[Paper Review] Fall-back crust around a quark-nova compact remnant I: The degenerate shell case with applications to SGRs, AXPs and XDINs

Rachid Ouyed, D. A. Leahy+1 more|Aug 24, 2006

Pulsars and Gravitational Waves Research36 references18 citations

TL;DR

This paper proposes that Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are powered by quark-nova remnants consisting of a degenerate iron-rich shell suspended by a quark star's magnetic field. As the shell drifts inward due to magnetic field decay, fragments fall onto the quark star, releasing energy as bursts; this model explains multiple observed features including burst types, spin-up/down cycles, iron lines, and X-ray/infrared correlations.

ABSTRACT

We explore the formation and evolution of debris ejected around quark stars in the Quark Nova scenario, and the application to Soft Gamma-ray Repeaters (SGRs) and Anomolous X-ray Pulsars (AXPs). If an isolated neutron star explodes as a Quark Nova, an Iron-rich shell of degenerate matter forms out of the fall-back (crust) material. Our model can account for many of the observed features of SGRs and AXPs such as: (i) the two types of bursts (giant and regular); (ii) the spin-up and spin-down episodes during and following the bursts with associated persistant increases in $\dot{P}$; (iii) the energetics of the boxing day burst, SGR1806$+$20; (iv) the presence of an Iron line as observed in SGR1900$+$14; (v) the correlation between the far-Infrared and the X-ray fluxes during the bursting episode and the quiescent phase; (vi) the hard X-ray component observed in SGRs during the giant bursts, and (vii) the discrepancy between the ages of SGRs/AXPs and their supernova remnants. We also find a natural evolutionary relationship between SGRs and AXPs in our model which predicts that only the youngest SGRs/AXPs are most likely to exhibit strong bursting. Many features of X-ray Dim Isolated Neutron stars (XDINs) are also accounted for in our model such as, (i) the two-component blackbody spectra; (ii) the absorption lines around 300 eV; and (iii) the excess optical emission.

Motivation & Objective

• To explain the origin of recurrent and giant bursts in SGRs and AXPs through a quark-nova remnant model.
• To resolve the age discrepancy between SGRs/AXPs and their supernova remnants by linking them to a quark-nova event.
• To account for the spectral features of X-ray Dim Isolated Neutron Stars (XDINs), including blackbody components, absorption lines, and optical excess.
• To establish a natural evolutionary sequence from SGRs to AXPs to XDINs based on age and shell evolution.
• To provide a physical mechanism for burst energetics and spin evolution via magnetic field-coupled shell dynamics.

Proposed method

• Models the formation of a degenerate, iron-rich shell from the neutron star crust after a quark-nova explosion.
• Applies the propeller mechanism and magnetic pressure confinement to sustain a corotating shell around the quark star.
• Uses magnetic field decay coupled to the quark star's period (via Niebergal et al. 2006) to drive inward drift of the shell.
• Simulates fragment breakoff above the line of neutrality where magnetic and gravitational fields align, enabling accretion and energy release.
• Estimates burst energetics using energy conversion from infalling shell material into quark matter (Lugones & Horvath 2002).
• Derives a luminosity-spin-down rate relation in quiescence and links optical excess to shell geometry via solid angle and scattering.

Experimental results

Research questions

• RQ1 How can the two types of bursts (regular and giant) in SGRs/AXPs be explained by a single physical mechanism?
• RQ2 What causes the observed spin-up and spin-down episodes during burst activity?
• RQ3 Why is there a correlation between X-ray and far-infrared fluxes during bursts and quiescence?
• RQ4 How do the hard X-ray spectra and iron K-alpha lines observed in giant flares arise?
• RQ5 What explains the spectral features and optical excess in X-ray Dim Isolated Neutron Stars (XDINs)?

Key findings

• The model explains the two-burst type behavior in SGRs/AXPs via fragment accretion from a drifting, magnetically confined shell.
• The observed spin-up and spin-down during bursts arise from angular momentum transfer during fragment accretion and magnetic braking.
• The correlation between X-ray and infrared fluxes is naturally explained by the shell’s reprocessing of X-ray emission into thermal radiation.
• The hard X-ray component in giant flares results from high-energy radiation released during the conversion of infalling shell material into CFL quark matter.
• The iron K-alpha line observed in SGR1900+14 is reproduced as a photospheric feature from the iron-rich shell.
• The optical excess in XDINs like RX J1856.5-3754 is quantitatively matched by the model’s prediction of a shell subtending ~7 times the blackbody tail flux, consistent with observed excess factors of 5–7.