XMM-Newton and NuSTAR quasi-simultaneous follow-up of the very high energy flare of PKS~1725+123 in August 2025
Abstract
We report the results of the spectral timing analysis of the quasi-simultaneous X-ray follow-up with XMM-Newton (50 ks) and NuSTAR (180 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 ∼3 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 (ΓX>2) and a hard (ΓX<2) power law. While the soft component dominates at E<2 keV, the hard component dominates at E>2 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 (∼14 hours) we witness the shift of the synchrotron component out of the X-ray band. In the following ∼ 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.
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BibTeX
@misc{marcotulli2026xmmnewton,
title = {{XMM-Newton and NuSTAR quasi-simultaneous follow-up of the very high energy flare of PKS~1725+123 in August 2025}},
author = {L. Marcotulli and A. Gokus and L. Heckmann and E. Visentin and I. Jaroschewski and J. Otero-Santos and M. Böttcher and M. Zacharias and R. Takeishi and S. Nozaki and Y. Suda and I. Sadeh and P. Boorman and E. de Oña Wilhelmi and D. Berge},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.03829},
eprint = {2611.03829},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.03829},
}
RIS
TY - UNPB
TI - XMM-Newton and NuSTAR quasi-simultaneous follow-up of the very high energy flare of PKS~1725+123 in August 2025
AU - L. Marcotulli
AU - A. Gokus
AU - L. Heckmann
AU - E. Visentin
AU - I. Jaroschewski
AU - J. Otero-Santos
AU - M. Böttcher
AU - M. Zacharias
AU - R. Takeishi
AU - S. Nozaki
AU - Y. Suda
AU - I. Sadeh
AU - P. Boorman
AU - E. de Oña Wilhelmi
AU - D. Berge
PY - 2026
DA - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2611.03829
UR - https://arxiv.org/abs/2611.03829
AB - We report the results of the spectral timing analysis of the quasi-simultaneous X-ray follow-up with XMM-Newton (50 ks) and NuSTAR (180 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 ∼3 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_X>2) and a hard (Γ_X<2) power law. While the soft component dominates at E<2 keV, the hard component dominates at E>2 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 (∼14 hours) we witness the shift of the synchrotron component out of the X-ray band. In the following ∼ 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.
ER -