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GRB 050709 was the first short Gamma-ray Burst (sGRB) with an identified optical counterpart. Here we report a reanalysis of the publicly available data of this event and the discovery of a Li-Paczynski macronova/kilonova that dominates the optical/infrared signal at t >2.5 days. Such a signal would arise from 0.05 r-process material launched by a compact binary merger. The implied mass ejection supports the suggestion that compact binary mergers are significant and possibly main sites of heavy r-process nucleosynthesis. Furthermore, we have reanalysed all afterglow data from nearby short and hybrid GRBs (shGRBs). A statistical study of shGRB/macronova connection reveals that macronova may have taken place in all these GRBs, although the fraction as low as 0.18 cannot be ruled out. The identification of two of the three macronova candidates in the I -band implies a more promising detection prospect for ground-based surveys. A macronova is a clear signature that a short gamma-ray burst has been emitted by a compact-binary merger, but evidence of these events is so far scarce. Here, the authors report signs of a macronova in the optical afterglow of GRB050709, and find similar evidence in other three short bursts.

Long-duration (>2 s) γ-ray bursts that are believed to originate from the death of massive stars are expected to be accompanied by supernovae. GRB 060614, that lasted 102 s, lacks a supernova-like emission down to very stringent limits and its physical origin is still debated. Here we report the discovery of near-infrared bump that is significantly above the regular decaying afterglow. This red bump is inconsistent with even the weakest known supernova. However, it can arise from a Li-Paczyński macronova—the radioactive decay of debris following a compact binary merger. If this interpretation is correct, GRB 060614 arose from a compact binary merger rather than from the death of a massive star and it was a site of a significant production of heavy r-process elements. The significant ejected mass favours a black hole–neutron star merger but a double neutron star merger cannot be ruled out. The gamma-ray burst GRB 060614 was an unusual astrophysical event whose origins are still unclear. This study re-examines the burst’s afterglow data and finds an excess in the spectrum that appears to be consistent with a weak macronova, suggesting that GRB 060614 originated from a compact binary merger.

Stars could produce our heavy elements through a rapid neutron-capture process during a supernova or merger of binary stars, but which is it? A study of 244 Pu reveals that a rare event with a high yield is more likely, favouring mergers. The origin of heavy elements produced through rapid neutron capture (‘r-process’) by seed nuclei is one of the current nucleosynthesis mysteries 1 , 2 , 3 . Core collapse supernovae (cc-SNe; ref.  4 ) and compact binary mergers are considered as possible sites 5 , 6 , 7 . The first produces small amounts of material at a high event rate whereas the latter produces large amounts in rare events. Radioactive elements with the right lifetime can break the degeneracy between high-rate/low-yield and low-rate/high-yield scenarios. Among radioactive elements, most interesting is 244 Pu (half-life of 81 million years), for which both the current accumulation of live 244 Pu particles accreted via interstellar particles in the Earth’s deep-sea floor 8 and the Early Solar System (ESS) abundances have been measured 9 . Interestingly, the estimated 244 Pu abundance in the current interstellar medium inferred from deep-sea measurements is significantly lower than that corresponding to the ESS measurements. Here we show that both the current and ESS abundances of 244 Pu are naturally explained within the low-rate/high-yield scenario. The inferred event rate remarkably agrees with compact binary merger rates estimated from Galactic neutron star binaries 10 and from short gamma-ray bursts 11 . Furthermore, the ejected mass of r-process elements per event agrees with both theoretical 12 , 13 , 14 and observational 15 , 16 , 17 macronova/kilonova estimates.

