Sky & Telescope - February 2023 - 11

NASA / SWIFT / A. BEARDMORE (UNIVERSITY OF LEICESTER)
STARS
Brightest Gammaray
Burst Yet
AN UNPRECEDENTEDLY
BRIGHT gamma-ray burst
(GRB) lit up the sky on
October 9th. Cataloged
as GRB 221009A, it
exploded in a galaxy 1.9
billion light-years away, which is close
for a GRB.
This event likely belongs to the class
of long GRBs, the end-of-life phase of
rare, rapidly rotating massive stars. As
such a star goes supernova, it ejects
material into jets at speeds just shy of
the speed of light. If one of those jets
points toward us, we see a minutes-long
burst of gamma rays. A longer-lived
fading afterglow, observable at other
wavelengths, often follows.
This explosion is the brightest and
among the most energetic GRB yet
detected, so intense that it temporarily
blinded multiple sensitive gamma-ray
detectors in space. China's Large High
Altitude Air Shower Observatory also
MILKY WAY
Astronomers Map Our
Galaxy's " Underworld "
ASTRONOMERS HAVE DRAWN the
first map of the distribution of dead
stars in and around our Milky Way.
This stellar cemetery of neutron stars
and black holes takes a different shape
from the galaxy we can see.
The most ancient of these compact
corpses are the most difficult to track
down, both because of the complex
changes the galaxy has undergone over
billions of years, and because of the
" kicks " these objects received when they
were born. Uneven supernovae explosions
can boot remnants out of the
Milky Way's disk. While young remnants
won't have moved far since their
demise, older ones are long gone. Based
on calculations, the team found that
40% of neutron stars and 2% of black
holes get enough of a kick to leave the
galaxy entirely.
GRB 221009A's X-ray afterglow
has a ring-like appearance
because some of these
distant photons scatter off
dust in our own galaxy.
reported several tera-electron-volt
photons pre
sumably originating from
the GRB. Among them is
the most energetic photon
from any GRB detected to date.
Such high-energy photons usually
don't survive long treks through space,
but when they do, they offer a unique
glimpse into the physical processes that
occur in extreme environments that we
can't replicate on Earth.
The burst's high-energy photons even
impacted Earth, ionizing its upper atmosphere
and modifying the radio-propagation
properties of the ionosphere.
Even after the initial burst, the X-ray
afterglow remained about 1,000 times
brighter than typical GRBs. Observatories
have also detected the afterglow all
the way down to radio waves - it will
likely shine for months to come.
¢JURE JAPELJ
The remainder are still caught in the
Milky Way's gravitational clutches, and
David Sweeney (University of Sydney)
and his team set out to find out where
they are. They constructed a simulated
map of their current locations using a
statistical model that factored in the
birth, death, and ejection of ancient
stars and their remains. They found
that, rather than being confined to a
disk, the compact stellar corpses occupy
a nearly spherical cloud that stretches
three times the disk's height. The
results appear in the November Monthly
Notices of the Royal Astronomical Society.
Astronomer Karen Masters (Haverford
College) is impressed: " I think this
work is a really clever use of well-established
models to investigate the Milky
Way, " she says.
But actually observing ancient black
holes and neutron stars is challenging.
" The most exciting part of this research
is still ahead of us, " Sweeney says.
¢COLIN STUART
IN BRIEF
Amateur Finds New
Images of Uranus Rings
Previous comparisons between Voyager
2's 1986 images of Uranus' dusty ring system
and ground-based images from 2007
suggested that ring material might move
around, with one of the rings possibly
having changed in brightness and position.
But it was hard to be sure because
of the different points of view. Then Ian
Regan, an image-processing enthusiast,
asked himself whether Voyager might have
captured the dusty rings in " lookback "
images. By stacking data stored in NASA's
Planetary Data System, he found three
faint dust rings not previously observed in
the 36-year-old images. Regan shared his
newly processed images on Twitter, where
they found their way to planetary-rings
scientist Matthew Hedman (University of
Idaho). Hedman presented the images
(with Regan as coauthor) at October's
Division for Planetary Sciences meeting.
" These data not only confirm that the
location of the peak brightness in the zeta
ring did indeed change between 1986 and
2007, " Hedman says, " but also indicate
that the total amount of material in this ring
changed substantially over that time. "
¢EMILY LAKDAWALLA
See images and how-to details at
https://is.gd/UranusRings.
SOFIA's Final Flight
The Stratospheric Observatory for Infrared
Astronomy (SOFIA) has reached the end
of its flight plan. Although SOFIA was
designed to last 20 years, NASA decommissioned
it after only eight, citing high
cost and low scientific output. The abrupt
end left scientists devastated and projects
unfinished. SOFIA flew above 99% of the
infrared-blocking water vapor in Earth's atmosphere,
enabling, for example, the mapping
of magnetic fields in stellar nurseries
and distant galaxies (see https://is.gd/
MagneticGallery) and the discovery of water
on the sunlit portions of the Moon (S&T:
Feb. 2021, p. 11). But the telescope cost
NASA more than $80 million a year to run
- nearly as much as the Hubble Space
Telescope - and it didn't result in nearly
as many findings. As such, the mission
earned a low ranking in the 2020 Decadal
Survey. " I think if we had known how much
it would have cost when we started, " says
Paul Hertz (NASA), " we would have chosen
to do this science some other way. "
¢SHANNON HALL
sk yand tele scope .o r g * FEBRUARY 2023 11
https://www.is.gd/UranusRings https://www.is.gd/ http://skyandtelescope.org
Sky & Telescope - February 2023

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