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The SkyMapper Transient survey (SMT) is exploring variability in the southern sky by performing (a) a rolling search to discover and study supernovæ, and (b) a Target of Opportunity programme that uses the robotic SkyMapper Telescope at Siding Spring Observatory. The supernova survey is obtaining a non-targeted sample of Type Ia supernovæ (SNe Ia) at low redshifts, z < 0.1, and studying other interesting transients found with the search strategy. We have a Target of Opportunity programme with an automatic response mechanism to search for optical counterparts to gravitational-wave and fast radio-burst events; it benefits from SkyMapper’s large field of view of 5.7 sq. deg. and a rapid data reduction pipeline.
We present first results of the SMT survey. The SMT pipeline can process and obtain potential candidates within 12 hours of observation. It disentangles real transients from processing artefacts using a machine-learning algorithm. To date, SMT has discovered over 60 spectroscopically confirmed supernovæ, several peculiar objects, and over 40 SNe Ia including one (SNIa 2016hhd) which was found within the first few days of explosion. We have also participated in searches for optical counterparts of gravitational waves, fast radio bursts and other transients, and have published observations of the optical counterpart of the gravitational-wave event GW170817. We also participate in coordinated observations with the Deeper Wider Faster programme, and the Kepler K2 cosmology project.
The Berkeley Visible Image Tube (BVIT) has been a user instrument on the SALT 10-m telescope for the past six years. It can observe transient astrophysical phenomena occurring on time-scales of micro-seconds. This overview presented some recent observations of a dMe flare star, and discussed the recent results of our optical Search for Extraterrestrial Intelligence (OSETI) around nearby exoplanet-hosting stars.
In addition to its main goal of measuring the parallaxes of 109 stars in the Galaxy, ESA’s Gaia mission is also probing the time domain on a range of astrophysically interesting time-scales. The photometric measurements are processed for transient variability, and events that are discovered are published as a public alerts stream, known as Gaia Science Alerts. This talk gave an overview of the project to date, and highlighted its unique characteristics as a transient survey. It discussed briefly some of the recent scientific results which illustrate the broad range of science investigations enabled by the data. Gaia Alerts is particularly suited to the study of Galactic populations, as it covers the Galactic Plane with high astrometric precision and photometric accuracy.
As shown by recent gravitational wave detections, galaxies harbour an unknown population of black holes at high masses. In our Galaxy such dark objects can be found and studied solely via gravitational microlensing methods. This paper described our search for black-hole lenses both in archived OGLE data and among on-going microlensing events found by OGLE and Gaia. That combination of superb time-domain astrometry and photometry will enable us to derive masses and distances to these dark lenses uniquely, and to describe the demographics of the unseen component of the Milky Way.
Radio astronomy is currently exploring an intriguing new phase-space that probes the dynamic Universe on time-scales of milliseconds. Recent developments of sensitive, high-time-resolution instruments has made possible the discovery of millisecond-duration fast radio bursts (FRBs). The FRB class encompasses a number of single pulses, each unique in its own way, hindering a consensus for their origin. The key to de-mystifying FRBs lies in discovering many of them in real time in order to identify commonalities. The recently upgraded UTMOST, in Australia, has undergone a digital back-end transformation to rise as a fast-transient detection machine. The talk presented the first interferometric detections of FRBs made by this telescope at less that a quarter of its target sensitivity, placing their origin beyond the near-field region of the telescope and thus ruling out local sources of interference as a possible origin. Despite rigorous follow-ups, none of the FRBs observed with the upgraded UTMOST has been seen to repeat, suggesting the possibility of there being two independent classes of FRBs with two classes of possible progenitors. The talk then discussed the recent developments in the field, some of the open questions in FRB astronomy, and how the next-generation telescopes are vital in the quest to understand this enigmatic population.
The Transiting Exoplanet Survey Satellite (TESS) is a NASA Astrophysics Explorer-class mission that will perform an all-sky survey to search for planets transiting nearby bright stars. The primary goal is to search for planets smaller than Neptune that are amenable to follow-up spectroscopic observations that will yield planet masses, thereby providing prime targets for future atmospheric characterization studies. In its two-year prime mission, TESS will monitor more than 200,000 stars with four wide-field optical CCD cameras that will tile more than 90% of the sky. TESS will also obtain full-frame images (FFIs) of the entire field of view with a cadence of 30 minutes to facilitate additional science. These FFIs will provide photometry for more than 30 million objects brighter than magnitude I =16 during the two-year prime mission. The TESS legacy will be a catalogue of the nearest and brightest main-sequence stars hosting transiting exoplanets. The TESS Mission will also have a robust Guest Investigator (GI) Programme that will be managed by the TESS Science Support Center at NASA Goddard Space Flight Center. Under the GI programme, the astrophysics community may propose new 2-minute cadence targets and investigations using the 30-minute cadence FFI data. TESS GI calls for proposals will occur once per year, and about 20,000 targets will be available for each GI programme cycle.
