transient

A transient is an observed phenomenon in the sky involving a change, such as the appearance of an astronomical object or a change in ones appearance. Their general study is termed transient astronomy (which can be thought of as the same, or as a portion of time-domain astronomy). Transients are detected by observing an area of the celestial sphere more than once and noting the differences. Some projects, missions, and surveys are specifically to discover them, for further study. Numerous observed occurrences and event types fall under the broad category of transients, thus a survey aimed at finding one phenomenon often yields discoveries of unrelated phenomena:

• novae.
• supernovae.
• kilonovae, etc.
• gravitational microlensing.
• transits.
• occultations.

The movement of minor planets and comets through the celestial sphere are less-often referred to as transients, but surveys aimed at changes do discover and record them as well, and surveys hunting for them discover transients. Transients are also categorized by the type of EMR under which they were observed, radio, infrared, etc. Some terms:

• radio transient, such as a fast radio burst (FRB) or a rotating radio transient (RRAT).
• X-ray transient (X-ray burst) such as a black hole X-ray transient (BHXRT or BHXT) or soft X-ray transient (SXT, typically from a binary star that incomes a compact object).
• gamma-ray bursts.

The term transient also includes GW events and neutrino bursts. Current technological advances are increasing our ability to detect and observe transients, in particular, the advancement of automation: telescopes under computer control imaging the sky repeatedly, with computers identifying changes (and distinguishing weather and airplanes from actual changes in the celestial sphere). There has been an increase in the number and the efficiency of surveys aiming to find and record them, with more in development, such as Rubin Observatory.