Sky and Telescope - February 2016 - 22

Galactic Center Goings-On

Brightness

Pulsar

Time

Original Pulse

Original image

Brightness

Cloud of
Plasma

Time

S&T: GREGG DINDERMAN; SOURCE: G. BOWER

Pulse proﬁle
observed at Earth

Image
observed at Earth

PLASMA-SMEARED PULSE When a spinning neutron star emits
an earthward pulse, some radio waves travel directly to us. But most
of its photons encounter free electrons ﬂoating in clouds of plasma,
which force radio waves on detours. By the time scattered photons
converge on an earthbound detector, the pulsar's point-like image has
smeared out. Moreover, the longer paths of scattered photons delay
their arrival times, smudging the pulse proﬁle in time as well.

Februar y 2016 sky & telescope

supernova phase and result in a neutron star or black
hole. Some of these stars have been used to study the
gravitational ﬁeld of Sgr A* as they orbit the black hole.
Theorists have suggested that there might be thousands
of pulsars close to Sgr A*, leading to numerous searches
at radio wavelengths.
But none of these searches has uncovered a pulsar any
closer to the center than about 80 light-years, a distance
too far away for them to be gravitationally bound to Sgr
A*. This absence of radio pulsars has proved puzzling and
made the magnetar's discovery all the more surprising.
For the past two decades, the leading explanation
has been not the absence of pulsars but the cloaking of
their signal. Dust obscures the galactic center at visible
wavelengths even as radio waves pass right through. But
plasma, ionized gas between Earth and the galactic center,
scatters radio waves and blurs images of radio sources
such as Sgr A* (see box at left). The blurring has the eﬀect
of smearing individual pulses of radiation. If that smearing in time is longer than the pulse period of a particular
pulsar, then the pulsar will cease to appear as a pulsed
source and we won't detect it in our searches.
The smearing eﬀect is strongest at long radio wavelengths and diminishes rapidly at shorter wavelengths.
But perversely, typical pulsars become fainter at shorter
wavelengths, making them much harder to detect.
One of us (Bower) used the Very Long Baseline Array,
a transcontinental network of radio telescopes, to measure the eﬀects of scattering on the radio waves coming
from Sgr A* and the magnetar. We have long known that
Sgr A* is one of the most heavily scattered objects in the
galaxy, and new observations show that the magnetar
suﬀers exactly the same fate, leading to a blurred image
of the magnetar. This isn't surprising because the two
objects are so close together.
But the second measurement, measuring how much
the magnetar's individual pulses smeared in time, was
quite surprising. Current understanding suggested
that pulses might smear by as much as 100 seconds at a
wavelength of 30 cm, much longer than the typical pulsar
period. But our observations showed that the magnetar's
pulses are smudged by only 1 second at 30 cm, and even
less at shorter wavelengths. If the "missing" pulsars are in
the galactic center and behind the same amount of material, previous surveys should have easily seen through the
fog of interstellar plasma to ﬁnd them.
So why have past searches failed? And why was
the ﬁrst pulsar discovered in the galactic center a rare
magnetar? Perhaps these facts tell us that the extreme
conditions in the galactic center, such as strong magnetic ﬁelds and dense gas, drive the formation of stars
and neutron stars that are more highly magnetized than
their ordinary cousins throughout the Milky Way. Some
have suggested far more exotic ideas: neutron stars
might accrete dark matter, which should be prevalent

Sky and Telescope - February 2016

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