Question: What is the  period of low X-ray emission called?
About the article "Black hole", section "X-ray binaries".

Candidate passages:
Soft X-ray emission spectroscopy is an experimental technique for determining the electronic structure of materials.
It is a form of X-ray spectroscopy. 
Centaurus X-3 (4U 1118-60) is an X-ray pulsar with a period of 4.84 seconds.
It was the first X-ray pulsar to be discovered, and the third X-ray source to be discovered in the constellation Centaurus.
The system consists of a neutron star orbiting a massive, O-type supergiant star dubbed Krzeminski's star after its discoverer.
Matter is being accreted from the star onto the neutron star, resulting in X-ray emission. 
The Galactic ridge is a region of the inner galaxy that is coincident with the galactic plane of the Milky Way.
This feature has a width of 5° latitude (b) and ±40° longitude (l) in the Galactic coordinate system.
It is an intense source of X-ray emission known as the Galactic Ridge X-ray Emission (GRXE).
Initially this emission was thought to be caused by low energy cosmic rays interacting with cold gas in the region, which heated up the gas and caused it to emit X-rays.
However, it is now thought that about 80% of the emission comes from discrete sources such as white dwarfs and stars with active coronae. 
The observed X-ray background is thought to result from, at the "soft" end (below 0.3 keV), Galactic X-ray emission (the "galactic" X-ray background), and, at the "hard" end (above 0.3keV), from a combination of many unresolved X-ray sources outside of the Milky Way (the "cosmic" X-ray background).
The galactic X-ray background is produced largely by emission from hot gas in the Local Bubble within 100 parsecs of the Sun.
Deep surveys with X-ray telescopes, such as the Chandra X-ray Observatory, have demonstrated that around 80% of the cosmic X-ray background is due to resolved extra-galactic X-ray sources, the bulk of which are unobscured ("type-1") and obscured ("type-2") active galactic nuclei (AGN). 
From the observed X-ray spectrum, combined with spectral emission results for other wavelength ranges, an astronomical model addressing the likely source of X-ray emission can be constructed.
For example, with Scorpius X-1 the X-ray spectrum steeply drops off as X-ray energy increases up to 20 keV, which is likely for a thermal-plasma mechanism.
In addition, there is no radio emission, and the visible continuum is roughly what would be expected from a hot plasma fitting the observed X-ray flux.
The plasma could be a coronal cloud of a central object or a transient plasma, where the energy source is unknown, but could be related to the idea of a close binary.
In the Crab Nebula X-ray spectrum there are three features that differ greatly from Scorpius X-1: its spectrum is much harder, its source diameter is in light-years (ly)s, not astronomical units (AU), and its radio and optical synchrotron emission are strong.
Its overall X-ray luminosity rivals the optical emission and could be that of a nonthermal plasma.
However, the Crab Nebula appears as an X-ray source that is a central freely expanding ball of dilute plasma, where the energy content is 100 times the total energy content of the large visible and radio portion, obtained from the unknown source.
The "Dividing Line" as giant stars evolve to become red giants also coincides with the Wind and Coronal Dividing Lines.
To explain the drop in X-ray emission across these dividing lines, a number of models have been proposed:  low transition region densities, leading to low emission in coronae,  high-density wind extinction of coronal emission,  only cool coronal loops become stable,  changes in a magnetic field structure to that an open topology, leading to a decrease of magnetically confined plasma, or  changes in the magnetic dynamo character, leading to the disappearance of stellar fields leaving only small-scale, turbulence-generated fields among red giants.