The cosmic origin of elements heavier than iron has long been uncertain. Theoretical modelling shows that the matter that is expelled in the violent merger of two neutron stars can assemble into heavy elements such as gold and platinum in a process known as rapid neutron capture (r-process) nucleosynthesis. The radioactive decay of isotopes of the heavy elements is predicted to power a distinctive thermal glow (a 'kilonova'). The discovery of an electromagnetic counterpart to the gravitational-wave source GW170817 represents the first opportunity to detect and scrutinize a sample of freshly synthesized r-process elements. Here we report models that predict the electromagnetic emission of kilonovae in detail and enable the mass, velocity and composition of ejecta to be derived from observations. We compare the models to the optical and infrared radiation associated with the GW170817 event to argue that the observed source is a kilonova. We infer the presence of two distinct components of ejecta, one composed primarily of light (atomic mass number less than 140) and one of heavy (atomic mass number greater than 140) r-process elements. The ejected mass and a merger rate inferred from GW170817 imply that such mergers are a dominant mode of r-process production in the Universe.

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: day-long optical (“blue”) emission from lanthanide-free components of the ejecta; 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.

The coalescence of double neutron star (NS–NS) and black hole (BH)–NS binaries are prime sources of gravitational waves (GW) for Advanced LIGO/Virgo and future ground-based detectors. Neutron-rich matter released from such events undergoes rapid neutron capture (r-process) nucleosynthesis as it decompresses into space, enriching our universe with rare heavy elements like gold and platinum. Radioactive decay of these unstable nuclei powers a rapidly evolving, approximately isotropic thermal transient known as a “kilonova”, which probes the physical conditions during the merger and its aftermath. Here I review the history and physics of kilonovae, leading to the current paradigm of day-timescale emission at optical wavelengths from lanthanide-free components of the ejecta, followed by week-long emission with a spectral peak in the near-infrared (NIR). These theoretical predictions, as compiled in the original version of this review, were largely confirmed by the transient optical/NIR counterpart discovered to the first NS–NS merger, GW170817, discovered by LIGO/Virgo. Using a simple light curve model to illustrate the essential physical processes and their application to GW170817, I then introduce important variations about the standard picture which may be observable in future mergers. These include hour-long UV precursor emission, powered by the decay of free neutrons in the outermost ejecta layers or shock-heating of the ejecta by a delayed ultra-relativistic outflow; and enhancement of the luminosity from a long-lived central engine, such as an accreting BH or millisecond magnetar. Joint GW and kilonova observations of GW170817 and future events provide a new avenue to constrain the astrophysical origin of the r-process elements and the equation of state of dense nuclear matter.

We present Chandra X-ray observations of four optically-selected tidal disruption events (TDEs) obtained 4-9 years after their discovery. Three sources were detected with luminosities between 9×10 40 and 3 × 10 42 erg s −1 . The spectrum of PTF09axc is consistent with a power law with index of 2.5±0.1, whereas the spectrum of PTF09ge is consistent with the Wien tail of a soft black body best described over the 0.3-7 keV range with a power law of index 3.9±0.5 (the best-fit black body temperature is 0.18 ± 0.02 keV). The power law spectrum of PTF09axc may signal that TDEs transition from an early-time soft state to a late-time low-hard state many years after disruption. The mismatch in Eddington fractions of these sources (≈ 5% for PTF09axc; ≈ 0.2% for PTF09ge) could indicate that, as is the case for X-ray binaries, mass accretion rate is not the sole parameter responsible for TDE state changes. These detections can be used to shed light on the difference between optically selected vs. X-ray selected TDEs. We propose that the time to peak luminosity for optical and X-ray emission may differ substantially in an individual TDE, with X-rays being produced or becoming observable later. This delay can serve to explain the differences in observed properties such as L opt /L X of optically and X-ray selected TDEs. Using our observations to calibrate simple models for TDE X-ray light curves, we update predictions for the soon-to-be-launched eROSITA instrument, finding an eROSITA TDE detection rate of 3 yr −1 to 990 yr −1 , a range that depends sensitively on (i) the distribution of black hole spins and (ii) the typical time delay between disruption and peak X-ray brightness. We further predict an asymmetry in the number of retrograde and prograde disks in samples of optically and X-ray selected TDEs, even if the intrinsic number of stars on pro-and retrograde TDE orbits is equal. X-ray selected TDE samples will have a strong bias towards prograde disks (up to 1-2 orders of magnitude if most supermassive black holes spin rapidly, and less so if most spin slowly). On the other hand, in flux-limited samples of optically-selected TDEs, there seems to exist a more modest (typically factor of a few) bias for either retrograde or prograde disks, depending on the underlying optical emission mechanism and regime of loss cone repopulation. These observational biases can contribute to observed differences between optically and X-ray selected TDEs (with optically selected TDEs being fainter in X-rays if the TDE disk is retrograde).

For the first ∼3 yrs after the binary neutron star merger event GW 170817, the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n < 0.01 cm−3. We report on observational evidence for an excess of X-ray emission at δt > 900 days after the merger. With L x ≈ 5 × 1038 erg s−1 at 1234 days, the recently detected X-ray emission represents a ≥3.2σ (Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance ≥3.1σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥3.7σ. A lack of detectable 3 GHz radio emission suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e., a kilonova afterglow. In this context, we present a set of ab initio numerical relativity binary neutron star (BNS) merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.