Hyperactive Magnetars

• Hyperactive magnetars are young neutron stars with internal fields ≳10^16 G that experience rapid, large-scale eruptions driven by ambipolar diffusion.
• Their eruptions eject baryon-rich crustal matter, produce giant gamma-ray flares, radioactive afterglows, and can generate fast radio bursts via shock coherent emission.
• Numerical simulations capture the dynamic interplay of field evolution, reconnection, and mass ejection, aligning observed transient energetics with FRB production mechanisms.

Hyperactive magnetars are young neutron stars with internal magnetic fields $B \gtrsim 10^{16}$ G that undergo extreme, recurrent, large-scale eruptive events during rapid field evolution. These eruptions drive a complex, multi-phase sequence of electromagnetic transients—including giant gamma-ray flares, the ejection of baryonic crustal material, radioactively powered afterglows, and, under suitable magnetospheric conditions, fast radio bursts (FRBs). Their activity is governed by magnetic field transport and dissipation mechanisms (chiefly ambipolar diffusion) operating on characteristic timescales orders of magnitude shorter than the evolutionary phases observed in older Galactic magnetars. Hyperactive eruptions are proposed to play a pivotal role in explaining both the energetics and phenomenology of the most powerful magnetar flares as well as the central engines of extragalactic FRBs (Bransgrove et al., 19 Aug 2025).

1. Magnetic Field Dynamics and Ambipolar Diffusion

In nascent neutron stars with $B_{\rm int} \gtrsim 10^{16}$ G, the internal magnetic field is not static but evolves primarily through ambipolar diffusion. In the liquid core, the coupled evolution of the proton–electron plasma relative to the neutron background is driven by the Ampère force, $\mathbf{j} \times \mathbf{B}/c$, resisted by proton–neutron friction and weak-interaction pressure gradients. The matter remains near chemical equilibrium owing to rapid modified Urca reactions. The characteristic ambipolar diffusion timescale is

$t_{\rm amb} \sim 10^9\,{\rm s},$

and the induction equation in the core is

$$\frac{\partial \mathbf{B}}{\partial t} = \nabla \times (\mathbf{v}_p \times \mathbf{B}),$$

with $\mathbf{v}_p = \mathbf{v}_n + \mathbf{v}_{\rm amb}$, separating the bulk neutron velocity from the ambipolar drift (Bransgrove et al., 19 Aug 2025).

In the deep crust (with densities above neutron drip, $\rho \gtrsim 4 \times 10^{11}$ g cm⁻³), a similar evolution obtains: compression by the growing field alters the electron Fermi energy, and electron captures (or $\beta$-decays) facilitate localized matter reorganization, allowing magnetic flux to move through the otherwise rigid crust. Weak nuclear reactions thus play an essential role in coupling field evolution to local crustal dynamics.

2. Eruption Mechanism and Crustal Ejecta

Ambipolar diffusion acts to transport and concentrate the internal field into toroidal “loops” or “bubbles.” Poloidal field contraction toward the core equator, accompanied by compositional changes and pressure adaptations, leads to rising, twisted toroidal flux bundles. As a magnetic loop expands into the outer crust, it lifts and entrains significant neutron-rich crustal mass. When the lower interface of the loop reaches a region with fluid-to-magnetic pressure ratio $\beta \lesssim 1$, the field energy and tension trigger a catastrophic, dynamical eruption.

Magnetic reconnection mediates the detachment of the loop, essential for “dressing” the heavy baryon-rich ejecta with hot, baryon-clean magnetic field loops. The typical energetics are set by the local surface field and the volume or mass of the ejected region: $E_{\rm flare} \approx V \frac{B^2}{8\pi},$ with simulation parameters yielding $E_{\rm flare} \sim 5 \times 10^{49}$ erg and ejected mass $M_{\rm ej} \sim 4 \times 10^{28}$ g (Bransgrove et al., 19 Aug 2025).

Once ejected, the crustal material decompresses on millisecond timescales; neutron emission and $\beta$-decays driven by the cessation of high-density pressure release nuclear energy and render the ejecta temporarily radioactive.

3. Energetics, Feedback, and Observable Transients

The magnetic eruption couples multiple electromagnetic and baryonic energy release channels:

• Prompt Gamma-ray Flare: Magnetic reconnection converts $\sim$10% of the available pre-eruption energy into a thermalized gamma-ray burst. Computed thermal equilibrium, $aT^4 \sim E_{\rm flare}/r^3$ with $a$ the radiation constant and $r$ the local radius, leads to temperatures in the MeV regime. If portions of the hot plasma achieve high Lorentz factors via geometrical expansion, the observed spectral peak is Doppler-boosted into the hundreds-of-keV domain, accounting for classical “giant” magnetar flares.
• Radioactive Afterglow: The ejected, neutron-rich debris undergoes decompression and nuclear heating. As the ejecta expand, they become transparent to MeV photons on $\sim 10^3$ s timescales, followed by optical transients once recombination temperatures $T \sim 10^5$ K are reached.
• Radio Afterglow: External shocks as the ejecta interact with the ambient interstellar medium give rise to synchrotron radio emission on much longer timescales, as observed following the 2004 Dec SGR 1806–20 flare.

