PoGOLite Collaboration Home Page

Member Institutions

Stanford U., U. Hawaii (US); Royal Inst. of Tech., Stockholm U. (Sweden), Tokyo Inst. of Tech., Hiroshima U., ISAS, Yamagata U. (Japan); Ecole Polytechnique (France)

News (January 2009)                        

A) PoGOLite polarimeter for the path finder flight assemble in November 2008. Click here for a larger image.

B) PoGOLite pressure vessel for the path finder flight assembled in November 2008. Click here for a larger image.

C) 19-PDC assembly tested a polarized gamma-ray beam at KEK in February 2008.  Cliclk here for a larger image.

D) A prototype gondola to test Attitude Control System designed in May.  Cliclk here for a larger image.

A) B) C)   D)

Scientific Objectives

Polarized gamma-ray emission is expected from a wide variety of astrophysical sources including rotation-powered pulsars, accreting black-holes and neutron stars, and jet-dominated active galaxies. Polarization measurements provide a powerful probe into the gamma-ray emission mechanism and the distribution of magnetic and radiation fields, as well as the distribution of matter, around these sources. We plan to observe northern sky sources including the Crab Nebula, Crab Pulsar, Cygnus X-1, and Hercules X-1. The observation will bring valuable information for understanding the pulsar emission mechanism, particle acceleration in the pulsar wind nebulae, the geometry of reprocessing material around the black hole, and photon propagation in the strongly magnetized neutron star surface, respectively. 

Key science issues PoGOLite will address in its initial balloon flights with the 61-PDC PoGOLite and the 217-PDC PoGOLite    

Crab Nebula: Polarization position angle and emission mechanism in the torus (or torii). 

             Position angle to be measured to ~4.7 deg in the Path Finder Flight with 61 PDCs and  ~2.4 deg in a 6-hour flight with the 217-PDC PoGOLite.                 

Crab Pulsar: Determine the pulsar emission  mechanism and determine the emission site in the magnetosphere.

             We will need the 217-PDC PoGOLite for this study.

Cygnus X-1: Determine the geometry around the black-hole and acceretion disk arount it

             Polarization of 10% level in the hard state will be detected in the Path Finder Flight with 61 PDCs.

             To study the soft state the 217-PDC PoGOLite will be needed. 

Hercules X-1: Study the surface of this highly magnetized neutron star and the cyclotron resonance

             absorption feature.

Publications

Overview paper (Kamae et al. 2008 Astroparticle Physics 30, 72) describes more about PoGOLite science capability.

Meas. of energy dependent polariz with PoGOLite (simulation) (Axelsson et al. 2007 Astroparticle Physics 2007 28, 327)

Results from 2005 beam test at KEK-PF (Kanai et al. 2007 Nucl. Instr. Meth. A 570, 158)

Low energy response of PoGO (Kataoka et al. 2005 SPIE Vol. 5898, 5898)

Results from 2004 beam test at ANL-APS (Mizuno et al. 2005 Nucl. Instr. Meth. A 540, 158)

Last modified on March 20, 2009 by T. Kamae

Instrument Description

The PoGOLite instrument is based on the well-type phoswich detector technology that has proven to be very effective in reducing large background with the built-in active collimator through the WELCOME balloon experiment and the Suzaku Hard X-ray Detector. The instrument uses Compton scattering and photo-absorption in a closely packed array of 61 PDCS (the "Path finder flight") or              217 phoswich detector cells made of plastic and BGO scintillators.  While each unit (shown in the right figure) is capable of selecting gamma-rays coming from a narrow aperture (field-of-view ~ 1.25 msr), additional shields for neutrons and gamma-rays protect the closely packed array.