TESS was launched in April 2018, and will observe from a unique elliptical high-Earth orbit that will provide an unobstructed view of its field to obtain continuous light-curves.
This talk introduced and described the Next Generation Transit Survey (NGTS), which is a new ground-based transit survey operating at the ESO Paranal Observatory. NGTS has been designed to achieve better photometric precision than previous ground-based surveys; it aims to detect Neptune-sized planets around Sun-like stars, and sub-Neptunes around M dwarfs that are sufficiently bright for radial-velocity confirmation and mass determination. NGTS is also optimised for ground-based follow up of exoplanet candidates from TESS and PLATO. I presented early results from the survey, and described the status of our HARPS radial-velocity and SAAO photometric follow-ups of exoplanet candidates.
Time-Domain astronomy exercises all aspects of the Virtual Observatory framework and Astroinformatics. Applications of machine learning and statistics to the analysis of large numbers of light-curves will increasingly yield new results as the data accumulate. However, the most challenging problems remain in the arena of rapid classification of transient events and their automated follow-up prioritisation. The talk illustrated those issues with examples from recent or ongoing synoptic sky surveys.
The radio sky is full of transients, their time-scales ranging from nanoseconds to decades. Recent developments in technology sensitivity and computing capabilities have opened up the short end of that range, and are revealing a plethora of new phenomenologies. Studies of radio transients were previously restricted to analyses of archived data, but are now including real-time analyses. We focus here on Fast Radio Bursts, discuss and compare the properties of the population, and describe what is to date the only known repeating Fast Radio Burst and its host galaxy. We also review what will be possible with the new instrumentation coming online.
Explosive stellar transients arise from diverse situations, including deaths of massive stars, a variety of thermonuclear outbursts, and compact-object mergers. Stellar interactions are heavily implicated in explaining the observed populations of events, and not only those where binarity is obviously involved. Relationships between these classes probably help to elucidate our understanding; for example; the production of double neutron-star mergers from field binaries is thought to be heavily biased towards routes involving stripped core-collapse supernovæ. As we gain an ever more synoptic view of the changing sky, theorists should be mindful of developing an ability to take robust quantitative advantage of the available population information to help constrain the physics. This is complementary to aiming for deep understanding of individual events.
As the era of the Square Kilometre Array approaches, astronomers are investigating how to make good use of its facilities for studying radio transients. This talk presented two different methods for radio transient discovery – ‘triggered’ and ‘targeted’ observations – which can be used to supplement the blind survey approach. Both techniques focus on performing radio observations of sky regions in which we expect to find radio transients. ‘Triggered’ observations are obtained by telescopes capable of responding rapidly to transient alerts; they automatically repoint and begin collecting data within minutes of the alert being given. ‘Targeted’ observational techniques involve radio monitoring of specific sources or regions such as nearby, face-on galaxies, globular clusters, and the Galactic Plane. Such observations are sensitive to transient radio jets from black holes accreting at, or above, the Eddington limit, with the additional benefit of providing many potential sources within a single field of view. Both observing strategies illustrate important techniques for radio transient discovery that can be employed by the SKA.
Astrophysical jets have been detected in objects as diverse as protostellar objects and supermassive black holes, yet we still have not answered the key question of what system properties are necessary to launch a jet. This talk described multi-wavelength time-domain studies to determine if two classes of objects at opposite ends of the energy scale are launching jets. First, Cataclysmic Variables (binaries with mass accretion rates of ≤ 10−8 M⊙y−1) were previously thought not to launch jets, and have been used to constrain jet launching models. Nevertheless, recent radio observations have indicated a jet in one system, and have shown that that system is not unique. As regards the other end of the energy scale, we still do not know if the most powerful stellar explosions (Super-Luminous Supernovæ) launch jets. Recent improvements in sensitivity (particularly at radio wavelengths), higher-cadence transient surveys, significantly improved telescope response times and longer-term monitoring have led to substantial advances in these fields. The talk discussed how we are using multi-wavelength studies (with different cadences and coverage times) of these two extremely different classes of object to determine if they launch jets, thereby to constrain the properties necessary to do so.
