INSIGHTS | PERSPECTIVES
science.org SCIENCEorbital plane. Under the most extreme cir-
cumstances, this kick can even unbind the
binary system. In cases in which the binary
system survives the supernova explosion
and remains gravitationally bound, mass
transfer from the smaller star to the black
hole might ensue. This stripped-away mass
from the star would take form in the shape
of an accretion disk, which lies in the or-
bital plane around the black hole. Only part
of this material would disappear below the
event horizon—the point of no return—and
falls into the black hole. The rest of the ma-
terial is ejected at relativistic speeds in two
symmetric, collimated jets, following the
strong magnetic field along the spin axis of
the black hole ( 3 , 4 ). This mechanism is un-
stable and undergoes erratic and powerful
outbursts, producing radio and x-ray emis-
sions that can be and have been observed
(see the figure).
When the black hole was formed by the
exploding massive star, as it collapsed in-
ward its angular momentum was likely
conserved. Therefore, as in the case of a
ballet dancer bringing her arms closer to
her body to increase the rotation velocity,
the resulting spinning speed of the col-
lapsing star is greatly increased and can
reach more than a thousand rotations per
second. This has important relativistic
consequences, such as the modification
of space-time around the eventual black
hole. In the absence of external perturba-
tions—for example, the gravitational pull
from other stars in a multistar system or
within a stellar cluster—mass transfer and
tidal forces tend to align the rotation axes
and keep the spin axis of everyone perpen-
dicular to the orbital plane. This geometry
is assumed, for example, when calculating
the mass of the black hole.
However, the aforementioned alignment
hypothesis is all but easy to verify in obser-
vational astronomy. Poutanen et al. have
devised an ingenious technique that makes
use of linear polarimetry and applied it to
the x-ray binary system known as MAXI
J1820+070. This system was discovered
during an outburst by the All-Sky Auto-
mated Survey for Supernovae (ASAS-SN),
and x-ray measurements were performed
by the MAXI imager on board the Inter-
national Space Station in July 2018 ( 5 ). At
about 10,000 light-years from Earth, the
system hosts a black hole with ~8 Msun,
which orbits around a star with ~0.5 Msun.
The axial offset between the black hole
and the orbiting star in MAXI J1820+070
is more than 40°—the largest offset ever
reported. This has crucial implications for
theories of black hole formation ( 6 ). A sub-
stantial natal kick might either split the
binary system or tilt the orbital plane with
respect to its initial orientation ( 7 ). Be-
cause mass accretion ( 8 ) and tides ( 9 ) tend
to align the spin axis to the orbital angular
momentum of the binary system, the natal
kick from the supernova is the main if not
the only mechanism that can produce such
a misalignment ( 10 ).
Although this explanation is very plau-
sible, it is still a matter of debate ( 11 ). Al-
ternatively, there are cases in which mass
accretion does not necessarily result in
spin alignment ( 12 ). Hence, measuring a
substantial axial offset is to be considered
as the smoking gun of either a large natal
kick or a dynamical formation scenario for
the binary system ( 13 ). Furthermore, find-
ing a large axial offset in this system is par-
ticularly surprising because the evolution
after the supernova explosion can only re-
duce the misalignment induced by the kick
( 8 ). This implies that it must have been
even larger when the black hole was born.
When compared with the theoretical pre-
dictions, the reported lower limit of the mis-
alignment angle is very high; this may call
for the need of revising the existing models
for these systems. Given the low mass of the
companion star ( 6 ), MAXI J1820+070 will
not evolve into a binary black hole. Nev-
ertheless, the spin-orbit misalignment in
MAXI J1820+070 is a crucial step forward
for interpreting the observed distribution
of spin tilts in Laser Interferometer Grav-
itational-wave Observatory (LIGO)–Virgo
event candidates ( 14 , 15 ). jREFERENCES AND NOTES- J. Poutanen et al., Science 375 , 874 (2022).
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ACKNOWLEDGMENTS
The authors thank S. Piranomonte for her help during the
preparation of this article. M.M. acknowledges support
from the European Research Council Consolidator grant
DEMOBLACK, under contract 77001.10.1126/science.abn5290(^1) European Southern Observatory, D-85748 Garching,
Germany.^2 Physics and Astronomy Department “G. Galilei,”
University of Padova, 35122 Padova, Italy.
Email: [email protected]
PHOTONICS
Flashing
light with
nanophotonics
Manipulation and
enhancement of
scintillation is achieved in
nanophotonic structures
B y Renwen Yu and Shanhui Fan
W
hen a material is bombarded
with high-energy particles, such
as free electrons or x-ray pho-
tons, it may emit light in the
visible frequency. This process,
known as scintillation, is im-
portant for applications such as medical
imaging and nondestructive inspection
( 1 ). For these applications, enhancing and
controlling the light emission is of critical
importance to improve their capabilities
involving precision and resolution. Toward
this goal, researchers are constantly look-
ing for better scintillator materials ( 2 ) and
better ways to control the scintillation
processes. One way to improve the func-
tionalities of scintillator materials is by in-
troducing photonic structures to enhance
scintillation ( 3 ). On page 837 of this issue,
Roques-Carmes et al. ( 4 ) report a method
to optimize scintillation using nanopho-
tonic structures that can achieve orders-of-
magnitude enhancement.
The scintillation process that occurs in-
side a solid material can be broken down
into three smaller steps. In the first step of
the process, the incoming high-energy par-
ticles create a nonequilibrium distribution
of electrons inside the material. In the sec-
ond step, the now-excited electrons diffuse
through the material to create a steady-
state density distribution of secondary
electrons. In the final step, these second-
ary electrons radiatively decay at certain
sites in the material, shedding their excess
energy as light.
There is extensive research on how to ma-
nipulate electromagnetic fields using nano-
photonic structures, such as nanostructures
with periodically varying refractive indices,
for controlling and enhancing these light
emissions ( 5 ). Consequently, by integrating
the scintillator with or patterning it into a
nanophotonic structure, it should be pos-822 25 FEBRUARY 2022 • VOL 375 ISSUE 6583