Re-detection and a possible time variation of soft X-ray polarization from the Crab

Abstract

The Crab nebula is so far the only celestial object with a statistically significant detection in soft X-ray polarimetry<SUP>1-4</SUP>, a window that has not been explored in astronomy since the 1970s. However, soft X-ray polarimetry is expected to be a sensitive probe of magnetic fields in high-energy astrophysical objects, including rotation-powered pulsars<SUP>5-7</SUP> and pulsar wind nebulae<SUP>8</SUP>. Here we report the re-detection of soft X-ray polarization after 40 years from the Crab nebula and pulsar with PolarLight<SUP>9</SUP>, a miniature polarimeter utilizing a novel technique<SUP>10,11</SUP> onboard a CubeSat. The polarization fraction of the Crab in the on-pulse phases was observed to decrease after a glitch of the Crab pulsar on 23 July 2019, while that of the pure nebular emission remained constant within uncertainty. The phenomenon may have lasted about 100 days. If the association between the glitch and polarization change can be confirmed with future observations, it will place strong constraints on the physical mechanism of the high-energy emission<SUP>12-14</SUP> and glitch<SUP>15-17</SUP> of pulsars.