[Paper Review] Short gamma-ray bursts from binary neutron star mergers: the time-reversal scenario

R. Ciolfi, Daniel M. Siegel|arXiv (Cornell University)|May 6, 2015

Gamma-ray bursts and supernovae4 citations

TL;DR

This paper proposes a 'time-reversal' scenario in which a binary neutron star merger first forms a supramassive neutron star (SMNS) that powers long-lasting X-ray afterglow via spin-down radiation, followed by its delayed collapse to a black hole that launches a relativistic jet, producing the prompt short gamma-ray burst (SGRB). The model reconciles long X-ray afterglows with SGRB emission by inverting the standard BH-torus jet model timeline.

ABSTRACT

After decades of observations the physical mechanisms that generate short gamma-ray bursts (SGRBs) still remain unclear. Observational evidence provides support to the idea that SGRBs originate from the merger of compact binaries, consisting of two neutron stars (NSs) or a NS and a black hole (BH). Theoretical models and numerical simulations seem to converge to an explanation in which the central engine of SGRBs is given by a spinning BH surrounded by a hot accretion torus. Such a BH-torus system can be formed in compact binary mergers and is able to launch a relativistic jet, which can then produce the SGRB. This basic scenario, however, has recently been challenged by Swift satellite observations, which have revealed long-lasting X-ray afterglows in association with a large fraction of SGRB events. The long durations of these afterglows (from minutes to several hours) cannot be explained by the $\sim ext{s}$ accretion timescale of the torus onto the BH, and, instead, suggest a long-lived NS as the persistent source of radiation. Yet, if the merger results in a massive NS the conditions to generate a relativistic jet and thus the prompt SGRB emission are hardly met. Here we consider an alternative scenario that can reconcile the two aspects and account for both the prompt and the X-ray afterglow emission. Implications for future observations, multi-messenger astronomy and for constraining NS properties are discussed, as well as potential challenges for the model.

Motivation & Objective

• To resolve the observational conflict between long-lasting X-ray afterglows (minutes to hours) and the short-lived accretion timescale of a BH-torus system in standard SGRB models.
• To explain both the prompt SGRB and the long X-ray afterglow in a unified phenomenological framework, avoiding the dichotomy between magnetar and BH-torus models.
• To provide testable predictions for multi-messenger astronomy, particularly linking gravitational wave (GW) and electromagnetic (EM) signals from BNS mergers.
• To constrain neutron star equation of state (EOS) and magnetic field properties through joint GW and EM observations.
• To explore the viability of jet formation from a uniformly rotating, magnetized SMNS that collapses to a BH.

Proposed method

• The scenario assumes a binary neutron star merger forms a supramassive neutron star (SMNS) supported by uniform rotation, with mass above the nonrotating maximum but below the uniformly rotating maximum.
• Phase I: The differentially rotating SMNS ejects a baryon-loaded, mildly relativistic wind via magnetic winding and neutrino processes, lasting ~1 s.
• Phase II: As differential rotation dissipates, the SMNS becomes uniformly rotating and emits spin-down radiation, inflating a photon-pair plasma nebula that drives a shock through the ejecta, producing long-lasting X-ray afterglow.
• Phase III: The SMNS collapses to a black hole, forming a BH-torus system that launches a relativistic jet, which drills through the nebula and ejecta to produce the prompt SGRB.
• The model uses the SMNS lifetime (determined by spin-down) to predict the time delay between GW merger peak and SGRB, enabling precise GW-EM coincidence.
• Constraints on NS mass, spin period, and magnetic field strength are derived from the SMNS lifetime and spin-down luminosity, assuming dipole radiation.

Experimental results

Research questions

• RQ1 Can the long-lasting X-ray afterglows observed in many SGRB events be explained by spin-down radiation from a long-lived supramassive neutron star (SMNS) formed in a BNS merger?
• RQ2 Is it possible for a uniformly rotating, magnetized SMNS to launch a relativistic jet upon collapse to a black hole, consistent with SGRB production?
• RQ3 Can the time delay between the peak of gravitational wave emission (from merger) and the SGRB be used to measure the SMNS lifetime and constrain NS properties?
• RQ4 What constraints can be placed on the neutron star equation of state (EOS) and magnetic field strength using joint GW and X-ray afterglow observations?
• RQ5 What observational signatures, such as pre-SGRB X-ray emission or 'orphan' afterglows, can distinguish this time-reversal scenario from standard models?

Key findings

• The time-reversal scenario provides a unified explanation for both the prompt SGRB and long-lasting X-ray afterglow by inverting the standard BH-torus jet model timeline.
• The model predicts that the peak of gravitational wave emission from the BNS merger occurs significantly earlier than the SGRB, with a delay equal to the SMNS spin-down timescale.
• For a typical SMNS spin-down timescale of ~10^3 s, the time delay between GW peak and SGRB can be measured with ~0.1% precision, enabling accurate lifetime determination.
• The model allows for constraints on the neutron star equation of state: for a 2.0 M⊙ SMNS, the H4 EOS is excluded, while APR4 is allowed; for 2.61 M⊙, both are excluded.
• The initial magnetic field strength of the SMNS can be constrained to a narrow range (~10^14–10^15 G) if the initial spin period is known (e.g., from GW observations), assuming dipole spin-down.
• The scenario predicts observable pre-SGRB X-ray afterglow emission and the possibility of 'orphan' afterglows without a SGRB, providing key observational tests.