Sky & Telescope - August 2024 - 9

DASCH SCANNING: KAYLEIGH MACDONALD; SGR A*: EHT COLLABORATION
SOLAR SYSTEM
A Century of Sky, Digitized
DEEP WITHIN the photographic plate
stacks at the Harvard College Observatory,
curator of astronomical photographs
Thom Burns slips a delicate glass
plate out of its brown paper sheath. He
places it on a large, specialized scanner
- custom-built for this purpose
- preparing it for digitization, the
astronomical markings encoded on its
surface soon to be forever preserved in
an online database. While there isn't
anything unique about this particular
glass negative, its pale, filmy surface
speckled with dark stars, there is nonetheless
cause for tension.
This moment is the culmination
of a project that has spanned two
decades: Digital Access to a Sky Century
@ Harvard, or DASCH. Within the
labyrinthian walls and filing cabinets
of the Harvard College Observatory's
Plate Stacks reside some 600,000 glass
plates, collected from 1880 to 1990.
BLACK HOLES
Watch Hotspots Orbit Our
Galaxy's Black Hole
OUR RESIDENT SUPERMASSIVE black
hole, dubbed Sgr A*, emits bursts of
radiation at infrared, X-ray, and radio
wavelengths. Such flares were a nuisance
to the Event Horizon Telescope
(EHT) team in 2017, as they worked to
image Sgr A*'s dark silhouette.
Now, Aviad Levis (Caltech) and collaborators,
including several members
of the EHT team, have utilized some of
that 2017 data to examine not the black
hole but one of those pesky flares. The
results were published April 22nd in
Nature Astronomy.
The researchers started with the idea
that flares come from hotspots within
gas flowing around the central maw.
Magnetic fields snap-crackle-popping
close to the black hole can heat and
energize this gas. A hotspot would look
different from its surroundings because,
while gas flowing into the black hole
won't change much over a few hours,
pThe EHT team took this image of Sgr A*; Levis's
team used part of these data to generate
the 3D video of the moving hotspots. See the
video at https://is.gd/SgrAhotspot.
hot pockets of gas shear out and fade
more quickly. The radio waves' polarization,
which tells us at what angle
the light waves are oscillating, is also
expected to be higher for light coming
from a hot bubble of plasma.
The team used data from the Atacama
Large Millimeter/submillimeter
Array (ALMA) in Chile, which was
imaging Sgr A* on April 11, 2017, when
the black hole had also just emitted
Members of DASCH have now digitized
the more than 430,000 plates from
which accurate brightness data can be
obtained. With more than 400 terabytes
of imaging data now at their
fingertips, astronomers now have access
to a century of sky.
As Burns sets the plate in place and
flicks off the lights, the room falls
silent. The machine begins its work
- red lights flickering as the scanner
measures the position and brightness
of each tiny region, then shifts the
plate over with microscopic precision.
The entire process usually takes about
85 seconds, according to the project's
operation lead Peter Williams; when the
project was underway, the team would
complete on average around 200 to
400 plates a day.
The collection is more than just a
repository of research data - the early
years of photography and the progress
of astronomical data collection can be
seen through these images. The archive
also traces the work of astronomers
pThom Burns places a glass plate on DASCH's
custom-built scanner.
at the Harvard College Observatory,
including that of the women " computers "
who stewarded its collection (S&T:
Dec. 2021, p. 12).
On March 28th, the final plate was
scanned into DASCH - finishing one
project and paving the way for many
others.
¢ARIELLE FROMMER
What's next? Read more about future
projects: https://is.gd/DASCH
an X-ray flare. To incorporate 2D data
(brightness and polarization) into a 3D
rendering of events, the team employed
a relatively new form of artificial intelligence
known as neural radiance fields,
developed in part by team member
Pratul Srinivasan (Google Research). In
addition to feeding this neural network
the ALMA data, the algorithm was also
given rules of sorts, according to the
physics that the astronomers know or
assume to be operating near Sgr A*.
The result is a 3D video showing two
hot pockets of gas moving around the
black hole, elongating as they go. The
brightest part is only 2 astronomical
units from the black hole's event horizon.
A fainter hotspot lags behind at a
somewhat greater distance.
This video isn't a simulation, but it
isn't a direct recording of events, either.
" It is a reconstruction based on our
models of black hole physics, " Levis
says. " There is still a lot of uncertainty
associated with it, because it relies on
these models being accurate. "
¢MONICA YOUNG
sk yand tele scope .o r g * AUGUST 2024
9
https://www.is.gd/DASCH https://www.is.gd/SgrAhotspot https://skyandtelescope.org/
Sky & Telescope - August 2024

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