[Paper Review] Constraining the long-lived supramassive neutron stars by magnetar boosted kilonovae

Hao Wang, Paz Beniamini+1 more|arXiv (Cornell University)|Aug 17, 2023

Gamma-ray bursts and supernovaePhysics and Astronomy89 references3 citations

TL;DR

This paper models magnetar-boosted kilonovae—extremely luminous transients powered by energy injection from long-lived supramassive neutron stars (SMNS) formed in binary neutron star mergers. Using detailed simulations of magnetar spindown, ejecta evolution, and X-ray photoionization, the authors show that the absence of observed such events implies SMNS energy injection is far below theoretical limits, requiring collapse within seconds to minutes, thus constraining SMNS lifetimes to be short and their occurrence to be rare in the universe.

ABSTRACT

Kilonovae are optical transients following the merger of neutron star binaries, which are powered by the r-process heating of merger ejecta. However, if a merger remnant is a long-lived supramassive neutron star supported by its uniform rotation, it will inject energy into the ejecta through spindown power. The energy injection can boost the peak luminosity of a kilonova by many orders of magnitudes, thus significantly increasing the detectable volume. Therefore, even if such events are only a small fraction of the kilonovae population, they could dominate the detection rates. However, after many years of optical sky surveys, no such event has been confirmed. In this work, we build a boosted kilonova model with rich physical details, including the description of the evolution and stability of a proto neutron star, and the energy absorption through X-ray photoionization. We simulate the observation prospects and find the only way to match the absence of detection is to limit the energy injection by the newly born magnetar to only a small fraction of the neutron star rotational energy, thus they should collapse soon after the merger. Our result indicates that most supramassive neutron stars resulting from binary neutron star mergers are short lived and they are likely to be rare in the Universe.

Motivation & Objective

• To understand the detectability of magnetar-boosted kilonovae, which could be orders of magnitude brighter than standard kilonovae due to energy injection from a long-lived supramassive neutron star (SMNS).
• To resolve the apparent contradiction between the high detectable volume of such events and their non-detection in optical surveys over many years.
• To constrain the energy injection efficiency from a newborn magnetar into the ejecta, using the absence of observed events as a key observational limit.
• To determine the implications for the equation of state and central engine physics in binary neutron star merger remnants.

Proposed method

• Developed a detailed boosted kilonova model incorporating the spindown evolution of a proto-magnetar, including rotational energy loss via magnetic dipole radiation.
• Tracked energy deposition into the ejecta via X-ray photoionization, accounting for radiative transfer and ionization state evolution.
• Simulated light curves and spectral energy distributions across optical and near-infrared bands, including the effects of ejecta expansion and cooling.
• Constrained the energy injection fraction by comparing model predictions with the non-detection of such events in deep optical surveys.
• Performed both model-dependent and model-independent analyses of detection rates to assess the statistical significance of non-detection.
• Incorporated instability mechanisms such as Rayleigh-Taylor growth in the magnetar wind bubble to assess blast wave formation and afterglow emission.

Figure 1: Illustration of the structure of the post-merger system. The system is composed by an inner pulsar wind nebula (PWN) inflated by a magnetar, and an outer ejecta shell. The PWN is composed by X-ray photons and electron-positron pairs. The X-ray radiation ionize and heat the ejecta, leading

Experimental results

Research questions

• RQ1 What fraction of rotational energy from a newborn magnetar must be injected into the ejecta to produce a detectable magnetar-boosted kilonova?
• RQ2 Why have no magnetar-boosted kilonovae been detected despite their potentially large detectable volume?
• RQ3 How does the stability and evolution of a supramassive neutron star affect the luminosity and light curve of a boosted kilonova?
• RQ4 What are the implications of non-detection for the lifetime and formation rate of long-lived supramassive neutron stars?
• RQ5 Can the formation of a relativistic blast wave from a breakout of the magnetar wind into the ejecta explain the absence of orphan afterglows?

Key findings

• The absence of detected magnetar-boosted kilonovae implies that only a small fraction—less than 1%—of the magnetar's rotational energy can be injected into the ejecta, ruling out efficient energy transfer.
• Supramassive neutron stars formed in binary neutron star mergers must collapse within seconds to minutes after formation, indicating they are short-lived and not stable for long durations.
• The model predicts that the peak luminosity of boosted kilonovae is highly sensitive to the SMNS survival timescale, with longer lifetimes leading to significantly brighter and more detectable transients.
• The non-detection of orphan afterglows from blast wave breakout suggests such events are rare, further supporting the conclusion that SMNS collapse occurs quickly.
• The gravitational wave emission from a long-lived, rapidly rotating SMNS is unlikely to be detectable, as the required ellipticity and timescale are inconsistent with the observed non-detection.
• The results imply that SMNS formation is rare in the universe, as only a small fraction of mergers can produce such short-lived remnants.

Figure 2: The Monte Carlo simulation result of absorption, reflection, and transmission rate of a slab caused by both absorption and scattering processes. The red lines show the contour plot of $A^{\rm tra}$ as a function of optical depth of two processes, and the blue lines are the $A^{\rm ref}$ .