A Proposed Student Built and Operated Satellite: The Gamma Ray Burst Polarization Observer (PolOSat)

ABSTRACT

The Polarization Observer (PolOSat) is small satellite mission whose goal is to measure the polarization of bright gamma-ray bursts (GRBs). A precise measurement of the polarization of GRBs will constrain the models of radiative mechanisms associated with GRBs as supermassive stars undergo collapse into black holes. The primary goal of PolOSat is the detection of strongly linearly polarized GRBs (≥20 %) and/or to set upper limits on polarization for a few GRBs (≤30 %). PolOSat is designed to have a sensitivity to polarization that exceeds all prior experiments. The primary scientific instrument, the Gamma-ray Polarization Monitor (GPM) is based on a CMOS hybrid array that is optimized for performance in the low energy gamma-ray band (20-200 keV). The GPM has two passive Beryllium (Be) scattering elements which provide signal gamma-rays within a large field of view (two 45 degree radius cones). Gamma-rays impinge on the Be scatterers and are then Compton scattered into the CZT arrays and detected. A bright GRB (occurring ~5 times a year) will produce 100,000s of direct gamma-rays and 1000s of Compton scattered gamma-rays detected by the CZT array. The PolOSat satellite with the GPM is rotated (~1 Hz) inducing a strong temporal component at twice the spin frequency that is proportional to the linear polarization in the GRB signal. The team includes the University of California, Berkeley, the Kentucky Space Program including the Kentucky Science and Technology Corporation, the University of Kentucky, Morehead State University, Sonoma State University, the Rochester Institute of Technology, the University of Rochester and the Lawrence Livermore National Laboratory. PolOSat features significant participation by undergraduate and graduate students in all phases of development and operation of the spacecraft and instruments in and data analysis. PolOSat was initially proposed as a small complete NASA Mission of Opportunity and is currently seeking funding.

Funding and CreditsThis project was submitted to NASA in 2008 as a small complete NASA Mission of Opportunity. The project was not recommended for funding under NASA MO and is currently seeking other sources funding. Background image courtesy of Kentucky Space– taken by Kentucky Space Balloon-1 Edge of Space Mission July 2008. Kentucky Space EOS and suborbital missions will provide flight heritage for PolOSat subsystems.

Mission OverviewThe evolution of microelectronics has facilitated a trend toward the use of small spacecraft as science platforms, with emphasis on reducing cost and development timescales and maximizing science return. Since 2003, 17 CubeSats (picosats 1-10 kg) have flown and recently NASA had outstanding success with GeneSat (NASA 2007), a 3U CubeSat flying a DNA replication experiment. NASA plans a follow-on experiment (PharmaSat) to be launched in 2008. PolOSat is a microsat (~20kg) that leverages the successes and technology developments resulting from these picosatellite missions, and will, for a very low cost of development, launch, and operation, potentially provide insight into a fundamental question in astrophysics, namely constraining the models of radiative mechanisms associated with Gamma Ray Bursts (GRBs) as supermassive stars undergo collapse into black holes.

Simulated Detection Signals.A bright GRB (occurring ~5 times a year) will produce 100,000s of direct gamma-rays and 1000s of Compton scattered gamma-rays detected by the CZT array. The science signal is a strong component at a frequency of twice the spin frequency of PolOSat (~1 Hz). The fact that PolOSat is specifically designed to detect linear polarization gives a ten fold sensitivity advantage over systems such as RHESSI and INTEGRAL which are not optimized for polarization measurements and generate many secondary gamma-rays which form a large unwanted background. Simulations have shown that PolOSat will be able to detect >5 bright GRBs per year and will be able to detect linear polarization effects greater than 20%.

Spacecraft OverviewA view of the PolOSat Spacecraft is shown at left. The instrument (Listener and GBM) is shown in the center of the figure housed in the shield canister which is 200mm tall and 200mm in diameter. The sides of the canister are composed of three layers designed to block gamma rays and x-rays (2mm Lead, 2mm Copper, and 2mm Aluminum). End caps provide a light-tight enclosure that allows the more energetic X-rays to penetrate and instrument sensors. The view in the figure is shown with the end caps removed.

