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.

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.

Workshop 11 covered the substantial recent progress in studies of supernovæ (SNe), tidal disruption events (TDEs), and other types of luminous transients occurring within the nuclear regions of galaxies. In the past, such transients have largely been missed owing to the substantial extinction of those regions, and to the problems of contrast against the bright (and often complex) nuclear background – or mistaken for normal active galactic nucleus (AGN) variability.

Despite the expectation of a high supernova rate in luminous infra-red galaxies (LIRGs), a deficit has been discovered in optical surveys that is due to high levels of extinction by dust and to issues of contrast against the bright nuclear background. Searching in the near infra-red enables observations to penetrate that dust, while using adaptive-optics achieves the resolution required to observe supernovæ close to the nuclei of those galaxies. Over the last decade multiple observing programmes using the best AO instrumentation mounted on large telscopes have accumulated a dataset of many LIRGs, and met with much greater success in discovering nuclear supernovæ. However, a significant proportion is still being missed. By using techniques to evaluate our detection efficiency in these data, and simulations of the supernovæ occuring in the galaxies, we can evaluate the nature of these transients.

With so many spectroscopic surveys, both past and upcoming, such as SDSS and LAMOST, the number of accessible stellar spectra is continuously increasing. There is therefore a great need for automated procedures that will derive estimates of stellar parameters. Working with spectra from SDSS and LAMOST, we put forward a hybrid approach of Kernel Principal Component Analysis (KPCA) and Support Vector Machine (SVM) to determine the stellar atmospheric parameters effective temperature, surface gravity and metallicity. For stars with both APOGEE and LAMOST spectra, we adopt the LAMOST spectra and APOGEE parameters, and then use KPCA to reduce dimensionality and SVM to measure parameters. Our method provides reliable and precise results; for example, the standard deviation of effective temperature, surface gravity and metallicity for the test sample come to approximately 47–75 K, 0.11–0.15 dex and 0.06–0.075 dex, respectively. The impact of the signal:noise ratio of the observations upon the accuracy of the results is also investigated.

In the framework of this project, the K2 RR Lyrae Survey, we proposed to observe thousands of RR Lyrae stars along the Ecliptic in Kepler’s K2 Mission. The high photometric precision and the 80-to-90-day continuous coverage enabled us to investigate in unprecedented detail the light variations of these variable stars which can trace galactic structure. The survey enabled us to conduct a thorough statistical study of RR Lyrae pulsation dynamics, including both old and more recently discovered dynamical phenomena such as resonances, non-radial modes, period doubling and the Blazhko effect. This talk described the K2 RR Lyrae Survey, and discussed the prospects of combining our endeavour with Gaia, LSST and other surveys in the context of studies of Galactic archeology.

Ambitious X-ray observatories have enabled a rapid expansion in our knowledge of the X-ray time domain. With state-of-the-art facilities like Chandra, XMM Newton, and Swift performing surveys for over a decade (and counting), variability catalogues are becoming increasingly rich. Meanwhile, high time-resolution from the likes of NuSTAR and NICER (and RXTE before them) continue to uncover the richness of individual systems. These efforts have revealed a likely pulsar-ULX connection and possible magnetar oscillations, and have enabled reverberation mapping of AGN – to name only a few results. The talk reviewed recent highlights from the X-ray time domain, and described briefly what we hope to achieve with up-coming and proposed X-ray missions including HEX-P, Athena, XARM, eROSITA, STROBE-X, eXTP and TAP.

Photographic plate archives contain a wealth of information about the positions and brightness of celestial objects decades ago. Plate digitization is necessary to make this information accessible, but extracting it is a technical challenge. We have developed algorithms to extract photometry with an accuracy of better than ∼0.1 mag. in the range 13 < B < 17 mag from photographic images obtained in 1948–1996 with the 40-cm Sternberg Institute astrograph (30 × 30 cm plate size, 10 × 10 deg field of view) and digitized using a flatbed scanner. The extracted photographic light-curves are used to identify thousands of new high-amplitude variable stars (>0.2 mag). The algorithms are implemented in the free software VaST available at http://scan.sai.msu.ru/vast/

Wide-angle surveys at different wavelengths are already providing triggers for very different kinds of transients. The most interesting science is produced when new sources are followed-up and characterised by using the right instrumentation, telescopes and observing strategies. In the coming years, with new large-scale surveys such as ZTF and LSST, the amount of triggers is expected to scale up massively. Furthermore, new observational windows, such as gravitational waves or neutrinos, are now opening and adding complexity to the picture. The instrumentation and strategies that we have been using over recent years may just not be appropriate for those new situations. In this Workshop we discussed the present and projected future of transient discovery, the instrumentation that will be needed for the follow-up of those targets, and the observing strategies, data analysis and community efforts that will be required to tackle the challenges that lie ahead of us.

