The past few weeks of the NuSTAR program has been heavily dominated by target-of-opportunity (ToO) investigations, including observations of X-ray binaries in outburst, Galactic novae, variable stars, and magnetars. As part of this regular diet of ToOs, over the past week NuSTAR observed 4U 1630-47—a candidate black hole X-ray binary in our Galaxy—as part of a comprehensive spectroscopic campaign coordinated with the IXPE, XMM-Newton, and XRISM observatories. 4U 1630-47 is an intriguing source that exhibits variable disk wind signatures, abnormally high X-ray polarization, and multiple classes of outbursts on a quasi-annual recurrence time. NuSTAR observations are crucial for determining the temperature and distribution of the hot plasma of electrons near the black hole known as the corona, and these observations have already sampled three different states of the central engine. Meanwhile, coordinated lower-energy X-ray observations by XMM and XRISM will measure atomic transitions, allowing study of the corona's reflection off the encircling accretion disk as well as the associated wind of ionized plasma. 
 
Of particular note, NuSTAR had its first detection of 4U 1630-47 transitioning between a high-energy-dominated “heartbeat" state and a more stable low-energy-dominated state. The NuSTAR heartbeat of 4U 1630-47 appears very different from the famed heartbeat of the Galactic black hole binary GRS 1915+105, which is generally seen at lower X-ray energies. NuSTAR's unique broadband capabilities will be essential for testing whether the two oscillation modes have the same physical origin, and the multi-facility coordinated campaign will provide insight into how matter falls into stellar mass black holes in our Galaxy, and how the energy released leads to material being ejected in a powerful wind.

Author: Maxime Parra (JSPS Postdoctoral Fellow, Ehime University, Japan)