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On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves (GWs) emanating from a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) telescopes detected a gamma-ray transient, GRB 170817A. At 10.9 hours after the GW trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and GW observations.

SN 2005E: untrue to type The novel properties of the faint supernova SN 2005E mean that it does not fit readily into the established supernova categories. Types Ib, Ic and II, core-collapse supernovae, are thought to form when a massive star explodes at the end of its life, and type Ia as a result of the thermonuclear explosion of an accreting white dwarf. From spectroscopic data, Perets et al . conclude that SN 2005E is helium rich, like a type Ib, and lacks the hydrogen, silicon and sulphur spectral lines typical of type Ia. But based on its presence in an 'old' stellar environment, and with a low derived ejected mass, they argue against a core-collapse origin and for an origin from a low-mass, old progenitor, probably a helium-accreting white dwarf in a binary system. Kawabata et al . see it differently. SN 2005E resembles SN 2005cz, they say, a type Ib supernova that is unusual in being found in an elliptical galaxy. Both SN 2005E and SN 2005cz, they suggest, are best explained as products of the core collapse of massive stars at the low (6–12 solar mass) end of massiveness. In the accompanying News & Views, David Branch discusses these two models in the context of the latest thinking on how stars explode. Supernovae are thought to arise through one of two processes. Type Ib/c and type II supernovae are produced when the cores of massive, short-lived stars undergo gravitational core collapse and eject a few solar masses. Type Ia supernovae are thought to form by the thermonuclear detonation of a carbon-oxygen white dwarf. Here a faint type Ib supernova, SN 2005E, is reported that seems not to have had a core-collapse origin, but perhaps arose from a low-mass, old progenitor, probably a helium-accreting white dwarf in a binary. Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae 1 , 2 . Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (∼0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous 3 , 4 or a regular 1 type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44 Ti.

On 17 August 2017, gravitational waves (GWs) were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB 170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a),was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days postmerger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in rapid neutron capture (r-process) nucleosynthesis in the universe.

On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after the merger. Over the first hour of observations, the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measured the photosphere cooling from 11,000 − 900 + 3400 to 9300 − 300 + 300 kelvin, and determined a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a began displaying broad features after 1.46 days and evolved qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process–enriched neutron star ejecta, whereas the blue component requires high-velocity, lanthanide-free material.

Eleven hours after the detection of gravitational wave source GW170817 by the Laser Interferometer Gravitational-Wave Observatory and Virgo Interferometers, an associated optical transient, SSS17a, was identified in the galaxy NGC 4993. Although the gravitational wave data indicate that GW170817 is consistent with the merger of two compact objects, the electromagnetic observations provide independent constraints on the nature of that system. We synthesize the optical to near-infrared photometry and spectroscopy of SSS17a collected by the One-Meter Two-Hemisphere collaboration, finding that SSS17a is unlike other known transients. The source is best described by theoretical models of a kilonova consisting of radioactive elements produced by rapid neutron capture (the r-process). We conclude that SSS17a was the result of a binary neutron star merger, reinforcing the gravitational wave result.

Some types of core-collapse supernovae are known to produce a neutron star (NS). A binary NS merger was recently detected from its gravitational wave emission, but it is unclear how such a tight binary system can be formed. De et al. discovered a core-collapse supernova with unusual properties, including the removal of the outer layers of the star before the explosion. They interpret this as the second supernova in an interacting binary system that already contains one NS. Because the explosion probably produced a second NS (rather than a black hole) in a tight orbit, it could be an example of how binary NS systems form. Science , this issue p. 201 An unusual core-collapse supernova appears to have formed a binary neutron star in a tight orbit. Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a type Ic supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.2 solar masses) and low kinetic energy (≈2 × 10 50 ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope, likely ejected in an intense pre-explosion mass-loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.

Shifting the focus of Type Ia supernova (SN Ia) cosmology to the near infrared (NIR) is a promising way to significantly reduce the systematic errors, as the strategy minimizes our reliance on the empirical width-luminosity relation and uncertain dust laws. Observations in the NIR are also crucial for our understanding of the origins and evolution of these events, further improving their cosmological utility. Any future experiments in the rest-frame NIR will require knowledge of the SN Ia NIR spectroscopic diversity, which is currently based on a small sample of observed spectra. Along with the accompanying paper, Phillips et al., we introduce the Carnegie Supernova Project-II (CSP-II), to follow-up nearby SNe Ia in both the optical and the NIR. In particular, this paper focuses on the CSP-II NIR spectroscopy program, describing the survey strategy, instrumental setups, data reduction, sample characteristics, and future analyses on the data set. In collaboration with the Harvard-Smithsonian Center for Astrophysics (CfA) Supernova Group, we obtained 661 NIR spectra of 157 SNe Ia. Within this sample, 451 NIRspectra of 90 SNe Ia have corresponding CSP-II follow-up light curves. Such a sample will allow detailed studies of the NIR spectroscopic properties of SNe Ia, providing a different perspective on the properties of the unburned material; the radioactive and stable nickel produced; progenitor magnetic fields; and searches for possible signatures of companion stars.