This presentation addressed some aspects of photometric standardisation and calibration that have a very significant effect on the accuracy of long time-baseline photometry. The difficulties were illustrated by examples of combinations of vintage photographic magnitudes with photomultiplier and CCD photometry, and with photometry from space. The case studies involved variability on time-scales of hours, years, decades and centuries. The examples went beyond classical problems of combining incongruent and ambiguous passbands, non-linear detectors and poor standardisation.

This Workshop covered a cornucopia of topics that were featured in short formal presentations, followed by a round-table discussion. G. Hosseinzadeh and H. Kuncarayakti presented the results of their recent researches into interacting supernovæ. They included both the intriguing Type Ibn supernova subclass, and SN 2017dio, which appears to be the first Type Ic supernova to be seen to exhibit signatures of hydrogen-rich circumstellar interaction at all phases. M. Sullivan provided a summary relating to the future of transient science in the era of Big Data, and participants discussed strategies to determine which targets and fields should be selected for spectroscopic follow-up. The Workshop concluded with a rather heated discussion regarding the need for the IAU Supernovæ Working Group to consider modifying the current criterion for a confirmed supernova in order for it to receive an official IAU designation.

The launch of NASA’s next exoplanet mission, the Transiting Exoplanet Survey Satellite (TESS), took place successfully on 2018 April 18, and has now commenced science operations. TESS is specifically designed to search for exoplanets transiting the closest and brightest stars using high-cadence photometric measurements. The images employed for detecting those planets can also be used for a wide variety of time-domain astronomy, especially when considering the full-frame images that TESS takes every 30 minutes. This (pre-launch) workshop familiarised participants with the details of how TESS will operate, described the expected data products and how to access them, introduced the software suite PyKE, which can be used to analyse TESS data, and highlighted ways for participants to request additional TESS targets.

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.

The Thai National Observatory (TNO) is equipped with a 2.4-m Ritchey-Chretien telescope and the high-speed versatile Ultraspec camera. The instrument employs a low-noise frame-transfer EMCCD, suitable for the observation of faint objects and for high-time-resolution astrophysics. We present some of the results obtained in the first four years of operation, focusing particularly on fast photometry of lunar and stellar occultations, and follow-up efforts on a few white-dwarf binaries. Among the latter is the polar cataclysmic variable UZ For. This system displays period changes and is suspected of hosting circumbinary planets. Our high-speed photometry data show a decreasing trend in the O–C diagram of UZ For. Using our new data set, we will investigate whether the period change in this binary is due to a possible third body, or to other mechanism(s).

Astrosat is a multi-instrument orbiting observatory that was launched in 2015 by the Indian Space Research Organization (ISRO). The same field of view is observed simultaneously at wavelengths ranging from gamma ray to the optical blue. This talk described the observatory’s performance, with emphasis on time-domain studies, and gave examples of results.

The Deeper Wider Faster programme (DWF) is a project that coordinates more than 30 multi-wavelength and multi-messenger facilities worldwide and in space, in order to detect and study fast transients (durations of milliseconds to hours). DWF has four main components: (1) simultaneous observations, where ∼10 major facilities, from radio to gamma-ray, are coordinated to perform short-cadence, deep, wide-field observations of the same field at the same time. Radio telescopes search for fast radio bursts, while optical imagers and high-energy instruments search for transient events whose time-scales are seconds to hours, (2) supercomputer data processing and candidate identification in real time (seconds to minutes), along with human inspection of candidates, also in real time (minutes), using sophisticated visualisation technology, (3) rapid-response (minutes) follow-up spectroscopy and imaging, and conventional ToO observations, and (4) long-term follow up by a global network of 1-m to 4-m telescopes. The principal goals of DWF are to discover and study counterparts to fast radio bursts and gravitational-wave events, as well as transients at all wavelengths that have durations of milliseconds to hours.

The contribution of citizens to research is irrefutable. Especially this century with the outburst of all-sky surveys, professional astronomers use citizen-science projects to engage the public in analysing and sorting large quantities of data, often leading to noteworthy discoveries. From crowdsourcing to acquiring data, citizens are leaving a significant mark in the science landscape, assisting professional astronomers with their work. In turn, citizen science is a means of increasing science literacy and public understanding of science. At the same time, the time domain enables a more active engagement of backyard observers in research. Citizen astronomers not only take data, but also reduce and analyse them, and participate in preparing scientific manuscripts.

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.

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.

The Optical Gravitational Lensing Experiment (OGLE) started at Las Campanas Observatory in 1992 with a pilot monitoring programme of two million stars in the Galactic Bulge. It is still operating today, collecting time-domain photometric data of a billion stars from the densest regions in the southern sky. Among its main achievements are discoveries of thousands of microlensing events, a few dozen extrasolar planets and candidates for black holes, a million variable stars, and thousands of quasars and supernovæ. It has made a major contribution to the studies of the dark-matter content of the Milky Way halo, the structure of the Galactic Bulge, the Magellanic Clouds, and new classes of variable stars. In this its 25th anniversary year, we presented a selection of the major scientific highlights of OGLE.