[Paper Review] Thirteenth Marcel Grossmann Meeting, Summary of the session, White Dwarf Pulsars and Rotating White Dwarf Theory

Y. Terada|arXiv (Cornell University)|Jun 18, 2013

Pulsars and Gravitational Waves Research3 citations

TL;DR

This paper proposes that magnetized white dwarfs, or 'white dwarf pulsars,' can act as significant sources of galactic cosmic rays through particle acceleration in their rotating magnetospheres, analogous to neutron star pulsars. Key evidence comes from X-ray and gamma-ray observations of AE Aquarii, which show non-thermal pulsations consistent with particle acceleration, with a hard X-ray spectrum (photon index 2.28) and 0.03% efficiency relative to spin-down luminosity.

ABSTRACT

This is the summary of the parallel session entitled "White Dwarf Pulsars and Rotating White Dwarf Theory", chaired by Yukikatsu Terada in Thirteenth Marcel Grossmann Meeting. The origin of cosmic rays remains a mystery, even over 100 years since their discovery. Neutron stars (NSs) are considered textbook cases of particle acceleration sites in our Galaxy, but many unresolved numerical problems remain. Searches for new acceleration sites are crucial for astrophysics. The magnetized white dwarfs (MWDs) have the same kind of rotating magnetosphere as NSs, and may be the source of up to 10% of galactic cosmic ray electrons. In the parallel session of the "white dwarf pulsars and rotating white dwarf theory", we focus on the current observational results on white dwarf pulsars, related theories of the radiation process both in white dwarfs and neutron stars, and the origin and rule of white dwarf pulsars, as well as surveying on the current theories of the internal structure and the equation of state of white dwarfs.

Motivation & Objective

• To investigate the role of magnetized white dwarfs as potential sources of galactic cosmic rays.
• To assess observational and theoretical evidence for white dwarf pulsars, particularly in the context of particle acceleration mechanisms similar to those in neutron star pulsars.
• To examine the internal structure and equation of state of white dwarfs under extreme conditions, including rotation, magnetic fields, and temperature effects.
• To evaluate whether white dwarf pulsars could explain the high-energy emission observed in sources like SGRs and AXPs.
• To determine the contribution of white dwarf pulsars to high-energy electron-positron pairs detected by PAMELA.

Proposed method

• Utilized particle-in-cell simulations to model the rotating magnetosphere of white dwarfs, adapting methods originally developed for neutron stars.
• Analyzed X-ray and gamma-ray data from AE Aquarii using spectral fitting with power-law and thermal (MEKAL) models to distinguish non-thermal emission.
• Applied the Hillas diagram to assess the particle acceleration potential of white dwarfs based on magnetic field strength, size, and spin period.
• Evaluated the spin-down luminosity and magnetic field strength of AE Aquarii (P = 33.0767 s, L_sd = 6×10³³ erg/s, B = 50 MG) to estimate particle acceleration efficiency.
• Investigated the equation of state of white dwarfs under rotation, magnetic fields, and temperature using general relativistic and effective geometry formalisms.
• Compared the energy budget of white dwarf pulsars with that of neutron star pulsars and magnetars to assess their relative contributions to high-energy emission.

Experimental results

Research questions

• RQ1 Can magnetized white dwarfs with strong magnetic fields and rapid rotation serve as viable particle accelerators for galactic cosmic rays?
• RQ2 What observational signatures—such as non-thermal X-ray or gamma-ray pulsations—can confirm the existence of white dwarf pulsars?
• RQ3 How does the efficiency of particle acceleration in white dwarf pulsars compare to that in neutron star pulsars?
• RQ4 Can white dwarf pulsars explain the high-energy emission observed in sources like SGRs and AXPs without invoking magnetar-like fields?
• RQ5 How do rotation, magnetic fields, and temperature affect the equation of state and maximum mass of white dwarfs?

Key findings

• AE Aquarii exhibits hard X-ray pulsations with a photon index of 2.28 ± 0.08, consistent with a non-thermal power-law spectrum, indicating particle acceleration.
• The pulsed X-ray luminosity of 1.8×10³⁰ erg/s in the 0.5–10 keV band corresponds to 0.03% of the object’s spin-down luminosity, suggesting efficient but subdominant particle acceleration.
• The X-ray spectrum is better fit by a power-law model than a thermal model (MEKAL), with a low metal abundance of 0.08 solar, further supporting a non-thermal origin.
• Magnetic white dwarfs can generate electric potentials of 10¹⁴–10¹⁶ V, sufficient to accelerate particles to cosmic ray energies, making them plausible sources of galactic cosmic ray electrons.
• Theoretical models suggest that white dwarf pulsars could contribute up to 10% of galactic cosmic ray electrons, with potential contributions comparable to neutron star pulsars.
• Numerical simulations show that rotating white dwarfs can sustain accelerating regions in their magnetospheres, with a defined 'death line' analogous to neutron star pulsars.