The spacecraft design exhibits symmetry around the z-axis ad xy-plane to encourage desirable gyroscopic tendencies at the high spin rates (0.5-1Hz) required for the science instrument. The top and bottom faces of the satellite slope down at 45° to an outer diameter of 450mm. Six structural ribs radiate from the canister providing a hexagonal frame for the satellite body. The 45° angle of the spacecraft body relative to the xy-plane optimize the solar power generation and internal payload volume while maintaining the profile desirable for the gyroscopic characteristics of the system. The satellite bus utilizes the CubeSat PC104 form factor in 100mm x 100mm x 50 mm “half” cube bays in each of the six facets of the satellite. The choice of this design maintains symmetry in the design to avoid balance issues during spin-up or operation on orbit. In addition, the PC-104 form factor bus module design offers many advantages and flexibility in composing a modular bus. The development time and costs are minimized through re-use and leveraging COTS subsystems. The primary spacecraft subsystems are summarized in the table below.

0000

Figure 1.0 PolOSat Exterior and Cutaway View

AuthorsBenjamin K. Malphrus1, J. G. Jernigan2, J. S. Bloom2, S. Boggs2, N. R. Butler2, L. R.

Cominsky3, T. J. Doering4, J. P. Doty5, D. M. Erb4, D. F. Figer6, K. C. Hurley2, K. W. Kimel7, J. E. Lumpp4, S. Labov,8, F. Marshall9, R. J. Twiggs10, W. C. C. Hutchison, III11, A. H. Djamshidpour12, M. B. Gailey4, S. F. Hishmeh,4, D. Pontillo6, Y. S. Yu6, Jeffrey Kruth1,

Michael Combs1 ,P. R. Eluru1

1Morehead State Univ., 2University of California Berkeley, 3Sonoma State University, 4University of Kentucky, 5Noqsi Aerospace, Ltd., 6Rochester Institute of Technology,

7Kentucky Science and Technology Corporation, 8Lawrence Livermore National Laboratory, 9University of Rochester, 10Stanford University, 11University of Louisville, 12San Jose State

University

Figure 3.0 The LLNL CZT detector module (left) and H2RG Arrays (right)

Core Student Collaboration Since PolOSat is a “small complete mission,” many of the mission sub-systems that would normally be subcontracted to companies are being designed by students working with more senior members of the PolOSat team. PolOSat therefore has a strong student component, including student investigators, student mission operations and Earth station operations through the Kentucky Satellite Enterprise (KySat), and student engineers at San Jose State University and the Rochester Institute of Technology. The KySat students will be intimately involved in the design, fabrication, testing, and operation of many of the PolOSat spacecraft subsystems, and will also develop, test and manage the dedicated Earth stations, assist with commanding the spacecraft and will perform other additional and significant mission support activities.

Scientific ObjectivesScientific interest in GRBs remains strong as the topic matures as a branch of high-energy astrophysics. NASA’s dedicated GRB mission Swift remains operational and was recently joined by GLAST (now the Fermi Gamma Ray Telescope) which also has a dedicated GRB instrument, the Fermi Burst Monitor (FBM). Following the tantalizing reports of ~80% polarization in at least one GRB (Coburn & Boggs 2006), PolOSat will undertake a systematic search for linear polarization in new GRBs. Despite the small size and low cost, PolOSat will have a sensitivity to polarization that exceeds all prior experiments.

The primary goal of the PolOSat GPM instrument is the detection of a single strongly linear polarized GRB (20 per cent or greater) and/or upper limits on polarization for a few GRBs (30 per cent or less). Either or both outcomes will help illuminate one of the major uncertainties in high-energy astrophysics: the physical origin of emission of gamma-ray bursts. As an observational benchmark close in space and time to the central engine driving the explosion, this particular type of measurement is an important clue to the nature and environment that accompanies the formation a new black hole.

PolOSat Science InstrumentsPolOSat’s two scientific instruments, the Gamma-ray Polarization Monitor (GPM) and the Listener, are based on CMOS hybrid CZT arrays that are optimized for performance in the Hard X-ray (20-70 keV) or the low energy Gamma-ray band (20-200 keV). The CZT arrays are provided by the Lawrence Livermore National Laboratory (LLNL). These sensors have been in development for over ten years and have now been packaged into a small format with operational low power electronics. The Si CMOS arrays with a photoconverter layer that are the science sensors for the Listener are optimized for the Hard X-ray band (20-70 keV).

The ListenerThe Listener supports the primary science goal of PolOSat by detecting the direct Gamma-ray emission from bright GRBs. This additional information about the light curve of the GRB allows for a small but needed correction in the analysis of the Compton scattered Gamma-rays detected by the CZT array. Hard X-rays are detected by either the up or down looking Si arrays that comprise the Listener instrument. The Listener field of view is defined by a cone with a 150 degree opening angle which is wider than the GPM field of view. Therefore any GRB that is sufficiently bright to allow a polarization measurement of the prompt Gamma-ray emission will be easily observed by the Listener. Final linear polarization analysis of the GPM data requires approximate knowledge of the location of the GRB (~10 degrees) that will be provided by the InterPlanetary Network (IPN). PolOSat will become a component of the IPN. The Listener will determine many of the basic properties of the GRBs including the temporal profile and spectrum (20-200 keV).

