SALT ANNUAL REPORT 2017 BEGINNING of a NEW ERA Multi-Messenger Events: Combining Gravitational Wave and Electromagnetic Astronomy
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SALT ANNUAL REPORT 2017 BEGINNING OF A NEW ERA Multi-messenger events: combining gravitational wave and electromagnetic astronomy A NEW KIND OF SUPERSTAR: KILONOVAE − VIOLENT MERGERS OF NEUTRON STAR BINARIES On 17 August 2017 the LIGO and Virgo gravitational wave observatories discovered their first candidate for the merger of a neutron star binary. The ensuing explosion, a kilonova, which was observed in the lenticular galaxy NGC 4993, is the first detected electromagnetic counterpart of a gravitational wave event. One of the earliest optical spectra of the kilonova, AT However, a simple blackbody is not sufficient to explain the 2017gfo, was taken using RSS on SALT. This spectrum was data: another source of luminosity or opacity is necessary. featured in the multi-messenger summary paper by the Predictions from simulations of kilonovae qualitatively full team of 3677 collaborators. Combining this spectrum match the observed spectroscopic evolution after two with another SALT spectrum, as well as spectra from the days past the merger, but underpredict the blue flux in Las Cumbres Observatory network and Gemini–South, the earliest spectrum from SALT. From the best-fit models, Curtis McCully from the Las Cumbres Observatory and the team infers that AT 2017gfo had an ejecta mass of his colleagues were able to follow the full evolution of 0.03 solar masses, high ejecta velocities of 0.3c, and a the kilonova. The spectra evolved very rapidly, from low mass fraction ~0.0001 of high-opacity lanthanides blue (~6400K) to red (~3500K) over the three days they and actinides. One possible explanation for the early observed. The spectra are relatively featureless − some excess of blue flux is that the outer ejecta is lanthanide- weak features exist in the latest spectrum, but they are poor, while the inner ejecta has a higher abundance of likely due to the host galaxy. high-opacity material. MASTER–Net full frame composite of GW170817. Filters used are B, R, I, and W; MASTER telescopes in South Africa and Argentinia were used. The kilonova position is marked by white lines on the left part of the composed image. The right (large) image is the MASTER main telescope’s usual FOV. Image credit: MASTER–Net/NRF/SAAO Shara, M., et al. 2017, GCN 21610: LIGO/VIRGO G298048: SALT optical spectra of the candidate optical/NIR counterpart of the gravitational wave G298048 in NGC4993 Abbott, B.P., et al., 2017/10, ApJ Letters 848, 12L: Multi-messenger observations of a binary neutron star merger McCully, C., et al., 2017/10, ApJ Letters 848, 32L: The rapid reddening and featureless optical spectra of the optical counterpart of GW170817, AT 2017gfo, during the first four days SALT ANNUAL REPORT 2017 Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programs 11 Figure 7.: The rapid spectral evolution of AT2017gfo. The ANU 2.3m WiFeS, SALT RSS (2 spectra), and AAT AAOmega+2dF spectra obtained at 0.93 d, 1.18 d, 2.16 d, and 6.92 d, respectively, after GW detection are shown and labelled. Vertical grey bands denote telluric features that are not well removed in some spectra. Blackbody model fits (red curves) over the full spectra result in temperatures of 6275 K (WiFeS), 6475 K and 4700 K (RSS), Ph.D. student Igor Andreoniand 2080 from K (AAOmega).Swinburne Peaks University in the WiFeS, of Technology RSS, and AAOmega and his continua collaborators correspond to studied6400 K, the5600 rapid K, spectral evolution of the 4400 K, and < 3200 K, respectively. ⇠ ⇠ electromagnetic counterpart⇠ AT2017gfo using the two SALT spectra together with spectra obtained at two Australian telescopes. Vertical grey bands denote telluric features that are not well removed in some spectra. Blackbody model fits (red curves) over the full spectra result in temperaturesand granted via of Director’s 6 275 Discretionary K (WiFeS), Time 6 475 while and ac- 4 700(Childress K (RSS), et al. and2017). 2 Four 080 exposures K (AAOmega). were analyzed, Peaks in the WiFeS, RSS, and tivating the newly commissioned AAT 2dF Target Of revealing an E/S0-like galaxy spectrum (Figure 8) with AAOmegaOpportunity continua (ToO) mode. correspond Fully configuring to ~6400 all 392 fi- K, ~5600a weak redK, ~4400 flux enhancement K, and <3200 (Andreoni K, et respectively. al. 2017b). bres takes 40 minutes and is too long for rapid follow- The source was isolated by subtracting the host galaxy ⇠ up of short-lived transient phenomena. In rapid ToO using the SALT host galaxy spectrum (McCully et al. mode, the 2dF software determines, from an existing fi- 2017) extracted from the region of the galaxy near the bre configuration, which fibres need to move to place a source. The SALT spectrum was cleaned over chip gaps single fibre on the target and one on a guide star. This and telluric line regions using the average