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs) 1 , sources of high-frequency gravitational waves (GWs) 2 and likely production sites for heavy-element nucleosynthesis by means of rapid neutron capture (the r -process) 3 . Here we present observations of the exceptionally bright GRB 230307A. We show that GRB 230307A belongs to the class of long-duration GRBs associated with compact object mergers 4 – 6 and contains a kilonova similar to AT2017gfo, associated with the GW merger GW170817 (refs.  7 – 12 ). We obtained James Webb Space Telescope (JWST) mid-infrared imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns, which we interpret as tellurium (atomic mass A  = 130) and a very red source, emitting most of its light in the mid-infrared owing to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r -process elements across a broad atomic mass range and play a central role in heavy-element nucleosynthesis across the Universe. Observations from the JWST of the second brightest GRB ever detected, GRB 230307A, indicate that it belongs to the class of long-duration GRBs resulting from compact object mergers, with the decay of lanthanides powering the longlasting optical and infrared emission.

We present the science case for the proposed Daksha high energy transients mission. Daksha will comprise of two satellites covering the entire sky from 1 keV to > 1  MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can address a wide variety of science cases. With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes.

We present a comprehensive study of neutrino shock acceleration in core-collapse supernova (CCSN). The leading players are heavy leptonic neutrinos, \\(\\nu_{\\mu}\\) and \\(\\nu_{\\tau}\\); the former and latter potentially gain the energy up to \\(\\sim 100\\) MeV and \\(\\sim 200\\) MeV, respectively, through the shock acceleration. Demonstrating the neutrino shock acceleration by Monte Carlo neutrino transport, we make a statement that it commonly occurs in the early post bounce phase (\\(\\lesssim 50\\) ms after bounce) for all massive stellar collapse experiencing nuclear bounce and would reoccur in the late phase (\\(\\gtrsim 100\\) ms) for failed CCSNe. This opens up a new possibility to detect high energy neutrinos by terrestrial detectors from Galactic CCSNe; hence, we estimate the event counts for Hyper(Super)-Kamiokande, DUNE, and JUNO. We find that the event count with the energy of \\(\\gtrsim 80\\) MeV is a few orders of magnitude higher than that of the thermal neutrinos regardless of the detectors, and muon production may also happen in these detectors by \\(\\nu_{\\mu}\\) with the energy of \\(\\gtrsim 100\\) MeV. The neutrino signals provide a precious information on deciphering the inner dynamics of CCSN and placing a constraint on the physics of neutrino oscillation; indeed, the detection of the high energy neutrinos through charged current reaction channels will be a smoking gun evidence of neutrino flavor conversion.

The LIGO/Virgo Scientific Collaboration recently announced the detection of a compact object binary merger, GW190412, as the first asymmetric binary black hole (BBH) merger with mass ratio \\(q\\approx0.25\\). Other than the mass ratio, this BBH has shown to have a positive effective spin of around \\(\\chi_{\\rm eff}\\approx0.28\\). Assuming a field formation channel, associating this effective spin to either the primary or the secondary BH each has its implications: If the spin of the BBH comes form the primary BH, it has consequences for the efficiency of angular momentum transport in the formation of the BH. If, on the other hand, the spin is due to the secondary BH through tidal spin-up processes, one has to note that (i) such processes have very short delay-times, and (ii) subsequently, their local merger rate is determined by local star formation rate at assumed formation metallicity of the BBH. We show that the predicted merger rate density from this channel is \\(\\lesssim 0.3~\\rm Gpc^{-3} yr^{-1}\\) and in tension with the rather high local merger rate of such systems which we estimate from this single event to be \\(\\sim 1.7^{+2.5}_{-1.4}~\\rm Gpc^{-3} yr^{-1}\\) (90\\% confidence interval, and assuming 50 days of observing time). Large natal kicks (\\(v\\gtrsim 500\\,{\\rm km/s}\\)) would be required to get such BBHs with an in-plane spin component to account for the marginal detection of precession in GW190412. However, this would only exacerbate the tension as the estimated local merger rate would be further decreased. Similarly, the formation of such systems through the dynamical assembly is exceedingly rare, leaving this system a dilemma hard to account for with the currently accepted paradigms of BBH formation.

