What is a hard spectral state vs. a soft spectral state? In X-ray astronomy, the source is considered to be in the hard or soft spectral state. So what is the meaning of the hard spectral state? What are the soft state and hard state in spectroscopy?
Active galaxies are known to change state as seen by a change in slope of their X-ray and gamma-ray spectra.  We say that a spectrum has become harder (or changed to its hard state) when the slope changes so that there are relatively more high energy photons, and it becomes softer when the ratio of low energy photons to high energy photons increases.  The physics of why they change state is not yet understood.
If the high energy spectrum is in a thermally dominated state, ie a Planck-like spectrum, then it is certainly soft.  If it is in a steep powerlaw dominated state with low  $\alpha &lt; 1, I_\nu = f^{-\alpha}$, then it is still usually considered soft. But if $\alpha$ is higher, then it is hard.   But, it is also a relative term, so a system cycling between any two different values of $\alpha$ will be going between its hard state and its low state.