Spectral states in Be/X-ray pulsars

In the last quarter of a century, a unified characterization of the spectral evolution of low-mass X-ray binaries, both containing a neutron star and a black hole, was possible. In this context, the notion of source states characterizing the X-ray emission from black-hole binaries and neutron-star low-mass X-ray binaries revealed to be a very useful tool to disentangle the complex spectral and aperiodic phenomenology displayed by those classes of accreting objects. Be/X-ray binaries constitute another major class of transient accreting binaries, for which very little work has been done on the correlated timing and spectral variability. Especially, no definition of source states exists for this class, in spite of their highly variable X-ray emission. When active, Be/X-ray binaries are among the brightest objects in the X-ray sky and are characterized by dramatic variability in brightness on timescales ranging from seconds to years. It is then worth it to ask whether a definition of spectral states is possible for these systems. In this work, we try to address such a question, investigating whether accreting X-ray pulsars display source states and characterizing those states through their spectral properties. Our results show that Be/X-ray pulsars trace two different branches in their hardness-intensity diagram: the horizontal branch, a low-intensity state, and the diagonal branch, a high-intensity state that only appears when the X-ray luminosity exceeds a critical limit. We propose that the two branches are the phenomenological signature of two different accretion modes -- in agreement with recently proposed models -- depending on whether the luminosity of the source is above or below a critical value.