Time-domain astronomy is the study of howastronomical objects change with time. Said to have begun with Galileo'sLetters on Sunspots, the field has now naturally expanded to encompass variable objects beyond theSolar System. Temporal variation may originate from movement of the source, or changes in the object itself. Common targets includenovae,supernovae,pulsating stars,flare stars,blazars andactive galactic nuclei. Optical time domain surveys includeOGLE,HAT-South,PanSTARRS,SkyMapper,ASAS,WASP,CRTS,GOTO, and theLSST at theVera C. Rubin Observatory.
Time-domain astronomy studiestransient astronomical events ("transients"), which include various types of variable stars, includingperiodic,quasi-periodic, highproper motion stars, and lifecycle events (supernovae,kilonovae) or other changes in behavior or type. Non-stellar transients includeasteroids,planetary transits andcomets.
Transients characterizeastronomical objects or phenomena whose duration of presentation may be from milliseconds to days, weeks, or even several years. This is in contrast to the timescale of the millions or billions of years during which thegalaxies and their componentstars in theuniverse have evolved. The term is used for violentdeep-sky events, such assupernovae,novae,dwarf nova outbursts,gamma-ray bursts, andtidal disruption events, as well asgravitational microlensing.[1]
Time-domain astronomy also involves long-term studies ofvariable stars and their changes on the timescale of minutes to decades. Variability studied can beintrinsic, including periodic orsemi-regularpulsating stars,young stellar objects,stars with outbursts,asteroseismology studies; orextrinsic, which results fromeclipses (inbinary stars,planetary transits),stellar rotation (inpulsars,spotted stars), orgravitational microlensing events.
Modern time-domain astronomy surveys often usesrobotic telescopes, automatic classification of transient events, and rapid notification of interested people.Blink comparators have long been used to detect differences between two photographic plates, andimage subtraction became more used when digital photography eased the normalization of pairs of images.[2] Due to large fields of view required, the time-domain work involves storing and transferring a huge amount of data. This includesdata mining techniques, classification, and the handling of heterogeneous data.[3]
The importance of time-domain astronomy was recognized in 2018 byGerman Astronomical Society by awarding aKarl Schwarzschild Medal toAndrzej Udalski for "pioneering contribution to the growth of anew field of astrophysics research, time-domain astronomy, which studies the variability of brightness and other parameters of objects in the universe in different time scales."[4]Also the 2017Dan David Prize was awarded to the three leading researchers in the field of time-domain astronomy:Neil Gehrels (Swift Gamma-Ray Burst Mission),[5]Shrinivas Kulkarni (Palomar Transient Factory),[6]Andrzej Udalski (Optical Gravitational Lensing Experiment).[7]
Before the invention oftelescopes, transient events that were visible to thenaked eye, from within or near theMilky Way Galaxy, were very rare, and sometimes hundreds of years apart. However, such events were recorded in antiquity, such as thesupernova in 1054 observed by Chinese, Japanese and Arab astronomers, and the event in 1572 known as "Tycho's Supernova" afterTycho Brahe, who studied it until it faded after two years.[8] Even though telescopes made it possible to see more distant events, their small fields of view – typically less than 1square degree – meant that the chances of looking in the right place at the right time were low.Schmidt cameras and otherastrographs with wide field were invented in the 20th century, but mostly usedto survey the unchanging heavens.
Historically time domain astronomy has come to include appearance ofcomets and variable brightness ofCepheid-type variable stars.[2] Old astronomical plates exposed from the 1880s through the early 1990s held by theHarvard College Observatory are being digitized by theDASCH project.[9]
The interest in transients has intensified when largeCCD detectors started to be available to the astronomical community. As telescopes with larger fields of view and larger detectors come into use in the 1990s, first massive and regular survey observations were initiated - pioneered by the gravitational microlensing surveys such asOptical Gravitational Lensing Experiment and theMACHO Project. These efforts, beside the discovery of the microlensing events itself, resulted in the orders of magnitude more variable stars known to mankind.[10][11]Subsequent, dedicated sky surveys such as thePalomar Transient Factory, the spacecraftGaia and theLSST, focused on expanding the coverage of the sky monitoring to fainter objects, more optical filters and better positional and proper motions measurement capabilities. In 2022, theGravitational-wave Optical Transient Observer (GOTO) began looking for collisions between neutron stars.[12]
The ability of modern instruments to observe inwavelengths invisible to thehuman eye (radio waves,infrared,ultraviolet,X-ray) increases the amount of information that may be obtained when a transient is studied.
Inradio astronomy theLOFAR is looking for radio transients. Radio time domain studies have long includedpulsars and scintillation. Projects to look for transients in X-ray and gamma rays includeCherenkov Telescope Array,eROSITA,AGILE,Fermi,HAWC,INTEGRAL,MAXI,Swift Gamma-Ray Burst Mission andSpace Variable Objects Monitor.Gamma ray bursts are a well known high energy electromagnetic transient.[13] The proposedULTRASAT satellite will observe a field of more than 200 square degrees continuously in an ultraviolet wavelength that is particularly important for detecting supernovae within minutes of their occurrence. The proposed Argus Array would act as a smaller Northern Hemisphere counterpart to the Rubin Observatory with a field of 8,000 square degrees and a focus on transients, and would be part of the futureEric Schmidt Observatory System that includes an array of small radio dishes similar to theSquare Kilometre Array, a similarly designed array of telescopes linked by fiber to a spectrograph, and aHubble-scaled visible-light space telescope.[14][15]