The astronomical event GW170817, detected in gravitational and electromagnetic waves, is used to determine the expansion rate of the Universe, which is consistent with and independent of existing measurements. Hubble constant from colliding neutron stars The gravitational-wave signature of merging black holes or neutron stars yields the distance to the merger. If a counterpart is observed and its recession velocity arising from the Hubble flow is known, then a calibration of the Hubble constant that is entirely independent of the usual 'distance ladder' is possible. The gravitational-wave event of 17 August 2017 (GW170817) corresponded to the merger of two neutron stars, and an associated 'kilonova' was seen. Daniel Holz and the LIGO–Virgo collaboration, along with a group of astronomers involved with the search for the counterpart, have determined that the Hubble constant calculated this way is about 70 kilometres per second per megaparsec. This is consistent with other determinations, but independent of them. On 17 August 2017, the Advanced LIGO 1 and Virgo 2 detectors observed the gravitational-wave event GW170817—a strong signal from the merger of a binary neutron-star system 3 . Less than two seconds after the merger, a γ-ray burst (GRB 170817A) was detected within a region of the sky consistent with the LIGO–Virgo-derived location of the gravitational-wave source 4 , 5 , 6 . This sky region was subsequently observed by optical astronomy facilities 7 , resulting in the identification 8 , 9 , 10 , 11 , 12 , 13 of an optical transient signal within about ten arcseconds of the galaxy NGC 4993. This detection of GW170817 in both gravitational waves and electromagnetic waves represents the first ‘multi-messenger’ astronomical observation. Such observations enable GW170817 to be used as a ‘standard siren’ 14 , 15 , 16 , 17 , 18 (meaning that the absolute distance to the source can be determined directly from the gravitational-wave measurements) to measure the Hubble constant. This quantity represents the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Here we report a measurement of the Hubble constant that combines the distance to the source inferred purely from the gravitational-wave signal with the recession velocity inferred from measurements of the redshift using the electromagnetic data. In contrast to previous measurements, ours does not require the use of a cosmic ‘distance ladder’ 19 : the gravitational-wave analysis can be used to estimate the luminosity distance out to cosmological scales directly, without the use of intermediate astronomical distance measurements. We determine the Hubble constant to be about 70 kilometres per second per megaparsec. This value is consistent with existing measurements 20 , 21 , while being completely independent of them. Additional standard siren measurements from future gravitational-wave sources will enable the Hubble constant to be constrained to high precision.

We present pre- and post-explosion observations of the Type II-P supernova (SN~II-P) 2019mhm located in NGC~6753. Based on optical spectroscopy and photometry, we show that SN\\,2019mhm exhibits broad lines of hydrogen with a velocity of \\(-8500\\pm200\\)~km~s\\(^{-1}\\) and a \\(111\\pm2\\)~day extended plateau in its luminosity, typical of the Type II-P subclass. We also fit its late-time bolometric light curve and infer that it initially produced a \\({}^{56}\\)Ni mass of \\(1.3 \\times 10^{-2}\\)~M\\(_{\\odot} \\pm 5.5 \\times 10^{-4}\\)~\\(M_\\odot\\). Using imaging from the Wide Field Planetary Camera 2 on the {\\it Hubble Space Telescope} obtained 19~years before explosion, we aligned to a post-explosion Wide Field Camera 3 image and demonstrate that there is no detected counterpart to the SN to a limit of \\(>\\)24.53~mag in F814W, corresponding to an absolute magnitude limit of \\(M_{\\rm F814W} < -7.7\\)~mag. Comparing to massive-star evolutionary tracks, we determine that the progenitor star had a maximum zero-age main sequence mass \\(<\\)17.5~M\\(_{\\odot}\\), consistent with other SN~II-P progenitor stars. SN\\,2019mhm can be added to the growing population of SNe~II-P with both direct constraints on the brightness of their progenitor stars and well-observed SN properties.

There is a wide consensus that type Ia supernovae (SN Ia) originate from the thermonuclear explosion of CO white dwarfs (WD), with the lack of hydrogen in the observed spectra as a distinctive feature. Here, we present SN 2016jae, which was classified as a Type Ia SN from a spectrum obtained soon after the discovery. The SN reached a B-band peak of -17.93 +- 0.34 mag, followed by a fast luminosity decline with sBV 0.56 +- 0.06 and inferred Dm15(B) of 1.88 +- 0.10 mag. Overall, the SN appears as a \"transitional\" event between \"normal\" SNe Ia and very dim SNe Ia such as 91bg-like SNe. Its peculiarity is that two late-time spectra taken at +84 and +142 days after the peak show a narrow line of Halpha (with full width at half-maximum of ~650 and 1000 kms-1, respectively). This is the third low-luminosity and fast-declining Type Ia SN after SN 2018cqj/ATLAS18qtd and SN 2018fhw/ASASSN-18tb, found in the 100IAS survey that shows resolved narrow Halpha line in emission in their nebular-phase spectra. We argue that the nebular Halpha emission originates in an expanding hydrogen-rich shell (with velocity < 1000 kms-1). The hydrogen shell velocity is too high to be produced during a common envelope phase, while it may be consistent with some material stripped from an H-rich companion star in a single-degenerate progenitor system. However, the derived mass of this stripped hydrogen is ~0.002-0.003 Msun, which is much less than that expected (>0.1 Msun) for standard models for these scenarios. Another plausible sequence of events is a weak SN ejecta interaction with a H-shell ejected by optically thick winds or a nova-like eruption on the C/O WD progenitor some years before the supernova explosion.