Gamma Ray Large Area Space Telescope Balloon Flight: Data Handling Overview

1. Abstract: The GLAST Balloon Flight Engineering Model (BFEM) represents one of 16 towers that constitute the Gamma-ray Large Area Space Telescope (GLAST) [1], scheduled for launch in March 2006. The prototype tower consists of a Pb/Si pair-conversion tracker (TKR), a CsI hodoscopic calorimeter (CAL), an anti-coincidence detector (ACD) and an autonomous data acquisition system (DAQ). The self-triggering capabilities and performance of the detector elements have been previously characterized using positron, photon and hadron beams. External target scintillators were placed above the instrument to act as sources of hadronic showers. This paper provides an overview of the BFEM data reduction process, from receipt of the flight data from telemetry through event reconstruction and background rejection cuts. The goals of the ground analysis presented here are to verify the functioning of the instrument and to validate the reconstruction and background-rejection scheme. Further results will be presented in a separate paper giving an overview of the BFEM program. Gamma Ray Large Area Space Telescope Balloon Flight: Data Handling Overview • E. do Couto e Silva, R. Dubois, D. Flath, I. Gable,T. Kamae, A. Kavelaars,T. Lindner, M. Ozaki, L. S. Rochester, T. Usher, K. Young,Stanford Linear Accelerator Center • T. Mizuno, Hiroshima University, Japan • M. Kuss, N. Lumb, G. Spandre, INFN-Pisa and University of Pisa, Italy • R. Hartman, H. Kelly, T. Kotani, A. Moiseev, R. Schaefer,D. J. Thompson,NASA Goddard Space Flight Center • Chekhtman, J. E. Grove, Naval Research Laboratory • D. Lauben, Stanford University • T. H. Burnett, University of Washington • on behalf of the GLAST Large Area Telescope Collaboration The BFEM does not produce images; the data from the tracker, calorimeter and ACD must be reconstructed into particle tracks, which then must be identified as photons that have converted into electron-positron pairs, or charged particle or other background. The quality of the reconstruction determines our ability to resolve sources, and our success in identifying photons and charged particles determines the level of background contaminating our sample. The analysis process is facilitated by converting the raw or simulated data into ROOT [2] format, which is well matched to the C++ reconstruction code. (LEFT) Raw (IVTE) or simulated (IRF) data are converted to a uniform ROOT format. These data are then reconstructed, yielding an output ROOT file and a smaller summary file. These result files provide the data for event selection. (RIGHT) ROOT provides for an object-oriented hierarchical data structure, well suited to our C++ programming environment. This example shows the structure of the digitized data for a single event. Particle tracks are reconstructed in two orthogonal projections in the thirteen layers of the silicon tracker using the method of the Kalman filter [3], which provides a convenient way to include the significant effects of multiple scattering in the total of ½ radiation length of lead contained in the conversion foils. It also allows us to calculate the measurement resolution of the track parameters at any point on the trajectory (in particular, the conversion point). The projections are associated by matching lengths and starting positions, when possible. The calorimeter, eight layers of ten CsI crystals in a hodoscopic arrangement, measures the energy of the showering particle, as well as the position, direction, and topological features of the showers. Several algorithms have been developed to take into account the leakage of shower energy out the back of this ten-radiation-length detector. The thickness of the detector is limited by the weight constraints of the satellite launch. A display program (implemented in ROOT) allows us to examine events in detail. To the right, we see a passing cosmic ray. Note the single track in the tracker, the energy deposit in the calorimeter along the line of the track and the ACD tile along the track trajectory. Much of the analysis described here was developed in connection with the test of a similar instrument in the particle beams at the Stanford Linear Accelerator [4], [5]. With charged particle fluxes several orders of magnitude greater than that of gamma rays, shower fluctuations in background interactions can mimic photon showers in non-negligible numbers. We have developed a set of simple and intuitive cuts to reject such events, based on our test beam experience, and also on previous experience with EGRET [6]. We first require that no ACD tile fire and that the reconstructed tracks have sufficiently high quality. We then ask that there be a downward-pointing “vee” in both views and that both tracks extrapolate to the calorimeter. About 0.3% of the triggered events survive this cut. The number of photon candidates in rough agreement with previously measured upper-atmosphere gamma-ray fluxes. Almost all of the candidates are visually consistent with being pairs produced from photons. The event to the left is a photon that converted in the tracker. Note the vee in the tracker, the energy in the calorimeter, and the absence of any signals in the ACD. Additional cuts, based on extra tracks in the tracker and on the spatial distribution of energy in the calorimeter, should help us to refine our analysis. References: [1] N. Gehrels and P. Michelson, "GLAST: the next-generation high energy gamma-ray astronomy mission," Astroparticle Physics, Vol. 11, Issue 102, pp. 277-282, 1999 [2] http://root.cern.ch [3] P. Billoir, " Track fitting with multiple scattering: a new method," Nucl. Instr. And Meth. A, vol. 225, pp. 352-366, 1984. [4] W. B. Atwood, "Beam test of Gamma ray Large Area Space Telescope components," Nucl. Instum. Meth. A, vol. 446, pp. 444-460, 2000. [5] E. do Couto e Silva et al., "Results from the beam test of the engineering model of the GLAST large area telescope,” Nuclear Inst. Meth. A, Vol 474/1, pp 19-37, 2001. [6] D. J. Thompson et al., "Energetic Gamma Ray Experiment Telescope (EGRET) for the Compton Gamma Ray Observatory," Ap. J. Sup., Vol. 86, pp. 629-656, 1993. [7] D. J. Thompson et al., " Gamma ray Large Area Space Telescope Balloon Flight Engineering Model: Overview," this conference. Summary--The data-handling sequence for the BFEM has allowed us to verify that the instrument functioned correctly and that the structures put in place provide a useful framework for analyzing the data. Many elements of this sequence will be used to process the data from the flight instrument. Some results of processing the BFEM data are discussed in another paper at this conference [7].