This paper presented very early, high-cadence photometric observations of the nearby Type Ia SN 2017cbv. The light-curve is unique in that during the first five days of observations it has a blue bump in the U, B, and g bands which is clearly resolved by virtue of our photometric cadence of 5.7 hr during that time span. We modelled the light-curve as the combination of an early shock of the supernova ejecta against a non-degenerate companion star plus a standard Type Ia supernova component. Our best-fit model suggested the presence of a subgiant star 56 R⊙ from the exploding white dwarf, although that number is highly model-dependent. While the model matches the optical light-curve well, it over-predicts the flux expected in the ultraviolet bands. That may indicate that the shock is not a blackbody, perhaps because of line blanketing in the UV. Alternatively, it could point to another physical explanation for the optical blue bump, such as interaction with circumstellar material or an unusual distribution of the element Ni. Early optical spectra of SN 2017cbv show strong carbon absorption as far as day –13 with respect to maximum light, suggesting that the progenitor system contained a significant amount of unburnt material. These results for SN 2017cbv illustrate the power of early discovery and intense follow-up of nearby supernovæ for resolving standing questions about the progenitor systems and explosion mechanisms of Type Ia supernovæ.
This general overview of our understanding of the Galaxy followed the lines of its main structures (halo, disc, bulge/bar) and emphasized some time-domain astronomy contributions. On the one hand the distance and tangential motions of the stars are essential to that understanding, and are obtained through multi-epoch surveys. On the other hand the chemistry of the stars and their radial velocities are also key elements for mapping the Galactic (sub-)structures, and unravelling their history and evolution. Contemporary surveys are revolutionizing our view of the Milky Way and of galaxies in general. Among those, the Gaia mission excels through its precise astrometry of 1.3 billion stars that populate the Milky Way and beyond, providing the first 3-D view of a major part of the Milky Way.
On 17th August 2017 a strong source of gravitational waves was detected by the LIGO-Virgo collaboration. The signal lasted for 60 seconds, and the event was followed just 2 seconds later by a short burst of gamma-rays that was detected by Fermi and INTEGRAL. The gravitational-wave and gamma-ray source had consistent sky positions to within about 30 square degrees. Within 10 hours of the gravitational-wave source event, a fast fading optical and near-infrared counterpart was discovered, which was subsequently followed-up and studied intensively for several weeks and months by numerous facilities. This talk presented the results from our optical and near-infrared imaging and spectroscopic follow-up campaign of this unprecedented discovery, which was the first electromagnetic counterpart of a gravitational-wave source, the first identification of a neutron star–neutron star merger, and the first direct evidence of the source of r-process elements. It focussed on the results of the GROND and ePESSTO teams, showing that this remarkable transient truly opened up the era of multi-messenger astronomy.
This talk presented and discussed some recent results obtained from a photometric and spectroscopic optical follow-up survey of bright classical novæ. The survey concerned the role of those objects in Galactic chemical evolution, with particular attention to the production of lithium.
This paper presented the discovery of an energetic nuclear transient from near-infrared monitoring of nearby starburst and luminous infrared galaxies. The transient radiated at least 1.5E+52 erg in the infrared but remained elusive at optical and X-ray wavelengths. We interpret its properties as arising from a stellar tidal disruption event (TDE) close to a supermassive black hole. Much of its emission must have been reprocessed by dense gas and re-radiated at infrared wavelengths by dust, suggesting a way for reducing the tension between theoretical luminosity predictions and observations of TDEs. Such events are not detectable by optical, UV or soft X-ray observations, and might represent just the tip of the iceberg of a missed TDE population in the local Universe. That population could be more numerous at higher redshifts where luminous infrared galaxies are more common.
Supernovae (SNe) are cosmic explosions which are usually represented in a small region of the luminosity–time-scale diagram when discussing the variable sky. However, there are different time-scales involved in the evolution of SNe that are not reflected by that representation. This talk reviewed some of the physical mechanisms driving the SN light-curve diversity, especially at early times. It then discussed our efforts in the astroinformatics laboratory at CMM and at MAS to discover very young SNe using large etendue telescopes such as Blanco/DECam; those efforts led to the real-time discovery of more than one hundred SNe, some of them very young, under the High cadence Transient Survey (HiTS). We showed that, by comparing hydrodynamical models in the literature with HiTS SNe using Markov Chain Monte Carlo to sample from the posterior in a Bayesian approach, we can constrain the physical parameters that are driving the early time-evolution of these events. We also discussed how these data are being used for different projects, such as the discovery of asteroids and variable stars, and for testing different machine-learning algorithms in an interdisciplinary approach.
About 30,000 astronomical photographic plates were digitised between 2012–2017 with a special digitising machine that has high precision in both astrometry and photometry. All the images from the plates, together with plate information and measured coordinates of all the objects on the plates, have been stored in the Chinese Virtual Observatory.
New observations of Kepler δ Scuti stars show that our understanding of pulsation in these stars is incomplete. A large fraction of A and B stars exhibit rotational modulation in light, suggesting that spots exist in stars with radiative envelopes. Flares are seen in some A stars, as may be expected if starspots are present. Differential rotation shear increases from M to F but decreases for A stars; it reaches a maximum among the γ Doradus variables. Current views of stars with radiative envelopes may need to be reviewed in the light of these observations.