TY - JOUR

T1 - Measuring the Hubble constant with kilonovae using the expanding photosphere method

AU - Sneppen, Albert

AU - Watson, Darach

AU - Poznanski, Dovi

AU - Just, Oliver

AU - Bauswein, Andreas

AU - Wojtak, Radoslaw

N1 - Publisher Copyright:
© 2023 The Authors.

PY - 2023/10/1

Y1 - 2023/10/1

N2 - While gravitational wave (GW) standard sirens from neutron star (NS) mergers have been proposed to offer good measurements of the Hubble constant, we show in this paper how a variation of the expanding photosphere method (EPM) or spectral-fitting expanding atmosphere method, applied to the kilonovae (KNe) associated with the mergers, can provide an independent distance measurement to individual mergers that is potentially accurate to within a few percent. There are four reasons why the KN-EPM overcomes the major uncertainties commonly associated with this method in supernovae: (1) the early continuum is very well-reproduced by a blackbody spectrum, (2) the dilution effect from electron scattering opacity is likely negligible, (3) the explosion times are exactly known due to the GW detection, and (4) the ejecta geometry is, at least in some cases, highly spherical and can be constrained from line-shape analysis. We provide an analysis of the early VLT/X-shooter spectra AT2017gfo showing how the luminosity distance can be determined, and find a luminosity distance of DL., =., 44.5 ± 0.8 Mpc in agreement with, but more precise than, previous methods. We investigate the dominant systematic uncertainties, but our simple framework, which assumes a blackbody photosphere, does not account for the full time-dependent three-dimensional radiative transfer effects, so this distance should be treated as preliminary. The luminosity distance corresponds to an estimated Hubble constant of H0 = 67.0 ± 3.6 km s-1 Mpc-1, where the dominant uncertainty is due to the modelling of the host peculiar velocity. We also estimate the expected constraints on H0 from future KN-EPM-analysis with the upcoming O4 and O5 runs of the LIGO collaboration GW-detectors, where five to ten similar KNe would yield 1% precision cosmological constraints.

AB - While gravitational wave (GW) standard sirens from neutron star (NS) mergers have been proposed to offer good measurements of the Hubble constant, we show in this paper how a variation of the expanding photosphere method (EPM) or spectral-fitting expanding atmosphere method, applied to the kilonovae (KNe) associated with the mergers, can provide an independent distance measurement to individual mergers that is potentially accurate to within a few percent. There are four reasons why the KN-EPM overcomes the major uncertainties commonly associated with this method in supernovae: (1) the early continuum is very well-reproduced by a blackbody spectrum, (2) the dilution effect from electron scattering opacity is likely negligible, (3) the explosion times are exactly known due to the GW detection, and (4) the ejecta geometry is, at least in some cases, highly spherical and can be constrained from line-shape analysis. We provide an analysis of the early VLT/X-shooter spectra AT2017gfo showing how the luminosity distance can be determined, and find a luminosity distance of DL., =., 44.5 ± 0.8 Mpc in agreement with, but more precise than, previous methods. We investigate the dominant systematic uncertainties, but our simple framework, which assumes a blackbody photosphere, does not account for the full time-dependent three-dimensional radiative transfer effects, so this distance should be treated as preliminary. The luminosity distance corresponds to an estimated Hubble constant of H0 = 67.0 ± 3.6 km s-1 Mpc-1, where the dominant uncertainty is due to the modelling of the host peculiar velocity. We also estimate the expected constraints on H0 from future KN-EPM-analysis with the upcoming O4 and O5 runs of the LIGO collaboration GW-detectors, where five to ten similar KNe would yield 1% precision cosmological constraints.

KW - Cosmological parameters

KW - Distance scale

KW - Radiation mechanisms: thermal

KW - Stars: neutron

UR - http://www.scopus.com/inward/record.url?scp=85174161546&partnerID=8YFLogxK

U2 - 10.1051/0004-6361/202346306

DO - 10.1051/0004-6361/202346306

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AN - SCOPUS:85174161546

SN - 0004-6361

VL - 678

JO - Astronomy and Astrophysics

JF - Astronomy and Astrophysics

M1 - A14

ER -