Over the past week, NuSTAR observed the X-ray binary GX 13+1 in coordination with an observation by NASA’s Imaging X-ray Polarimetry Explorer (IXPE). This is a peculiar star system in which a neutron star accretes matter from a companion having mass smaller than the Sun through a disk of plasma. X-ray emission from this system originates near the neutron star, where the accretion flow of material terminates in "boundary/spreading" layers, impacting on the surface of the neutron star. A wind of gas above the accretion disk is also sometimes observed in this system forming a cloud of high energy electrons in an extended corona above the disk. The nature and the geometry of such an extended accretion disk corona are not yet fully understood. Previous IXPE observations of GX 13+1 have provided measurements of the corona during periodic dipping periods in which the total X-ray flux is lower when the central region is obscured by the accretion stream and the X-ray emission is then dominated by the contribution from an extended corona. The simultaneous NuSTAR observation of GX 13+1 last week was crucial to obtain high-resolution observations at energies above 6 keV, outside the capabilities of IXPE. This was a missing piece from two previous IXPE observations, and will provide a more accurate probe into the accretion disk and corona geometry.

Authors: Alessandro Di Marco (Researcher, National Institute for Astrophysics, Italy)