Hypervariable galactic nuclei
http://phys.org/news/2016-10-hypervariable-galactic-nuclei.html
Extreme variability in the intensity of the optical light of galaxies, by factors of two or more, is of great interest to astronomers. It can flag the presence
of rare types of supernovae, for example, or spot sudden accretion activity around quiescent black holes or around the supermassive black hole at the galaxy's
nucleus. In recent years systematic searches for such variability have been made using instruments that can survey wide swaths of the sky. One, the Panoramic
Survey Telescope & Rapid Response System (PanSTARRS), is a facility capable very wide-field imaging using a combination of relatively small mirrors coupled
with very large digital cameras, and it can observe the entire sky accessible to it several times a month.
CfA astronomer Martin Elvis was part of a team of scientists that looked for variability in galaxies by comparing PanSTARRS images of the sky with images taken
by an earlier survey, the Sloan Digital Sky Survey, about ten years before; the results were followed up with several other telescopes. Their comparison spanned
nearly one-third of the whole sky. After sifting through thousands of apparent transients per month to check, among other things, for accurate spatial coincidences,
that the candidates were galaxies, and that multiple observations confirmed the variability, the team reports finding seventy-six reliable objects. Spectroscopic
followups and other observations were then able to classify these into nine categories, including supernovae and radio-emitting galaxies. In the end, the team
found fifteen hypervariable sources that have brightened by almost a factor of ten in the past decade; light from the most distant one of these has been
traveling for about nine billion years. The galaxies' light is blue in color and has been steadily changing, usually getting weaker.