Abstract

ABSTRACT We predict linear polarization for a radioactively powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3D Monte Carlo radiative transfer simulations for two different models, both featuring a lanthanide-rich dynamical ejecta component from numerical-relativity simulations while only one including an additional lanthanide-free disc-wind component. We calculate polarization spectra for nine different orientations at 1.5, 2.5, and 3.5 d after the merger and in the $0.1\!-\!2\, \mu$m wavelength range. We find that both models are polarized at a detectable level 1.5 d after the merger while show negligible levels thereafter. The polarization spectra of the two models are significantly different. The model lacking a disc wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching $\sim 1\!-\!6{{\ \rm per\ cent}}$ at $2\, \mu$m depending on the orientation. The model with a disc-wind component, instead, features a characteristic ‘double-peak’ polarization spectrum with one peak in the optical and the other in the infrared. Polarimetric observations of future events will shed light on the debated neutron richness of the disc-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disc-wind component, while polarization increasing from zero in the optical to a peak in the infrared would suggest a lanthanide-rich composition for the whole ejecta. Future polarimetric campaigns should prioritize observations in the first ∼48 h and in the $0.5\!-\!2\, \mu$m range, where polarization is strongest, but also explore shorter wavelengths/later times where no signal is expected from the kilonova and the interstellar polarization can be safely estimated.

Highlights

  • Compact object mergers involving at least one neutron star (NS) were long regarded as the most promising scenario for a simultaneous detection of gravitational waves (GWs) and light

  • The detection of GRB 170817A provided the smoking gun for the long-thought association between short GRBs and binary neutron star (BNS) mergers, while the discovery of AT 2017gfo, a KN powered by the radioactive decay of r−process elements synthesized during the coalescence (Li & Paczyński 1998), confirmed that BNS mergers are the prime sites for the production of heavy elements in the Universe (e.g. Kasliwal et al 2019; Watson et al 2019)

  • Assuming an idealized two-component model for the material ejected in BNS mergers, Bulla et al (2019) showed that the presence of two distinct ejecta components with different geometries and compositions leads to an overall polarization signal

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Summary

INTRODUCTION

Compact object mergers involving at least one neutron star (NS) were long regarded as the most promising scenario for a simultaneous detection of gravitational waves (GWs) and light. Following a 19 months break, the Advanced LIGO (LIGO Scientific Collaboration et al 2015) and Advanced Virgo (Acernese et al 2015) interferometers began their third observing run in April, 2019 During this run, the LIGO/Virgo Collaboration (LVC) released 14 real-time public alerts thought to involve at least one NS: 6 BNS mergers and 8 black-hole (BH) - NS mergers. Assuming an idealized two-component model for the material ejected in BNS mergers, Bulla et al (2019) showed that the presence of two distinct ejecta components with different geometries and compositions leads to an overall polarization signal. They predicted a maximum polarization level of ∼ 0.8% at ∼ 7000 Å and 1.5 d after the merger under favourable (edge-on) inclinations of the system.

MODELS
RADIATIVE TRANSFER SIMULATIONS
RESULTS
Dynamical ejecta
DISCUSSION AND CONCLUSIONS
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