value on ei- capability enables configuration and observation within ther edge of each feature. Finally, the SALT host and An otherwise normal aFriday, few minutes the and, 18 inth the August case of AT2017gfo, 2017 quickly 5 minutes AAT AAOmega host+event spectra were scaled and between ToO activation and the commencement of the subtractedSALT (Figure 7). SubtractingCATCHES two spectra with A became an exciting observations.day at SALT. An urgent phone call relative flux calibrations introduces uncertainties in the around midday from MikeAT2017gfo Shara observations alerted began the onteam 2017-08-24 about at scalar o↵set. Such subtractions do not significantly af- 08:55:07 to 09:41:28 with exposure times of 600 s each. fect the form of the residual spectrum, but can provide a possible electromagneticThe data counterpart were processed using to a the gravitational OzDES pipeline aGRAVITATIONAL small a↵ect on blackbody model fit results. Although wave detection. The TechOps team rushed to prepare the PASA (2017) telescope for observationsdoi:10.1017/pas.2017.xxx that night while Petri Väisänen, WAVE the SALT Astronomer on duty, waited for details about the target. As it turned out, he would only have a small window Spectra were attempted the following nights as well before during early twilight to catch observations of the transient the object completely disappeared from the SALT visibility object in NGC 4993, some 130 million light-years away. window after 20th August. Combined with observations from other observatories, these data helped track how the With support from Cape Town, including Encarni Romero kilonova rapidly cooled (McCully et al.). SALT was the third Colmenero, Steve Potter, Ted Williams, and Steve facility to obtain a spectrum of the event and provided Crawford, Petri obtained a low resolution spectrum of important coverage between observations made in the kilonova that resulted from GW 170817, a merger of Australia and Chile (Valenti et al., Andreoni et al.). The two neutron stars. The spectrum, obtained barely a day SALT spectrum was highlighted in the multi-messenger after the initial detection, was heavily contaminated by paper (Abbott et al.) that announced the beginning of twilight and blended with the host galaxy. After careful a new era of astrophysics that combines two different extraction, the spectrum revealed the still hot fireball that carriers of information: electromagnetic radiation and resulted from the explosion. gravitational waves. Front page image: Four early spectra: SALT (at t=1.2d), ESO–NTT and SOAR (at t=1.4d) and ESO–VLT (at 2.4d). Each spectrum is normalised to its maximum and shifted arbitrarily along the linear y–axis. The high background in the SALT spectrum below 4500Å prevents the identification of spectral features in this band (Abbott et al. 2017) Valenti, S., et al., 2017/10, ApJ Letters 848, 24L: The discovery of the electromagnetic counterpart of GW170817: Kilonova AT 2017gfo/DLT17ck Andreoni, I., et al., 2017/12, PASA 34, 69: Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programmes Buckley, D.A.H., et al., 2018/02, MNRAS 474, 71L: A comparison between SALT/SAAO observations and kilonova models for AT 2017gfo: the first electromagnetic counterpart of a gravitational wave transient − GW170817 SALT ANNUAL REPORT 2017 SALT ANNUAL REPORT 2017 SOUTHERN AFRICAN LARGE TELESCOPE PO Box 9, Observatory, 7935, South Africa Phone: +27 (0)21 447 0025 Email: [email protected] SALT ANNUAL REPORT 2017 SALT ANNUAL REPORT 2017 CONTENTS 07 About SALT 09 Vision & Mission 10 Chairperson’s Overview 13 SALT Partners: An Introduction 25 Science Highlights 26 Extragalactic astronomy 38: Stellar and Galactic astronomy 48 Ongoing research 51 Student projects 55 SALT conference 59 Operations 60 Astronomy operations 63 Technical operations 65 Instrument news 70 Software updates 75 Outreach & Education 76 SALT Collateral Benefits Programme activities 81 Activities by other institutions 83 Visiting SALT 87 Corporate Governance 91 List of Publications 92 Refereed publications 93 Other 97 Glossary of Acronyms SALT ANNUAL REPORT 2017 ABOUT SALT SALT ANNUAL REPORT 2017 07 SALT ANNUAL REPORT 2017 Like diamond dust on velvet, the myriad stars of the Milky Way are strewn across the evening sky with the Magellanic Clouds gleaming below. The last glow of sunset is fading behind the SALT dome as the astronomers get ready to reap their nightly harvest of answers to the equally myriad questions of science. The Board of the Southern African Large Telescope (SALT) is proud to present its Annual Performance Report for the period 1st January 2017 to 31st December 2017. This report offers an overview of the activities and performance of SALT, highlights a selection of SALT research projects, and introduces the SALT partner institutions/consortia. SALT is the largest single optical telescope in the southern Universities of Central Lancashire, Keele, Nottingham hemisphere and amongst the largest in the world.