AbstractWe describe development of a Thomson X-ray polarimeter for a small satellite mission of the Indian Space Research Organisation (ISRO). A laboratory model was made and tested successfully and it was redesigned for a flight unit. Fabrication of an engineering model is in final stages. The instrument consists of position sensitive proportional counter detectors and works in the 5-30 keV energy band. This instrument will be suitable for X-ray polarisation measurement in about 50 hard X-ray sources. The accretion powered X-ray pulsars, black hole X-ray binaries, rotation powered pulsars and non-thermal SNR will be the prime targets for this mission. In spite of its moderate sensitivity, this experiment can give us unique opportunity to expand the field of X-ray astronomy into a hitherto unexplored dimension. This instrument will complement 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. We describe the mission specifications, its sensitivity, the mechanical and electronic design aspects of the complete system, the signal processing pipe-line and test results.

Development of a Thomson X-ray Polarimeter for a Small Satellite Mission Rishin P.V., Gopala Krishna M.R., Biswajit Paul, Duraichelvan R., Chandreyee Maitra, Ateequlla C.M.,

Rajagopala G., Ezhilarasi M.S., Sandhya P., Mamatha T.S., Nagaraja H.N.Raman Research Institute, Bangalore, India

AcknowledgementWe would like to thank Dhiraj Dedhia (TIFR), Parag Shaw (TIFR), Vadawale S.V. (PRL), Cowsik R. (Washington University, USA), and Seetha S (ISAC, ISRO) for all the timely help and support. Special thanks to the members of RRI Mechanical Engineering Services.

References[1] M. C.Weisskopf et al., “Measurement of the X-ray Polarisation of the Nebula”, ApJ, 208, L125-L128 (1976)[2] M. C.Weisskopf et al.,“The prospects for X-ray polarimetry and its potential use for understanding neutron stars”, astro-ph/0611483, (2006)[3] Kallman T., “Astrophysical motivation for X-ray polarimetry”, Advances in Space Research 34, 2673-2677 (2004)[4] Chandreyee Maitra et al., “ Prospect of polarisation measurements from black hole binaries in their thermal state with a scattering polarimeter”, arXiv:1103.2639 [astro-ph.HE], (2011)[5] Rishin, P.V. et al., “ Development of a Thomson X-ray Polarimeter”, arXiv:1009.0846v1 [astro-ph.IM], (2009)[6] Vadawale et al., “Comparative study of different scattering geometries for the proposed Indian X-ray polarization measurement experiment using Geant4”, Nuclear Instruments and Methods in Physics Research - A, 618, 182, arXiv:1003.0519v1,(2010)

For details contact : rishinpv/gkrishna/bpaul@rri.res.in

Introduction

The space X-ray astronomy experiments carried out during the last three decades have brought a few order of magnitude improvement in sensitivity for Timing, Spectroscopy and Imaging. However, in the whole history of X-ray astronomy, the Crab Nebula is the only X-ray source for which definite polarisation measurements exist1. 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 5-30 keV 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 progress. The instrument has been proposed for an independent small satellite mission of the Indian Space Research Organisation (ISRO).

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. Polarised X-ray will produce an azimuthal modulation in the count rate. The total configuration will be rotated about the viewing axis.

Instrument Specifications

Description ValuePhoton collection area 1018 cm2 Energy range 5-30 keVField of view 33 degree with ±0.2 degree flat topped response

Detectors Proportional countersTotal weight ~100 kgs.Overall dimension ~ 650 x 650 x 550 mm3 Power 50 Watt

Data generation rate 4.2 Gbits per day

Scattering element Beryllium/LithiumLife time 3-5 yearsModulation factor ~40%Sensitivity 2-3% Minimum Detectable Polarisation (MDP)

for 50 mCrab sources with 1 mega sec exposureSatellite requirements Equatorial orbit (preferred) of altitude 500-600km,

with pointing accuracy of 0.1 deg, and with spin along roll axis of about 0.5 rpm

Instrument Configuration

Signal Processing Electronics

Fabricated Prototype Collimator

Fabricated Engineering Model Detector

Integration of one engineering model detector and testing with the signal processing electronics chain is successfully completed. Position determination using charge division principle is demonstrated. Work towards fabricating the remaining detectors along with signal processing electronics is in progress.

Front-end electronics

Processing electronics

One of the engineering model detectors has been fabricated and tested. Figure shows the position determination by charge division for this detector when shined with radioactive source. Peaks in the figure show the position of the radioactive source when shined on the detector.

Results

Future Plans

The table below describes the specifications of the Thomson X-ray polarimeter. This instrument will complement 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.

We are using position sensitive proportional counters employing 12 nos. of resistive Nichrome wires split into two channels. This helps to improve the modulation factor and achieve better background rejection at the edges of the detector. The position sensitivity is achieved by charge division method as shown in figure. The processing electronics logic is implemented in Field Programmable Gate Array (FPGA).

In order to to compensate for inaccuracy in satellite pointing and to attain constant effective area, a collimator with a flat-topped response is required. For the Engineering model, we plan to use solid aluminum block with hexagonal tapered holes, machined using wire-cutting technology to achieve a flat-topped response. Figure shows both faces of the sample piece machined for the engineering model collimator.

Figure shows the configuration of the Thomson Polarimeter. Here the detector elements are flat multi-wire proportional counters placed on four sides of a disk-like scattering element made of Be or Li. The geometry of the scatterer is optimized using Geant4 simulation to get highest sensitivity6.

FE

PE

Mechanical analysis

Finite Element Analysis (FEA) has been carried out on the above design based on the Environmental Test Level Specifications (ETLS) specified by the Indian Space Research Organization (ISRO) of the small satellite platform to parametrically optimize the design for minimum mass and maximum stiffness. The natural frequency of the structure and the maximum stress/displacement developed in the model meets the qualification model requirements with ample factor of safety.

Polarimeter electronics block diagram