- Email: [email protected]
PoGOLite: Opening a new window on the universe with polarized gamma-rays
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879 www.elsevier.com/locate/nima
PoGOLite: Opening a new window on the universe with polarized gamma-rays M. Kiss, M. Pearce Physics Department, Royal Institute of Technology (KTH), Stockholm, Sweden For the PoGOLite Collaboration Available online 5 July 2007
Abstract PoGOLite (the Polarized Gamma-ray Observer, light-weight version) is a balloon-borne instrument that will measure the polarization of soft gamma-rays in the energy range 25–100 keV from various astronomical sources such as pulsars, active galactic nuclei, galactic X-ray binaries and accreting black holes. The polarization properties of such radiation can reveal important new information about the geometry, magnetic fields and the emission mechanisms of the observed sources. The first flight is scheduled for 2009. In this paper, we present the current state of the project. r 2007 Elsevier B.V. All rights reserved. PACS: 95.55.Ka; 95.55.Qf; 95.75.Hi Keywords: Polarimetry; X-rays; Gamma-rays; Well-type phoswich detectors
1. Introduction Polarimetry at optical and radio wavelengths has enormous diagnostic capabilities [1]. Similarly, much new information is expected from X-ray and gamma-ray polarimetry, e.g. on the physical state and geometry of compact objects and the intervening material. Polarimetric information (degree and direction), in addition to time variability and spectral energy distributions, doubles the parameter space that can be used to investigate emitting objects. Although the potential targets for such measurements are numerous, polarimetry in this energy range is still in its infancy. So far, only one significant polarization detection in the X-ray band has been reported, from an experiment where the polarization of 2.6 keV photons and 5.2 keV photons from the Crab nebula was studied [2,3]. The PoGOLite instrument is designed to measure 10% polarization from a 100 mCrab source in a single 6–8 h high-altitude flight. It will use coincident detection of Compton scatterings and photoelectric absorptions to measure the polarization of photons in the energy Corresponding author.
E-mail address: [email protected] (M. Kiss). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.06.082
range 25–100 keV [4]. Photons have a higher probability to scatter perpendicularly to the incident polarization vector [5]. A measurement of the modulation of the photon scattering angles in a segmented detector therefore allows the polarization of the incoming photons to be determined [6]. The instrument consists of 217 well-type Phoswich Detector Cells (PDCs) arranged in a close-packed hexagonal array, which is surrounded by a Side Anticoincidence Shield (SAS) made of BGO crystals (Fig. 1). Each PDC is 84 cm long and consists of a solid fast scintillator (20 cm), a hollow slow scintillator (60 cm) and a BGO crystal (4 cm), viewed by a single photomultiplier tube. The fast scintillator is the active detector component where the Compton scatterings and photoelectric absorptions take place, whereas the slow scintillator acts as an active collimator with a field of view of about 5 square degrees and is used to detect charged particles and photons entering the detector cell off-axis. Signals from the different components are distinguished using pulse shape discrimination [4] (Fig. 2). PDCs such as these have been used in hard X-ray balloon spectrometers [7] and satellite spectrometers [8] and have proven to be very effective for background suppression.
ARTICLE IN PRESS M. Kiss, M. Pearce / Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879
877
200
Final Energy [keV]
g
60
de
g
90
on
ot
100 d
re
te
at
Sc
ph
120
deg
ron
ect
l el
coi
Re
de
eg
90 d
eg
60 d
0 20
Fig. 1. Sketch of the PoGOLite instrument. The total length of the detector array is about 1 m. For clarity, only part of the side anticoincidence shield is shown.
Fig. 2. Using pulse shape discrimination, signals from the fast scintillator and signals from the slow scintillator and BGO crystal can be separated due to the different scintillation decay times of the materials. These are 2, 230 and 300 ns for the fast scintillator, slow scintillator and BGO crystal, respectively [4].
