During its August 2024 outburst, the magnetar 1E 1841−045 underwent a spin-up glitch, a flux enhancement dominated by a newly formed hard non-thermal power-law component (photon index Γ≈1.5 extending above 20 keV), and — most distinctively — the appearance of a narrow new pulse peak that migrated toward the main pulse at ≈4.6×10⁻³ rad per day, only the second such pulse-peak migration observed after SGR 1830−0645. Existing theory cannot disentangle the two candidate drivers: slow plastic motion of the neutron-star crust, and relaxation (untwisting) with polar migration of a twisted magnetospheric field bundle. Nor is the radiative mechanism of the new hard component — resonant Compton upscattering versus synchrotron from pair cascades — definitively identified, despite new X-ray polarimetry. The problem is to determine, through quantitative modeling tied to the measured pulse-shape, spectral, and polarization evolution, the site, dynamics, and emission physics of the new emitting region: the relative roles of crustal and magnetospheric processes and the radiative mechanism, established jointly across all three observable classes.

Background

Magnetars are isolated neutron stars whose emission is powered by the decay of an ultra-strong magnetic field (surface dipole of order 101410^{14}–101510^{15} G) rather than by rotational energy loss. Their defining activity is the outburst: a storm of short hard X-ray bursts followed by a weeks-to-months enhancement of the persistent X-ray flux, interpreted as the release of magnetic stress accumulated in the solid stellar crust together with the twisting-up of magnetospheric field lines (Thompson, Lyutikov & Kulkarni 2002). A glitch is a sudden step-like change in the star's spin frequency ν\nu, usually accompanied by a step in the spin-down rate ν˙\dot{\nu}; in magnetars, spin-up glitches are frequently but not always accompanied by radiative outbursts (Dib & Kaspi 2014), and why some magnetar glitches are radiatively loud while others are quiet is itself an open population-level question.

1E 1841−045 is a persistently X-ray-bright magnetar (spin period ≈11.8 s, dipole field ≈7×10147\times10^{14} G) at the center of the supernova remnant Kes 73, whose soft X-ray measurements are contaminated by remnant emission. Its August 2024 outburst — the first well-observed radiative outburst from this source — was monitored for 82 days with the NICER, NuSTAR, and IXPE X-ray observatories (Younes et al. 2025). Three coupled phenomena were measured: (i) a spin-up glitch at outburst onset with Δν=(6.1±0.4)×10−8\Delta\nu=(6.1\pm0.4)\times10^{-8} Hz and Δν˙=(−1.4±0.1)×10−14\Delta\dot{\nu}=(-1.4\pm0.1)\times10^{-14} Hz s−1^{-1}; (ii) a flux enhancement of a factor ≈1.6 over the historical level, dominated by a newly formed non-thermal power-law component with photon index Γ≈1.5\Gamma\approx1.5 extending to ≳20\gtrsim20 keV; and (iii) a new, narrow (width ≈0.1 in pulse phase) peak in the 2–10 keV pulse profile that migrated toward the persistently present main pulse at a rate 4.6(3)×10−34.6(3)\times10^{-3} rad day−1^{-1}, with the migration somewhat better fit by a saturating exponential (τ≈43\tau\approx43 days) than by a constant drift. This was only the second observed case of pulse-peak migration in a magnetar, after SGR 1830−0645 during its 2020 outburst (Younes et al. 2022), which showed a comparable phase-drift rate; the phenomenon is therefore expected in the broader magnetar population.

Two families of explanation compete for the migration. In the crustal scenario, magnetic stress in the crust (built up by Hall drift, the advection of magnetic field by the conducting electron fluid) exceeds the elastic yield stress of the solid, and the crust fails and flows plastically; the migrating peak then traces the surface velocity of this flow, set by how far the stress exceeds yield and by the viscosity of the plastic phase (Lander & Gourgouliatos 2019; Gourgouliatos & Lander 2021). In the magnetospheric scenario, the outburst twists up a bundle of closed field lines — a current-carrying region with a toroidal magnetic-field component, concentrated in a quasi-toroidal j-bundle (Beloborodov 2009); as the activation abates, Ohmic dissipation shrinks the current-carrying bundle and its footpoints migrate toward the magnetic pole on a nominal untwisting timescale of up to about a year, so the emitting region drifts in pulse phase (Chen & Beloborodov 2017; Ntotsikas et al. 2023). The two are dynamically coupled — the same field lines anchor the crustal footpoints and carry the magnetospheric twist — and, crucially, essentially all quantitative theory on both sides (magneto-plastic crust evolution, force-free twisted-magnetosphere models — force-free meaning that the electromagnetic field, not the plasma, carries the dominant stress — and j-bundle decay) is axisymmetric, while the observed phenomenon is intrinsically a longitude-dependent, time-dependent effect.

