Sky and Telescope - February 2018 - 13

BLACK HOLES

Decoding a Black Hole Jet

Brightness (convenient units)

BL ACK HOLE JE T: N ASA / JPL- CA LTECH; TRITON: RUI GONÇA LV ES & DAVID DUNH A M / IOTA

VISIBLE LIGHT AND X-RAYS helped
astronomers paint a detailed picture
of the relativistic jet in V404 Cygni,
a black hole-star system. The results
appear in Nature Astronomy, published
online on October 30th.
Poshak Gandhi (University of Southampton, UK) and colleagues employed
NASA's NUSTAR X-ray satellite and the
super-fast UltraCam on the William
Herschel Telescope in La Palma, Spain,
to track emissions from V404 Cygni.
The black hole tugs gas away from its
stellar partner as they whip around each
other, resulting in ﬂares. In June 2015
V404 Cygni underwent the brightest
binary outburst in the 21st century.

Astronomers had previously monitored visible light from black hole
systems, but they hadn't been able to
pinpoint its origin - the photons could
arise in the gaseous disk that feeds the
black hole, the stellar companion that
feeds the disk, or the jets that the black
hole-disk system powers.
The addition of X-ray data resolves
this ambiguity. By exactly timing
incoming X-rays and visible photons,
Gandhi and colleagues discovered that
visible-light ﬂashes trailed X-ray ﬂares
by 0.1 second. So the visible-emitting
region has to be in the jet, some
30,000 km (19,000 miles) downstream
from its X-ray-emitting origin. The two
regions bookend the acceleration and collimation zone, a poorly understood space
where the jet's plasma narrows and
achieves relativistic speeds.
The results mesh nicely with previous studies of some supermassive black
holes, says Alan Marscher (Boston
University), but not all of them: The
supermassive black hole at the center of
the galaxy M87, for example, exhibits
a different delay, suggesting that some
details remain to be worked out.

15,000

10,000

5,000

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Time

SOLAR SYSTEM

Preliminary Results from Triton Cover-Up
WHEN A STAR disappeared behind
Triton on October 5th, astronomers
watched to monitor the atmosphere
around Neptune's largest moon.
The 12.6-magnitude star UCAC4
410-143659 was the brightest star
occulted by the moon in 20 years. The
last such occultation, in 1997, had
showed that Triton had experienced

Cassini's Saturn Surprises
During October's meeting of the American
Astronomical Society's Division for Planetary Sciences, a parade of Cassini scientists explained some of the insights they
gained during the unprecedented scrutiny
offered by the spacecraft's "Grand Finale"
orbits. Those last orbits sent the spacecraft dashing at 35 km per second (78,000
mph) through a planet-ring gap about
2,400 km wide, giving Cassini the chance
to directly sample the compounds present in Saturn's uppermost atmosphere.
In theory, material is leaking from the
innermost threads of the water-ice ring
and drifting toward the planet. But the
craft's mass spectrometer swept up many
heavier compounds, which might include
methane and carbon monoxide - gases
that would be chemically out of place
in the water-ice rings, and shouldn't be
percolating up from the atmosphere either.
Cassini's close-in flybys also gave mission
scientists a chance to examine the rings
in extreme detail, including new views of
structures that astronomers still struggle
to explain. Watch S&T's interview with
Cassini project scientist Linda Spilker at
https://is.gd/LindaSpilker.
■ J. KELLY BEATTY

■ MONICA YOUNG

= Light from Triton and occulted star
= Light from comparison star

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IN BRIEF

seasonal global warming since the
Voyager 2 ﬂyby in 1989. Now, these new
observations will determine if the trend
has continued.
Observers recorded the October 5th
event from more than 75 stations in
mainland Europe, the UK, northern
Africa, and the United States, making
it the best-recorded occultation by any

object with an atmosphere. The plot at
left shows some of these observations,
revealing the decrease in light when the
moon blocks the background star.
Moreover, NASA's Stratospheric
Observatory for Infrared Astronomy, as
well as 25 observers in Europe close to
the centerline of the occultation's path,
recorded the central ﬂash (seen as the
central peak at left), when the moon's
atmosphere brieﬂy focused the background star's light. Observations of the
central ﬂash probe a deeper level of the
atmosphere than the gradual disappearances and reappearances of the star
recorded by observers elsewhere in the
path of Triton's shadow.
Visit https://is.gd/tritonoccultation
for more details on the observations.
Ongoing analysis should reveal how
Triton's atmosphere has changed with
Neptune's seasons.
■ DAVID DUNHAM
sk yandtelescope.com * FE B RUA RY 2 018

https://www.is.gd/LindaSpilker https://www.is.gd/tritonoccultation http://www.skyandtelescope.com/

Sky and Telescope - February 2018

Table of Contents for the Digital Edition of Sky and Telescope - February 2018