# PolarLight: a CubeSat X-ray Polarimeter based on the Gas Pixel Detector

## TL;DR

PolarLight is a compact CubeSat X-ray polarimeter utilizing the gas pixel detector technology to enable space-based X-ray polarization measurements of bright celestial sources, demonstrating the feasibility of small satellite missions for high-sensitivity X-ray astronomy.

## Contribution

This paper introduces PolarLight, the first CubeSat-based X-ray polarimeter using GPD technology, showcasing its design, deployment, and initial testing in space.

## Key findings

- Successfully launched and tested in orbit in 2018.
- Demonstrated capability to observe bright X-ray sources like the Crab nebula.
- Provides valuable data for future X-ray polarization missions.

## Abstract

The gas pixel detector (GPD) is designed and developed for high-sensitivity astronomical X-ray polarimetry, which is a new window about to open in a few years. Due to the small mass, low power, and compact geometry of the GPD, we propose a CubeSat mission Polarimeter Light (PolarLight) to demonstrate and test the technology directly in space. There is no optics but a collimator to constrain the field of view to 2.3 degrees. Filled with pure dimethyl ether (DME) at 0.8 atm and sealed by a beryllium window of 100 micron thick, with a sensitive area of about 1.4 mm by 1.4 mm, PolarLight allows us to observe the brightest X-ray sources on the sky, with a count rate of, e.g., ~0.2 counts/s from the Crab nebula. The PolarLight is 1U in size and mounted in a 6U CubeSat, which was launched into a low Earth Sun-synchronous orbit on October 29, 2018, and is currently under test. More launches with improved designs are planned in 2019. These tests will help increase the technology readiness for future missions such as the enhanced X-ray Timing and Polarimetry (eXTP), better understand the orbital background, and may help constrain the physics with observations of the brightest objects.