Sky and Telescope - August 2017 - 11

SUPERNOVAE

Cosmic Lens Provides
Unique View

p Gravitationally lensed SN 2016geu

ASTRONOMERS HAVE discovered a

gravitationally lensed Type Ia supernova
that will hopefully give them a new
measure of the universe's expansion.
Type Ia supernovae are standard
candles, events with known intrinsic

luminosities from which observers can
calculate their distances. But initial
images of SN 2016geu didn't seem to
follow the rules. "I was bafﬂed," Mansi
Kasliwal (Caltech) says of her ﬁrst
impression of the supernova. "It was
much brighter than it should have been
given its distance from us."
Follow-up space- and ground-based
observations revealed three identical
supernovae near the ﬁrst one. Turns out
a foreground galaxy's gravity bent and
focused the light of SN 2016geu during
its 4.3-billion-year journey toward Earth,
dialing up its brightness by a factor of 52
and splitting its light into four images
just 0.3 arcsecond apart. The result is a
classic Einstein Cross (shown), the ﬁrst
ever for a Type Ia supernova.
As Ariel Goobar (Stockholm University, Sweden), Kasliwal, and their colleagues report in the April 21st Science,

BLACK HOLES

Gravitational Waves Dethrone Supermassive Black Hole

p The starlike quasar 3C 186. Its former host
galaxy is the faint, extended object behind it.

SUPER NOVA: ESA / HUBBLE / N ASA; BL ACK HOLES: N ASA /
ESA / M. CHIA BERG E (STSCI / ESA)

ASTRONOMERS HAVE discovered a

supermassive black hole not sitting in its
customary seat at the center of its galaxy. Gravitational waves from a recent
merger might have ejected the object,
Marco Chiaberge (Space Telescope Science Institute) and colleagues suggest in
the April 2017 Astronomy & Astrophysics.
The black hole powers the quasar
3C 186, a brilliant beacon of light emitted by gas that's plummeting down into
the black hole. But new Hubble Space
Telescope images reveal that the quasar's location is offset from its galaxy's

center by about 35,000 light-years, farther than the Sun lies from our galaxy's
center. The galaxy itself looks like it's
been distorted by a merger.
The team also analyzed the quasar's
visible-light spectrum and found that
the black hole that powers it is ﬂying
away from the galaxy's core at 2,140
km/s (4.8 million mph).
Together, the quasar's incredible
speed and its offset from the galaxy's
center point to one likely conclusion,
the team says: This black hole is the
product of two unequal black holes,
whose lopsided merger emitted asymmetric gravitational waves that kicked
the resulting object out of its galactic
home. This isn't the ﬁrst such candidate
discovered, but it's the ﬁrst to have two
signatures of gravitational-wave recoil -
previous candidates have shown spatial
offset or incredible speed but not both.
There is a chance that the quasar lies
in a second, less luminous galaxy that's
behind the galaxy that Hubble imaged.
Chiaberge's team is pursuing follow-up
observations to rule out this alternative.

the object could help resolve a debate
in cosmology about how quickly the
universe is expanding. Researchers using
supernovae in the local universe calculate a different rate than those looking
at the cosmic microwave background.
Since the light from SN 2016geu was
split into four images, each of those
images took a slightly different path to
Earth. The team is now using the delays
between the arrival times of the different images to calculate the rate of cosmic expansion: Since the astronomers
didn't need to assume any cosmological
model to measure the supernova's distance, luminosity, or amount of magniﬁcation, they can use that information
and the time delays to calculate the
expansion rate, instead of vice versa. "It
would be neat if we could contribute to
resolving that quest," Goobar says.
■ MONICA YOUNG

IN BRIEF
Supernova Discovered
in "Fireworks Galaxy"
On May 13th, Utah amateur Patrick
Wiggins discovered a core-collapse
(Type IIP) supernova in the spiral galaxy NGC 6946 in Cygnus. The explosion, designated SN 2017eaw, is the
10th supernova found in this galaxy in
the past century, reaffirming its reputation for fireworks of the grandest kind.
Pegged at magnitude 12.8 at its
discovery, SN 2017eaw is located at
right ascension 20h 34m 44.24s, declination +60° 11′ 35.9″, which is 61″
west and 143″ north of the galaxy's
nucleus and not far from two stars of
similar brightness. The galaxy lies on
the border of Cygnus and Cepheus,
within a degree of the open cluster
NGC 6939. It is approximately 15 million light-years away.
Wiggins found the supernova by
comparing a CCD image made on
May 14th UT through his 0.35-m f/5.5
reflector with one taken several years
ago and another from May 12th. In
fact, SN 2017eaw marks his third
supernova. The explosion hit peak
brightness around May 19th, reaching
magnitude 12.6 or 12.7.
■ BOB KING

■ MONICA YOUNG
s k y a n d t e l e s c o p e .c o m

* AUGUST 2017

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Sky and Telescope - August 2017

Table of Contents for the Digital Edition of Sky and Telescope - August 2017