Figure 8.
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 (col-
ored curve). Open, gray,
and black circles repre-
sent previous, rederived,
and new measurements,
respectively. Panel C:
residuals from the best-
fitting velocity model.
Figure adapted from Do
et al., Science, 365: 664,
2019.
34
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 conceptually, the test was
incredibly exacting from a technical per-
GeminiFocus
spective. GR predicts that luminous objects
in strong gravitational fields should exhibit
relativistic redshifts. This means that a star
moving towards us in the vicinity of a black
hole should appear to have a smaller blue-
shift, and one moving away from us should
have a larger redshift, than would be the
case if the law of Newtonian gravity pre-
vailed. In the most stringent test of this pre-
diction to date, the team analyzed over two
decades of astrometric and spectroscopic
data, obtained using adaptive optics, on a
star known as S0-2 as it followed its eccen-
tric 16-year orbit around Sagittarius A* (Sag
A*), the supermassive black hole at the cen-
ter of our Galaxy. Figure 8 shows the full set
of positional and velocity data.
The star reached its closest approach to Sag
A* in May 2018, when it was at a distance of
only 120 AU and moving at 2.7% of the speed
of light. During the critical months surround-
ing pericenter passage, the team used three
different spectroscopic instruments at three
different observatories, including the Near-
infrared Integral Field Spectrometer (NIFS)
on Gemini North, the OH-Suppressing Infra-
Red Imaging Spectrograph (OSIRIS) on the
Keck II telescope, and the Infrared Camera
and Spectrograph (IRCS) on the Subaru tele-
scope. "The velocity of the star was chang-
ing quickly every
night! So having all
three observatories
participate was es-
sential," said Tuan
Do (also of UCLA),
the lead author of
the study. Combin-
ing data from mul-
tiple instruments
also allowed the
team to carefully
check for instru-
mental biases.
January 2020 / 2019 Year in Review