Since a photon deposits all its energy upon absorption, whereas only part of the energy is deposited when the photon is scattered (Fig. 3), absorption sites and scattering sites can be distinguished in the detector array. The path of photons entering the array can thus be reconstructed by comparing the relative energy deposition in the different detector cells, see ‘‘fully contained events’’ in Fig. 4. Activity in the slow scintillators, the side anticoincidence shield or the bottom BGO crystals corresponds to background events, which are discarded during data analysis. As shown in Fig. 4, photons can scatter in several different detector cells before being absorbed. Scatterings involving more than two cells can be treated by assuming that the highest energy deposition corresponds to photo-
100 Incident Photon Energy [keV]
200
Fig. 3. Compton scattering kinematics in the PoGOLite energy range. Since the energy of a recoil electron is well separated from the energy of the scattered photon in this energy range, absorption sites and scattering sites can be distinguished, even with a limited energy resolution.
Fig. 4. Simplified sketch of the detector array (not all phoswich detector cells shown, not to scale) and different types of events.
electric absorption, the second highest energy is from the dominating Compton scattering event and that the other interactions correspond to scattering events with a lowenergy deposition, i.e. events leading to only a negligible change in the direction of the photon. From simulation studies, we note that polarization can be reliably reconstructed using three-cell events [9] and that at PoGOLite energies less than 20% of the events are from photons interacting in more than three different detector cells (Fig. 5). 2. The PoGOLite prototype We assembled a 7-unit prototype instrument at the Stanford Linear Accelerator Center (SLAC) during the second half of 2005. This prototype consists of a full PDC
ARTICLE IN PRESS 878
M. Kiss, M. Pearce / Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879
Fig. 5. Integrated relative number of counts as a function of the number of hit detector cells for a simulated observation of the Crab.
(the ‘‘central unit’’) surrounded by six incomplete units with fast scintillators only (the ‘‘peripheral units’’) and has undergone extensive testing, both laboratory-based tests at SLAC [10] and accelerator-based ones at the KEK ‘‘Photon Factory’’ in Tsukuba, Japan [4]. During the tests at SLAC in 2005–2006, a polarized beam of 53.3 keV photons was directed through the hollow slow scintillator of the central unit of the prototype. Due to the polarization of the beam, some scattering angles are more probable than others. The relative number of detected counts in the peripheral units gives the distribution of azimuthal scattering angles. From this distribution, we could study the polarization properties of the beam and evaluate the performance of the instrument. The results were found to be consistent with our simulations, demonstrating that the PoGOLite concept is feasible [10]. The tests at the KEK ‘‘Photon Factory’’ in December 2005 were performed with a polarized beam of synchrotron photons. We tried several photon energies: 25 [11], 30, 50 and 70 keV. The beam was again directed through the hollow slow scintillator and into the solid fast scintillator of the central unit. During the tests, the prototype detector array was rotated 360 around the beam axis in 15 steps in order to eliminate systematic bias. The flight-version of the instrument will be rotated in a similar fashion. During the rotation and all the subsequent measurements, the polarization direction of the incident photon beam was kept constant. We examined the relative number of counts in each peripheral unit as a function of the rotation angle. For 30, 50 (Fig. 6) and 70 keV, the modulation factor (amplitude of the modulation curve divided by the average value of the curve) was found to be 0:344 0:004, 0:358 0:012 and 0:372 0:011, respectively. The polarization characteristics can subsequently be deduced from the modulation factor [4]. Simulated values obtained using the Geant4 toolkit are 0:388 0:003, 0:403 0:005 and 0:409 0:009, respectively. The values agree within 10%, thereby validating our simulation model. The small discrepancy is believed to be due to misalignment of the instrument or non-linearities in the detector units and is currently under investigation.
Fig. 6. The polarization causes a modulation in the observed relative count rates in the peripheral units as the detector array is rotated. This modulation is more pronounced, the higher the polarization degree of the incident beam is. Results seen here have been pairwise averaged for opposing peripheral detectors in order to account for the intrinsic differences between the units and sinusoidal curves have been fitted to the data points.