The radiative mechanism of the new hard component is similarly unresolved between two candidates. In resonant Compton upscattering (resonant inverse-Compton scattering, RICS), relativistic charges accelerated along twisted field lines resonantly upscatter ~0.5 keV surface thermal photons, producing the hard spectra known from persistently bright magnetars. If the charges reach Lorentz factors at which single-photon pair creation ignites in the ultra-strong field, electron–positron pair cascades develop, and pairs created in high Landau (quantized gyration) states emit synchrotron radiation from the outer parts of the twisted loops (Harding, Wadiasingh & Baring 2025). Spectropolarimetry from the Imaging X-ray Polarimetry Explorer (IXPE), obtained ~40 days after outburst onset, found the polarization degree rising from ≈15% at 2–3 keV to ≈55% (Rigoselli et al. 2025, at 5.5–8 keV, energy-resolved, model-independent analysis) – ≈70% (Stewart et al. 2025, at 6–8 keV, model-independent analysis) with a nearly constant polarization angle aligned close to celestial north; the hard power-law component is polarized at ≈40–57% (Stewart et al. 2025) or ≳65% (Rigoselli et al. 2025), the latter pointing to a synchrotron/curvature origin. Both RICS and pair-cascade synchrotron give reasonable spectropolarimetric agreement with the data — the latter somewhat more naturally — but neither is definitively identified, and no model has yet combined the phase migration, the spectrum, and the polarization in one time-dependent picture of the emitting region.

Problem Statement

Determine the physical origin and dynamics of the newly formed, migrating, non-thermal emitting region in the 2024 outburst of the magnetar 1E 1841−045, in the form of a self-consistent quantitative model that can be confronted with the measured data (Younes et al. 2025). Specifically:

(1) Establish the relative roles of (i) slow plastic motion (flow) of the neutron-star crust and (ii) relaxation (untwisting) and polar migration of magnetospheric twisted-field bundles in producing the observed pulse-peak migration — the new narrow peak (width ≈0.1 cycles) drifting toward the main pulse at a rate ≈4.6×10−3\approx4.6\times10^{-3} rad day−1^{-1}, a drift somewhat better fit over the 82-day window by a saturating exponential (τ≈43\tau\approx43 days) than by a constant rate — together with the associated energy-dependent pulse-profile evolution. The migration is intrinsically a longitude-dependent, time-dependent effect, whereas existing quantitative theory on both sides is axisymmetric; the source paper notes that future detailed simulations of non-axisymmetric twist set-ups and their temporal evolution, assessing the connections between pulse-peak phase separations, evolution timescales, and the twist enhancement to the net field energy and geometry, are highly motivated (Younes et al. 2025).

(2) Identify the radiative mechanism of the newly formed hard non-thermal component (power-law photon index Γ≈1.5\Gamma\approx1.5 extending to ≳20\gtrsim20 keV in the blackbody-plus-power-law decomposition of the NuSTAR monitoring by Younes et al. 2025) — resonant Compton upscattering of surface thermal photons by charges in the twisted bundle, versus synchrotron (or curvature) emission from pair cascades in extended loops, versus a quantified combination of these — consistently with the measured energy dependence of the polarization degree (rising from ≈15% at 2–3 keV to ≈55–70% at the hard end in the two IXPE analyses of Rigoselli et al. 2025 and Stewart et al. 2025, which use different hard-end energy bands, 5.5–8 keV and 6–8 keV respectively) and the approximately constant polarization angle.

These two determinations concern the same emitting region and must be answered jointly: a complete answer supplies an identified geometry and time evolution that reproduces the measured pulse-shape, spectral, and polarization evolution together, and thereby disentangles the crustal and magnetospheric contributions — or quantifies their combination, as the source data themselves allow (Younes et al. 2025).

References

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