Conclusions and ImplicationsThe PolOSat mission has the potential to produce a significant GRB science return as a small satellite because only a modest aperture is needed to detect polarization from a bright GRB. Since we expect about 5 bright GRBs each year, only modest amounts of telemetered data are needed. Since GRBs are isotropic and occur as singular events there is no need for an expensive pointing system or precision attitude or aspect measurements. PolOSat will detect about five bright GRBs per year over a three year period of science operations. The detection of a clear linear polarized gamma-ray signal greater than 20 per cent from a single GRB will define a fully successful mission. Also if a few GRBs are shown to have upper limits of linear polarization less than 30 per cent then the PolOSat mission will be a full science success. In addition to GRB science, PolOSat will monitor a few bright X-ray pulsars, providing a luminosity versus torque history of these sources. There is also a significant chance that PolOSat could discover a new transient pulsating X-ray source during its mission life.

The GPM supports the primary science goal of PolOSat, the measurement of the linear polarization of bright GRBs. This instrument has two passive Be scattering elements with a total on axis projected aperture of 80 cm2. Gamma-rays are detected from GRBs within a large field of view (two 45 degree radius cones). Gamma-rays from GRBs impinge on the Be scatterers and are then Compton scattered into the CZT arrays and detected. Overall a few percent of the photons that undergo a Compton scatter reach the CZT arrays which are rotated normal to the viewing axis to reduce any direct detection of gammy-rays from a GRB. The direct gamma-rays are further reduced by a blocking cap made of a graded Z material. This concept is a lower cost version of the design of the collimation walls of the BAT instrument on board Swift.

Figure 2.0 Simulated polarization signal for a very bright, 5.5 s long GRB 021206 and a typical, 25s bright GRB. In the period folded plots on the left, the signal contribution from the DXRB and Earth reflected GRB are plotted in addition to the total detected flux. In the periodograms (right), the expected regions for a signal at half the spin period of 1 s are marked with red horizontal hatches.

Gamma-ray Polarization Monitor

PolOSat Mission Overview

Mission ObjectivesMeasure Polarization of GRBsMeasure Phase and Amplitude Tracking of X- ray PulsarsProvide New S/C for IPN

CharacteristicsSmall Complete Mission of OpportunityEquatorial Orbit (635 km)3 year mission life (5 year SC option)Student Built and OperatedIPN produces GRB localizations

Mission Team

PolOSat is a science mission with a focused set of goals that can be achieved in a small complete mission. The PI Dr. G. Jernigan has extensive experience in NASA high energy science investigations and was a member of the NASA’s HETE-2 team designed to investigate Gamma-ray Bursts (GRBs). The science team includes J. S. Bloom, S. Boggs, N. R. Butler and K.C. Hurley. The Kentucky Space team including Prof. B. Twiggs, the PolOSat System's engineer, and Dr. Malphrus, Dr. Lumpp, will design, fabricate, test and operate the spacecraft. Kentucky Space and B. Twiggs in particular, has extensive experience in small satellite systems including the design, fabrication and testing of all required S/C components. The CZT arrays for the GPM are provided by CoI Dr. Labov. The Si arrays are developed and manufactured by Teledyne and integrated and tested by CoI Dr. Figer. Dr. F. Marshall also serves on the science instrument development team. J. P. Doty serves as advisor to both the science and engineering teams. Prof. Cominsky will lead the E/PO effort.

Mission Team

Principal Investigator: Dr. J Garrett JerniganProject Scientist: Prof. Benjamin K. Malphrus Project Manager: Kris W. Kimel Systems Engineer: Prof. Robert TwiggsS/C Engineer: Prof. James E. Lumpp, Jr.GPM Instrument Scientist: Dr. Nathaniel R. ButlerListener Instrument Scientist: Prof. Donald F. FigerE/PO Lead Scientist: Prof. Lynn R. Cominsky

Science Team Co-Is

Prof. Joshua S. Bloom Prof. Steve Boggs Dr. John P. Doty Dr. Kevin C. Hurley Dr. Simon Labov Dr. Frederick Marshall Student Researchers Student Engineers

PolOSat SCIENCE INSTRUMENTS

GPM Science Instrument 80 cm2 aperture on-axis 20-200 keV energy range Up and Down 90° Conical FOV

GPM detects a signal at twice spin with S/N>5

Listener Science Instrument Two units – 16 cm2 each 20-70 keV energy range Up and Down 150° Conical FOV