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.

12 pages, 2 figures; published in ApJ

References in corpus (38)

• Neutron Star Observations: Prognosis for Equation of State Constraints
• The Physics of Gamma-Ray Bursts and Relativistic Jets
• An online repository of Swift/XRT light curves of GRBs
• 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
• A decade of Gamma-Ray Bursts observed by $Fermi$-LAT: The 2${}^{nd}$ GRB catalog
• Quasi-blackbody component and radiative efficiency of the prompt emission of gamma-ray bursts
• QPOs during magnetar flares are not driven by mechanical normal modes of the crust
• On the theory of magnetar QPOs
• Possible Evidence for Free Precession of a Strongly Magnetized Neutron Star in the Magnetar 4U 0142+61
• Alfvén QPOs in Magnetars
• Relativistic models of magnetars: structure and deformations
• 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
• 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
• Kilohertz quasiperiodic oscillations in short gamma-ray bursts
• Determining the efficiency of converting magnetar spin-down energy into gamma-ray burst X-ray afterglow emission and its possible implications
• The Peculiar Precursor of a Gamma-Ray Burst from a Binary Merger Involving a Magnetar
• Starquakes in millisecond pulsars and gravitational waves emission
• 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
• 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
• Precession of magnetars: dynamical evolutions and modulations on polarized electromagnetic waves
• Constraining the ellipticity of new-born magnetar with the observational data of Long gamma-ray bursts
• 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