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.

We have compiled historical observations, spanning ∼100 years, for a dozen of the best-studied LBVs in the Local Group. We described how we prepared structure functions for their light-curves and calculated two parameters (the structure function slope and the characteristic time-scale) to describe the behaviour of the LBVs. The sensitivity of those parameters to the variability behaviour of the stars was tested with a number of photometric data sets. The slope of the structure function may anticorrelate with the time-scale. These types of variable stellar objects are crucial to studies of stellar variability and the final stages of stellar evolution.

The Hubble Source Catalog (HSC) combines lists of sources detected on images obtained with the WFPC2, ACS and WFC3 instruments aboard the Hubble Space Telescope (HST) and now available in the Hubble Legacy Archive. The catalogue contains time-domain information for about two million of its sources detected using the same instrument and filter on at least five HST visits. The Hubble Catalog of Variables (HCV) aims to identify HSC sources showing significant brightness variations. A magnitude-dependent threshold in the median absolute deviation of photometric measurements (an outlier-resistant measure of light-curve scatter) is adopted as the variability detection statistic. It is supplemented with a cut in χred2 that removes sources with large photometric errors. A pre-processing procedure involving bad image identification, outlier rejection and computation of local magnitude zero-point corrections is applied to the HSC light-curves before computing the variability detection statistics. About 52 000 HSC sources have been identified as candidate variables, among which 7,800 show variability in more than one filter. Visual inspection suggests that ∼70% of the candidates detected in multiple filters are true variables, while the remaining ∼30% are sources with aperture photometry corrupted by blending, imaging artefacts or image processing anomalies. The candidate variables have AB magnitudes in the range 15–27m, with a median of 22m. Among them are the stars in our own and nearby galaxies, and active galactic nuclei.

Evolved low- and intermediate-mass stars that have reached the Asymptotic Giant Branch (AGB) phase tend to show pronounced long-period variability due to large-amplitude pulsations. Those pulsations are considered to play a key role in triggering mass loss through massive dusty winds. The winds enrich the surrounding interstellar medium with newly-produced chemical elements and dust grains, providing building blocks for new generations of stars and planets. Considerable efforts are being made to understand the physics of AGB stars, and to develop quantitative models. This talk gave a brief summary of recent developments, with references to the literature.

This talk discussed the basics of gravito-inertial asteroseismology as recently developed for stars born with a convective core. Photometric space missions originally built for exoplanet hunting, notably Kepler, have opened up the low-frequency regime of stellar oscillations and revealed a larger diversity in variability than anticipated prior to the era of high-precision space photometry. The talk explained the basics of forward seismic modelling based on gravito-inertial modes, which probe the deep stellar interior. It described how a hierarchical fitting approach allows us to derive the near-core rotation period, the amount and shape of convective core overshooting, and the level of chemical mixing in the radiative envelope for stars born with a convective core and burning hydrogen in their core. A summary of the current status, covering the mass range 1.4 ≲ M ≲ 5 M⊙, is provided here through references to numerous recent papers.

Over an 11-year cycle the Sun changes its brightness by less than 0.1%. However, it is an open question how strong the Sun’s photometric variability was in the distant past. One way to answer that is to study other Sun-like stars and compare their photometric variability with that of the Sun. In a recent paper, we presented ground-based spectroscopic observations of a 7.4-year cycle in the solar analogue HD 173701. Complemented with observations from the Kepler space telescope, those data constitute the most complete set of observations of a stellar cycle ever obtained for any Sun-like star. They reveal that HD 173701 has strong solar-like differential rotation and a magnetic cycle comparable to the cycle generated by the solar dynamo, but with a resulting variability twice the amplitude of that observed in the Sun.

Recent studies have revealed the existence of a significant population of Ap stars with extremely long rotation periods, and the frequent occurrence of Ap stars in wide binaries. Those results represent new constraints on the understanding of the origin and evolution of Ap stars, and (by extension) of all upper-main-sequence stars. Current knowledge of Ap stars with the longest rotation and orbital periods remains incomplete, on the one hand because in many cases the periods of interest are longer than the time-spans over which relevant observations have been obtained, and on the other hand because some important subsets of Ap stars have been omitted from the studies that have been carried out until now. Additional observations over time-scales of decades to centuries are needed to complement the current incomplete picture. Securing them with the required accuracy and time coverage, and ensuring that their full exploitation will ultimately be possible, represents a unique challenge in time-domain astronomy.

The period variations of rotating, pulsating and eclipsing variable stars bear valuable astrophysical information about the presence of companions, evolutionary effects, and the inner structure of the stars. This talk described a universal method for de-trending and re-scaling precise photometric data (Kepler, MOST, CoRoT, OGLE, …) appropriate for period-change diagnostics of periodic variables. We demonstrated the potential of the method by analysing the period variability of one of the newly-identified Kepler magnetic chemically-peculiar (mCP) stars. We showed that, surprisingly, our target star displays near-sinusoidal changes in its observed light-variations, with a period of 2.85(6) years, which is apparently the result of the presence of a nearby stellar companion. The expected long-term changes of the rotational period, as have been observed in several mCP stars, have not been identified among the sample of Kepler mCP stars.

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.

