Kilonovae

Abstract

The mergers of double neutron star (NS-NS) and black hole (BH)-NS binaries are promising gravitational wave (GW) sources for Advanced LIGO and future GW detectors. The neutron-rich ejecta from such merger events undergoes rapid neutron capture (r-process) nucleosynthesis, enriching our Galaxy with rare heavy elements like gold and platinum. The radioactive decay of these unstable nuclei also powers a rapidly evolving, supernova-like transient known as a "kilonova" (also known as "macronova"). Kilonovae are an approximately isotropic electromagnetic counterpart to the GW signal, which also provides a unique and direct probe of an important, if not dominant, r-process site. I review the history and physics of kilonovae, leading to the current paradigm of week-long emission with a spectral peak at near-infrared wavelengths. Using a simple light curve model to illustrate the basic physics, I introduce potentially important variations on this canonical picture, including: [Formula: see text]day-long optical ("blue") emission from lanthanide-free components of the ejecta; [Formula: see text]hour-long precursor UV/blue emission, powered by the decay of free neutrons in the outermost ejecta layers; and enhanced emission due to energy input from a long-lived central engine, such as an accreting BH or millisecond magnetar. I assess the prospects of kilonova detection following future GW detections of NS-NS/BH-NS mergers in light of the recent follow-up campaign of the LIGO binary BH-BH mergers.

Keywords: Black holes; Gravitational waves; Neutron stars; Nucleosynthesis; Radiative transfer.

Fig. 1

Timeline of the development kilonova models in the space of peak luminosity and peak timescale. The wavelength of the predicted spectral peak are indicated by color as marked in the figure

Fig. 2

Luminosity versus time after the merger of a range of heating sources relevant to powering kilonovae. Left sources of radioactive heating include the decay of $\sim {10}^{-2}{M}_{\odot }$ of r-process nuclei, as first modeled in a parameterized way by Li and Paczyński (1998) (Eq. 2, grey band) and then more accurately by Metzger et al. (2010b), plotted here using the analytic fit of Korobkin et al. (2012) (Eq. 22, black line) and applying the thermalization efficiency of Barnes et al. (2016) (Eq. 23). The outer layers of the merger ejecta may contain ${10}^{-4}{M}_{\odot }$ free neutrons (red line), which due to their anomalously long half-life produce significant heating on a timescale of tens of minutes if they exist in the ejecta (Sect. 4.1.3). Right sources of central engine heating. These include fall-back accretion (blue lines), shown separately for the case of a NS–NS merger (solid line) and BH–NS merger (dashed line), based on the SPH simulations of Rosswog (2007) for an assumed jet efficiency ${\mathit{ϵ}}_j=0.1$ (Eq. 30). Also shown is the energy input due to the spin-down of a stable central magnetar remnant with an initial spin period of $P=0.7\text{ms}$ dipole field strengths of $B={10}^{15}\text{G}$ (brown lines) and $B={10}^{16}\text{G}$ (orange lines). We show separately the total spin-down luminosity $L_{\mathrm{sd}}$ (dashed lines; Eq. 32), as well as the effective luminosity accounting also for the suppression of thermalization of the magnetar energy by the high opacity of $e^{\pm }$ pairs in the nebula (solid lines; see Eq. 35 and surrounding discussion; Metzger and Piro 2014). The isotropic X-ray luminosity of the extended emission following the short GRB 080503 is shown with a green line for an assumed redshift $z=0.3$ (Perley et al. ; see also bottom panel of Fig. 9)

Fig. 3

Different components of the ejecta from NS–NS mergers and the dependence of their kilonova emission on the observer viewing angle, ${\mathit{\theta }}_{\mathrm{obs}}$, relative to the binary axis, in the case of prompt BH formation (top panel) and a long-lived magnetar remnant (bottom panel). In both cases, the dynamical ejecta in the equatorial plane is highly neutron-rich ($Y_e\lesssim 0.1$), producing lanthanides and correspondingly “red” kilonova emission peaking at NIR wavelengths. Mass ejected dynamically in the polar directions by shock heating may be sufficiently neutron-poor ($Y_e\gtrsim 0.3$; Wanajo et al. 2014b) to preclude Lanthanide production, instead powering “blue” kilonova emission at optical wavelengths (although this component may be suppressed if BH formation is extremely prompt). The outermost layers of the polar ejecta may contain free neutrons, the decay of which powers a UV transient lasting a few hours following the merger (Sect. 4.1.3). The innermost layers of the ejecta originate from accretion disk outflows, which are likely to emerge more isotropically. When BH formation is prompt, this matter is also mainly neutron-rich, powering red kilonova emission (Just et al. ; Wu et al. 2016). If the remnant is instead long-lived, then neutrinos from the NS remnant can increase the electron fraction of the disk outflows to suppress Lanthanide production and result in blue disk wind emission (Metzger and Fernández ; Perego et al. ; Martin et al. 2015). Energy input from the central accreting BH (top panel) or magnetar remnant (bottom panel) enhance the kilonova luminosity compared to the purely radioactive-powered case (Sect. 4.2)

