Quasars are variable and their variability can both constrain their physical
properties and help to identify them. We look for ways to efficiently identify
quasars exhibiting consistent variability over multi-year time-scales, based on
a small number of epochs. Using infrared (IR) is desirable to avoid bias
against reddened objects. We compare the apparent brightness of known quasars
that have been observed with two IR surveys, covering up to a twenty-year
baseline: the Two Micron All Sky Survey (2MASS; 1997-2001) and the VISTA
Hemisphere Survey (VHS; 2009-2017). We look at the previous studies of the
selected variable quasars to see if their variable behaviour is known and when
available we use multi-epoch monitoring with the Zwicky Transient Facility
(ZTF) to obtain a measure of optical variability of individual objects. Nous
build a sample of ~2500 quasars that show statistically significant variability
between the 2MASS and VHS. À propos 1500 of these come from the new Quaia sample
based on Gaia spectra and about 1/3 of these have hardly been studied. The
Quaia sample constitutes the main product of this work. Based on ensemble
variability and structure function analysis we demonstrate that the selected
objects in our sample are representative of the typical quasar population and
show behaviour, consistent with other quasar samples. Our analysis strengthens
previous results, for example that variability decreases with the rest-frame
wavelength and that it exhibits peaks for certain absolute magnitudes of the
quasars. Similarly, the structure function shows an increase in variability for
rest-frame time lags below ~1500 d and a decrease for longer lags, just like in
previous studies. Our selection, even though it is based on two epochs only,
seems to be surprisingly robust, showing up to ~11% contamination by quasars
that show stable non-variable behaviour in ZTF.

Cet article explore les excursions dans le temps et leurs implications.

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