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.

We report the discovery of a group of galaxies at redshift 2.38. We imaged about 10% of a claimed supercluster of QSO absorption-lines at z=2.38 (Francis & Hewett 1993). In this small field (2 arcmin radius) we detect two Ly-alpha emitting galaxies. The discovery of two such galaxies in our tiny field supports Francis & Hewett's interpretation of the absorption-line supercluster as a high redshift "Great Wall". One of the Ly-alpha galaxies lies 22 arcsec from a background QSO, and may be associated with a multi-component Ly-alpha absorption complex seen in the QSO spectrum. This galaxy has an extended (50kpc) lumpy Ly-alpha morphology, surrounding a compact IR-bright nucleus. The nucleus shows a pronounced break in its optical-UV colors at about 4000 A (rest-frame), consistent with a stellar population of mass about 7E11 solar masses, an age of more than 500 Myr, and little on-going star-formation. C IV emission is detected, suggesting that a concealed AGN is present. Extended H-alpha emission is also detected; the ratio of Ly-alpha flux to H-alpha is abnormally low (about 0.7), probable evidence for extended dust. This galaxy is surrounded by a number of very red (B-K>5) objects, some of which have colors suggesting that they too are at z=2.38. We hypothesize that this galaxy, its neighbors and a surrounding lumpy gas cloud may be a giant elliptical galaxy in the act of bottom-up formation.Comment: Accepted for publication in ApJ (Feb 1st 1996 issue). 23 pages, uuencoded compressed tar postscript figures (1.4 Mbytes). Text and figures also available from http://www.ph.unimelb.edu.au/~pjf/blob.html in ps forma

We have used an Imaging Fabry-Perot Spectrophotometer with the Adaptive Optics Bonnette on the Canada-France-Hawaii Telescope to measure stellar radial velocities in the globular cluster M15 (NGC 7078). An average seeing of 0.15 ′′ full-width at half maximum, with the best-seeing image having 0.09 ′′ , allowed us to measure accurately the velocities for five stars within 1 ′′ of the center of M15.

We present the spectroscopic evolution of AT 2017gfo, the optical counterpart of the first binary neutron star (BNS) merger detected by LIGO and Virgo, GW170817. While models have long predicted that a BNS merger could produce a kilonova (KN), we have not been able to definitively test these models until now. From one day to four days after the merger, we took five spectra of AT 2017gfo before it faded away, which was possible because it was at a distance of only 39.5 Mpc in the galaxy NGC 4993. The spectra evolve from blue (∼ 6400K) to red (∼ 3500K) over the three days we observed. The spectra are relatively featureless -some weak features exist in our latest spectrum, but they are likely due to the host galaxy. However, a simple blackbody is not sufficient to explain our data: another source of luminosity or opacity is necessary. Predictions from simulations of KNe qualitatively match the observed spectroscopic evolution after two days past the merger, but underpredict the blue flux in our earliest spectrum. From our best-fit models, we infer that AT 2017gfo had an ejecta mass of 0.03M , high ejecta velocities of 0.3c, and a low mass fraction ∼ 10 −4 of high-opacity lanthanides and actinides. One possible explanation for the early excess of blue flux is that the outer ejecta is lanthanide-poor, while the inner ejecta has a higher abundance of high-opacity material. With the discovery and follow-up of this unique transient, combining gravitational-wave and electromagnetic astronomy, we have arrived in the multi-messenger era. arXiv:1710.05853v1 [astro-ph.HE]

We present the results of an in-depth optical study of the core-collapse supernova remnant G292.0+1.8 using the Rutgers Fabry-Perot (RFP) imaging spectrometer. Our observations provide a detailed picture of the supernova remnant in the emission lines of [O iii] k5007, H , and [N ii] k6548. The [O iii] Fabry-Perot scans reveal a bright crescent-shaped spur of previously known high-velocity (V radial $ 1500 km s À1 ) O-rich ejecta located on the eastern side of the remnant. The spur consists of a semicoherent structure of mostly redshifted material, along with several clumps that have apparently broken out of the more orderly shell-like expansion. The high-velocity (k600 km s À1 ) component of the spur also displays a scalloped morphology characteristic of Rayleigh-Taylor instabilities. We also find a large number of fast-moving knots (FMKs) of O-rich ejecta undetected in prior photographic plate images and similar to features seen in Cas A. The FMKs are distributed sparsely in the interior of G292.0+1.8 and are seen mostly in blueshifted emission out to V radial % À1700 km s À1 . The position-velocity distribution of the FMKs can be kinematically described as a shell 3A4 in radius expanding at a velocity of 1700 km s À1. Another feature apparent in the [O iii] scans is an equatorial belt consisting of both a barlike structure at zero radial velocity and a clumpy, highvelocity ejecta component seen in projection along the line of sight. Portions of the zero-velocity bar are spatially well correlated with a similar structure seen in the Chandra X-ray image of G292.0+1.8. The bar is also detected in our H RFP images at zero radial velocity, providing further evidence that this structure is of circumstellar origin. We find that the optical and X-ray properties of the bar are consistent with incomplete (partially radiative) shocks in material of moderate densities. There are also a number of faint, elongated structures seen in H at zero radial velocity across the interior of G292.0+1.8 that lack [O iii] and X-ray counterparts. These filaments may be low-density H i clouds photoionized by hard radiation from the interior of the remnant. Overall, these results suggest that G292.0+1.8 is currently interacting with a low-density environment. We find no evidence for high-velocity H or [N ii] emission over the dynamical range sampled by the RFP. Assuming a distance of 6 kpc for G292.0+1.8, we estimate a kinematic age of (3000-3400)d 6 yr for this remnant.