{
  "abstract": "We present an X-ray study of the high-synchrotron-peaked blazar Mrk 421 using ten days of XPoSat/XSPECT observations (2025 January 20-29; 202 ks) and contemporaneous NICER data. The 0.8-15 keV light curve has a fractional variability amplitude of 19.6 +/- 0.3 per cent, and Bayesian blocks resolve 26 flux states spanning a factor of 2.3. The best-constrained flare has a fast rise and a slow decay, with e-folding times of about 3 h and 30 h. For typical jet parameters, the X-ray-emitting electrons cool faster than this, suggesting that the decay traces a gradual weakening of their acceleration. The count-rate distribution is best described by two lognormal components, consistent with multiplicative variability. The power spectrum is a red-noise power law with a slope of about 1.7 over timescales of 25 min to 10 d, typical of blazars. XSPECT and NICER count rates are well correlated over eight matched epochs (r = 0.941). We fitted log-parabola models to day-resolved spectra from XSPECT, NICER and both jointly, and to flux-resolved XSPECT spectra. Both show spectral hardening with brightness. In the flux-resolved spectra, the photon index at 1 keV decreases from 2.24 in the faintest state to 1.96 in the brightest, and the synchrotron peak moves from about 0.66 to 1.06 keV, while the curvature ($β\\approx 0.7$) shows no significant change. Power-law fits to Bayesian blocks show that bright blocks at the same rate can differ in photon index by up to $6.4σ$, so count rate alone does not fix the spectral state.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2611.02772",
  "author_total": 6,
  "authors": [
    {
      "name": "Sikandar Akbar"
    },
    {
      "name": "Zahir Shah"
    },
    {
      "name": "Zahoor Malik"
    },
    {
      "name": "Faheem Aalijah"
    },
    {
      "name": "P Sreekumar"
    },
    {
      "name": "Vivek Kumar"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/00aydg3n0vpqt.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2611.02772",
  "fulltext_url": null,
  "id": "00aydg3n0vpqt",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2611.02772",
  "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/2611.02772",
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-02",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/00aydg3n0vpqt.ris",
  "sources": [
    {
      "first_seen": "2026-11-03T12:20:59Z",
      "native_id": "2611.02772",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2611.02772"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "X-ray Variability of Mrk 421 in 2025 January: A Ten-Day XPoSat XSPECT Campaign with Contemporaneous NICER Coverage",
  "title_text": "X-ray Variability of Mrk 421 in 2025 January: A Ten-Day XPoSat XSPECT Campaign with Contemporaneous NICER Coverage",
  "type": "preprint",
  "updated_at": "2026-11-05T15:20:18Z",
  "url": "https://research.draconitic.com/paper/00aydg3n0vpqt",
  "venue": "arXiv",
  "versions": []
}