[Paper Review] Assessing Millisecond Proto-Magnetars as GRB Central Engines

Todd A. Thompson|arXiv (Cornell University)|Nov 12, 2006

Gamma-ray bursts and supernovae1 references11 citations

TL;DR

This paper evaluates millisecond proto-magnetars as central engines for long-duration gamma-ray bursts (GRBs), proposing that their relativistic, Poynting-flux-dominated winds—driven by rapid rotation and strong magnetic fields—can power relativistic jets. The key contribution is showing that the transition from non-relativistic to relativistic winds, shaped by hoop stress in the mass-loaded phase, naturally produces collimated outflows consistent with GRB observations.

ABSTRACT

Magnetars are a sizable subclass of the neutron star census. Their very high magnetic field strengths are thought to be a consequence of rapid (millisecond) rotation at birth in a successful core-collapse supernova. In their first tens of seconds of existence, magnetars transition from hot, extended ``proto-''magnetars to the cooled and magnetically-dominated objects we identify $\sim10^4$ years later as Soft Gamma-ray Repeaters (SGRs) and Anamolous X-ray Pulsars (AXPs). Millisecond proto-magnetar winds during this cooling phase likewise transition from non-relativistic and thermally-driven to magneto-centrifugally-driven, and finally to relativistic and Poynting-flux dominated. Here we review the basic considerations associated with that transition. In particular, we discuss the spindown of millisecond proto-magnetars throughout the Kelvin-Helmholtz cooling epoch. Because of their large reservoir of rotational energy, their association with supernovae, and the fact that their winds are expected to become highly relativistic in the seconds after their birth, proto-magnetars have been suggested as the central engine of long-duration gamma ray bursts. We discuss some of the issues and outstanding questions in assessing them as such.

Motivation & Objective

• To evaluate whether millisecond proto-magnetars can serve as viable central engines for long-duration gamma-ray bursts.
• To understand the transition from non-relativistic, thermally driven winds to relativistic, Poynting-flux-dominated winds in proto-magnetars.
• To investigate how magnetic fields and rapid rotation during the Kelvin-Helmholtz cooling phase influence jet collimation and energy extraction.
• To assess the implications of proto-magnetar wind dynamics for supernova remnant asymmetries and nucleosynthesis, particularly 56Ni yields.

Proposed method

• Analyzes the spindown of proto-magnetars during the Kelvin-Helmholtz cooling timescale (10–100 s), using rotational energy reservoirs of ~10^51 ergs.
• Models the evolution of wind properties through five phases: (1) hot, non-relativistic, neutrino-heated wind; (2) transition to magnetically dominated flow; (3) mass-loaded, non-relativistic wind with σ < 1; (4) transition to relativistic, Poynting-flux dominated wind; (5) fully relativistic, Poynting-flux dominated wind.
• Applies the hoop stress effect in the σ < 1 phase to show that energetic flux is preferentially channeled along the rotation axis, creating a pre-processed, hollow, asymmetric cavity.
• Uses the condition for maximum energy flux at zenith angle increasing with σ to explain how the non-relativistic wind shapes the environment for subsequent relativistic outflows.
• Evaluates the energy extraction efficiency and timescales for rotational energy release, comparing them to supernova shockwave traversal timescales.
• Considers the implications of wind-driven asymmetries on nucleosynthesis, particularly 56Ni production, and compares to observed hyper-energetic supernovae like SN 1998bw and SN 2003dh.

Experimental results

Research questions

• RQ1 Can the relativistic, Poynting-flux-dominated winds of millisecond proto-magnetars naturally produce the collimated jets observed in long-duration GRBs?
• RQ2 How does the transition from non-relativistic to relativistic wind phases influence jet collimation, and what role does hoop stress play in shaping the outflow geometry?
• RQ3 What is the timescale for rotational energy extraction in proto-magnetars, and how does it compare to the timescale for supernova shockwave propagation in Type-Ibc and Type-II progenitors?
• RQ4 To what extent can proto-magnetar winds modify the nucleosynthetic yields—particularly 56Ni—of supernova remnants, and can this explain hyper-energetic events?
• RQ5 Can the observed asymmetries in supernova remnants like Cassiopeia A be explained by proto-magnetar-driven winds, and is there evidence linking such remnants to magnetars?

Key findings

• The timescale for extracting ~10^51 ergs of rotational energy from a millisecond proto-magnetar is comparable to or shorter than the shockwave traversal timescale in Type-Ibc supernovae, enabling significant energy coupling to the ejecta.
• In the σ < 1 phase, hoop stress directs the energetic flux along the rotation axis, creating a pre-processed, asymmetric, and elongated cavity that can collimate subsequent relativistic winds.
• The relativistic Poynting-flux dominated wind (phase 5) emerges into a pre-conditioned environment, which may naturally lead to jet-like collimation without requiring fine-tuned initial conditions.
• For fiducial parameters, the proto-magnetar wind can extract enough energy to enhance 56Ni yields, potentially explaining the high 56Ni masses inferred in hyper-energetic supernovae like SN 1998bw and SN 2003dh.
• The Cassiopeia A supernova remnant exhibits a jet/counter-jet morphology and nucleosynthetic asymmetries consistent with a proto-magnetar origin, and its central X-ray source is consistent with an Anomalous X-ray Pulsar.
• The model suggests that proto-magnetar winds may significantly alter the angular distribution of nucleosynthetic products, particularly in asymmetric supernova remnants.