Alan Garner

The Globe Orbiting Soft X-ray Polarimeter (GOSoX) is a spectropolarimeter for the soft X-ray band. GOSoX is based on the Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX∗), a NASA-funded sounding rocket payload. Like REDSoX, GOSoX consists of Wolter I X-ray optics from NASA/MSFC, critical-angle transmission (CAT) gratings made at MIT, and multilayer (ML) coated mirrors from LBNL that polarize the X-rays. Colleagues at U. Tübingen will adapt commercially available sCMOS sensors for the focal plane. The grating dispersion is matched to the lateral grading of ML mirrors set at 45°. GOSoX can measure polarization across the entire 0.2-0.4 keV band with a spectral resolution E/dE ∼100. Minimum detectable polarizations (MDPs) of 3-10% are expected for over a dozen targets in a one-year mission. The mission was selected by NASA for launch in 2030.

Conditions in a black hole outburst: The binary system V404 Cygni consists of a red giant star orbiting a black hole. In 2015, a surge of accretion by the black hole caused the surrounding plasma to brighten suddenly for the first time since 1989, briefly becoming the brightest x-ray source in the sky. Dallilar et al. combined observations from radio, infrared, optical, and x-ray telescopes taken during the outburst. They compared how fast the flux decayed at each wavelength, which allowed them to constrain the size of the emitting region, determine that the plasma within it cooled through synchrotron radiation, and measure the magnetic field around the black hole. Science , this issue p. 1299