A Brief Introduction to Magnetars

Presented by Lerong Liu
2025.12.30

What is a magnetar?

In brief: neutron stars powered by the decay of ultra-strong magnetic fields.

The Historical Puzzle

Two distinct classes (Pre-1990s):

  • SGRs (Soft Gamma Repeaters):
    Discovered 1979; Violent bursts; Originally thought to be Gamma-Ray Bursts.
     
  • AXPs (Anomalous X-ray Pulsars):
    Discovered 1980s; Long periods (\(2\sim12\text{ s}\)); Stable but bright.
     

The Historical Puzzle

Problems:

  • Energy Crisis:
    X-ray Luminosity \(\gg\) Spin-down Luminosity (\(L_X\gg\dot{E}\))
     
  • Cannot be powered by rotation (like the Crab Pulsar) or accretion (no binary companion).
     

The Magnetar Model

The Unification (Duncan & Thompson, 1992):

  • SGRs and AXPs are the same class of object.
  • Powered by the decay of an ultra-strong internal magnetic field.
  • \(B\sim10^{14}\text{ - }10^{15}\text{ Gs}\)

Key Prediction:

  • Strong \(B\)-field \(\longrightarrow\) Rapid Braking \(\longrightarrow\) Long Periods
  • Confirmed in 1998 (Kouveliotou et al.) via SGR 1806-20 spin-down measurement.

Observational Methods

Detection: All-sky monitors (Swift BAT, Fermi GBM) for bursts.

Timing Analysis:

  • Phase-coherent timing (tracking every rotation).
  • The \(P\text{ - }\dot{P}\) Diagram:
    Shows Magnetars in the top right (Long \(P\), high \(\dot{P}\)).
  • Dipole Formula: \(B\propto\sqrt{P\dot{P}}\)

Spectroscopy: Soft X-rays (\(0.5\text{ - }10\text{ keV}\)) and Hard X-rays (\(>10 \text{ keV}\)).

Temporal Phenomenon (Variability)

Hierarchy of Activity:

  • Short Bursts: ms to sec duration, common.
  • Outbursts: Sudden flux increase, slow decay (months).
  • Giant Flares: Extreme energy (\(10^{44}\text{ - }10^{46}\text{ erg}\)). Only 3 observed in history (1979, 1998, 2004).

Glitches: Sudden spin-up events (common).

Anti-Glitches: Sudden spin-down (rare, unique to magnetars).

Radiation Spectrum

The "j-bundle" Model:

  • Twisted magnetic field lines support electric currents.

Two Spectral Components:

  • Thermal (Soft X-ray): Surface emission, \(k_BT\sim0.5\text{ keV}\).
  • Non-Thermal (Hard X-ray): Resonant Compton Scattering.

Graph: Spectrum showing the "upturn" at high energies (\(>10 \text{ keV}\)).

Internal Mechanism (The Engine)

Structure of a Neutron Star: Core (Liquid) vs. Crust (Solid).

Dynamics:

  • Core: Ambipolar Diffusion (field moves through plasma).
  • Crust: Hall Drift (field rearranges itself).

The "Starquake":

  • Magnetic stress > Crust yield strength.
  • Crust fractures \(\longrightarrow\) Energy release \(\longrightarrow\) Flare.

Findings & Limits

Population: \(\sim30\) known sources (at time of this presentation).

Transient Magnetars: Some have "low" dipole fields (\(B<7.5\times10^{12}\text{ Gs}\)) but still burst.
  Implies strong internal toroidal fields.

Radio Emission: rare, pulsed radio emission detected in only \(\sim4\) magnetars.

Bias: We only find magnetars when they burst. Many "quiet" ones likely exist.

Future Research

  • Finding the "Hidden" Population: Monitoring the Galactic Plane.
     
  • X-ray Polarimetry: Testing Quantum Electrodynamics (QED) and Vacuum Birefringence.
     
  • Connection to FRBs: Are Magnetars the source of Fast Radio Bursts?
     
  • 3D Simulations: Moving from 2D to full 3D modeling of starquakes.

Reference

[1] Duncan, R. C. and Thompson, C.: 1992, The Astrophysical Journal 392, L9
[2] Kaspi, V. M. and Beloborodov, A.: 2017, Annual Review of Astronomy and Astrophysics 55(1), 261
[3] Kouveliotou, C., Dieters, S., Strohmayer, T., van Paradijs, J., Kommers, J., Smith, I., Frail, D., and Murakami, T.: 1998
[4] Kramer, M., Liu, K., Desvignes, G., Karuppusamy, R., and Stappers, B. W.: 2023, Nature Astronomy 8(2), 230
[5] Mazets, E. P. and Golenetskii, S. V.: 1981, Astrophysics and Space Science 75(1), 47
[6] Younes, G., Baring, M. G., et al.: 2023, Nature Astronomy 7(3), 339

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