shown in the left panel of the figure below). In order measure the actual collimator response, a collimator calibration jig has been fabricated (see the right panel), which can scan the collimator in front of a X-ray source at different angles.
Scientific objectives
Emission of X-rays involving magnetic field, scattering or beaming can give rise to linear polarisation. Polarisation measurement gives insight about the strength and the distribution of magnetic field in the source, geometric anisotropies in the source and their alignment with respect to the line of sight, as well as the nature of the accelerator responsible for energizing the electrons taking part in radiation and scattering. This experiment will be suitable for X-ray polarisation measurement of hard X-ray sources like accretion powered pulsars, black hole candidates in low-hard states, etc. For about 50 brightest X-ray sources a Minimum Detectable Polarisation (MDP) of 2-3% will be achieved.
Courtesy: NASA GSFC
Accretion disk around a black hole
Current status
The figure below shows the overall status of the development of the X-ray Polarimeter.
Courtesy: ESA
X-ray pulsar in a binary system
Principle of operation
The instrument is based on anisotropic Thomson scattering of X-ray photons X-rays from the source are made to undergo Thomson scattering and the intensity distribution of the scattered photons is measured as a function of azimuthal angle as shown in the left panel of the figure below. Polarised X-rays will produce an azimuthal modulation in the count rate as opposed to uniform azimuthal distribution of count rate for unpolarised X-rays. The test results from a laboratory unit are shown in the right panel. The total configuration will be rotated about the viewing axis.
In order to compensate for inaccuracy in satellite pointing and to attain constant effective area, a collimator with a flat topped response as shown in figure below is required.
Polarised
FOV of 3 deg x 3 deg
Hexagonal tapered slots
Flat top response of 0.2 deg
Unpolarised
References
Instrument Specifications
The table below describes the specifications of the Thomson X-ray polarimeter.
To minimize the effect of photoelectric absorption of photons in the scattering element, a low atomic mass scatterer (lithium/beryllium) is preferred. Monte Carlo simulations have been carried out to optimize the scatterer geometry so as to achieve best scattering efficiency6. Figure below shows the different scatterer geometries in consideration.
[1] Weisskopf, M. C. et al., “Measurement of the X-ray Polarization of the Crab Nebula”, 1976, ApJ, 208, L125
[2] Weisskopf, M. C. et al., “X-Ray Polarimetry and Its Potential Use for Understanding Neutron Stars”, 2009, Astrophysics and Space Science Library, 357, 589
[3] Kallman, T., “Astrophysical motivation for X-ray polarimetry”, 2004, Advances in Space Research, 34, 2673
[4] Maitra, C. et al., “ Prospect of polarisation measurements from black hole binaries in their thermal state with a scattering polarimeter”, 2011, MNRAS, 414, 2618
[5] Rishin, P. V. et al., “Development of a Thomson X-ray Polarimeter”, 2010, in "X-ray Polarimetry: A New Window in Astrophysics", Eds. R. Bellazzini, E. Costa, G. Matt and G. Tagliaferri, Cambridge University Press, p83 (arXiv: )
[6] Vadawale, S. V. et al., “Comparative study of different scattering geometries for the proposed Indian X-ray polarization measurement experiment using Geant4”, 2010, Nuclear Instruments and Methods in Physics Research - A, 618, 182 (arXiv: )
Description
Value
Photon collection area
640 cm2, Detector area=4 x1080 cm2
Energy range
5-30keV
Field of view
33 degree with ± 0.2 degree flat topped response
Detectors
Position sensitive proportional counters
Total weight
~125kg
Overall dimension
~ 650 x 650 x 600 mm3
Power
80 Watt
Data generation rate
6 Gbits per day max
Scattering element
Beryllium/Lithium
Life time
3-5 years
Modulation factor
~40%
Sensitivity
2-3% Minimum Detectable Polarisation (MDP) for a mCrab sources with 1 mega sec exposure
Acknowledgements
We would like to thank Dhiraj K. Dedhia (TIFR), Parag B. Shah (TIFR), Vadawale S.V. (PRL), Cowsik R. (Washington University, USA) and the members of the ISAC (SAID) group for all the help and support.
Source: Vadawale et al.
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