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When reading about hardness ratio, it shows that depending on the hardness ratio spectral states of a binary star can be divided into soft, hard states. So how the HR value changes with respect to the spectral state? And when telling about hard state and soft state, what do we mean by this statement.

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  • $\begingroup$ Hardness ratio is the way to measure color index in high-energy photon astronomy. In this case, the hardness ratio = flux_from_high_energy_photon / flux_from_lower_energy. HR > 1 implies more fluxes coming out at higher energy. If you assume a spectral shape and renormalize it with the observed fluxes at two different energy levels, you will also get a low-order spectral profile. $\endgroup$ Jan 29, 2021 at 19:22

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