{
  "abstract": "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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2611.03829",
  "author_total": 15,
  "authors": [
    {
      "name": "L. Marcotulli"
    },
    {
      "name": "A. Gokus"
    },
    {
      "name": "L. Heckmann"
    },
    {
      "name": "E. Visentin"
    },
    {
      "name": "I. Jaroschewski"
    },
    {
      "name": "J. Otero-Santos"
    },
    {
      "name": "M. Böttcher"
    },
    {
      "name": "M. Zacharias"
    },
    {
      "name": "R. Takeishi"
    },
    {
      "name": "S. Nozaki"
    },
    {
      "name": "Y. Suda"
    },
    {
      "name": "I. Sadeh"
    },
    {
      "name": "P. Boorman"
    },
    {
      "name": "E. de Oña Wilhelmi"
    },
    {
      "name": "D. Berge"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/dvrkka2mvh34k.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2611.03829",
  "fulltext_url": null,
  "id": "dvrkka2mvh34k",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2611.03829",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.HE"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2611.03829",
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-02",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/dvrkka2mvh34k.ris",
  "sources": [
    {
      "first_seen": "2026-11-04T15:21:25Z",
      "native_id": "2611.03829",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2611.03829"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "XMM-Newton and NuSTAR quasi-simultaneous follow-up of the very high energy flare of PKS~1725+123 in August 2025",
  "title_text": "XMM-Newton and NuSTAR quasi-simultaneous follow-up of the very high energy flare of PKS~1725+123 in August 2025",
  "type": "preprint",
  "updated_at": "2026-11-06T18:21:01Z",
  "url": "https://research.draconitic.com/paper/dvrkka2mvh34k",
  "venue": "arXiv",
  "versions": []
}