The Generalized Hardness–Intensity Diagram for Black Hole and Neutron Star X-Ray Binaries

Abstract

Abstract Over the past half-century, X-ray and radio observations of accreting neutron stars and stellar-mass black holes have yielded a rich observational picture, with common features including state transitions and jet formation, but also sharp differences. While black hole X-ray binaries overwhelmingly suppress jets in so-called soft states, accreting neutron stars are less restrictive, with a soft state wind observed in some sources to coexist with a jet. We propose an explanation for these differences that leads to a generalization of a foundational element, the hardness–intensity diagram of Fender et al. The inverse relation between jets and winds fits into a picture that connects to prograde accretion, while the possibility of counterrotation between accretion disk and compact object accounts for observed differences in accreting neutron stars. This picture comes with a surprising twist, which is that neutron stars embody the small-scale analog of FRII quasars, an idea that allows us to complete the scale-invariant picture for the jet–disk connection.