Fig. 4

Schematic illustration of the opacity of the NS merger ejecta as a function of photon energy near peak light. The free–free opacity (red line) is calculated assuming singly-ionized ejecta of temperature $T=2\times {10}^4\text{K}$ and density $\mathit{\rho }={10}^{-14}\text{g}{\text{cm}}^{-3}$, corresponding to the mean properties of ${10}^{-2}{M}_{\odot }$ of ejecta expanding at $v=0.1\text{c}$ at $t=3$ days. Line opacities of Fe-like elements and lanthanide-rich elements are approximated from Figs. 3 and 7 of Kasen et al. (2013). Bound-free opacities are estimated as that of neutral Fe (Verner et al. 1996), which we expect to crudely approximate the those of heavier r-process elements. Electron scattering opacity accounts for the Klein–Nishina suppression at energies $\gg m_e{c}^2$ and (very schematically) for the rise in opacity that occurs above the keV energy scale due to all electrons (including those bound in atoms) contributing to the scattering opacity when the photon wavelength is smaller than the atomic scale. At the highest energies, opacity is dominated by pair creation by $\mathit{\gamma }$-rays interacting with the electric fields of nuclei in the ejecta (shown schematically for Xenon, $A=131$, $Z=54$). Not included are possible contributions from r-process dust; or $\mathit{\gamma }$–$\mathit{\gamma }$ pair creation opacity at energies $\gg m_e{c}^2$, which is important for high compactness $\ell \gg 1$ (Eq. 9)

Fig. 5

Kilonova light curves in AB magnitudes for a source at 200 Mpc, calculated using the toy model presented in Sect. 4, assuming a total ejecta mass $M={10}^{-2}$ and minimum velocity $v_0=0.1\text{c}$. The top panel shows a standard “red” kilonova, corresponding to very neutron-rich ejecta with Lanthanide elements, while the bottom panel shows a “blue” kilonova produced by ejecta without Lanthanides. Shown for comparison in the red kilonova case with dashed lines are models from Barnes et al. (2016) for $v=0.1\text{c}$ and $M={10}^{-2}{M}_{\odot }$. Depending on the viewing angle of the observer, both red and blue emission components may be present in a single merger, if they originate from different locations in the ejecta (Fig. 3)

Fig. 6

Kilonova light curves, including the presence of free neutrons in the outer $M_{\mathrm{n}}={10}^{-4}{M}_{\odot }$ mass layers of the ejecta (“neutron precusors”), calculated for the same parameters of total ejecta mass $M={10}^{-2}$ and velocity $v_0=0.1\text{c}$ used in Fig. 5. The top panel shows a calculation with an opacity appropriate to lanthanide-bearing nuclei, while the bottom panel shows an opacity appropriate to lanthanide-free ejecta. Models without a free neutron layer ($M_n=0$; Fig. 5) are shown for comparison with dashed lines

Fig. 7

Kilonova light curves powered by fall-back accretion, calculated for the same parameters of total ejecta mass $M={10}^{-2}$ and velocity $v_0=0.1\text{c}$ used in Fig. 5, and for an opacity appropriate to lanthanide-bearing nuclei. We adopt an ejecta heating rate from Eq. (30) for a fixed efficiency ${\mathit{ϵ}}_{\mathrm{j}}=0.1$. We normalize the mass fall-back rate to a value of ${\dot{M}}_{\mathrm{fb}}(t=0.1)={10}^{-3}{M}_{\odot }{\text{s}}^{-1}$ in the case of NS–NS mergers (top panel), and to a value 10 times higher in BH–NS mergers (bottom panel), based on Rosswog (2007)

