Detection of polarized gamma-ray emission from the Crab nebula with the Hitomi Soft Gamma-ray Detector

We present the results from the Hitomi Soft Gamma-rayDetector (SGD) observation of the Crab nebula. The main part of SGD is a Compton camera, which in addition to being a spectrometer, is capable of measuring polarization of gamma-rayphotons. The Crab nebulais one of the brightest X-ray / gamma-raysources on the sky, and, the only source from which polarized X-ray photons have been detected. SGD observed the Crab nebuladuring the initial test observation phase of Hitomi. We performed the data analysis of the SGD observation, the SGD background estimation and the SGD Monte Carlo simulations, and, successfully detected polarized gamma-rayemission from the Crab nebulawith only about 5 ks exposure time. The obtained polarization fraction of the phase-integrated Crab emission (sum of pulsar and nebulaemissions) is (22.1% +/- 10.6%), and, the polarization angle is 110.degrees 7 + 13.degrees 2 /-13.degrees 0 in the energy range of 60-160 keV (The errors correspond to the 1 sigma deviation). The confidence level of the polarization detection was 99.3%. The polarization angle measured by SGD is about one sigma deviation with the projected spin axis of the pulsar, 124.degrees 0 +/- 0.degrees 1.