3. Background suppression The excellent background suppression achieved using active collimation and vetoing in the side and bottom BGO shields is one of the main features of PoGOLite. A charged particle background, an order of magnitude greater than that expected at float altitude, has been simulated in the laboratory by directly irradiating the slow scintillator of one of the detector cells with electrons from a radioactive source, 90Sr, while the fast scintillator was irradiated with 59.5 keV photons from 241Am. Using pulse shape discrimination, we could extract photoelectric absorption events in the fast scintillator (Fig. 7). During an in-flight observation, the incoming radiation is not monoenergetic and several sources of background are present. The response of the instrument to realistic spectra and the associated background levels has therefore been studied in great detail using Geant4 simulations [9,12].
ARTICLE IN PRESS M. Kiss, M. Pearce / Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879
879
and the emission mechanisms of this system. Hercules X-1 is an accretion-powered pulsar. In this case, a polarization measurement would provide information about the radiative processes in the magnetic accretion columns of the system [14]. Long-duration flights from Northern Sweden to Western Canada at constant latitude are also planned. During such flights, multiple targets can be acquired, better statistics can be obtained and time variations can be studied. PoGOLite uses a simple instrumental design, enabling it to quickly be brought from stand-by to operation. Future observations may therefore also focus on ‘‘targets of opportunity’’, e.g. transient events detected by satellite experiments such as GLAST or SWIFT or other groundbased instruments. Fig. 7. When the slow scintillator is irradiated with electrons from 90Sr, which have kinetic energies up to about 2 MeV, the number of counts in the corresponding region is approximately doubled (cf. Fig. 2), but the signal from 241Am is still visible.
4. Mission plans and outlook A 1:11 106 m3 balloon will be used to carry PoGOLite to the operating altitude, about 41 km. The first flights will be short (6 h) proof-of-principle flights and will focus on the Crab pulsar, the prime northern hemisphere target. There are currently three main models describing the highenergy emission from the Crab, each one predicting different polarization characteristics. With PoGOLite, one of these short flight will be enough to identify the correct model [12]. Other targets of great interest on the northern hemisphere are Cygnus X-1 and Hercules X-1. Cygnus X-1 is an X-ray binary system where the compact object is believed to be a black hole [13]. Polarimetry with PoGOLite will shed light on both the accretion geometry
References [1] J. Tinbergen, Astronomical Polarimetry, Cambridge University Press, Cambridge, NY, 2005. [2] M.C. Weisskopf, et al., Astro. Phys. J. 208 (1976) L125. [3] M.C. Weisskopf, et al., Astro. Phys. J. 220 (1978) L117. [4] Y. Kanai, et al., Nucl. Instr. and Meth. A 570 (2007) 61. [5] T. Mizuno, et al., Nucl. Instr. and Meth. A 540 (2005) 158. [6] F. Lei, et al., Space Sci. Rev. 82 (1997) 309. [7] T. Takahashi, et al., SPIE 1734 (1992) 2. [8] T. Kamae, et al., SPIE 2806 (1996) 314. [9] O. Engdega˚rd, Studies of energy dependent X-ray polarisation with PoGOLite—Monte Carlo simulations with Geant4, KTH Master’s Thesis, hwww.particle.kth.se/pogolitei. [10] M. Kiss, Construction and laboratory tests of the PoGO-Lite prototype, KTH Master’s Thesis, hwww.particle.kth.se/pogolitei. [11] T. Ylinen, Construction and accelerator-based tests of the PoGO-Lite prototype, KTH Master’s Thesis, hwww.particle.kth.se/pogolitei. [12] V. Andersson, et al., Proceedings of the 22nd Texas Symposium on Relativistic Astrophysics, Stanford, December 13–17, 2004. [13] D.R. Gies, C.T. Bolton, Astro. Phys. J. 304 (1986) 371. [14] P. Me´sza´ros, et al., Astro. Phys. J. 324 (1988) 1056.
Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879 www.elsevier.com/locate/nima
PoGOLite: Opening a new window on the universe with polarized gamma-rays M. Kiss, M. Pearce Physics Department, Royal Institute of Technology (KTH), Stockholm, Sweden For the PoGOLite Collaboration Available online 5 July 2007
Abstract PoGOLite (the Polarized Gamma-ray Observer, light-weight version) is a balloon-borne instrument that will measure the polarization of soft gamma-rays in the energy range 25–100 keV from various astronomical sources such as pulsars, active galactic nuclei, galactic X-ray binaries and accreting black holes. The polarization properties of such radiation can reveal important new information about the geometry, magnetic fields and the emission mechanisms of the observed sources. The first flight is scheduled for 2009. In this paper, we present the current state of the project. r 2007 Elsevier B.V. All rights reserved. PACS: 95.55.Ka; 95.55.Qf; 95.75.Hi Keywords: Polarimetry; X-rays; Gamma-rays; Well-type phoswich detectors
1. Introduction Polarimetry at optical and radio wavelengths has enormous diagnostic capabilities [1]. Similarly, much new information is expected from X-ray and gamma-ray polarimetry, e.g. on the physical state and geometry of compact objects and the intervening material. Polarimetric information (degree and direction), in addition to time variability and spectral energy distributions, doubles the parameter space that can be used to investigate emitting objects. Although the potential targets for such measurements are numerous, polarimetry in this energy range is still in its infancy. So far, only one significant polarization detection in the X-ray band has been reported, from an experiment where the polarization of 2.6 keV photons and 5.2 keV photons from the Crab nebula was studied [2,3]. The PoGOLite instrument is designed to measure 10% polarization from a 100 mCrab source in a single 6–8 h high-altitude flight. It will use coincident detection of Compton scatterings and photoelectric absorptions to measure the polarization of photons in the energy Corresponding author.
E-mail address: [email protected] (M. Kiss). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.06.082
range 25–100 keV [4]. Photons have a higher probability to scatter perpendicularly to the incident polarization vector [5]. A measurement of the modulation of the photon scattering angles in a segmented detector therefore allows the polarization of the incoming photons to be determined [6]. The instrument consists of 217 well-type Phoswich Detector Cells (PDCs) arranged in a close-packed hexagonal array, which is surrounded by a Side Anticoincidence Shield (SAS) made of BGO crystals (Fig. 1). Each PDC is 84 cm long and consists of a solid fast scintillator (20 cm), a hollow slow scintillator (60 cm) and a BGO crystal (4 cm), viewed by a single photomultiplier tube. The fast scintillator is the active detector component where the Compton scatterings and photoelectric absorptions take place, whereas the slow scintillator acts as an active collimator with a field of view of about 5 square degrees and is used to detect charged particles and photons entering the detector cell off-axis. Signals from the different components are distinguished using pulse shape discrimination [4] (Fig. 2). PDCs such as these have been used in hard X-ray balloon spectrometers [7] and satellite spectrometers [8] and have proven to be very effective for background suppression.
ARTICLE IN PRESS M. Kiss, M. Pearce / Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879
877
200
Final Energy [keV]
g
60
de
g
90
on
ot
100 d
re
te
at
Sc
ph
120
deg
ron
ect
l el
coi
Re
de
eg
90 d
eg
60 d
0 20
Fig. 1. Sketch of the PoGOLite instrument. The total length of the detector array is about 1 m. For clarity, only part of the side anticoincidence shield is shown.
Fig. 2. Using pulse shape discrimination, signals from the fast scintillator and signals from the slow scintillator and BGO crystal can be separated due to the different scintillation decay times of the materials. These are 2, 230 and 300 ns for the fast scintillator, slow scintillator and BGO crystal, respectively [4].