Fig. 8

Maximum extractable rotational energy from the magnetar remnant of a NS–NS merger as a function of its gravitational mass $M_{\mathrm{ns}}$ (black line, left axis). Below the maximum mass of a non-rotating NS of $M_{\max }(\mathit{\Omega }=0)$, this is just the rotational energy at the mass-shedding limit. For $M_{\mathrm{ns}}\gtrsim M_{\max }(\mathit{\Omega }=0)$, the extractable rotational energy is the difference between the mass-shedding limit at the rotational energy at the point of collapse into a black hole. Also show with a red dashed line is the time to spin-down via magnetic dipole to the point of collapse, in units of the characteristic spin-down time $t_{\mathrm{sd}}$ (Eq. 33). The remnant mass of a merger consisting of two NSs of mass $\approx 1.3-1.4M_{\odot }$ is typically $\approx 2.3-2.5M_{\odot }$, after accounting for neutrino losses and mass ejection (Ruffert et al. ; Belczynski et al. ; Kaplan et al. 2014). The structure of the solid-body rotating NS is calculated using the rns code (Stergioulas and Friedman 1995) assuming a parameterized piecewise polytropic EOS with an adiabatic index $\mathrm{\Gamma }=3$ above the break density of ${\mathit{\rho }}_1={10}^{14.7}\text{g}{\text{cm}}^{-3}$ at a pressure of $P_1=3.2\times {10}^{34}\text{dyn}{\text{cm}}^{-2}$ (Margalit et al. 2015). The chosen EOS results in a $1.4M_{\odot }$ NS radius of 10.6 Km and maximum non-rotating mass of $M_{\max }(\mathit{\Omega }=0)\approx 2.24M_{\odot }$. This figure is modified from a related figure in Metzger et al. (2015b)

Fig. 9

Left schematic illustration of a possible scenario by which accretion onto the magnetar remnant of a NS–NS merger could power an ultra-relativistic short GRB jet. Strong magnetic fields in the polar region confine the hot atmosphere of the proto-NS (Thompson 2003), preventing the formation of a steady neutrino-driven wind in this region. Open magnetic field lines, which thread the accretion disk or shear boundary layer, carry the Poynting flux powering the GRB jet. These field lines are relatively devoid of baryonic matter due to the large centrifugal barrier, enabling the outflow to accelerate to attain high asymptotic Lorentz factors. At larger radii in the disk, outflows will be more heavily mass-loaded and form a potential collimating agent for the jet. Right X-ray and optical light curves following the short duration GRB 080503. Note the sharp drop, by over 6 orders of magnitude, in the X-ray flux, within hours following the burst. This ‘steep decay phase’, often observed following the prompt emission in long duration GRBs, is probably not related to the ‘afterglow’ (forward or reverse shock created by the jet interacting with the circumburst medium), instead requiring ongoing central engine activity. Image reproduced with permission from Perley et al. (2009), copyright by AAS

Fig. 10

Kilonova light curves, boosted by spin-down energy from an indefinitely stable magnetar ($t_{\mathrm{collapse}}=\infty$). We assume an ejecta mass $M=0.1M_{\odot }$ (Metzger and Fernández 2014), initial magnetar spin period $P_0=0.7\text{ms}$, thermalization efficiency ${\mathit{ϵ}}_{\mathrm{th}}=1$ and magnetic dipole field strength of ${10}^{15}\text{G}$ (left panel) or ${10}^{16}\text{G}$ (right panel)

Fig. 11

Same as Fig. 10, but calculated for ejecta opacities corresponding to lanthanide-free matter

Fig. 12

I band light curves for a magnetar with a field strength of ${10}^{15}\text{G}$ and a variable lifetime $t_{\mathrm{col}}$ in units of the spin-down timescale $t_{\mathrm{sd}}\simeq 150\text{s}$ (shown as different colored line). Other ejecta parameters are identical to those in Figs. 10, 11. The calculation shown in the left panel assumes opacities appropriate to Lanthanide matter, while the right panel assumes Lanthanide-free ejecta

Fig. 13

Schematic illustration of the mapping between mergers and kilonova light curves. The top panel shows the progenitor system, either an NS–NS or an NS–BH binary, while the middle plane shows the final merger remnant (from left to right: an HMNS that collapses to a BH after time $t_{\mathrm{collapse}}$, a spinning magnetized NS, a non-spinning BH and a rapidly spinning BH). The bottom panel illustrates the relative amount of UV/blue emission from an neutron precursor (purple), optical emission from lanthanide-free material (blue) and IR emission from lanthanide containing ejecta (red). We caution that the case of a NS–NS merger leading to a slowly spinning black hole is very unlikely, given that at a minimum the remnant will acquire the angular momentum of the original binary orbit. Image reproduced with permission from Kasen et al. (2015), copyright by the authors