In such a binary system, the gravitational pull of the dense neutron star is stripping gas away from its stellar companion (a low-mass star, not shown in these images). The gas forms an accretion disk and spirals towards the neutron star.
Observations of the Rapid Burster using three X-ray space telescopes -- NASA's NuSTAR and Swift, and ESA's XMM-Newton -- have revealed what happens around the neutron star before and during a so-called "type-II" burst. These bursts are sudden, erratic and extremely intense releases of X-rays that liberate enormous amounts of energy during periods when very little emission occurs otherwise.
Before the burst, the fast-spinning magnetic field of the neutron star keeps the gas flowing from the companion star at bay, preventing it from reaching closer to the neutron star and effectively creating an inner edge at the center of the disk (Figure 1, panel 1). During this phase, only small amounts of gas leak towards the neutron star.
However, as the gas continues to flow and accumulate near this edge, it spins faster and faster (Figure 1, image 2), and eventually catches up with the spinning velocity of the magnetic field (Figure 1, images 3 and 4).
NuSTAR is a Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC Berkeley, and the official data archive is at NASA's High Energy Astrophysics Science Archive Research Center. ASI provides the mission's ground station and a mirror archive. JPL is managed by Caltech for NASA.