Abstract
Abstract We present the results from the Hitomi Soft Gamma-ray Detector (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-ray photons. The Crab nebula is one of the brightest X-ray/gamma-ray sources on the sky, and the only source from which polarized X-ray photons have been detected. SGD observed the Crab nebula during the initial test observation phase of Hitomi. We performed data analysis of the SGD observation, SGD background estimation, and SGD Monte Carlo simulations, and successfully detected polarized gamma-ray emission from the Crab nebula with only about 5 ks exposure time. The obtained polarization fraction of the phase-integrated Crab emission (sum of pulsar and nebula emissions) is (22.1% ± 10.6%), and the polarization angle is ${110{^{\circ}_{.}}7}$ +${13{^{\circ}_{.}}2}$/−${13{^{\circ}_{.}}0}$ in the energy range of 60–160 keV (the errors correspond to the 1 σ 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, ${124{^{\circ}_{.}}0}$ ± ${0{^{\circ}_{.}}1}$.
Highlights
In addition to spectral, temporal, and imaging information gleaned from observations of any astrophysical sources, polarization of electromagnetic emission from those sources provides the fourth handle on understanding the radiative processes involved
The Soft Gamma-ray Detector (SGD) on board the Hitomi satellite observed the Crab nebula during the initial test observation period of Hitomi. Even though this observation was not intended for scientific analysis, the gamma-ray radiation from the Crab nebula was detected by combining careful data analysis, background estimation, and SGD Monte Carlo simulations
Polarization measurements were performed for the data obtained with SGD Compton cameras, and polarization of soft gamma-ray emission was successfully detected
Summary
Temporal, and imaging information gleaned from observations of any astrophysical sources, polarization of electromagnetic emission from those sources provides the fourth handle on understanding the radiative processes involved. One of the brightest X-ray sources on the sky, with appreciable polarization measured in the radio and optical bands, is the Crab nebula It has been detected by (probably) every orbiting X-ray astronomy mission (for a recent summary, see Hester 2008). The measurement, performed at 2.6 keV, measured polarization at roughly ∼20 ± 1% level It was some 30 years later that the INTEGRAL mission observed the Crab nebula and detected significant polarization of its hard X-ray / soft γ -ray emission (Forot et al 2008; Chauvin et al 2013). The Japanese mission Hitomi (Takahashi et al 2018), launched in 2016, included the Soft Gamma-ray Detector (SGD), an instrument sensitive in the 60–600 keV range, and capable of measuring polarization (see Tajima et al 2018) since it employs a Compton camera as a gammaray detector. The data analysis of the Crab pulsar with Hitomi’s instruments have been published (Hitomi Collaboration 2018b)
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