www.sciencemag.org SCIENCE VOL 344 11 APRIL 2014 159
Pulsar Beams—Big and Bright
Roger W. Romani
Recent studies of gamma-ray beams emitted
from rotating neutron stars are providing
a better understanding of the mechanism
therefore be interesting to see in future studies
how the ESF1 peptides contribute mechanis-tically to the apical-basal patterning process
and whether temporal or spatial information
is transduced by these signals.
Cell-to-cell signaling is a universal mechanism in plant development, and signals from
the endosperm have been implicated in several
developmental processes in the seed (6–8).
Although the precise genetic program is not
yet clear, there is increasing evidence that the
endosperm is involved in epidermis specification during embryogenesis (7). Loss of a sub-tilisin-like protease in the endosperm or loss
of candidate receptor kinases in the embryo
lead to epidermal defects, strongly suggesting
an involvement of endosperm-derived peptide
signals in controlling epidermal cell identity.
Angiosperm seeds consist principally of
three distinct compartments: the embryo,
In recent years, it has become clear that the
growth of these structures is coordinated and
that the endosperm has a profound impact
on seed size (9). The discovery of the ESF1
peptides demonstrates that the endosperm
also influences suspensor proliferation. By
regulating suspensor length, ESF1 signal-
ing might ensure an optimal position of the
embryo within the surrounding endosperm,
which seems to be crucial for fast develop-
mental progression of the embryo (10).
Signals from the endosperm regulate the
coordinated growth of the seed and influence
the development and patterning of the embryo
and the suspensor. In this emerging picture,
the endosperm seems to play a central role as
a signaling hub to orchestrate various growth
processes in the seed. Cysteine-rich peptides
are a large class of potential receptor ligands
that have an important function in diverse
developmental processes (8). Given the tem-
poral expression changes of many genes cod-
ing for such peptides during seed develop-
ment (2), a broader role for these ligands in
coordinating growth processes in the seed
appears to be likely.
1. P. Maheshwari, An Introduction to the Embryology of
Angiosperms (McGraw-Hill, New York, 1950).
2. L. M. Costa et al., Science 344, 168 (2014).
3. S. Lau et al., Annu. Rev. Plant Biol. 63, 483 (2012).
4. T. Kawashima, R. B. Goldberg, Trends Plant Sci. 15, 23
5. E. D. Supena et al., J. Exp. Bot. 59, 803 (2008).
6. I. De Smet et al., Nat. Cell Biol. 11, 1166 (2009).
7. M. Javelle et al., New Phytol. 189, 17 (2011).
8. E. Marshall et al., J. Exp. Bot. 62, 1677 (2011).
9. J. Li, F. Berger, New Phytol. 195, 290 (2012).
10. Y. Babu et al., Plant Physiol. 162, 1448 (2013).
The standard picture of a pulsar, or rota- tion-powered neutron star, has narrow beams of radiation lancing out of the
magnetic poles and sweeping across the sky
as the star spins. For nearly 50 years, astronomers have been observing radio flashes from
these objects and, on the whole, this lighthouse model does a good job of explaining
pulsar phenomenology. However, we have
not fully deciphered the pulsar mechanism—
although we understand how pulsars pulse,
we still seek to understand how they shine.
When gamma-ray pulses were seen from
the Crab and Vela pulsars in 1974 and 1975,
it was thought that this would advance our
understanding. Unlike the energetically small
radio beacons, the gamma rays represent several percent of the pulsars’ mechanical energy
loss and thus provide a more robust probe of
particle acceleration within the pulsar.
It was, and is, widely believed that the
coherent radio emission arises from insta-
bilities in a dense plasma of electron-positron
pairs in auroral zones just above the mag-
netic poles. The high-energy processes gen-
erating these pairs must inevitably produce
gamma rays as well, and so it was natural to
identify the observed gamma-ray pulsations
with this polar cap zone. One complication
was that, while for the Crab the gamma-ray
beams followed the radio emission, for Vela
the pulse structure was quite different, with a
double pulse well offset from the single radio
beacon. Of course, models were proposed to
interpret these data, but two contradicting
examples left the situation unclear.
The launch of the Energetic Gamma Ray
Experiment Telescope (EGRET) on NASA’s
Compton Gamma Ray Observatory in 1991
allowed detection of a half-dozen additional
pulsars, with a variety of gamma-ray pulse
shapes. None followed the radio emission
patterns. Even more important, aided by an
x-ray observation (1), EGRET was able to
find pulsations from a long-known mysteri-
ous gamma-ray source, Geminga. Here was a
spin-powered neutron star, with a typical dou-
ble gamma ray pulse, but no radio emission
visible at all. These data led us (2) to argue
that the gamma rays are produced in a zone
well separated from the radio-emitting polar
cap. The best model had wide fans of gamma-
ray emission arising near the so-called light
cylinder radius, where particles attempting C R
Department of Physics and Kavli Institute for Particle
Astrophysics and Cosmology, Stanford University, 382 Via
Pueblo Mall, Stanford, CA 94305–4060, USA. E-mail: rwr@
Beaming broad and bright.
The neutron star is surrounded by dipolar magnetic
field lines, which extend to
the light-cylinder radius. The
narrow radio beams (green)
arise near the surface and
propagate along the magnetic axes. Gamma rays are
generated high in the magnetosphere (magenta) and
are directed in broad beams
toward the spin equator. This
gamma-ray emission may
extend even beyond the light
cylinder into the magnetic
wind, whose separatrix layer
is indicated by the waving
blue surface, extending away
from the pulsar.