This guest post was written by Jay Vijay Kalinani. He is a PhD student in Physics at the University of Padova, Italy. His research is primarily based on working with numerical relativity codes to perform binary neutron star merger simulations on supercomputers. When not in front of a computer, he sometimes go out searching and clicking abandoned umbrellas in and around the city.

Title: Short gamma-ray bursts in the “time-reversal” scenario

Authors: Riccardo Ciolfi, Daniel M. Siegel

First author’s institution: University of Trento, Italy

Status: Accepted to ApJL, open access on arXiv

Recent detection of gravitational waves (GW) from the merger of two neutron stars by the LIGO–Virgo interferometers along with their electromagnetic counterparts across the entire spectrum, has opened the floodgates for multi-messenger astrophysics. This event, famously known as GW170817, was also coincident with a short gamma-ray burst (SGRB) GRB 170817A, giving us smoking-gun evidence connecting binary neutron star (BNS) mergers to SGRBs. However, the nature of remnants left behind by BNS mergers that could act as possible central engines to power such highly relativistic SGRB jets still remains an open question.

Remnants from a binary neutron star merger

Depending on the ratio of the total mass of the BNS system ‘M_BNS’ and the maximum mass allowed for a neutron star (NS) ‘M_max’, and on the NS equation of state, a BNS merger can lead to the following possible outcomes (see Figure 1):

1. If M_BNS/M_max ≳ 1.5: prompt collapse to a black hole (BH) with a surrounding accretion disk which gets eaten by the BH within a few seconds.
2. If 1 ≲ M_BNS/M_max ≲ 1.5: formation of a hypermassive NS (HMNS) which ultimately collapses to a BH in about 1-100 milliseconds (ms).
3. If M_BNS/M_max ≲ 1: formation of a long-lived NS, which could be either supramassive (SMNS), eventually collapsing to a black hole in seconds, minutes or more, or be indefinitely stable.

Consequently, the most-likely candidates for SGRB central engines are BHs and supramassive/hypermassive NSs possibly surrounded by an accretion disk.

Central engine: a black hole or a magnetar?

SGRBs are one of the most violent, energetic and bright explosions in the entire universe. These bursts, lasting anywhere from a few milliseconds to 2 seconds (with an average of around 0.2 seconds), are generally characterised by a collimated relativistic outflow powered by a central engine.  The leading SGRB central engine models favour the classic BH-disk system, capable of launching a relativistic jet via the Blandford-Znajek mechanism. Such a system very well explains the prompt SGRB emission. But what do observations tell us? A large fraction of SGRB events are accompanied by afterglows lying in the X-ray band of the electromagnetic spectrum that last between 100-10000 second. This is inconsistent with the survival timescales of accretion disks around BHs (roughly 1 second).

Thus, we move on to the ‘magnetar model’ which suggests that the spin-down radiation from a highly magnetised NS (or so-called ’magnetar’) could power a relativistic jet and also can produce the required X-ray afterglows, in particular, the characteristic X-ray plateau (see Figure 2) for much longer times. But this model also has its own caveat: baryon pollution.

A BNS merger is an explosive event, ejecting a lot of baryonic matter which can comprise a few percent of a solar mass in total. This ejecta makes up the remnant environment. In the case of BH as the remnant, ejecta along the spin-axis gets sucked back in by the BH. This baryon-free funnel provides a jet-friendly neighbourhood (Figure 3 top-panel). While in the case of magnetar model, the funnel is highly baryon-loaded (Figure 3 bottom-panel) which can make it difficult for the jet to break through.

Entering ‘time-reversal’

In this paper, the authors propose a third alternative called the ’time-reversal’ scenario, where a supramassive NS is formed after the BNS merger, which then collapses to a BH-disk system to produce an SGRB jet. The three main phases of this scenario are as follows:

1. In the first few milliseconds after the merger, the formed SMNS ejects baryon-loaded, highly-isotropic winds (Figure 4, left-panel).
2. The SMNS then cools down by emitting spin-down radiation, generating a photon-pair plasma nebula that develops a strong shock in the nearby ejecta constraining it into a thin shell. This cleans up the surrounding environment around the remnant, making it good-to-go for jet launching (Figure 4, center-panel).
3. The NS then collapses to a BH-disk system, and the resulting jet drills through the nebula and the ejecta shell producing the prompt SGRB. The spin-down emission continues to diffuse outward, possibly explaining the observed X-ray plateau in SGRB light-curves (Figure 4, right-panel).

The prompt SGRB jet and the spin-down emission powering the X-ray plateau are observed in the opposite order with respect to their emission from the central engine, hence the scenario is named ‘time-reversal.’

To validate their model, the authors compute diffusion time-scales of the photons generated by spinning-down of the SMNS just before collapse over a wide range of physical parameters (see section 2-4 of the paper for the gory theory), to explain the delay in the signal of X-ray radiation. It turns out that their analysis can successfully account for emission delays to time-scales of about 10⁵ seconds, making ‘time-reversal’ scenario an elegant addition to the list of current SGRB models.

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