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

Abstract

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 $1.29\pm 0.02$; and (3) a $6.37\pm 0.05$ 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 $P_0\simeq 1.38$ ms and a surface field of $B_{\mathrm{hexa}}\simeq 2.04\times 10^{16}$ 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 $B_{\mathrm{hexa}}$ 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.

arXiv comment12 pages, 2 figures; published in ApJ

References in corpus (74)

• The Large Area Telescope on the Fermi Gamma-ray Space Telescope Mission
• Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs
• The Fermi Gamma-Ray Burst Monitor
• Gamma-Ray Bursts and the Fireball Model
• The Physics of Neutron Stars
• Neutron Star Observations: Prognosis for Equation of State Constraints
• Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole
• Physical processes shaping GRB X-ray afterglow lightcurves: theoretical implications from the Swift XRT observations
• The Physics of Gamma-Ray Bursts and Relativistic Jets
• An online repository of Swift/XRT light curves of GRBs
• Magnetically Arrested Disk: An Energetically Efficient Accretion Flow
• Gamma-ray Burst Afterglow with Continuous Energy Injection: Signature of a Highly-Magnetized Millisecond Pulsar
• Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars
• The Proto-Magnetar Model for Gamma-Ray Bursts
• Signatures of magnetar central engines in short GRB lightcurves
• Low frequency QPO spectra and Lense-Thirring precession
• Short Duration Gamma-Ray Bursts with Extended Emission from Proto-Magnetar Spin-Down
• Global General Relativistic MHD Simulation of a Tilted Black-Hole Accretion Disk
• Gravitational Waves from Neutron Stars with Large Toroidal B-fields
• Formation of Precessing Jets by Tilted Black-hole Discs in 3D General Relativistic MHD Simulations
• Gamma-ray burst early optical afterglows: implications for the initial Lorentz factor and the central engine
• A Simple Comptonization Model
• A decade of Gamma-Ray Bursts observed by $Fermi$-LAT: The 2${}^{nd}$ GRB catalog
• A physical model for the continuum variability and QPO in accreting black holes
• A test of the millisecond magnetar central engine model of GRBs with Swift data
• Quasi-blackbody component and radiative efficiency of the prompt emission of gamma-ray bursts
• A new low magnetic field magnetar: the 2011 outburst of Swift J1822.3-1606
• GRB Afterglows with Energy Injection from a spinning down NS
• Extremely hard GRB spectra prune down the forest of emission models
• Time evolution of pulsar obliquity angle from 3D simulations of magnetospheres
• Magnetar powered GRBs: Explaining the extended emission and X-ray plateau of short GRB light curves
• QPOs during magnetar flares are not driven by mechanical normal modes of the crust
• Observing Lense-Thirring Precession in Tidal Disruption Flares
• On the theory of magnetar QPOs
• Observationally constraining gravitational wave emission from short gamma-ray burst remnants
• Supernova Fallback onto Magnetars and Propeller-Powered Supernovae
• Possible Evidence for Free Precession of a Strongly Magnetized Neutron Star in the Magnetar 4U 0142+61
• Magnetic field evolution in magnetar crusts through three dimensional simulations
• Alfvén QPOs in Magnetars
• The fate of fallback matter around newly born compact objects
• Relativistic models of magnetars: structure and deformations
• A study of gamma ray bursts with afterglow plateau phases associated with supernovae
• Multipolar electromagnetic fields around neutron stars: exact vacuum solutions and related properties
• The braking index of millisecond magnetars
• Hyperaccreting Black Hole as Gamma-Ray Burst Central Engine. II. Temporal evolution of central engine parameters during Prompt and Afterglow Phases
• Alfven QPOs in magnetars in the anelastic approximation
• Fallback Accretion onto a Newborn Magnetar: short GRBs with Extended Emission
• Constraints on millisecond magnetars as the engines of prompt emission in gamma-ray bursts
• Neutron-star spindown and magnetic inclination-angle evolution
• Kilohertz quasiperiodic oscillations in short gamma-ray bursts
• Evolution of non-spherical pulsars with plasma-filled magnetospheres
• Determining the efficiency of converting magnetar spin-down energy into gamma-ray burst X-ray afterglow emission and its possible implications
• Oscillations of rotating magnetised neutron stars with purely toroidal magnetic fields
• The Peculiar Precursor of a Gamma-Ray Burst from a Binary Merger Involving a Magnetar
• Starquakes in millisecond pulsars and gravitational waves emission
• Fallback accretion on to a newborn magnetar: long GRBs with giant X-ray flares
• Constraining the Braking Index and Energy Partition of Magnetar spindown with {\em Swift}/XRT data
• Evaluating the bulk Lorentz factors of outflow material: lessons learned from the extremely-energetic outburst GRB 160625B
• Violation of synchrotron line of death by the highly polarized $GRB160802A$
• Magnetic-distortion-induced ellipticity and gravitational wave radiation of neutron stars: millisecond magnetars in short GRBs, Galactic pulsars, and magnetars
• Non-rigid precession of magnetic stars
• Early Evolution of a Newborn Magnetar with Strong Precession Motion in GRB 180620A
• Constraining the ellipticity of new-born magnetar with the observational data of Long gamma-ray bursts
• Precession of magnetars: dynamical evolutions and modulations on polarized electromagnetic waves
• Persistent Emission Properties of SGR J1935+2154 During Its 2020 Active Episode
• Evidence for the transition of a Jacobi ellipsoid into a Maclaurin spheroid in gamma-ray bursts
• Polarization in early optical afterglows of gamma-ray bursts driven by precessing jets
• Evidence for a brief appearance of gamma-ray periodicity after a compact star merger
• Multipolar Electromagnetic Emission of Newborn Magnetar
• Incidence of afterglow plateaus in gamma-ray bursts associated with binary neutron star mergers
• A two-hump spectrum in the prompt emission of GRB 240825A
• Gamma-Ray Burst Central Engines: Black Hole Vs. Magnetar
• Multimessenger signatures of a deformed magnetar in gamma-ray bursts
• Constraining the Neutron-Star Equation of State via Short Gamma-Ray Burst X-ray Afterglows