Since a photon deposits all its energy upon absorption, whereas only part of the energy is deposited when the photon is scattered (Fig. 3), absorption sites and scattering sites can be distinguished in the detector array. The path of photons entering the array can thus be reconstructed by comparing the relative energy deposition in the different detector cells, see ‘‘fully contained events’’ in Fig. 4. Activity in the slow scintillators, the side anticoincidence shield or the bottom BGO crystals corresponds to background events, which are discarded during data analysis. As shown in Fig. 4, photons can scatter in several different detector cells before being absorbed. Scatterings involving more than two cells can be treated by assuming that the highest energy deposition corresponds to photo-
100 Incident Photon Energy [keV]
200
Fig. 3. Compton scattering kinematics in the PoGOLite energy range. Since the energy of a recoil electron is well separated from the energy of the scattered photon in this energy range, absorption sites and scattering sites can be distinguished, even with a limited energy resolution.
Fig. 4. Simplified sketch of the detector array (not all phoswich detector cells shown, not to scale) and different types of events.
electric absorption, the second highest energy is from the dominating Compton scattering event and that the other interactions correspond to scattering events with a lowenergy deposition, i.e. events leading to only a negligible change in the direction of the photon. From simulation studies, we note that polarization can be reliably reconstructed using three-cell events [9] and that at PoGOLite energies less than 20% of the events are from photons interacting in more than three different detector cells (Fig. 5). 2. The PoGOLite prototype We assembled a 7-unit prototype instrument at the Stanford Linear Accelerator Center (SLAC) during the second half of 2005. This prototype consists of a full PDC
ARTICLE IN PRESS 878
M. Kiss, M. Pearce / Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879
Fig. 5. Integrated relative number of counts as a function of the number of hit detector cells for a simulated observation of the Crab.
(the ‘‘central unit’’) surrounded by six incomplete units with fast scintillators only (the ‘‘peripheral units’’) and has undergone extensive testing, both laboratory-based tests at SLAC [10] and accelerator-based ones at the KEK ‘‘Photon Factory’’ in Tsukuba, Japan [4]. During the tests at SLAC in 2005–2006, a polarized beam of 53.3 keV photons was directed through the hollow slow scintillator of the central unit of the prototype. Due to the polarization of the beam, some scattering angles are more probable than others. The relative number of detected counts in the peripheral units gives the distribution of azimuthal scattering angles. From this distribution, we could study the polarization properties of the beam and evaluate the performance of the instrument. The results were found to be consistent with our simulations, demonstrating that the PoGOLite concept is feasible [10]. The tests at the KEK ‘‘Photon Factory’’ in December 2005 were performed with a polarized beam of synchrotron photons. We tried several photon energies: 25 [11], 30, 50 and 70 keV. The beam was again directed through the hollow slow scintillator and into the solid fast scintillator of the central unit. During the tests, the prototype detector array was rotated 360 around the beam axis in 15 steps in order to eliminate systematic bias. The flight-version of the instrument will be rotated in a similar fashion. During the rotation and all the subsequent measurements, the polarization direction of the incident photon beam was kept constant. We examined the relative number of counts in each peripheral unit as a function of the rotation angle. For 30, 50 (Fig. 6) and 70 keV, the modulation factor (amplitude of the modulation curve divided by the average value of the curve) was found to be 0:344 0:004, 0:358 0:012 and 0:372 0:011, respectively. The polarization characteristics can subsequently be deduced from the modulation factor [4]. Simulated values obtained using the Geant4 toolkit are 0:388 0:003, 0:403 0:005 and 0:409 0:009, respectively. The values agree within 10%, thereby validating our simulation model. The small discrepancy is believed to be due to misalignment of the instrument or non-linearities in the detector units and is currently under investigation.
Fig. 6. The polarization causes a modulation in the observed relative count rates in the peripheral units as the detector array is rotated. This modulation is more pronounced, the higher the polarization degree of the incident beam is. Results seen here have been pairwise averaged for opposing peripheral detectors in order to account for the intrinsic differences between the units and sinusoidal curves have been fitted to the data points.
