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Observations of the transient associated with the gravitational-wave event GW170817 and γ-ray burst GRB 170817A reveal a bright kilonova with fast-moving ejecta, including lanthanides synthesized by rapid neutron capture. When neutron stars collide Merging neutron stars are potential sources of gravitational waves and have long been predicted to produce jets of material as part of a low-luminosity transient known as a 'kilonova'. There is growing evidence that neutron-star mergers also give rise to short, hard gamma-ray bursts. A group of papers in this issue report observations of a transient associated with the gravitational-wave event GW170817—a signature of two neutron stars merging and a gamma-ray flash—that was detected in August 2017. The observed gamma-ray, X-ray, optical and infrared radiation signatures support the predictions of an outflow of matter from double neutron-star mergers and present a clear origin for gamma-ray bursts. Previous predictions differ over whether the jet material would combine to form light or heavy elements. These papers now show that the early part of the outflow was associated with lighter elements whereas the later observations can be explained by heavier elements, the origins of which have been uncertain. However, one paper (by Stephen Smartt and colleagues) argues that only light elements are needed for the entire event. Additionally, Eleonora Troja and colleagues report X-ray observations and radio emissions that suggest that the 'kilonova' jet was observed off-axis, which could explain why gamma-ray-burst detections are seen as dim. The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical–near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process) 1 , 2 , 3 . Such transients, named ‘macronovae’ or ‘kilonovae’ 4 , 5 , 6 , 7 , are believed to be centres of production of rare elements such as gold and platinum 8 . The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst 9 , 10 at redshift z  = 0.356, although findings indicating bluer events have been reported 11 . Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source 12 GW170817 and γ-ray burst 13 , 14 GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models 15 , 16 . The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.

Observations of two slow-to-fade super-luminous supernovae are reported; both show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Magnetar-powered super-luminous supernovae Observations of two recently discovered slow-to-fade super-luminous supernovae, known as PTF12dam and PS1-11ap, reveal relatively fast rise times and blue colours that are incompatible with the pair-instability mechanism, hitherto believed to be the best explanation for superluminous events. The authors suggest a model in which the debris from these remarkably energetic supernovae is powered by magnetic neutron stars or magnetars. Super-luminous supernovae 1 , 2 , 3 , 4 that radiate more than 10 44 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1–4. Some evolve slowly, resembling models of ‘pair-instability’ supernovae 5 , 6 . Such models involve stars with original masses 140–260 times that of the Sun that now have carbon–oxygen cores of 65–130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron–positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56 Ni are synthesized; this isotope decays to 56 Fe via 56 Co, powering bright light curves 7 , 8 . Such massive progenitors are expected to have formed from metal-poor gas in the early Universe 9 . Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova 1 , 10 . Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae 2 , 11 , 12 , which are not powered by radioactivity. Modelling our observations with 10–16 solar masses of magnetar-energized 13 , 14 ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10 −6 times that of the core-collapse rate.

Nature 502, 346–349 (2013); doi:10.1038/nature12569 In this Letter, we have identified an important error affecting Fig. 4 and Extended Data Fig. 6, as well as the values of some parameters derived from our model fits. We stress that this error in no way affects the discussion or the conclusions. Inbuilding the bolometric light curve of the superluminous supernova PTF 12dam, our code assumed that photometry from the Swift satellite was calibrated in the Vega magnitude system.

Background. To evaluate associations between exposure to disinfection byproducts in drinking water and adverse birth outcomes, personal exposure to disinfection byproducts must take into consideration multiple routes of exposure. Methods. We assessed the reproducibility and validity of a questionnaire measuring water consumption, showering and bathing habits, use of chlorine-based products, and swimming pool attendance in 237 pregnant Italian women enrolled between June and December 1999. The questionnaire was completed during the last trimester of pregnancy (preQ) and again a few days after delivery (postQ). Data from postQ were compared with a 7-day diary completed during the last trimester. Results. According to postQ, total water intake was 2.6 liters per day, whereas tap water intake was 0.6 liters per day. Intraclass correlation coefficients of postQ compared with preQ were 0.85 for tap water daily intake and 0.77 for duration of showering and bathing. Pearson's correlation coefficients were 0.84 for tap water daily intake, 0.81 for frequency of showering, and 0.94 for bathing. The kappa statistics were 0.76 (95% confidence limits = 0.68, 0.85) for use of domestic chlorine-based products and 0.82 (0.70, 0.94) for indoor swimming. Pearson's coefficients for postQ compared with the diary were 0.78 for tap water daily intake, 0.62 for frequency of showering, and 0.64 for bathing. Compared with the diary, the sensitivity and specificity of postQ in assessing indoor swimming were 75% and 90%, respectively. Conclusions. The questionnaire appears to be a valid and reliable method for assessing exposure to disinfection byproducts in the last trimester of pregnancy.