Many distinct classes of high-energy variability have been observed in astrophysical sources, and over a range of time-scales. The widest range, spanning microseconds to decades, is found in accreting, compact, stellar-mass objects, including neutron stars and black holes. Neutron stars are of particular observational interest as they exhibit surface effects giving rise to phenomena – such as thermonuclear bursts and pulsations – not seen in black holes.

This talk reviewed briefly the present understanding of thermonuclear (Type-I) X-ray bursts – events that are powered by an extensive chain of nuclear reactions which in many cases are unique to the environments. Thermonuclear bursts have been exploited over the last few years as an avenue to measure a neutron star’s mass and radius, although the contribution of systematic errors to the measurements remains contentious. We described recent efforts to match burst models to observations better, with a view to resolving some of the astrophysical uncertainties relating to those events. Our efforts have good prospects for providing information that is complementary to nuclear experiments.

This contribution gave three examples of X-ray transients in the Magellanic Clouds and the Milky Way that have been observed as part of the SALT Transients Large Programme. The transients (SMC X-3, MAXI J1957+032 and ASASSN-16oh) have been triggered from both space-based wide-field monitoring facilities (Swift/XRT, MAXI) and ground-based ones (ASASSN, the All Sky Automated Survey for SN), providing insights into the physics of super-Eddington accretion onto neutron stars and white dwarfs, and also into the long-term properties of accreting millisecond X-ray pulsars.

We have identified a new population of luminous, optical, narrow-lined transients (FWHM ∼1000 km s−1) coincident with the nuclear region of Seyfert galaxies. According to extensive spectrophotometric follow-ups of the main event (PS1-10adi), we could exclude both normal active galactic nucleus activity and changing-look quasars as the origin. The integrated energy output and spectral evolution over a time-scale of several years point to two possible paths of origin: a tidal disruption of a star by a supermassive black hole, or an extremely energetic supernova occurring within the Seyfert galaxy’s narrow-line (or broad-line) region. The former model would require invoking a specific variant of a tidal disruption, while the latter would require an extremely efficient conversion of kinetic energy via shock interaction between the supernova ejecta and the dense ambient medium.

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.

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.

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.

MAXI (Monitor of All-sky X-ray Image) is an astronomical mission onboard the International Space Station. It started observations in August 2009. The Gas Slit Camera of MAXI is sensitive to X-rays in the energy range 2–30 keV. Most of the sky is scanned every 90 min with the orbital revolution of the ISS. With this unbiased monitoring, MAXI has detected numerous outbursts from known and unknown X-ray sources. MAXI discovered 18 X-ray novæ in seven years, including seven neutron star binaries, six black hole binaries (+candidates) and four unidentified sources. Other results include detections of superluminous stellar flares, a super-Eddington luminous flare from a white dwarf+Be Star binary near the SMC, and monitoring of recurrent outbursts from Be neutron-star binaries. Variations in X-ray-bright AGNs such as Cen A and Mrk 421 have been also monitored. This talk presented the highlights of the MAXI observations of variable sources, including the search for X-ray counterparts of gravitational-wave events.

eROSITA (the extended ROentgen Survey with an Imaging Telescope Array) onboard the Spectrum Roentgen Gamma mission will perform a deep all-sky X-ray survey. During the first four years of operation the satellite will scan the entire sky every once every half year, visiting any position between 8 and 500 times. The eROSITA scanning strategy will test a wide range of times-scales, from seconds to years, and thus provide a powerful window into the X-ray transient and variable sky. This contribution summarised the key science opportunities for time-domain studies with eROSITA, and presented strategies for finding transients in the all-sky survey data.

This talk presented a summary of our study of different types of long-term variability in the high-mass X-ray binary LMC X-4, by taking advantage of more than 43 years of measurements in the X-ray domain. In particular, we investigated the 30-day cycle of modulation of the X-ray emission from the source (super-orbital or precessional variability), and refined the orbital period and its first derivative. We showed that the precession period in the time-interval 1991–2015 is near its equilibrium value of Psup = 30.370 days, while the observed historical changes in the phase of this variability can be interpreted in terms of the ‘red noise’ model. We obtained an analytical law from which the precession phase can be determined to within 5% throughout the entire time-interval under consideration. Our analysis revealed for the first time that the source is displaying near-periodic variations of its spin period, on a time-scale of roughly 6.8 years, thus making LMC X-4 one of the (few) known binary systems that show remarkable long-term spin–torque reversals.

AGNs are an inherently variable population but the physical mechanisms behind optical variability remain poorly constrained or not well understood. The advent of large archival collections of astronomical time series together with new analysis techniques is now driving systematic explorations of these phenomena. They reveal both population behaviours and individual extreme sources. This talk reviewed how the Catalina Real-time Transient Survey is transforming our knowledge of AGN behaviour, mentioning in particular the new CRTS Southern Sky Quasar Catalogue (which covers −70 < Dec < 0 down to V ∼ 19), and the potential of the Zwicky Transient Facility for further discoveries.

LSST will uncover a few million time-domain alerts per night with a latency of a minute. This will provide a huge discovery space, where previously undiscovered rare phenomena may be revealed. The early detection of such previously unknown events from a minimal set of measurements is critical for discovery and follow-up. Analysis methods to recognise such events are being investigated and developed. A discussion of those approaches was presented.