Sky and Telescope - January 2016 - 16

News Notes

ESA / ROSETTA / NAVCAM

Rosetta observations conﬁrm that the
"rubber ducky" nucleus of Comet 67P/
Churyumov-Gerasimenko was likely
born when two individual objects in the
outer solar system gently collided and
stuck together.

This image from Rosetta's navigation
camera shows the two lobes and connecting "neck" of Comet 67P's nucleus.
Mission scientists have now found layers suggesting that the oddball nucleus
began as two bodies that later joined.

Matteo Massironi (University of
Padova, Italy) and colleagues used images
from Rosetta's Optical, Spectroscopic,
and Infrared Remote Imaging System
(OSIRIS) to investigate the nucleus's
origin. The scientists peered down into
pits and along terraces and cliﬀs across
the nucleus. They found strata like those
seen in sedimentary rock on Earth, built
up as the bodies formed one layer at a
time. In some places the layers reach
650 meters deep, a fair fraction of the
kilometers-long nucleus. You can think of
them like the layers in an onion, the team
explains September 28th in Nature.
If Comet 67P's layering arose during
the nucleus's formation (the most probable explanation), then the onion shells
should form concentric rings around
their parent body's center of mass. If the
nucleus began as one object, then the
strata in the duck's head and body would
encircle a single center; if it began as two,
then the body and head strata would have
centers in their respective lobes.
The team found that the layers do
encircle two diﬀerent centers, one in each

500m

ESA / ROSETTA / MPS FOR OSIRIS TEAM MPS / UPD / LAM / IAA /
SSO / INTA / UPM / DASP / IDA; M. MASSIRONI ET AL. 2015

COMETS I Churyumov-Gerasimenko Began as Two Comets

This image from the Rosetta spacecraft shows
terraces in the Seth region, near the nucleus's
neck. Green marks main terraces and red
dashed lines mark exposed layers.

lobe. Thus, Comet 67P's nucleus began as
two separate bodies. Layering observed in
other comet nuclei during spacecraft ﬂybys
supports the idea that these objects likewise formed by accreting layers over time.
■ CAMILLE M. CARLISLE

BLACK HOLES I New Evidence for Binary Quasar
Astronomers have conﬁrmed that the
quasar PG 1302-102 is probably a binary
supermassive black hole, its components
less than a tenth of a light-year apart.
Recently, Matthew Graham (Caltech)
and colleagues found several dozen quasars glowing with regular beats, one of
which was PG 1302-102 (S&T: May 2015,
p. 14). This discovery surprised them,
because black holes are usually ﬁckle
emitters. The best explanation for the
periodic behavior was that the quasars are
actually two black holes spiraling toward
each other. The years between signals
would then be the orbital period.
But scientists didn't know why the
periodicity exists. Daniel D'Orazio
(Columbia University) and colleagues
now think they have the answer. They
realized that, if a smaller black hole were
circling a larger one, we would see a Doppler boost in the quasar's emission.
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Doppler shifts are the change in light's
wavelength because its source is moving
toward (blueshifted) or away (redshifted)
from us. Doppler boosting is this eﬀect
on steroids. With PG 1302-102's black
holes so close together, the smaller one
is whipping around its bigger sibling
at 7% the speed of light. That dramatically shifts the wavelength by 14%. So if
we're observing the quasar at an optical
wavelength of 600 nm, that means that
when the smaller black hole is moving
toward us, we're actually seeing 700-nm
light that's been blueshifted, but when it's
moving away, we're seeing 530-nm light
that's been redshifted.
If the quasar's emission were the same
intensity at all wavelengths, then we
wouldn't notice a diﬀerence. But its intensity increases from optical to ultraviolet.
So a 14% change in wavelength samples
diﬀerent intensities, meaning that as the

small black hole orbits, we see a periodic
change - the mysterious beat.
If this scenario is correct, the team
reports in the September 17th Nature,
then PG 1302-102's variations ought to
be more than twice as large at ultraviolet
wavelengths as at optical ones. That's
because the quasar's emission doesn't just
get brighter from optical to ultraviolet - it
also brightens more rapidly with wavelength. So if the optical emission shifts
toward a shorter wavelength, we'll see a bit
of an increase in intensity, but if the ultraviolet emission shifts the same amount,
we'll see a greater increase in intensity.
The team compared PG 1302-102's
multiwavelength variations using archival spectra from the Hubble (optical) and
Galaxy Evolution Explorer (ultraviolet)
space telescopes. The astronomers found
the expected extra oomph in ultraviolet.
■ CAMILLE M. CARLISLE
Sky and Telescope - January 2016

Table of Contents for the Digital Edition of Sky and Telescope - January 2016
