On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

Final published version including significant revisions. Twenty four pages, fourteen figures. Original version April 2006; final version published in MNRAS August 2007We describe the goals, design, implementation, and initial progress of the UKIRT Infrared Deep Sky Survey (UKIDSS), a seven year sky survey which began in May 2005, using the UKIRT Wide Field Camera. It is a portfolio of five survey components covering various combinations of the filter set ZYJHK and H_2. The Large Area Survey, the Galactic Clusters Survey, and the Galactic Plane Survey cover approximately 7000 square degrees to a depth of K~18; the Deep Extragalactic Survey covers 35 square degrees to K~21, and the Ultra Deep Survey covers 0.77 square degrees to K~23. Summed together UKIDSS is 12 times larger in effective volume than the 2MASS survey. The prime aim of UKIDSS is to provide a long term astronomical legacy database; the design is however driven by a series of specific goals -- for example to find the nearest and faintest sub-stellar objects; to discover Population II brown dwarfs, if they exist; to determine the substellar mass function; to break the z=7 quasar barrier; to determine the epoch of re-ionisation; to measure the growth of structure from z=3 to the present day; to determine the epoch of spheroid formation; and to map the Milky Way through the dust, to several kpc. The survey data are being uniformly processed, and released in stages through the WFCAM Science Archive (WSA : http://surveys.roe.ac.uk/wsa). Before the formal survey began, UKIRT and the UKIDSS consortium collaborated in obtaining and analysing a series of small science verification (SV) projects to complete the commissioning of the camera. We show some results from these SV projects in order to demonstrate the likely power of the eventual complete survey. Finally, using the data from the First Data Release we assess how well UKIDSS is meeting its design targets so far

The intergalactic medium was not completely reionized until approximately a billion years after the Big Bang, as revealed by observations of quasars with redshifts of less than 6.5. It has been difficult to probe to higher redshifts, however, because quasars have historically been identified in optical surveys, which are insensitive to sources at redshifts exceeding 6.5. Here we report observations of a quasar (ULAS J112001.48+064124.3) at a redshift of 7.085, which is 0.77 billion years after the Big Bang. ULAS J1120+0641 has a luminosity of 6.3 × 10(13)L(⊙) and hosts a black hole with a mass of 2 × 10(9)M(⊙) (where L(⊙) and M(⊙) are the luminosity and mass of the Sun). The measured radius of the ionized near zone around ULAS J1120+0641 is 1.9 megaparsecs, a factor of three smaller than is typical for quasars at redshifts between 6.0 and 6.4. The near-zone transmission profile is consistent with a Lyα damping wing, suggesting that the neutral fraction of the intergalactic medium in front of ULAS J1120+0641 exceeded 0.1.

In the local Universe, most galaxies are dominated by stars, with less than ten per cent of their visible mass in the form of gas. Determining when most of these stars formed is one of the central issues of observational cosmology. Optical and ultraviolet observations of high-redshift galaxies (particularly those in the Hubble Deep Field) have been interpreted as indicating that the peak of star formation occurred between redshifts of 1 and 1.5. But it is known that star formation takes place in dense clouds, and is often hidden at optical wavelengths because of extinction by dust in the clouds. Here we report a deep submillimetre-wavelength survey of the Hubble Deep Field; these wavelengths trace directly the emission from dust that has been warmed by massive star-formation activity. The combined radiation of the five most significant detections accounts for 30-50 per cent of the previously unresolved background emission in this area. Four of these sources appear to be galaxies in the redshift range 2 < z < 4, which, assuming these objects have properties comparable to local dust-enshrouded starburst galaxies, implies a star-formation rate during that period about a factor of five higher than that inferred from the optical and ultraviolet observations. Recent years have seen the first meaningful attempts to determine the global star-formation history of the Universe, using the combined information provided by deep redshift surveys (for example, the Canada France Redshift Survey 1 ) reaching z Ϸ 1, and the statistics of Lyman-limit galaxies 2 at higher redshifts in, for example, the Hubble Deep Field (HDF) 3-5 . The results 6 imply that the starformation and metal-production rates were about 10 times greater at z Ϸ 1 than in the local Universe, that they peaked at a redshift in the range z Ϸ 1-1:5, and that they declined to values comparable to those observed at the present day at z Ϸ 4.These conclusions, derived from optical-ultraviolet data, may however be misleading, because the absorbing effects of dust within distant galaxies undergoing massive star-formation may have distorted our picture of the evolution of the high-redshift Universe in two ways. First, the star-formation rate (SFR) in known highredshift objects is inevitably underestimated unless some correction for dust obscuration is included in deriving the rest-frame ultraviolet luminosity. Second, it is possible that an entire population of heavily dust-enshrouded high-redshift objects, as expected in some models of elliptical galaxy formation 7 , have gone undetected in the optical-ultraviolet surveys. The extent of the former remains controversial 8-11 , while the possibility of the latter has until now been impossible to investigate. Submillimetre cosmologyAt high redshifts (z Ͼ 1), the strongly-peaked far-infrared radiation emitted by star-formation regions in distant galaxies is redshifted into the submillimetre waveband, and the steep spectral index of this emission on the long-wavelength side of the peak, at l Ϸ 100 m in the rest-frame, result...

Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of -1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90-140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

In this, the first in a series of three papers concerning the SuperCOSMOS Sky Survey (SSS), we give an introduction and user guide to the survey programme. We briefly describe other wide‐field surveys and compare them with our own. We give examples of the data, and make a comparison of the accuracies of the various image parameters available with those from the other surveys providing similar data; we show that the SSS data base and interface offer advantages over these surveys. Some science applications of the data are also described and some limitations discussed. The series of three papers constitutes a comprehensive description and user guide for the SSS.

The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies 1 . Previous candidate flares 2-6 have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two 'relativistic' candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet 7-10 . Here we report the discovery of a luminous ultraviolet-optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decline of its light curve follows the predicted mass accretion rate, and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about 2 million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.When the pericenter of a star's orbit (R p ) passes within the tidal disruption radius of a massive black hole, R T ≈ R ⋆ (M BH /M ⋆ ) 1/3 , tidal forces overcome the binding energy of the 1 star, which breaks up with roughly half of the stellar debris remaining bound to the black hole and the rest being ejected at high velocity 1 . For black holes above a critical mass,, the star becomes trapped within the event horizon of the black hole before being disrupted. The mass accretion rate (Ṁ ) in a tidal disruption event (TDE) can be calculated directly from the orbital return-times of the bound debris 1,11,12 . For the simplest case of a star of uniform density this yields,Ṁ = 2 3 ( f M⋆ t min )( t t min ) −5/3 , where f is the fraction of the star accreted and t min is the orbital period of the most tightly bound debris and, therefore, the time delay between the time of disruption and the start of the flare, which scales asThe radiative output of the accreted debris is less certain, and depends on the ratio of the accretion rate to the Eddington rate 13 . Table 2). No source is detected in a deep coadd of all the TDS epochs in 2009, with a 3σ upper limit of > 25.6 mag implying a peak amplitude of variability in the NUV of > 6.4 mag. See the Supplementary Information for details on the PS1 and GALEX photometry. PS1-10jh is coincident with the centre of a galaxy within the 3σ positional uncertainty (0.036 arcsec; Supplementary Information) with rest-frame u, g, r, i, and z photometry from SDSS 16 and K photometry from UKIDSS 17 fitted with a galaxy template 18 with M stars = (3.6 ± 0.2) × 10 9 M ⊙ and M r = −18.7 mag, where M stars is the galaxy stellar mass and M r is the absolute r-band...

Context. The infrared wide-field camera (WFCAM) is now in operation on the 3.8 m UK Infrared Telescope on Mauna Kea. WFCAM currently has the fastest survey speed of any infrared camera in the world, and combined with generous allocations of telescope time, will produce deep maps of the sky from Z to K band. The data from a set of public surveys, known as UKIDSS, will be initially available to astronomers in ESO member states, and later to the world. Aims. In order to maximise survey speed, the WFCAM field of view was required to be as large as possible while incorporating conventional infrared-instrument design features such as a cold re-imaged pupil stop and cryogenic optics and mechanisms. Methods. The solution adopted was to build a cryogenic Schmidt-type camera, mounted forward of the primary mirror, which illuminates a very large 0.9• diameter focal plane, containing four 2k × 2k HgCdTe Rockwell detectors. Results. Following several commissioning periods during which the camera, focal plane and telescope optical axes were successfully co-aligned, WFCAM now operates close to specifications, regularly achieving 0.7 FWHM images over the full field. Projects which already report excellent results include the detection of variability in young stellar clusters, as well as preliminary deep IR imaging of the Subaru and XMM-Newton deep field.