Apart from a billion Milky Way stars, ESA's Gaia spacecraft also
observes extragalactic objects. Its automated alert system notifies
astronomers whenever Gaia spots a transient event. A team of astronomers
from SRON, Radboud University and the University of Cambridge have
found out that by tweaking the existing automated system, Gaia can be
used to detect hundreds of peculiar transients in the centres of
galaxies. They found about 480 transients over a period of about a year.
Their new method will be implemented in the system as soon as possible
allowing astronomers to determine the nature of these events.
In 2013, ESA launched its Gaia spacecraft to measure the location of
a billion stars in our Milky Way and tens of millions of galaxies. Each
position on the sky enters Gaia's view once every month, for a total of
about seventy times during the mission. This allows the spacecraft to
spot transient events, such as supermassive black holes ripping stars
apart or stars exploding as a supernova. Gaia will notice a change in
brightness when it returns to the same patch of sky a month later.
Astronomers Zuzanna Kostrzewa-Rutkowska, Peter Jonker, Simon Hodgkin
and others searched the Gaia database for transient events around the
nuclei of galaxies in the period between July 2016 and June 2017. They
used a galaxy catalogue – from the Sloan Digital Sky Survey Release 12 –
and a custom-made mathematical tool. The new tool allows the
researchers to identify rare luminous events coming from galactic
centers. Of the 480 events found, only five had been picked up
previously by the alert system.
Rapidly alerting the astronomical community is key for following up
many of the events found. For about one hundred transients nothing out
of the ordinary was observed by Gaia during the months before and after
detection, indicating that the event leading to the enhanced emission of
light was short.
"Such events have great value because they could allow astronomers to
study for a brief period previously invisible supermassive black holes",
says Jonker. "Especially the short-duration events could point us to
the location of the so far elusive intermediate-mass black holes ripping
stars apart."
The leading explanation for most events is that supermassive black
holes residing in the nuclei of galaxies suddenly become much more
active as the amount of gas falling into the black hole surges and
lights up the close environment of the black hole. This fresh fuel may
be extracted from a star which is ripped apart by the enormous
gravitational pull of the black hole.
Zuzanna Kostrzewa-Rutkowska and Peter Jonker have recently started a
dense campaign to decipher the nature of the 480 new transients using
the William Herschel Telescope.
More information:
Z. Kostrzewa-Rutkowska, P.G. Jonker, S.T. Hodgkin, L. Wyrzykowski,
M. Fraser, D.L. Harrison, G. Rixon, A. Yoldas, F. van Leeuwen, A.
Delgado, M. van Leeuwen, S. E. Koposov, 2018, "Gaia transients in
galactic nuclei", MNRAS, 481, 307 [ ADS ].
