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they also have a powerful magnetic field
(at least 100 million times more than the
Earth’s) generated by their incredible rota-
tion speed. The matter that is caught in
the lines of force of that magnetic field is
instantly accelerated to relativistic speeds
and violently hits the pulsar’s ultra-flat sur-
face, near its magnetic poles. At the im-
pact points, two spots are generated that
are so hot that they emit X-rays, and these
are the X-ray photons that NICER collects
and measures.
The magnetic poles are generally tilted out
of alignment from the rotational axis and,
statistically speaking, this last will only
rarely be facing the Earth; it follows that
the vast majority of pulsars show us the
two surface sources of X-rays in rapid se-
quence, once every half rotation.
The signal coming from them is modulated
according to the rotational motion, and it
intensifies for the source in the hemi-
sphere moving closer to the observer, and
it weakens for the one moving away.
The scenario is almost complete at this
point. All we need is the involvement of
the principal actor (whom we already met),
the extraordinary gravitational force of
the pulsars, which, in addition to compact-
ing matter nearly to the point at which
black holes form, also has the power to
heavily bend the space around those de-
generate stars, forcing some radiation leav-
ing the surface to follow a path that we
perceive as curved. This has an important
consequence: we can sense part of the ra-
diation emitted in the hemisphere oppo-
site the one facing Earth at a determined
instant. Because there is a proportion be-
tween the surplus of visible surface and
the pulsar’s mass, if we use the two ‘hot
spots’ as tracers we can figure out how
much total surface is visible and thereby
find the values of the mass and diameter
with a maximum error of ±5%. Finally, since
various theoretical models calculate the
different states of the matter and a differ-
ent internal structure in the pulsars de-
pending on the mass/diameter ratio, it will
be possible to learn which model is most
realistic and use it to derive other proper-
ties of those exotic celestial objects.
jects the size of a city, hundreds of light
years away, that rotate on their axes hun-
dreds of times per second? It can because
its equipment can fix the events in time
with a precision better than 300 nanosec-
onds, and because it knows its orbital
position with a precision of ±5 metres. Of
course, the proper motions of the pulsars
themselves must be noted with precision,
otherwise the results would be flawed.
Something is still missing in explaining
how NICER can solve our problem of hav-
ing scant knowledge about the masses and
diameters of neutron stars. These stars not
only have a fearsome gravitational force,
!