The figure on the right-hand side shows the unit Phoswich Detector Cell (PDC) and the slow plastic scintillator tube (active cosmic-ray/gamma-ray shield), the fast plastic scintillator (Compton scattering and photo-absorption detector) and the bottom BGO (active gamma-ray/cosmic-ray shield). The closely packed array of either 61 or 217 phoswich detector cells is surrounded by Side Anticoincidence Shield (SAS) made of BGO crystals as shown in the figure below. A blanket of polyethylene (~15 cm thick) surrounds SAS as shown in the figure below in yellow. The combination of the BGO and polyethylene shields reduces the background events produced by albedo neutrons, albedo gamma-rays and cosmic-rays to a level equivalent to the soft gamma-ray flux from a 100mCrab astronomical source between 25 and 80 keV. The design is optimized for polarization measurement on point-like sources. Any potential bias to the polarization measurement will be reduced and corrected by rotating the instrument around its axis as well as by monitoring the cosmic-ray distribution with the BGO shileds. The design shown below (left) implements such rotation mechnism around the polarimeter telescope axis. The flight design is under development now.

Shown below (right) is the base plate and the SAS units for the Path Finder Flight with 61 PDCs for the scheduled in 2010; the 217 PoGOLite design with the neutron shield (polyethylene), 217 PDCs, Side Anti Shield (BGO), PMTs, and rotation mechanism.

Several prototypes (7-PDC and 19-PDC configurations) of the flight instrument equipped with the flight front-end electronics (shown below) have been tested in polarized gamma-ray beams, a background proton beam and combination of gamma-rays from Am-241, electrons from Sr-91, and accelerator protons. Details are described in our journal publication in 2005, 2007a, 2007b.

Since 2007 Fall, we began production of the 61-unit Path Finder Model (all components to be implemented including the attitude control system). The Base Plate for the Model is shown above. The flight analog and digital electronics boards have been produced for the Path Finder Model as shown in the photo below. 

Neutron background has also been studied using neutron sources  in Japan and in Sweden. Our simulation of neutron background events including detailed nuclear processes in BGO and plastic scintillators, neutron scattering and the scintillation light quenching have been validated through these tests. We found that our waveform sampling at 36MHz and fast electronics can discriminate neutron-induced background using the recorded pulse waveform as shown in the plot below. This feature will allow to reduce the neutron background further if needed. The overview of the PoGOLite instrument given in the publication 2008a. 

Preliminary results from the 2008 KEK beam test on a 19-PDC prototype with flight electronics results are shown below. Shown in the figure are the modulation for coincidence between PDC pairs forming nearest neighbors (black),   the second nearest neighbors (red) and the third nearest neighbors (green).

Through earlier tests and the 2008 test, as well as computer simulations, the 61-PDC PoGOLite (Path Finder Model)  is predicted to detect 10% polarization in the Crab Nebula. The 217-PDC PoGOLite will detect 10% polarization in a 200 mCrab soft gamma-ray emission in a 6 hour observation. 

Several persistent soft gamma-ray sources of prime astronomical interest as well as several target-of-opportunity flaring objects are observable from New Mexico/Texas in the USA and from Kiruna in Sweden.         

The goals of the PoGOLite project are:

• Complete the 61 PDCs and the SAS BGO modules for the Path Finder Flight and integrate to a polarimeter assembly in 2008.
• Test the attitude control sysytem, star tracker and ground support system in 2009.
• Complete the Path Finder Flight and observe the Crab Nebula and Cygnus X-1 in 2010.   
• Complete the 217-PDC PoGOLite in 2012.
• Long-duration flights in North America, Sweden or Australia with the 217-PDC PoGOLIte. 

Last modified on January 6, 2009 by T. Kamae

Gondola and Attitude Control System

The PoGOLite pointing/attitude control system and gondola design have been adopted from the High Energy Focusing Telescope (HEFT) experiment which has successfully completed two balloon flights. The BLAST collaboration has also kindly provided us with its star tracker desin. We have assemble a first star tracker with a short bafflle, image-intensified CCD camera and Nikon 200mm f2 lens as shown in the photos below.

The design of the Azimuth Fly Wheel and Decoupler assembly has been completed and production is in progress. The system will be tested in May 2008. A Python based program has been developed to simulate the attitude control  system with expected turbulance.

A conceptual design of the gondala and attitude control system is shown on the right-hand side. A prototype gondala (shown at the top of this page) will be completed in June 2008 and the attitude control system and strat tracker will be tested on it. 

Last modified on Nov 6, 2008 by T. Kamae

Publications and presentations

Protected contents

• Subsystems
• Meeting minutes
• Simulated event archive
• Beam test data