The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo

Bulla, Mattia; Covino, S.; Kyutoku, Koutarou; Tanaka, Masaomi; Maund, Justyn R.; Patat, F.; Toma, Kenji; Wiersema, K.; Bruten, J. R.; Jin, Zhi-Ping; Testa, V.

doi:10.1038/s41550-018-0593-y

Cited by 37 publications

(48 citation statements)

References 57 publications

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“…Q reaches values of −0.2 per cent in the infrared at 2.5 d after the merger and becomes negligible at later epochs. This behaviour is in good agreement with what was found in Bulla et al (2019), with differences in the absolute polarization levels due to the different choices for the opacities.…”

Section: Polarizationsupporting

confidence: 90%

“…As mentioned in Section 2, our radiative transfer code possis is well-suited to calculate synthetic observables for both SN and KN models. In this study, however, we choose to test the code possis by computing spectra, light curves and polarization for the two-component KN model of Bulla et al (2019). Fig.…”

Section: A Test Model For Kilonovaementioning

confidence: 99%

“…The overall Stokes vectors coming from the lanthanide-free component are aligned in the horizontal direction, thus resulting in a negative Q value (see also fig. 2 in Bulla et al 2019).…”

Section: Polarizationmentioning

confidence: 99%

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possis: predicting spectra, light curves, and polarization for multidimensional models of supernovae and kilonovae

Bulla

2019

Monthly Notices of the Royal Astronomical Society

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We present possis, a time-dependent three-dimensional Monte Carlo code for modelling radiation transport in supernovae and kilonovae. The code incorporates wavelength-and time-dependent opacities and predicts viewing-angle dependent spectra, light curves and polarization for both idealized and hydrodynamical explosion models. We apply the code to a kilonova model with two distinct ejecta components, one including lanthanide elements with relatively high opacities and the other devoid of lanthanides and characterized by lower opacities. We find that a model with total ejecta mass M ej = 0.04 M and half-opening angle of the lanthanide-rich component Φ = 30 • provides a good match to GW 170817/AT 2017gfo for orientations near the polar axis (i.e. for a system viewed close to face-on). We then show how crucial is the use of self-consistent multi-dimensional models in place of combining one-dimensional models to infer important parameters such as the ejecta masses. We finally explore the impact of M ej and Φ on the synthetic observables and highlight how the relatively fast computation times of possis make it well-suited to perform parameter-space studies and extract key properties of supernovae and kilonovae. Spectra calculated with possis in this and future studies will be made publicly available.

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Section: Polarizationsupporting

confidence: 90%

Section: A Test Model For Kilonovaementioning

confidence: 99%

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possis: predicting spectra, light curves, and polarization for multidimensional models of supernovae and kilonovae

Bulla

2019

Monthly Notices of the Royal Astronomical Society

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“…As was found in Matsumoto (2018), the degree of polarization strongly relies on the existence of a free neutrons in the fastest component of the ejecta. While previous works (Bulla et al 2018;Matsumoto 2018) concentrated to the context of NS-NS mergers, in this paper we focus on the BH-NS mergers, a case potentially more susceptible to a polarization signature.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…In this paper, we extend this line of work to the case of a BH-NS merger kilonova, which is expected to show more significant polarization than that of a NS-NS merger. Adopting the analytic model of Shapiro & Sutherland (1982), which is different from the Monte Carlo approach or result used by Matsumoto (2018) and Bulla et al (2018), we can give detailed constraints not only on the mass of free neutrons but also the morphology and components of the ejecta. This paper is structured as follows.…”

Section: Introductionmentioning

confidence: 99%

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

Shen

2019

ApJ

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A multi-messenger, black hole (BH) -neutron star (NS) merger event still remains to be detected. The tidal (dynamical) ejecta from such an event, thought to produce a kinonova, is concentrated in the equatorial plane and occupies only part of the whole azimuthal angle. In addition, recent simulations suggest that the outflow or wind from the post-merger remnant disk, presumably anisotropic, can be a major ejecta component responsible for a kilonova. For any ejecta whose photosphere shape deviates from the spherical symmetry, the electron scattering at the photosphere causes a net polarization in the kilonova light. Recent observational and theoretical polarization studies have been focused to the NS-NS merger kilonova AT2017gfo. We extend those work to the case of a BH-NS merger kilonova. We show that the degree of polarization at the first ∼ 1 hr can be up to ∼ 3% if a small amount (10 −4 M ) of free neutrons have survived in the fastest component of the dynamical ejecta, whose beta-decay causes a precursor in the kilonova light. The polarization degree can be ∼ 0.6% if free neutrons survived in the fastest component of the disk wind. Future polarization detection of a kilonova will constrain the morphology and composition of the dominant ejecta component, therefore help to identify the nature of the merger.

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