We review current understanding of kilonova/macronova emission from compact binary mergers (mergers of two neutron stars or a neutron star and a black hole). Kilonova/macronova is emission powered by ...radioactive decays of r -process nuclei and it is one of the most promising electromagnetic counterparts of gravitational wave sources. Emission from the dynamical ejecta of ~0.01 M ⊙ is likely to have a luminosity of ~1040–1041 erg s−1 with a characteristic timescale of about 1 week. The spectral peak is located in red optical or near-infrared wavelengths. A subsequent accretion disk wind may provide an additional luminosity or an earlier/bluer emission if it is not absorbed by the precedent dynamical ejecta. The detection of near-infrared excess in short GRB 130603B and possible optical excess in GRB 060614 supports the concept of the kilonova/macronova scenario. At 200 Mpc distance, a typical peak brightness of kilonova/macronova with 0.01 M ⊙ ejecta is about 22 mag and the emission rapidly fades to >24 mag within ~10 days. Kilonova/macronova candidates can be distinguished from supernovae by (1) the faster time evolution, (2) fainter absolute magnitudes, and (3) redder colors. Since the high expansion velocity ( v ~ 0.1 – 0.2 c ) is a robust outcome of compact binary mergers, the detection of smooth spectra will be the smoking gun to conclusively identify the gravitational wave source.
Mergers of binary neutron stars (NSs) are among the most promising gravitational wave (GW) sources. Next generation GW detectors are expected to detect signals from NS mergers within about 200 Mpc. ...The detection of electromagnetic wave (EM) counterparts is crucial to understanding the nature of GW sources. Among the possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, predictions so far have not taken into account detailed r-process element abundances in the ejecta. We perform for the first time radiative transfer simulations of the NS merger ejecta including all the r-process elements from Ga to U. We show that the opacity of the NS merger ejecta is about kappa = 10 cm super(2) g super(-1), which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ~100. As a result, the emission is fainter and lasts longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound-bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of the two NSs and a softer equation of state adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, the expected brightness of the emission is i = 22-25 AB mag, z = 21-23 AB mag, and 21-24 AB mag in the NIR JHK bands. Thus, observations with wide-field 4 m- and 8 m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.
Systematic opacity calculations for kilonovae Tanaka, Masaomi; Kato, Daiji; Gaigalas, Gediminas ...
Monthly notices of the Royal Astronomical Society,
08/2020, Letnik:
496, Številka:
2
Journal Article
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ABSTRACT
Coalescence of neutron stars (NSs) gives rise to kilonova, thermal emission powered by radioactive decays of freshly synthesized r-process nuclei. Although observational properties are ...largely affected by bound–bound opacities of r-process elements, available atomic data have been limited. In this paper, we study element-to-element variation of the opacities in the ejecta of NS mergers by performing systematic atomic structure calculations of r-process elements for the first time. We show that the distributions of energy levels tend to be higher as electron occupation increases for each electron shell due to the larger energy spacing caused by larger effects of spin–orbit and electron–electron interactions. As a result, elements with a fewer number of electrons in the outermost shells tend to give larger contributions to the bound–bound opacities. This implies that Fe is not representative for the opacities of light r-process elements. The average opacities for the mixture of r-process elements are found to be κ ∼ 20–30 cm2 g−1 for the electron fraction of Ye ≤ 0.20, κ ∼ 3–5 cm2 g−1 for Ye = 0.25–0.35, and κ ∼ 1 cm2 g−1 for Ye = 0.40 at $T = 5000\!-\!10\, 000$ K, and they steeply decrease at lower temperature. We show that, even with the same abundance or Ye, the opacity in the ejecta changes with time by one order of magnitude from 1 to 10 d after the merger. Our radiative transfer simulations with the new opacity data confirm that ejecta with a high electron fraction (Ye ≳ 0.25, with no lanthanide) are needed to explain the early, blue emission in GW170817/AT2017gfo while lanthanide-rich ejecta (with a mass fraction of lanthanides ∼5 × 10−3) reproduce the long-lasting near-infrared emission.
We perform radiative transfer simulations for kilonova in various situations, including the cases of prompt collapse to a black hole from neutron star mergers, high-velocity ejecta possibly ...accelerated by magnetars, and a black hole-neutron star merger. Our calculations are done employing ejecta profiles predicted by numerical-relativity simulations and a new line list for all the r-process elements. We found that: (i) the optical emission for binary neutron stars promptly collapsing to a black hole would be fainter by 1-2 mag than that found in GW170817, while the infrared emission could be as bright as that in GW170817 if the post-merger ejecta is as massive as 0.01 M ; (ii) the kilonova would be brighter than that observed in GW170817 for the case that the ejecta is highly accelerated by the electromagnetic energy injection from the remnant, but within a few days it would decline rapidly and the magnitude would become fainter than in GW170817; and (iii) the optical emission from a black hole-neutron star merger ejecta could be as bright as that observed in GW170817 for the case that sufficiently large amount of matter is ejected ( 0.02 M ), while the infrared brightness would be brighter by 1-2 mag at the same time. We show that the difference in the ejecta properties would be imprinted in the differences in the peak brightness and time of peak. This indicates that we may be able to infer the type of the central engine for kilonovae by observation of the peak in the multiple band.
