As shown in Figure 9, GR provides an ac-
curate description of the star’s positional
and velocity data throughout its very large
swing in velocity near its closest approach
to Sag A*. In contrast, the observations rule
out Newton’s law of gravity with a high sta-
tistical significance. “The GR model is 43,000
times more likely than the Newtonian mod-
el in explaining the observations,” the study
concludes. The measurements also provide
strong constraints on the black hole’s dis-
tance and mass, 8.0 kiloparsecs and 4.0 mil-
lion solar masses, respectively.
Of course, no one wins forever, and at some
point, namely the event horizon of a black
hole, GR must also fail. However, although
S0-2 plunged precipitously near Sag A*, the
minimum distance was roughly 1,000 times
larger than the radius of the event horizon.
Thus, it may be some time before observa-
tional limits encroach on the limits of GR’s
validity. Meanwhile, such observations con-
tinue to enlighten our understanding of the
dynamics and evolution of the center of our
Galaxy. The study appears in the journal
Science.
JULY 2019
Reverberations from an
Intermediate-mass Black Hole
in a Bulgeless Dwarf
For some, the term “reverberation mapping”
might suggest the idea of pinpointing the
locations of the various garage bands in
the neighborhood (all with their amplifiers
turned way up) based on the distribution
and intensity of the vibrations emanating
from one’s walls and window panes. But in
actuality, it denotes a powerful technique
for determining the masses of the black
holes embedded within the active galactic
nuclei (AGNs) at the centers of many galax-
ies. Interestingly, the two phenomena are
January 2020 / 2019 Year in Review
not entirely dissimilar. Like the perfect guitar
riff, reverberation mapping requires precise
timing and can be quite challenging to ex-
ecute in practice. In addition, the virtue of
both lies in their conceptual simplicity.
Reverberation mapping works by applying
the familiar virial theorem to the broad line
region (BLR) of an AGN. Assuming that the
motion of the gas in the BLR is primarily influ-
enced by the central black hole, the mass of
the black hole M BH will be proportional to σ 2 R,
where σ is the velocity dispersion determined
from the Doppler width of a broad emission
line and R is the characteristic radius of the
BLR. The radius is determined from the delay
time τ between variations in the intensity of
the continuum light from the AGN, which
excites the gas within the BLR, and the line
emission itself: R = c τ , where c is the speed
of light. Because lines of different ionization
show different delays, the same line should
be used for determining both σ and τ . Typical
AGNs powered by supermassive black holes
of millions of solar masses (M B ) have delay
times measured from Balmer lines ranging
from a few days to many months.
Figure 9.
Top: Zoom in on the
radial velocity data from
2018, encompassing the
maximum and minimum
of the observed radial
velocity. Measurements
from the three different
observatories are
indicated; Gemini/
NIFS and Keck/OSIRIS
each provided nine
measurements during
this critical period, over
which the observed
velocity changed by
6,000 km/s. Bottom:
radial velocity residuals
with respect to the
best-fitting General
Relativistic model.
Figure from Do et al.,
Science, 365: 664, 2019.
A new study published in Nature Astron-
omy has measured the mass of the black
hole associated with one of the lowest lu-
GeminiFocus
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