TY - JOUR
T1 - Simulations of early kilonova emission from neutron star mergers
AU - Banerjee, Smaranika
AU - Tanaka, Masaomi
AU - Kawaguchi, Kyohei
AU - Kato, Daiji
AU - Gaigalas, Gediminas
N1 - Funding Information:
We want to express our sincere gratitude to the anonymous referee for providing us with constructive comments. Numerical simulations presented in this paper were carried out with Cray XC50 at the Center for Computational Astrophysics, National Astronomical Observatory of Japan. This research was supported by the JSPS Bilateral Joint Research Project and the Grant-in-Aid for Scientific Research from JSPS (16H02183, 19H00694, 20H00158) and MEXT (17H06363).
Publisher Copyright:
© 2020. The American Astronomical Society. All rights reserved.
PY - 2020/9/20
Y1 - 2020/9/20
N2 - We present radiative transfer simulations for blue kilonovae hours after neutron star (NS) mergers by performing detailed opacity calculations for the first time. We calculate atomic structures and opacities of highly ionized elements (up to the 10th ionization) with atomic number Z = 20-56. We find that the bound-bound transitions of heavy elements are the dominant source of the opacities in the early phase (t < 1 day after the merger) and that the ions with a half-closed electron shell provide the highest contributions. The Planck mean opacity for lanthanide-free ejecta (with electron fraction of Ye = 0.30-0.40) can only reach around k ~ 0.5-1 cm2 g-1 at t = 0.1 days, whereas that increases up to k ~ 5-10 cm2 g-1 at t = 1 day. The spherical ejecta model with an ejecta mass of Mej = 0.05 Me gives the bolometric luminosity of ~2 ´ 1042 erg s-1 at t ∼ 0.1 days. We confirm that the existing bolometric and multicolor data of GW170817 can be naturally explained by the purely radioactive model. The expected early UV signals reach 20.5 mag at t ∼ 4.3 hr for sources even at 200 Mpc, which is detectable by the facilities such as Swift and the Ultraviolet Transient Astronomy Satellite (ULTRASAT). The early-phase luminosity is sensitive to the structure of the outer ejecta, as also pointed out by Kasen et al. Therefore, the early UV observations give strong constraints on the structure of the outer ejecta and the presence of a heating source besides r-process nuclei.
AB - We present radiative transfer simulations for blue kilonovae hours after neutron star (NS) mergers by performing detailed opacity calculations for the first time. We calculate atomic structures and opacities of highly ionized elements (up to the 10th ionization) with atomic number Z = 20-56. We find that the bound-bound transitions of heavy elements are the dominant source of the opacities in the early phase (t < 1 day after the merger) and that the ions with a half-closed electron shell provide the highest contributions. The Planck mean opacity for lanthanide-free ejecta (with electron fraction of Ye = 0.30-0.40) can only reach around k ~ 0.5-1 cm2 g-1 at t = 0.1 days, whereas that increases up to k ~ 5-10 cm2 g-1 at t = 1 day. The spherical ejecta model with an ejecta mass of Mej = 0.05 Me gives the bolometric luminosity of ~2 ´ 1042 erg s-1 at t ∼ 0.1 days. We confirm that the existing bolometric and multicolor data of GW170817 can be naturally explained by the purely radioactive model. The expected early UV signals reach 20.5 mag at t ∼ 4.3 hr for sources even at 200 Mpc, which is detectable by the facilities such as Swift and the Ultraviolet Transient Astronomy Satellite (ULTRASAT). The early-phase luminosity is sensitive to the structure of the outer ejecta, as also pointed out by Kasen et al. Therefore, the early UV observations give strong constraints on the structure of the outer ejecta and the presence of a heating source besides r-process nuclei.
KW - Neutron stars (1108)
KW - Radiative transfer (1335)
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U2 - 10.3847/1538-4357/abae61
DO - 10.3847/1538-4357/abae61
M3 - Article
AN - SCOPUS:85092545160
VL - 901
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 1
M1 - 29
ER -