3. Background suppression The excellent background suppression achieved using active collimation and vetoing in the side and bottom BGO shields is one of the main features of PoGOLite. A charged particle background, an order of magnitude greater than that expected at float altitude, has been simulated in the laboratory by directly irradiating the slow scintillator of one of the detector cells with electrons from a radioactive source, 90Sr, while the fast scintillator was irradiated with 59.5 keV photons from 241Am. Using pulse shape discrimination, we could extract photoelectric absorption events in the fast scintillator (Fig. 7). During an in-flight observation, the incoming radiation is not monoenergetic and several sources of background are present. The response of the instrument to realistic spectra and the associated background levels has therefore been studied in great detail using Geant4 simulations [9,12].
ARTICLE IN PRESS M. Kiss, M. Pearce / Nuclear Instruments and Methods in Physics Research A 580 (2007) 876–879
879
and the emission mechanisms of this system. Hercules X-1 is an accretion-powered pulsar. In this case, a polarization measurement would provide information about the radiative processes in the magnetic accretion columns of the system [14]. Long-duration flights from Northern Sweden to Western Canada at constant latitude are also planned. During such flights, multiple targets can be acquired, better statistics can be obtained and time variations can be studied. PoGOLite uses a simple instrumental design, enabling it to quickly be brought from stand-by to operation. Future observations may therefore also focus on ‘‘targets of opportunity’’, e.g. transient events detected by satellite experiments such as GLAST or SWIFT or other groundbased instruments. Fig. 7. When the slow scintillator is irradiated with electrons from 90Sr, which have kinetic energies up to about 2 MeV, the number of counts in the corresponding region is approximately doubled (cf. Fig. 2), but the signal from 241Am is still visible.
4. Mission plans and outlook A 1:11 106 m3 balloon will be used to carry PoGOLite to the operating altitude, about 41 km. The first flights will be short (6 h) proof-of-principle flights and will focus on the Crab pulsar, the prime northern hemisphere target. There are currently three main models describing the highenergy emission from the Crab, each one predicting different polarization characteristics. With PoGOLite, one of these short flight will be enough to identify the correct model [12]. Other targets of great interest on the northern hemisphere are Cygnus X-1 and Hercules X-1. Cygnus X-1 is an X-ray binary system where the compact object is believed to be a black hole [13]. Polarimetry with PoGOLite will shed light on both the accretion geometry
References [1] J. Tinbergen, Astronomical Polarimetry, Cambridge University Press, Cambridge, NY, 2005. [2] M.C. Weisskopf, et al., Astro. Phys. J. 208 (1976) L125. [3] M.C. Weisskopf, et al., Astro. Phys. J. 220 (1978) L117. [4] Y. Kanai, et al., Nucl. Instr. and Meth. A 570 (2007) 61. [5] T. Mizuno, et al., Nucl. Instr. and Meth. A 540 (2005) 158. [6] F. Lei, et al., Space Sci. Rev. 82 (1997) 309. [7] T. Takahashi, et al., SPIE 1734 (1992) 2. [8] T. Kamae, et al., SPIE 2806 (1996) 314. [9] O. Engdega˚rd, Studies of energy dependent X-ray polarisation with PoGOLite—Monte Carlo simulations with Geant4, KTH Master’s Thesis, hwww.particle.kth.se/pogolitei. [10] M. Kiss, Construction and laboratory tests of the PoGO-Lite prototype, KTH Master’s Thesis, hwww.particle.kth.se/pogolitei. [11] T. Ylinen, Construction and accelerator-based tests of the PoGO-Lite prototype, KTH Master’s Thesis, hwww.particle.kth.se/pogolitei. [12] V. Andersson, et al., Proceedings of the 22nd Texas Symposium on Relativistic Astrophysics, Stanford, December 13–17, 2004. [13] D.R. Gies, C.T. Bolton, Astro. Phys. J. 304 (1986) 371. [14] P. Me´sza´ros, et al., Astro. Phys. J. 324 (1988) 1056.