Quasi-periodic oscillations during magnetar giant flares in the strangeon star model

Kavli Affiliate: Lijing Shao

| First 5 Authors: Hong-Bo Li, Yacheng Kang, Zexin Hu, Lijing Shao, Cheng-Jun Xia

| Summary:

Soft gamma-ray repeaters (SGRs) are widely understood as slowly rotating
isolated neutron stars. Their generally large spin-down rates, high magnetic
fields, and strong outburst energies render them different from ordinary
pulsars. In a few giant flares (GFs) and short bursts of SGRs, high-confidence
quasi-periodic oscillations (QPOs) were observed. Although remaining an open
question, many theoretical studies suggest that the torsional oscillations
caused by starquakes could explain QPOs. Motivated by this scenario, we
systematically investigate torsional oscillation frequencies based on the
strangeon-star (SS) model with various values of harmonic indices and
overtones. To characterize the strong-repulsive interaction at short distances
and the non-relativistic nature of strangeons, a phenomenological Lennard-Jones
model is adopted. We show that, attributing to the large shear modulus of SSs,
our results explain well the high-frequency QPOs ($gtrsim 150,mathrm{Hz}$)
during the GFs. The low-frequency QPOs ($lesssim 150,mathrm{Hz}$) can also
be interpreted when the ocean-crust interface modes are included. We also
discuss possible effects of the magnetic field on the torsional mode
frequencies. Considering realistic models with general-relativistic corrections
and magnetic fields, we further calculate torsional oscillation frequencies for
quark stars. We show that it would be difficult for quark stars to explain all
QPOs in GFs. Our work advances the understanding of the nature of QPOs and
magnetar asteroseismology.

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