Theory and observations suggest that single-star evolution is not able to produce black holes (BHs) with masses in the range \\(3-5M_{\\odot}\\) and above \\(\\sim 45M_{\\odot}\\), referred to as the lower mass gap (LMG) and the upper mas gap (UMG), respectively. However, it is possible to form BHs in these gaps through merger of compact objects in dense clusters, e.g. the LMG and the UMG can be populated through binary neutron star- and BBH mergers, respectively. This implies that if binary mergers are observed in gravitational waves (GWs) with at least one mass gap object, then either clusters are effective in assembling binary mergers, or our single-star models have to be revised. Understanding how effective clusters are at populating both mass gaps have therefore major implications for both stellar- and GW astrophysics. In this paper we present a systematic study on how efficient stellar clusters are at populating both mass gaps through in-cluster GW mergers. For this, we derive a set of closed form relations for describing the evolution of compact object binaries undergoing dynamical interactions and GW merger inside their cluster. By considering both static and time evolving populations, we find in particular that globular clusters are clearly inefficient at populating the LMG in contrast to the UMG. We further describe how these results relate to the characteristic mass, time, and length scales associated with the problem.

We discuss prospects of identifying and characterizing black hole (BH) companions to normal stars on tight but detached orbits, using photometric data from the Transiting Exoplanet Survey Satellite (TESS). We focus on the following two periodic signals from the visible stellar component: (i) in-eclipse brightening of the star due to gravitational microlensing by the BH (self-lensing), and (ii) a combination of ellipsoidal variations due to tidal distortion of the star and relativistic beaming due to its orbital motion (phase-curve variation). We evaluate the detectability of each signal in the light curves of stars in the TESS input catalog, based on a pre-launch noise model of TESS photometry as well as the actual light curves of spotted stars from the prime Kepler mission to gauge the potential impact of stellar activity arising from the tidally spun-up stellar components. We estimate that the self-lensing and phase-curve signals from BH companions, if exist, will be detectable in the light curves of effectively \\(\\mathcal{O}(10^5)\\) and \\(\\mathcal{O}(10^6)\\) low-mass stars, respectively, taking into account orbital inclination dependence of the signals. These numbers could be large enough to actually detect signals from BHs: simple population models predict some 10 and 100 detectable BHs among these \"searchable\" stars, although the latter may be associated with a comparable number of false-positives due to stellar variabilities and additional vetting with radial velocity measurements would be essential. Thus the TESS data could serve as a resource to study nearby BHs with stellar companions on shorter-period orbits than will potentially be probed with Gaia.

We study the heating rate of r-process nuclei and thermalization of decay products in neutron star merger ejecta and macronova (kilonova) light curves. Thermalization of charged decay products, i.e., electrons, \\(\\alpha\\)-particles, and fission fragments is calculated according to their injection energy. The \\(\\gamma\\)-ray thermalization processes are also properly calculated by taking the \\(\\gamma\\)-ray spectrum of each decay into account. We show that the \\(\\beta\\)-decay heating rate at later times approaches a power-law decline as \\(\\propto t^{-2.8}\\), which agrees with the result of Waxman et al. (2019). We present a new analytic model to calculate macronova light curves, in which the density structure of the ejecta is accounted for. We demonstrate that the observed bolometric light curve and temperature evolution of the macronova associated with GW170817 are reproduced well by the \\(\\beta\\)-decay heating rate with the solar r-process abundance pattern. We interpret the break in the observed bolometric light curve around a week as a result of the diffusion wave crossing a significant part of the ejecta rather than a thermalization break. We also show that the time-weighted integral of the bolometric light curve (Katz integral) is useful to provide an estimate of the total r-process mass from the observed data, which is independent of the highly uncertain radiative transfer. For the macronova in GW170817, the ejecta mass is robustly estimated as \\(\\approx 0.05M_{\\odot}\\) for \\(A_{\\rm min}\\leq 72\\) and \\(85\\leq A_{\\rm min}\\leq 130\\) with the solar r-process abundance pattern. The code for computation of the heating rate and light curve for given initial nuclear abundances is publicly available.