INTRODUCTION
1.6 Kilonovae
Figure 1.15: The green region is the allowed relationship between the r-process event rate R0 and yield Mej per event to produce the total amount of r-process material in the galaxy. The blue region is the constraint obtained from a galactic mixing model and a measurement of244Pu in the deep sea crust. The two constraints intersect at an event rate that is much lower than the CCSN rate but compatible with the expected neutron star merger rate. Furthermore, the predicted ejecta mass is also compatible with what we expect from neutron star mergers and what has been inferred from possible kilonova/macronova observations (see Section 1.6). Figure 1 from Hotokezaka et al. (2015); see that reference for details. © 2015 Macmillan Publishers Limited
the stars in other ultrafaint dwarf galaxies. And the measured abundances in these stars match the universal r-process abundance pattern. This implies that Reticulum II was enriched in r-process elements by a single, rare event, such as a neutron star merger, that has not happened in other ultrafaint dwarf galaxies. Furthermore, Ji et al. (2016) compute the total europium yield in Reticulum II and find that it is three orders of magnitude higher than what would be expected from a CCSN but the yield is compatible with a neutron star merger.
24 predominantly produced in the r-process and Reticulum II (Ret II) is highly enhanced in both of them compared to the other ultrafaint dwarf galaxies. Furthermore, the abundance pattern observed in some of the Reticulum II stars (right) matches the universal r-process pattern (purple line) but not the s-process pattern (yellow line). This suggests that Reticulum II has been enhanced in r-process elements by a single, rare event, such as neutron star merger. If frequent events like CCSNe were the main r-process site, then the amount of r-process material should be the same in all ultrafaint dwarf galaxies. Figure 2 from Ji et al. (2016); see that reference for details. © 2016 Macmillan Publishers Limited
or macronova. If r-process nucleosynthesis does indeed happen in the ejecta of neutron star mergers, then a kilonova would be an electromagnetic counterpart to the gravitational wave signal of the merger. The Laser Interferometer Gravitational Wave Observatory (LIGO) has recently detected gravitational waves from a binary black hole merger for the first time (Abbott et al., 2016a) and so it should not be long until we detect gravitational waves from neutron star mergers as well (e.g., LIGO Scientific Collaboration et al.,2015; Abbott et al.,2016b). If a kilonova can be found and tied to a gravitational wave event from a neutron star merger, then this would be concrete evidence that r-process nucleosynthesis happens in the ejecta of neutron star mergers. Furthermore, neutron star mergers are also thought to be the progenitors of short gamma ray bursts (sGRBs), in which case we would expect to see triple coincidence of a gravitational wave signal, an sGRB (provided it is beamed toward us), and a kilonova (e.g., Lee and Ramirez-Ruiz,2007; Nakar,2007;
Gehrels et al.,2009; Fernández and Metzger,2016).
The exact observational signature of kilonovae is subject to active investigation.
Kasen et al. (2013) and Tanaka and Hotokezaka (2013) have pointed out that the presence of lanthanides (58 ≤ Z ≤ 71) and actinides (90 ≤ Z ≤ 103) significantly increases (by two orders of magnitude) the opacity of the material due to the open f-shells of those elements, which results in a huge number of absorption lines.
This enhanced opacity increases the time it takes for photons to diffuse out and hence the transient will be dim (few× 1040 erg s−1) and peak in the red/infrared band on the timescale of a week (e.g., Barnes and Kasen, 2013; Grossman et al., 2014; Lippuner and Roberts,2015). However, lanthanides and actinides are only produced ifYe . 0.25 (Lippuner and Roberts,2015) and so we expect them to be present in the dynamical ejecta. In the disk outflow, the electron fraction may be too high for the full r-process to happen in the bulk of the outflow, and hence that part of the ejecta might make a brighter, blue transient on the timescale of a few days (e.g., Kasen et al.,2015; Martin et al., 2015). Additionally, it is possible that a small fraction of the dynamical ejecta expands rapidly enough that the neutrons do not have enough time to capture on seed nuclei, thus producing a very short (hours) bright, ultraviolet precursor (Metzger et al.,2015). Example light curves of kilonovae with and without lanthanides and with and without a neutron precursor are shown in Figure1.17, which is Figure 6 from Metzger (2017). Finally, a fraction of theγ-rays produced in the β-decays of the radioactive isotopes can escape without thermalizing (e.g., Barnes et al., 2016). However, detecting this γ-ray emission with current instruments seems unlikely, unless the kilonova is sufficiently close (a
few Mpc, but the event rate in this volume is small,∼10−3yr−1, Hotokezaka et al., 2016).
There are reports of possible kilonova detections for three sGRBs: GRB130603B (Berger et al.,2013; Tanvir et al.,2013), GRB060614 (Jin et al.,2015; Yang et al., 2015), and GRB050709 (Jin et al., 2016). However, the observational data for these events seems too sparse to allow any strong conclusion about the exact nature of the observed transient. Nevertheless, kilonovae are actively being searched for and there exists a pipeline for searches in response to a gravitational wave trigger involving telescopes all around the world (e.g., Law et al.,2009; Singer et al.,2014;
Bellm, 2014; Kasliwal et al., 2016). To aid with the detection, classification, and information extraction of and from kilonovae, it is important to have accurate models for the light curves and an understanding of the important mechanisms that affect them. This requires more work in simulating neutron star mergers and their outflows, simulating the subsequent nucleosynthesis, and finally modeling and/or simulating the light curve given the nucleosynthesis results and outflow morphology.
27 panel, and so the kilonova peaks in the infrared at a few days. In the right panel, there are no lanthanides and hence the light curves peaks in the optical bands at around one day. The solid lines include a neutron precursor, whereas the dashed lines do not. The neutron precursor is especially visible in the ultraviolet band in the left panel, where the UV emission at a few hours is enhanced by about 1.5 magnitudes. Figure 6 from Metzger (2017). © 2017 Brian D. Metzger
C h a p t e r II