[Paper Review] Quark-nova remnants IV: Application to radio emitting AXP transients

Rachid Ouyed, D. A. Leahy+1 more|arXiv (Cornell University)|Sep 27, 2008

Magnetic confinement fusion research1 references4 citations

TL;DR

This paper extends the quark-nova model to explain transient anomalous X-ray pulsars (AXPs) XTE J1810−197 and 1E 1547.0−5408 by modeling X-ray outbursts from accretion of iron-rich degenerate ring material and radio emission from magnetic bubble dissipation at the light cylinder. It successfully reproduces observed X-ray light curves, temperature evolution, and radio delays, predicting a ~1-year delay for XTE J1810−197 and ~1 month for 1E 1547.0−5408, with spin-down rate changes linked to magnetic island drift dynamics.

ABSTRACT

(Abridged) XTE J1810-197 and 1E 1547.0-5408 are two transient AXPs exhibiting radio emission with unusual properties. In addition, their spin down rates during outburst show opposite trends, which so far has no explanation. Here, we extend our quark-nova model for AXPs to include transient AXPs, in which the outbursts are caused by transient accretion events from a Keplerian (iron-rich) degenerate ring. For a ring with inner and outer radii of 23.5 km and 26.5 km, respectively, our model gives a good fit to the observed X-ray outburst from XTE J1810-197 and the behavior of temperature, luminosity, and area of the two X-ray blackbodies with time. The two blackbodies in our model are related to a heat front (i.e. Bohm diffusion front) propagating along the ring's surface and an accretion hot spot on the quark star surface. Radio pulsations in our model are caused by dissipation at the light cylinder of magnetic bubbles, produced near the ring during the X-ray outburst. The delay between X-ray peak emission and radio emission in our model is related to the propagation time of these bubbles to the light cylinder. We predict a ~1 year and ~1 month delay for XTE J1810-197 and 1E 1547.0-5408, respectively. The observed flat spectrum, erratic pulse profile, and the pulse duration are all explained in our model as a result of X-point reconnection events induced by the dissipation of the bubbles at the light cylinder. The spin down rate of the central quark star can either increase or decrease depending on how the radial drift velocity of the magnetic islands changes with distance from the central star. We suggest an evolutionary connection between transient AXPs and typical AXPs in our model.

Motivation & Objective

• To explain the dual X-ray blackbody emission and transient outbursts in AXPs XTE J1810−197 and 1E 1547.0−5408 using a quark-nova framework.
• To account for the delayed radio emission relative to X-ray peaks, with distinct time delays for the two sources.
• To explain the opposite spin-down rate trends—decreasing in XTE J1810−197 and increasing in 1E 1547.0−5408—within a unified model.
• To reproduce the flat radio spectra, erratic pulse profiles, and high luminosity of these transient radio sources.
• To establish an evolutionary link between transient AXPs and typical AXPs via the quark-nova mechanism.

Proposed method

• Modeling X-ray outbursts via transient accretion from a Keplerian, iron-rich degenerate ring with inner and outer radii of 23.5 km and 26.5 km.
• Introducing two X-ray blackbody components: one from a heat front (Bohm diffusion front) propagating along the ring’s surface, and one from an accretion hot spot on the quark star surface.
• Simulating radio emission via magnetic bubble dissipation at the light cylinder, with propagation delay scaling as $ t_{ m prop.} /propto P^{7/2 - eta/2} / ilde{P}^{1/2} $, where $ eta $ defines magnetospheric density radial dependence.
• Using X-point reconnection events at the light cylinder to explain flat radio spectra, erratic pulse profiles, and variable pulse durations.
• Modeling spin-down rate evolution based on radial drift velocity changes of magnetic islands in the magnetosphere.
• Calibrating the model using observed X-ray luminosity decay timescales (280 days and 870 days) and radio flux density enhancements (50× and 16× pre-burst levels).

Experimental results

Research questions

• RQ1 What causes the X-ray outbursts in transient AXPs XTE J1810−197 and 1E 1547.0−5408, and why do they exhibit dual blackbody components?
• RQ2 Why is there a significant delay between X-ray peak and radio emission, and why does it differ between the two sources?
• RQ3 How can the model explain the flat radio spectra and erratic, variable pulse profiles observed in these sources?
• RQ4 What physical mechanism explains the opposite spin-down rate trends—decreasing in XTE J1810−197 and increasing in 1E 1547.0−5408—during post-outburst evolution?
• RQ5 Can the quark-nova model provide a unified evolutionary framework linking transient AXPs to typical AXPs?

Key findings

• The model reproduces the X-ray light curves of XTE J1810−197 with a hot blackbody (T ~ 0.65 keV) decaying over ~280 days and a warm blackbody (T ~ 0.3 keV) decaying over ~870 days.
• For XTE J1810−197, the model predicts a radio emission delay of ~1 year due to bubble propagation to the light cylinder, consistent with observations of radio emission starting ~1 year after X-ray peak.
• For 1E 1547.0−5408, the model predicts a shorter delay of ~1 month, matching the observed radio emission onset relative to X-ray peak.
• The flat radio spectrum and erratic pulse profiles are explained by X-point reconnection events induced by dissipation of magnetic bubbles at the light cylinder.
• The spin-down rate evolution—decreasing in XTE J1810−197 and increasing in 1E 1547.0−5408—depends on how the radial drift velocity of magnetic islands changes with distance from the quark star.
• The model self-consistently predicts that the heat front (BF) and accretion hot spot (HS) evolve over time, with the HS area on one pole initially ~120 km², decreasing as the BF expands.