Figure 3.
Panel A: Astrometric
measurements the star
S0-2 over its 16-year
orbit of the supermas-
sive black hole at the
center of the Milky Way,
compared with the best-
fitting projected General
Relativistic orbit model.
The black hole is located
at the origin of the coor-
dinate system, and the
dashed line shows the
intersection of the orbital
plane with the plane of
the sky. The black points
represent new observa-
tions from 2017-2018,
while the gray points are
earlier measurements.
Panel B: radial velocity
measurements over the
period 2000-2018 and
the best-fitting model
(colored curve). Open,
gray, and black circles
represent previous,
rederived, and new mea-
surements, respectively.
Panel C: residuals from
the best-fitting velocity
model. [Figure adapted
from Do et al., Science,
365: 664, 2019.]
order of magnitude. This large data set en-
abled the team to uncover surprising pat-
terns in Io’s volcanic activity. For instance, of
the 18 sites with the brightest eruptions, 16
are on the trailing hemisphere with respect
to Io’s orbital motion. This tendency remains
unexplained; the likelihood of it occurring
from a random spatial distribution is much
less than 1%.
In a companion paper published in Geo-
physical Research Letters, de Kleer and
colleagues show that the roughly 500-day
variations in the intensity of Loki Patera’s
activity may be related to periodic changes
in the shape of the moon’s orbit. Regular
gravitational perturbations from Europa
and Ganymede, which respectively have 2:1
and 4:1 orbital resonances with Io, prevent
the inner moon’s orbit from circularizing.
Instead, Io’s eccentricity and semimajor axis
vary cyclically with periods of 480 and 460
days, respectively. This evolution in Io’s orbit
is consistent with the timescale of the quasi-
periodic behavior of Loki Patera.
At first, this link between orbital evolution
and volcanic activity may seem surprising,
since the range in the tidal stresses over a
single orbit is larger than the variation in the
mean tides resulting from the change in or-
bital shape. However, the researchers note
that while magma is likely too viscous to
change its flow significantly on the timescale
of one orbit, it can adjust its flow over the
longer period associated with the change
in Io’s orbital shape. If there is a connection,
the peak in activity should coincide with the
time of maximum orbital eccentricity, and
the data confirm that this is indeed the case.
Higher cadence observations are needed to
test this hypothesis and rule out shorter pe-
riod drivers of Loki Patera's variability.
Three Maunakea Observatories
Track Relativistic Star around a
Black Hole
If Einstein were alive today, he might be one
of the few people tired of actually winning.
Setting aside his long quarrel with quantum
mechanics and all that business about a uni-
fied field theory, his formulation of General
Relativity (GR) has proven to be one of the
most successful descriptions of nature ever
proposed. From the deflection of starlight in
1919 to the detection of gravitational waves
in 2015, Einstein’s General Relativity has tri-
umphed over every observational test to
date. Now a team of researchers led by An-
drea Ghez at the University of California Los
Angeles has tested GR in a new regime, the
strong gravitational field near a supermassive
black hole. The result:
chalk up another one for
the iconic physicist.
Although simple con-
ceptually, the test was
incredibly
exacting
from a technical per-
spective. GR predicts
that luminous objects
in strong gravitational
fields should exhibit
relativistic redshifts. This
means that a star mov-
ing towards us in the vi-
16
GeminiFocus
October 2019