The link between the fate of the most massive stars and the resulting supernova (SN) explosion is still a matter of debate, in major part because of the ambiguity among light-curve powering mechanisms. When stars explode as SNe, the light-curve luminosity is typically sustained by a central engine (radioactive decay, magnetar spin-down, or fallback accretion). However, since massive stars eject considerable amounts of material during their evolution, there may be a significant contribution coming from interactions with the previously ejected circumstellar medium (CSM). Reconstructing the progenitor configuration at the time of explosion requires a detailed analysis of the long-term photometric and spectroscopic evolution of the related transient. In this paper, we present the results of our follow-up campaign of SN 2020faa. Given the high luminosity and peculiar slow light curve, it is purported to have a massive progenitor. We present the spectro-photometric dataset and investigate different options to explain the unusual observed properties that support this assumption. We computed the bolometric luminosity of the supernova and the evolution of its temperature, radius, and expansion velocity. We also fit the observed light curve with a multi-component model to infer information on the progenitor and the explosion mechanism. Reasonable parameters are inferred for SN 2020faa with a magnetar of energy Ep=1.5(+0.5,-0.2)x10^50 erg and spin-down time t(spin)=15+/-1 d, a shell mass M(shell)=2.4(+0.5,-0.4) Msun and kinetic energy Ekin(shell)=0.9(+0.5,-0.3)x 10^51 erg, and a core with M(core)=21.5(+1.4,-0.7) Msun and Ekin(core)=3.9(+0.1,-0.4)x10^51 erg. In addition, we need an extra source to power the luminosity of the second peak. We find that hidden interaction with either a CSM disc or delayed, choked jets is a viable mechanism for supplying the required energy to achieve this effect.

We present an astrometric and photometric wide-field study of the Galactic open star cluster M37 (NGC 2099). The studied field was observed with ground-based images covering a region of about four square degrees in the Sloan-like filters ugi. We exploited the Gaia catalogue to calibrate the geometric distortion of the large field mosaics, developing software routines that can be also applied to other wide-field instruments. The data are used to identify the hottest white dwarf (WD) member candidates of M37. Thanks to the Gaia EDR3 exquisite astrometry we identified seven such WD candidates, one of which, besides being a high-probability astrometric member, is the putative central star of a planetary nebula. To our knowledge, this is a unique object in an open cluster, and we have obtained follow-up low-resolution spectra that are used for a qualitative characterisation of this young WD. Finally, we publicly release a three-colour atlas and a catalogue of the sources in the field of view, which represents a complement of existing material.

We present photometric and spectroscopic observations of the Type Icn supernova (SN) 2021ckj. Spectral modeling of SN 2021ckj reveals that its composition is dominated by oxygen, carbon and iron group elements, and the photospheric velocity at peak is ~10000 km/s. From the light curve (LC) modeling applied to SNe 2021ckj, 2019hgp, and 2021csp, we find that the ejecta and CSM properties of Type Icn SNe are diverse. SNe 2021ckj and 2021csp likely have two ejecta components (an aspherical high-energy component and a spherical standard-energy component) with a roughly spherical CSM, while SN 2019hgp can be explained by a spherical ejecta-CSM interaction alone. The ejecta of SNe 2021ckj and 2021csp have larger energy per ejecta mass than the ejecta of SN 2019hgp. The density distribution of the CSM is similar in these three SNe, and is comparable to those of Type Ibn SNe. This may imply that the mass-loss mechanism is common between Type Icn (and also Type Ibn) SNe. The CSM masses of SN 2021ckj and SN 2021csp are higher than that of SN 2019hgp, although all these values are within the diversity seen in Type Ibn SNe. The early spectrum of SN 2021ckj shows narrow emission lines from C II and C III, without a clear absorption component, in contrast with that observed in SN 2021csp. The similarity of the emission components of these lines implies that the emitting regions of SNe 2021ckj and 2021csp have similar ionization states, and thus suggests that they have similar properties of the ejecta and CSM, which is inferred also from the LC modeling. Taking into account the difference in the strength of the absorption features, this heterogeneity may be attributed to viewing angle effects in otherwise common aspherical ejecta.

We present photometric and spectroscopic data of the unusual interacting supernova (SN) 2021foa. It rose to an absolute magnitude peak of \\(M_r=-18\\) mag in 20 days. The initial light curve decline shows some luminosity fluctuations before a long-lasting flattening. A faint source (\\(M_r\\sim -14\\) mag) was detected in the weeks preceding the main event, showing a slow-rising luminosity trend. The \\(r\\)-band absolute light curve is very similar to those of SN 2009ip-like events, with a faint and shorter duration brightening (`Event A') followed by a much brighter peak (`Event B'). The early spectra of SN 2021foa show a blue continuum with narrow (\\(v_{FWHM}\\sim\\)400 km s\\(^{-1}\\)) H emission lines, that, two weeks later, reveal a complex profile, with a narrow P Cygni on top of an intermediate-width (\\(v_{FWHM}\\sim\\)2700 km s\\(^{-1}\\)) component. At +12 days metal lines in emission appear, while \\Hei lines become very strong, with \\Hei~\\(\\lambda\\)5876 reaching half of the \\Ha luminosity, much higher than in previous SN 2009ip-like objects. We propose SN 2021foa to be a transitional event between the H-rich SN 2009ip-like SNe and the He-rich Type Ibn SNe.

We catalog the 443 bright supernovae discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) in \\(2018-2020\\) along with the 519 supernovae recovered by ASAS-SN and 516 additional \\(m_{peak}\\leq18\\) mag supernovae missed by ASAS-SN. Our statistical analysis focuses primarily on the 984 supernovae discovered or recovered in ASAS-SN \\(g\\)-band observations. The complete sample of 2427 ASAS-SN supernovae includes earlier \\(V\\)-band samples and unrecovered supernovae. For each supernova, we identify the host galaxy, its UV to mid-IR photometry, and the offset of the supernova from the center of the host. Updated light curves, redshifts, classifications, and host galaxy identifications supersede earlier results. With the increase of the limiting magnitude to \\(g\\leq18\\) mag, the ASAS-SN sample is roughly complete up to \\(m_{peak}=16.7\\) mag and is \\(90\\%\\) complete for \\(m_{peak}\\leq17.0\\) mag. This is an increase from the \\(V\\)-band sample where it was roughly complete up to \\(m_{peak}=16.2\\) mag and \\(70\\%\\) complete for \\(m_{peak}\\leq17.0\\) mag.