A Medieval Bestiary of Sources
A variable astrophysical phenomena catalogue resembles a Medieval bestiary of strange species.
There are sources displaying flux and/or spectral changes recurrent at regular intervals of time. This behavior is called "periodic variability"
We will talk of aperiodic variability for all remaining sources; but if the sources show small fluctuation in their emission (let's say a factor less than 10 of variability), they are considered simply "variable"; if their fluctuations are larger than a factor 10, the sources are called "transient".
Sources with aperiodic variability and/or transient behavior
Up to 50 years ago, the search for astronomical transients was limited to comets (becoming bright when approaching the Sun), novae (giant eruptions on low mass stars) and supernovae (catastrophic explosions of stars). The birth of high energy astrophysics opened a new window to explore time-variable phenomena, unveiling an extremely dynamic sky and leading to the discovery of new classes of objects, some of them completely unexpected. Among the new objects and astrophysical processes discovered thanks to their transient – or highly variable - nature at high energies, there are:
- gamma-ray bursts (GRBs): the most powerful cosmic explosions, likely produced by the collapse of massive stars to black holes or by the coalescence of two neutron stars;
- soft gamma-ray repeaters (SGRs): X-ray sources believed to be powered by magnetars, i.e. neutron stars with the strongest magnetic field in the Universe;
- transient X-ray binaries: black holes, neutron stars or white dwarfs accreting matter from their stellar companion;
- stellar flares: X-ray flares from magnetically active, late-type stars, either isolated or in binary systems;
- blazar flares: gamma-ray flares produced by the jets of supermassive black holes at the centre of galaxies;
- tidal disruption events: gravitational capture and disruption of a star by a supermassive black hole;
- supernova X-ray flashes: produced by the supernova shock emerging from the exploding star.
More generally speaking, since the very beginning of X-ray astronomy it became apparent that variability is the rule in the X-ray sky. Almost all source classes, from accreting systems to stellar coronae, display a characteristic variability in flux and/or spectral shape at some level, at different time scales, which yields important clues on the emission physics. For instance, early observations of active galaxies' rapid variability provided the first compelling argument for the standard black hole/accretion disk model.
Since the prehistorical efforts aimed at developing the calendar, the detection and investigation of periodic phenomena have played a major role in astronomy. Crucial information is obtained through the observation and measurement of periodicities in many classes of celestial bodies. In many cases these periodic signals arise from the rotation of a compact star or the orbital motion in a binary system.
Among the known high energy astronomical objects displaying coherent signals are:
- spinning up and down accreting magnetic neutron stars in binary systems;
- spinning down young neutron stars, the emission of which is powered by the dissipation of rotational, thermal or even magnetic energy (as in the cases of classical radio pulsars, the so-called "Magnificent Seven" neutron stars and magnetars, respectively);
- accreting magnetic white dwarf systems, such as polars and intermediate polars;
- orbital modulations (including periodic dips and eclipses) of the X-ray flux in various classes of X-ray binaries with accreting neutron stars, black holes or white dwarfs (especially if seen from a high inclination).
So far, a significant fraction of the discovered signals has been obtained in a serendipitous way through the visual inspection of the light curves of the sources which lie, by chance, in the field of view of the instruments.