Recent detection of gravitational waves from a binary neutron star merger (GW170817) and the subsequent observations of electromagnetic counterparts provide a great opportunity to study the physics ...of compact binary mergers. The optical and near-infrared counterparts to GW170817 (SSS17a, also known as AT 2017gfo or DLT17ck) are found to be consistent with a kilonova/macronova scenario with red and blue components. However, in most previous studies wherein the contribution from each ejecta component to the lightcurves is separately calculated and composited, the red component is too massive of a dynamical ejecta, and the blue component is too fast of a post-merger ejecta. In this Letter, we perform a two-dimensional radiative transfer simulation for a kilonova/macronova, consistently taking the interplay of multiple ejecta components into account. We show that the lightcurves and photospheric velocity of SSS17a can be reproduced naturally by a setup that is consistent with the prediction of the numerical-relativity simulations.
ABSTRACT Black hole-neutron star (BH-NS) mergers are among the most promising gravitational-wave sources for ground-based detectors, and gravitational waves from BH-NS mergers are expected to be ...detected in the next few years. The simultaneous detection of electromagnetic counterparts with gravitational waves would provide rich information about merger events. Among the possible electromagnetic counterparts from BH-NS mergers is the so-called kilonova/macronova, emission powered by the decay of radioactive r-process nuclei, which is one of the best targets for follow-up observations. We derive fitting formulas for the mass and the velocity of ejecta from a generic BH-NS merger based on recently performed numerical-relativity simulations. We combine these fitting formulas with a new semi-analytic model for a BH-NS kilonova/macronova lightcurve, which reproduces the results of radiation-transfer simulations. Specifically, the semi-analytic model reproduces the results of each band magnitude obtained by the previous radiation-transfer simulations within ∼1 mag. By using this semi-analytic model we found that, at 400 Mpc, the kilonova/macronova is as bright as 22-24 mag for cases with a small chirp mass and a high black hole spin, and >28 mag for a large chirp mass and a low black hole spin. We also apply our model to GRB 130603B as an illustration, and show that a BH-NS merger with a rapidly spinning black hole and a large neutron star radius is favored.
Spectropolarimetry is one of the most powerful methods to study the multi-dimensional geometry of supernovae (SNe). We present a brief summary of the spectropolarimetric observations of ...stripped-envelope core-collapse SNe. Observations indicate that stripped-envelope SNe generally have a non-axisymmetric ion distribution in the ejecta. Three-dimensional clumpy geometry nicely explains the observed properties. A typical size of the clumps deduced from observations is relatively large: 25% of the photosphere. Such a large-scale clumpy structure is similar to that observed in Cassiopeia A, and suggests that large-scale convection or standing accretion shock instability takes place at the onset of the explosion.
This article is part of the themed issue ‘Bridging the gap: from massive stars to supernovae’.
Abstract
Binary neutron star (NS) mergers have been expected to synthesize
r
-process elements and emit radioactively powered radiation, called kilonovae. Although
r
-process nucleosynthesis was ...confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound–bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr
ii
triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca
ii
triplet lines. This is natural because Ca and Sr are coproduced in the material with relatively high electron fraction and their ions have similar atomic structures with only one
s
-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb, and Th. Our results suggest that the line-forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr
ii
and Ca
ii
lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca
ii
line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is <0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥0.40 and entropy per nucleon (in units of Boltzmann constant) ≥25.
ABSTRACT
The nebular phase of lanthanide-rich ejecta of a neutron star merger (NSM) is studied by using a one-zone model, in which the atomic properties a represented by a single species, neodymium ...(Nd). Under the assumption that β-decay of r-process nuclei is the heat and ionization source, we solve the ionization and thermal balance of the ejecta under non-local thermodynamic equilibrium. The atomic data including energy levels, radiative transition rates, collision strengths, and recombination rate coefficients are obtained by using atomic structure codes, grasp2k
and hullac. We find that both permitted and forbidden lines roughly equally contribute to the cooling rate of Nd ii and Nd iii at the nebular temperatures. We show that the kinetic temperature and ionization degree increase with time in the early stage of the nebular phase, while these quantities become approximately independent of time after the thermalization break of the heating rate because the processes relevant to the ionization and thermalization balance are attributed to two-body collision between electrons and ions at later times. As a result, in spite of the rapid decline of the luminosity, the shape of the emergent spectrum does not change significantly with time after the break. We show that the emission-line nebular spectrum of the pure Nd ejecta consists of a broad structure from $0.5$ to $20\, {\rm \mu m}$ with two distinct peaks around $1$ and $10\, {\rm \mu m}$.
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 at t = 0.1 days, whereas that increases up to at t = 1 day. The spherical ejecta model with an ejecta mass of Mej = 0.05 M gives the bolometric luminosity of 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.
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