Modeling the in-orbit background of PolarLight

Abstract

PolarLight is a gas pixel X-ray polarimeter mounted on a CubeSat, which was launched into a Sun-synchronous orbit in October 2018. We build a mass model of the whole CubeSat with the Geant4 toolkit to simulate the background induced by the cosmic X-ray background (CXB) and high energy charged particles in the orbit. The simulated energy spectra and morphologies of event images both suggest that the measured background with PolarLight is dominated by high energy electrons, with a minor contribution from protons and the CXB. The simulation reveals that, in the energy range of 2-8 keV, there are roughly 28% of the background events are caused by energy deposit from a secondary electron with an energy of a few keV, in a physical process identical to the detection of X-rays. Thus, this fraction of background cannot be discriminated from X-ray events. The background distribution is uneven on the detector plane, with an enhancement near the edges. The edge effect is because high energy electrons tend to produce long tracks, which are discarded by the readout electronics unless they have partial energy deposits near the edges. The internal background rate is expected to be around 6 x 10^-3 counts/s/cm2 in 2-8 keV if an effective particle discrimination algorithm can be applied. This indicates that the internal background should be negligible for future focusing X-ray polarimeters with a focal size in the order of mm.

arXiv commentaccepted for publication in ApJ

References in corpus (12)

• Calibration of the Rossi X-ray Timing Explorer Proportional Counter Array
• The Spectrum of Diffuse Cosmic Hard X-Rays Measured with HEAO-1
• An efficient photoelectric X-ray Polarimeter for the study of Black Holes and Neutron Stars
• The enhanced X-ray Timing and Polarimetry mission - eXTP
• Calibration of the Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat
• Cosmic-Ray Background Flux Model based on a Gamma-Ray Large-Area Space Telescope Balloon Flight Engineering Model
• Re-detection and a Possible Time Variation of Soft X-ray Polarisation from the Crab
• PolarLight: a CubeSat X-ray Polarimeter based on the Gas Pixel Detector
• A Mass Model for Estimating the Gamma Ray Background of the Burst and Transient Source Experiment
• In-orbit Operation and Performance of the CubeSat Soft X-ray Polarimeter PolarLight
• Background estimation in a wide-field background-limited instrument such as Fermi GBM
• Estimation of the detected background by the future gamma-ray transient mission CAMELOT

Cited by in corpus (2)

• X-ray polarimetry of the Crab nebula with PolarLight: polarization recovery after the glitch and a secular position angle variation
• The HaloSat and PolarLight CubeSat Missions for X-ray Astrophysics