The nature of low-energy enhancement in the radiative strength function (RSF), which is known as the upbend resonance (UBR) and has a crucial role in the description of neutron-capture cross section and stellar nucleosynthesis, is still under debate. The present Letter extends the exact thermal pairing plus phonon damping model to explore the microscopic nature of the UBR and its thermodynamic origin over a wide mass range of odd-odd, odd-$A$, and even-even systems, from ${}^{44}\mathrm{Sc}$ to ${}^{153}\mathrm{Sm}$, whose experimental RSFs, including the UBRs, are available. The results of our calculations indicate that the UBR arises from noncollective particle-particle and hole-hole excitations with a strength three times stronger than that of the giant dipole resonance. Moreover, the emergence of the UBR at finite temperature provides additional theoretical evidence for the invalidity of the Brink-Axel hypothesis in the very low $E_{\gamma }$ region. Last, but not least, a global relation between the integrated strength of the RSF in the UBR region to that of the total RSF and the mass number is reported, for the first time, within the present study.

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• Gamma-ray strength functions
• Giant resonances
• Nuclear structure & decays
• Collective models
• Nuclear many-body theory
• Thermal & statistical models