NICER / ISS Science Nugget
for March 6, 2025

Horizontal or vertical? Yes!

A common feature in accreting systems, especially binaries in which a black hole captures matter from a low-mass companion star, is a hot electron plasma usually designated a "corona." The shape and location - even the number - of coronal regions have been the subject of considerable debate, but there is no doubt that the corona plays a central role in defining the observed X-ray properties of these systems. Other contributors to the overall X-ray emission are (1) a thermal component from the accretion disk, i.e., matter spiraling onto the black hole; (2) a "reflection" component of coronal X-rays irradiating the accretion disk; and (3) synchrotron emission thought to arise in a compact jet. The relative importance of the components varies with time, so that different states across an accretion outburst are evident: in the hard state, the corona dominates as the disk truncates at relatively large radii from the black hole, while in the soft state, the accretion disk extends all the way to the black hole and produces most of the X-ray emission. Transitions, such as the hard-intermediate and soft-intermediate states, can also be identified.

In recent years, a number of studies - most using NICER measurements of rapid brightness and spectroscopic variability - have shown that the size of the corona likely changes throughout an outburst, with several models proposed to explain this evolution based on different assumed geometries: a spherical corona, a corona "lamppost" illuminating the disk, a corona outflow, propagation of mass accretion-rate fluctuations, and instabilities in the disk flow, among others. A new model, called VKOMPTH, invokes two distinct but physically coupled coronal regions. A peer-reviewed paper by K. Alabarta (New York Univ. Abu Dhabi) and collaborators, recently published in The Astrophysical Journal, applies this model to NICER data of the full 2019 outburst, as well as a subsequent "re-flare," of the black-hole binary MAXI J1348-630.

The accreting object within MAXI J1348 was identified as a black hole from its evolution through the different spectral states and from the detection of the three types of quasi-periodic brightness oscillations (A-, B-, and C-type QPOs) typical of black-hole systems; the different types are distinguished by their frequencies and their relationships to less well-defined, noisy fluctuations in brightness. Prior work by the same team had established some constraints on coronal geometry based on type-A and type-B QPOs in the soft state, finding a small corona within the inner disk radius and a vertically-extended corona that could be related to the jet detected in the system. The new paper studied type-C QPOs, at frequencies between 0.4 and 3 Hz, wherever they were found in the remaining accretion states - hard during the outburst rise and most of the re-flare, intermediate during the initial decay - completing a picture of the evolution of coronal geometry. Alabarta et al. find a two-component corona in the hard state: a small one (~ 760 km) in the inner parts of the system, as in the other cases, and a large (~30,000 km) corona extended horizontally, along the plane of the accretion disk. When the source begins transitioning to the soft state, the large corona becomes vertically extended, as the previous studies found. Unlike most other corona models, this result agrees with recent findings from measurements of X-ray polarization with NASA's IXPE mission. Together, polarization and spectral-timing methods promise to resolve the persistently mysterious geometry of accretion coronae.