Sky & Telescope - December 2022 - 15

Gravity determines how fast gas moves around Sgr A*; plasma
conditions determine how that flow brightens and fades.
It's the latter that surprised the team. Simulations forecasted
big changes for Sgr A* - perhaps so big that researchers
wouldn't be able to take a decent long-exposure picture.
But Sgr A* proved calmer than any simulated image in the
library. " It's like making weather predictions for a planet that
you've never seen - you do your best, but you have no idea
what the conditions there are, " Psaltis says. " We predicted the
storm, and we got a beautiful sunny day. "
The discrepancy means we're missing something in our
understanding of what's happening in the gas flow. Some
team members (including Psaltis) think this is one of the
most intriguing results. Others are unperturbed. " It is true,
not a single model was able to reproduce the variability, " says
Ramesh Narayan (Center for Astrophysics, Harvard & Smithsonian),
who has studied black hole accretion for decades.
" For some reason that doesn't bother me, and I can't even tell
you why, " he adds, laughing.
Myriad solutions abound. Perhaps the gas is less turbulent
or thicker than we expected, its magnetic fields are less
tangled, or its electrons and ions behave in ways we haven't
fully grasped. Adjusting where the light we see comes from
- perhaps there's a jet as well as the accretion disk? - might
I Dub Thee Sagittarius A*
Codiscoverer Robert Brown (1943-2014) coined
the name Sagittarius A* several years after finding
the black hole. He later explained, " Scratching on
a yellow pad one morning I tried a lot of possible
names. When I began thinking of the radio source
as the 'exciting source' for the cluster of H II
regions seen in the VLA [radio] maps, the name Sgr
A* occurred to me by analogy brought to mind by
my PhD dissertation, which is in atomic physics
and where the nomenclature for excited-state
atoms is He*, or Fe*, etc. "
solve the conundrum, too.
One possibility lies in how gas falls onto the black hole.
Sgr A* eats only a billionth as much as a beast of its size is
capable of, fed by a trickle of gas blown off as winds from
the bright, hot stars that encircle the black hole. These stellar
winds billow toward Sgr A* in a disorderly breeze, coming
in from all directions, explains Sean Ressler (University of
California, Santa Barbara), who studies how such wind-fed
accretion works.
2009
2011
2009
Extensive
theoretical
work
Models built
to explain
the emission
observed from
Sgr A*.
MOSCIBRODZKA
ET AL. 2009,
DEXTER ET AL.
2009, YUAN
ET AL. 2009,
BRODERICK ET
AL. 2006 AND
2009
1.3 mm
2011
Feeding black
hole
Comparison of
observations
with simulations
shows that Sgr
A* is consistent
with a crescent
of the right size
and shape to
be an accreting
black hole.
BRODERICK ET
AL. 2011, 2016
1.3 mm
2015
Magnetic
fields detected
Quickly varying
polarization
patterns suggest
tangled, writhing
magnetic fields
near the event
horizon.
JOHNSON ET AL.
2015
1.3 mm
2016
Sgr A* is not
symmetric
Detection of
asymmetric
shape
FISH ET AL. 2016
1.3 mm
2015
2018
Ultracompact
and
asymmetric
structure
APEX
telescope in
Chile detects
structure in
emission from
Sgr A*
LU ET AL. 2018
1.3 mm
2019
2019
Scattering
removed
Astronomers
map and
subtract the
scattering
effect of clumpy
interstellar gas
on Sgr A*'s
light.
ISSAOUN ET AL.
2019
3.5 mm
2022
First EHT
image of
Sgr A*
Shadow
provides
first direct
evidence of the
supermassive
black hole at
the center of
our galaxy.
EHT
COLLABORATION
2022
1.3 mm
sk yand tele scope .o r g * DECEMBER 2022 15
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Sky & Telescope - December 2022

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