Polarization of Kilonova Emission from a Black Hole–Neutron Star Merger

Li, Yan; Shen, Rong-Feng

doi:10.3847/1538-4357/ab2387

Cited by 6 publications

(7 citation statements)

References 44 publications

(92 reference statements)

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“…Because the expansion occurs rapidly due to the high velocity, the emission from the dynamical ejecta is likely to be observed directly irrespective of the disk outflow. The polarization degree of $ 1% for the wavelength of J0:7lm might still be expected in a wide range of the viewing angle at 1-2 d after merger for a combination of ejecta components (Bulla et al 2021, see also Li and Shen 2019 for an early-time polarization with hypothetical existence of free neutrons).…”

Section: Kilonova/macronovamentioning

confidence: 94%

Coalescence of black hole–neutron star binaries

2021

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We review the current status of general relativistic studies for coalescences of black hole–neutron star binaries. First, high-precision computations of black hole–neutron star binaries in quasiequilibrium circular orbits are summarized, focusing on the quasiequilibrium sequences and the mass-shedding limit. Next, the current status of numerical-relativity simulations for the merger of black hole–neutron star binaries is described. We summarize our understanding for the merger process, tidal disruption and its criterion, properties of the merger remnant and ejected material, gravitational waveforms, and gravitational-wave spectra. We also discuss expected electromagnetic counterparts to black hole–neutron star coalescences.

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Section: Kilonova/macronovamentioning

confidence: 94%

Coalescence of black hole–neutron star binaries

2021

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“…The photosphere is initially in the layer of free neutrons, τ es ; 1, and the total polarization is ∼−0.4%. The timescale of the initial plateau can be used to effectively estimate the total mass of neutrons (e.g., Matsumoto 2018;Li & Shen 2019). When the photosphere recedes into the r-process element-rich ejecta, the main source of opacity is not electron scattering but boundbound transition, and the recombination of electrons to the rprocess elements causes τ es to drop rapidly, resulting in a rapid decrease in polarization degree.…”

Section: Lensed Kilonova Polarizationsmentioning

confidence: 99%

“…A fast expanding layer known as the fast velocity tail is ejected first and located at the head of the r-process ejecta (e.g., Kyutoku et al 2014;Ishii et al 2018;Radice et al 2018aRadice et al , 2018b. The fast tail has a short expansion timescale during which the neutron-capture reaction may not proceed efficiently, leaving free neutrons (Metzger et al 2015), which produce bright emission known as neutron precursors and exhibit a degree of polarization in the first hour after the merger (e.g., Matsumoto 2018;Li & Shen 2019).…”

Section: Introductionmentioning

confidence: 99%

“…The geometry and composition of the ejected material, as well as the interaction of the various ejecta components, are all factors that might affect the polarization signals that a distant observer receives. Matsumoto (2018) and Li & Shen (2019), focusing on the electron-scattering dominated period in the first hour after the merger, anticipated a polarization signal produced by free neutrons up to 3% for binary NS and BH-NS mergers, respectively. Bulla et al (2019) used a Monte Carlo simulation to examine the polarization produced by r-process ejecta in the binary NS merger scenario that were comparable to those of AT 2017gfo.…”

Section: Introductionmentioning

confidence: 99%

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Light Curves and Polarizations of Gravitationally Lensed Kilonovae

Liu

2023

ApJ

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Kilonovae are generally believed to originate from the ejecta of binary neutron stars (NSs) or black hole–NS mergers. Free neutrons might be retained in the outermost layer of the ejecta to produce a precursor via β decay. During the propagation of kilonovae to observers, a small percentage of them might be gravitationally lensed by foreground objects. In this paper, three lens models, i.e., the point-mass model, the singular isothermal sphere (SIS) model, and the Chang–Refsdal model, were taken into consideration to explore the light curves and polarizations of gravitationally lensed kilonovae. We found that, if the time delay between two images exceeds the ejecta-heating timescale for the lens mass ∼1010 M ⊙ in the SIS model, a tiny bump-like signal will be generated in the light curve, and the total luminosity will be magnified in all cases. The polarization of lensed kilonovae is significantly enhanced in most cases. Future detections of lensed kilonovae will impose constraints on the morphology of the ejecta and aid in the determination of the nature of compact object mergers and the search for strong gravitational lenses.

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“…Covino et al 2017;Lamb et al 2019;Troja et al 2019;Hajela et al 2019;Hotokezaka et al 2019), at different wavelengths and later times. Focusing on the electron-scattering dominated phase in the first few hours following the merger, Matsumoto (2018) and Li & Shen (2019) predicted a polarization signal up to ∼ 3% for BNS and BHNS mergers, respectively.…”

Section: Introductionmentioning

confidence: 99%

Polarized kilonovae from black hole-neutron star mergers

Bulla,

Kyutoku,

Tanaka

et al. 2020

Preprint

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We predict linear polarization for a radioactively-powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3-D 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 disk 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 µm 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 disk wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching ∼ 1% − 6% at 2 µm depending on the orientation. The model with a disk-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 disk-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disk-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 hours and in the 0.5 − 2 µm 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.

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Polarization of Kilonova Emission from a Black Hole–Neutron Star Merger

Cited by 6 publications

References 44 publications

Coalescence of black hole–neutron star binaries

Coalescence of black hole–neutron star binaries

Light Curves and Polarizations of Gravitationally Lensed Kilonovae

Polarized kilonovae from black hole-neutron star mergers

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