XMM-Newton and NuSTAR quasi-simultaneous follow-up of the very high energy flare of PKS~1725+123 in August 2025

We report the results of the spectral timing analysis of the quasi-simultaneous X-ray follow-up with XMM-Newton ($50\,\rm ks$) and NuSTAR ($180\,\rm ks$) of the very high energy flare of the blazar PKS 1725+123 ($z=0.586$) in August of 2025. The X-ray monitoring started $\sim3$ days after the announcement of the VHE flare by LST-1, MAGIC and H.E.S.S. and lasted for almost 5 days. The high sensitivity of XMM-Newton reveals two spectral components, a soft ($Γ_{\rm X}>2$) and a hard ($Γ_{\rm X}<2$) power law. While the soft component dominates at $E<2\,\rm keV$, the hard component dominates at $E>2\,\rm keV$, likely marking the transition between the synchrotron and the high-energy emission, the "valley" of this blazar's spectral energy distribution. Within the XMM-Newton pointing ($\sim14$ hours) we witness the shift of the synchrotron component out of the X-ray band. In the following $\sim 4$ days, NuSTAR further reveals the brightening by a factor of 1.5 of the hard component, following the shift of the high-energy part of the SED into the X-ray band. Overall, within a week (observed frame) of the VHE flare, PKS 1725+123 went from temporarily being an intermediate synchrotron peaked blazar to transitioning back to its `normal' state of a low synchrotron peaked one. This is the first time that the evolution of a VHE flare from a blazar jet has been followed continuously in X-rays over such an extended period and our results highlight the importance of long-term quasi-simultaneous X-ray follow-up of VHE blazar flares.