Sky & Telescope - May 2021 - 9

L AUER E T A L. / ASTR OPHYSICAL JOUR NAL 2021

MONICA YOUNG

slowly fading pulsation. Unfortunately, that telltale signal is too faint
to be visible outside our galaxy.
" The possibility that a subset of
short gamma-ray bursts is associated
with magnetar giant ﬂares was suggested long ago, " says Roberto Turolla
(University of Padua, Italy), who was
not part of this study. " This detection is key in proving it right. "
Scientists hope to ﬁnd and study
additional magnetar ﬂares with
StarBurst, a small gamma-raydetecting satellite in development as
part of NASA's Astrophysics Pioneers
program. Eric Burns (Louisiana
State University) says, " The study
of GRB 200415A is really laying the
groundwork for future research. "
GOVERT SCHILLING

COSMOLOGY

New Horizons Measures
" Extra " Visible Light
NASA'S NEW HORIZONS MISSION to

the outer solar system has turned its
cameras to far-off vistas - and found
more light than expected. This has
potential consequences for galaxy evolution and perhaps even cosmology.
Team member Tod Lauer (NSF's
NOIRLab) and colleagues used the Long
Range Reconnaissance Imager (LORRI)
camera to image regions above and
below the star-ﬁlled galactic plane and
measure the cosmic optical background.
The team subtracted all known sources
of visible light, including stars and
galaxies in the image, as well as light
from those outside the ﬁeld of view that
scattered into the camera. Computer
simulations aided the removal of stars
and galaxies too faint for the camera to
resolve. The team also subtracted Milky
Way starlight scattered off interstellar
dust, basing the latter on observations
of galactic cirrus.
Yet when all the calculations were
done, there was still " extra " light they
couldn't explain, Lauer and colleagues
report in the January 10th Astrophysical
Journal. " The total unknown amount
is more than the integrated ﬂux from
Sky brightness (nanowatts per square meter per steradian)

establish that the signal they see is
correlated across space.
" Here on Earth, we're bobbing in
an ocean of low-frequency gravitational waves, " Simon explains. The
passing waves slightly jostle our
planet, and as Earth moves toward
one pulsar, its signals arrive a little
sooner than expected. Signals from
a pulsar in the opposite direction
would come slightly later.
Gravitational waves are by their
nature quadrupolar, which means
that when they travel, say, along the
x-axis, spacetime squeezes along the
y-axis and stretches along the z-axis
(and vice versa). That same squeezestretch action happens along multiple axes, so the actual correlation
pattern is complex and difﬁcult to
suss out of the signal detected so far.
But scientists are conﬁdent they
will, and soon, with estimates ranging from within the year to a few
years. " Trying to detect gravitational
waves with a pulsar timing array
requires patience, " says NANOGrav
chair Scott Ransom (National Radio
Astronomy Observatory). " It's great
that these new results are exactly
what we would expect to see as we
creep closer to a detection. "

all known galaxies, " Lauer said at the
237th meeting of the American Astronomical Society.
Some of the excess may yet ﬁnd a
simple explanation. Shuji Matsuura
(Kwansei Gakuin University, Japan),
who also found an indication of " extra "
light with his colleagues in the Cosmic Infrared Background Experiment
(CIBER), suggested that the New Horizons team might have underestimated
the amount that scattered Milky Way
light contributes to the background.
" My opinion, " he adds, " is that we have
to be careful to claim the existence of
the diffuse cosmic optical background
at this stage. "
But if it pans out, the New Horizons
result has some interesting implications. The team sees twice as much
optical background as predicted by
existing galaxy catalogs. Faint galaxies
might account for some of the extra
light, but other sources could contribute, too. Faint stellar halos around
galaxies, " lost " stars torn away during
galactic mergers, undiscovered black
holes, or even axions, a hypothetical
dark matter particle (S&T: Jan. 2021,
p. 16), might be part of the explanation.
MONICA YOUNG

Read the full story at https://is.gd/
NewHorizonsCOB.

Milky Way light scattered by galactic dust, using two models to represent the dust

Scattered starlight

Faint stars
Scattered galaxy light

Faint galaxies
Visible-light background

35
30
25
20
15
10
5
0
OJ394

ZL138c

ZL138d

ZL138a

n3c61f

ZL138b

OE394

New Horizons field ID numbers

S Lauer and colleagues measured the visible-light background (black horizontal lines) in seven
regions outside the galactic plane. They then subtracted sources that make and scatter visible light
(colored bars). Known sources of light, including the total associated uncertainty shown by the error
bars, do not fully explain the background light New Horizons saw.
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 1

https://is.gd/NewHorizonsCOB https://is.gd/NewHorizonsCOB http://www.skyandtelescope.org

Sky & Telescope - May 2021

Table of Contents for the Digital Edition of Sky & Telescope - May 2021