Sky & Telescope - October 2020 - 9

GRAVITATIONAL WAVES

Black Hole Collision May
Have Caused Burst of Light
BLACK HOLES DON'T EMIT LIGHT.

BL ACK HOLES: R. HURT (IPAC) / CA LTECH; EROSITA:
J. SA NDERS / H. BRUNNER / ESASS TE A M (MPE ) / E.
CHUR A ZOV / M. GILFA NOV (ON BEH A LF OF IK I)

Yet in a candidate gravitational-wave
event recorded by the LIGO and Virgo
detectors, dubbed S190521g, a ﬂash of
light appears to have followed the collision of two black holes. (The putative
merger isn't ofﬁcially conﬁrmed yet;
the LIGO-Virgo team is working on the
paper now.)
Matthew Graham (Caltech) and his
colleagues have been following up on
candidate events using observations
from the Zwicky Transient Facility,
looking for ﬂares of visible light that
might be associated with black hole
mergers. Such a ﬂare could be produced
if two stellar-mass black holes collided
in the vicinity of a supermassive one.
The object they create could bulldoze its
way through the disk of gas feeding the
supermassive black hole, causing surrounding gas to heat up and brighten
days to weeks after the merger.
In the June 25th Physical Review
Letters, Graham's team says they've
found evidence for exactly this kind of
ﬂare. Of the 21 candidate events the
researchers investigated, one of them,
S190521g, matched up with a peculiar
month-long ﬂare from the active galactic nucleus (AGN) J1249+3449. The
ﬂare showed no signs of changing color,

of supermassive black holes. Other X-ray
sources include the hot gases released in
supernovae explosions and the far more
tenuous gas in and around our galaxy.
However, EROSITA's true quarry
is galaxy clusters. The hot gas spread
out in the vast space between galaxies
emits X-rays. So by looking for extended
X-ray-emitting blobs, mission scientists can ﬁnd galaxy clusters from back
when the universe was only half its
current age. Clusters mark junctures
in the cosmic web, so by plotting a
large number of them over billions of
years, astronomers can watch cosmic

An artist's concept shows a
pair of stellar-mass black holes
embedded in the gas disk of a
supermassive black hole.

as would be expected from a supernova
blast that's expanding and cooling. Nor
is it likely to come from the supermassive black hole's gas-guzzling activity,
based on the AGN's past ﬂickering.
After a careful rundown of alternatives, the astronomers conclude that a
merger-induced ﬂare is the most likely
explanation.
Given that the ﬂare didn't last long,
the team thinks that the merger kicked
the black hole up and out of the disk.
But the speed the researchers estimate
it took from the merger isn't enough

to escape the supermassive black hole.
Instead, it should loop back through
the disk and create a second ﬂare in
the next few years, the team says - a
speciﬁc prediction that will be stunning
if conﬁrmed.
Ryan Chornock (Northwestern), who
was not involved in the study, is excited
by the team's result and agrees that the
researchers did a good job ruling out
normal supermassive black hole activity.
"However," he warns, "AGN have a long
history of surprising astronomers."
■ CAMILLE M. CARLISLE

X EROSITA
has captured its
first full view of the X-ray sky.

structures evolve over time. With more
than 20,000 clusters spotted so far, the
mission is on track to ﬁnd more than
100,000 by the time its eight all-sky
surveys are complete.
The mission has already begun a
second round of sky-mapping and

will ultimately combine eight maps to
achieve unprecedented sensitivity to the
whole X-ray sky.
■ MONICA YOUNG

* Explore close-up regions of EROSITA's
all-sky map using the Worldwide Telescope at https://is.gd/EROSITA.
sk yandtelescope.org * OCTOBER 2 02 0

9
https://www.is.gd/EROSITA http://www.skyandtelescope.org
Sky & Telescope - October 2020

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