A summary table of the main energy and mass budgets from (Bransgrove et al., 19 Aug 2025):

Component	Typical Value	Physical Role
Flare energy	$\sim 5 \times 10^{49}$ erg	Prompt gamma-rays, internal field release
Ejecta mass	$\sim 4 \times 10^{28}$ g	Neutron-rich crust expelled
Kinetic energy (ejecta)	$\sim E_{\rm flare}$	Bulk motion, afterglow
Radioactive afterglow	$>10^3$ s post-flare	Nuclear energy release (MeV, then optical)

4. FRB Production from Hyperactive Eruptions

Each magnetic eruption launches an ultrarelativistic fast magnetosonic (FMS) pulse into the outer magnetosphere. Kinetic and MHD studies indicate that this pulse rapidly steepens into a relativistic shock that traverses the magnetar wind region. Two key FRB generation channels are proposed:

• Shock Coherent Emission: The FMS-driven blast wave propagating in a relativistic pair wind leads to cyclotron/maser instabilities at the shock front, converting a fraction ($10^{-5} - 10^{-6}$) of kinetic energy into coherent GHz emission. The observed FRB duration is compressed by the high Lorentz factor ($\Gamma \gtrsim 10^3$) of the shock.
• Alternative Plasmoid Reconnection Channel: The FMS pulse may collide with pre-existing magnetospheric current sheets, compressing and fragmenting them into chains of magnetic islands. Their subsequent mergers, in the regime of strong synchrotron cooling, stimulate plasma currents that radiate coherent FRBs (Mahlmann, 4 May 2024).

This model aligns both the energetics and the timescales of observed extragalactic FRBs with rare, dynamically extreme magnetar eruptions, without requiring continuous or persistent outflow.

5. Observational Diagnostics and Comparison with Data

The model provides a quantitative match to several classes of observed transients:

• The 2004 Dec SGR 1806–20 giant flare is interpreted as a hyperactive magnetic eruption: the required ejected mass ($10^{25} - 10^{27}$ g) and kinetic energies closely track the afterglow measurements and nebular energetics.
• Prompt gamma-ray emission (luminosity $\gtrsim 10^{47}$ erg s$^{-1}$, duration $\sim0.1-1$ s), delayed radioactive MeV/optical afterglow, and GHz radio afterglow/radio nebulae are natural consequences.
• The rarety of such events in the Galactic sample is attributed to the advanced age and lower internal magnetic field strength compared to the young and “hyperactive” population operating in high-$B$ systems at earlier times.
• Hyperactive extragalactic magnetars, through their frequent, high-energetic eruptions, are proposed as central engines for cosmological FRBs, with the FRB occurrence tightly linked to the FMS pulse and subsequent relativistic wind interactions.

6. Theoretical and Simulation Framework

The eruption process is treated through numerical simulation of axisymmetric (2D) magnetic field evolution, capturing the essential features of ambipolar diffusion (in both core and crust), loop formation, and mass entrainment. Out-of-plane effects, 3D field topology, and the full kinetic regime of FMS pulse propagation and shock emission require further development in future 3D MHD and kinetic-PIC studies.

Formulae central to the modeling include:

• Induction equation (core): $$\partial \mathbf{B}/\partial t = \nabla \times (\mathbf{v}_p \times \mathbf{B})$$.
• Eruption energy: $$E_{\rm flare} \sim (M_{\rm ej}/\overline{\rho}) \, B^2/(8\pi)$$.
• Thermal equilibrium in reconnection region: $aT^4 \sim E_{\rm flare}/r^3$.
• Timescale for prompt emission: $t_{\rm dyn} \sim$ milliseconds (set by local Alfvén speed and geometry).

7. Future Directions and Observational Strategies

Verification of the hyperactive magnetar eruption model requires:

• Targeted coordinated searches for coincident FRBs and high-energy gamma-ray flares in both young Galactic magnetars and extragalactic candidates.
• Systematic follow-up for delayed MeV/optical afterglow signatures, using sensitive, high-cadence transient facilities.
• Long-term radio monitoring for expanding nebular structures tracing crustal ejecta.
• Extension to full 3D MHD and kinetic treatments, which will clarify beaming effects, emission anisotropy, and the efficiency of FRB production per giant flare.

This model represents a unifying framework in which ambipolar diffusion–driven evolution in young, ultrastrong-field neutron stars powers a spectrum of eruptive phenomena. The hyperactive magnetar’s eruption cycle couples hard gamma-ray flares, baryon-rich and radioactive ejecta, and FRBs through a single, physically self-consistent dynamical sequence (Bransgrove et al., 19 Aug 2025).