Prompt and Afterglow Constraints on the Central Engine of GRB 240825A

@misc{saeed2026prompt,
  title = {{Prompt and Afterglow Constraints on the Central Engine of GRB 240825A}},
  author = {Daban Mohammed Saeed and Rahim Moradi and Chen-Wei Wang and Brwa Shukur Ismael and Marco Muccino},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.34816},
  eprint = {2609.34816},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.34816},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Prompt and Afterglow Constraints on the Central Engine of GRB 240825A
AU  - Daban Mohammed Saeed
AU  - Rahim Moradi
AU  - Chen-Wei Wang
AU  - Brwa Shukur Ismael
AU  - Marco Muccino
PY  - 2026
DA  - 2026/09/28
PB  - arXiv
DO  - 10.48550/arxiv.2609.34816
UR  - https://arxiv.org/abs/2609.34816
AB  - GRB 240825A shows unprecedented temporal and spectral features that constrain the properties of its inner engine: (1) a prompt emission three-component spectrum consisting of a Band function continuum, a quasi-thermal bump, and a hard MeV tail; and (2) an X-ray afterglow fitted by a power-law decay with index 1.29 ± 0.02; and (3) a 6.37 ± 0.05 Hz quasi-periodic oscillation (QPO) identified in the 100-300 keV band during the 2.07-3.25 s time interval, coinciding with the photospheric radiation phase. We compare two central-engine candidates: (a) a newborn millisecond magnetar undergoing multipolar spin-down, free precession, and global magnetoelastic oscillations of its interior; and (b) a Kerr black hole powering a Blandford-Znajek jet subject to Lense-Thirring disk precession. In principle, both channels can reproduce the observed QPO frequency but impose different constraints on the energetics, temporal evolution, and the origin of the oscillation. We first demonstrate that the X-ray afterglow is quantitatively reproduced by a hexapolar magnetar spin-down model with an initial spin period of P_0 ≃ 1.38 ms and a surface field of B_hexa ≃ 2.04 × 10^16 G. The decay index measures a braking index rather than the field multipolarity, does not exclude a fallback-regulated flow, and leaves P_0 and B_hexa uncertain by a factor of a few. In this scenario, magnetic or magnetoinertial dynamics of the star interior provide a plausible explanation of the QPO without invoking extreme stellar deformations. Although a black hole engine cannot be firmly excluded, the combined prompt and afterglow observational properties of GRB 240825A favor a long-lived magnetar central engine.
ER  -