Sky and Telescope - February 2018 - 38

Gravitational Waves

case, astronomers would have expected it to be at least 10,000
times more powerful in gamma rays than they detected,
given how close it was. Moreover, the jets should also have
produced prompt X-ray emission, which was not detected. So
maybe we observed the gamma-ray burst slightly from the
side? Many astronomers, including Kalogera and Eleonora
Troja (NASA Goddard), who led the X-ray follow-up, think
that is the most likely explanation for the weakness of the
gamma-ray burst. Troja also says that the delay in X-rays -
observed 9 days later - would be natural when looking at
the jet from an angle. The same holds for the radio waves
from the source, which didn't show up until early September. While the optical and infrared glow of the kilonova is
thermal radiation from the radioactive ﬁreball, the X-rays and
radio waves are produced by the energetic gamma-ray burst
jet after it started to broaden as it slowed down.
Mansi Kasliwal (Caltech) and colleagues suggest a more
complex version of this scenario, in which the jets get stuck
(either temporarily or permanently) in a thick cocoon of
material that the jets inﬂated as they drilled through the
collision ejecta. In this setup, the jet is still off-axis, but the
cocoon itself would emit the gamma rays, at a much weaker
level than seen from a jet pointed straight at Earth. Wijers
and his colleagues had put forward a similar scenario to
explain the strange behavior of the long gamma-ray burst
GRB 980425, which was also relatively close and surprisingly

weak, and coincided with a supernova explosion known as
SN 1998bw. Wijers also notes that this model neatly accounts
for the transition of the optical counterpart of GRB 170817A
from blue to red wavelengths within 48 hours.
A detailed analysis of all existing kilonova observations
may eventually solve the issue. And future observations of
the site of the cosmic catastrophe could also shed light on
another as-yet-unsolved mystery: What was the fate of the
two neutron stars? A small fraction of their combined mass
was ejected into space, but what happened to the rest? Did
the two city-sized stars merge into a hyper-massive neutron
star of a few solar masses, or did they collapse into a stellarmass black hole? Astronomers have only detected a few neutron stars that weigh in just above 2 solar masses - an upper
limit that might have implications for the physics of these
stars. The merger remnant might be extremely informative.
Unfortunately, the LIGO data can't provide a deﬁnitive answer: The ﬁnal stages of the merger event weren't
observed. With the earlier black hole crashes, LIGO could
detect hints of the collision's "ring-down phase," a brief
period in which the amplitude of the gravitational waves
rapidly dwindles to zero. From the characteristics of this
ring-down, astronomers were able to estimate the ﬁnal mass
of the merged black hole.
But in the case of GW170817, the rising wave frequency
moved out of LIGO's detection range before the two neutron

The Stellar Graveyard

65
60
55
50
45

35
30
25
20

X-ray black holes

15
10
5

Neutron stars

FE B RUA RY 2 018 * SK Y & TELESCOPE

GW170817

Masses

LE AH TISCIONE / S&T, LIGO SOURCES: LIGO A ND VIRGO COLL A BOR ATIONS
NEU TRON STA RS: F. ÖZEL A ND P. FREIRE / ANNUAL R E VIE WS OF ASTR ONOMY AND ASTR OPHYSICS 2016
BL ACK HOLES: G R ZEGOR Z WIK TOROWICZ A ND CHRIS BELCY NSK I / STELL A RCOLL A PSE.ORG

Gravitational-wave
black holes

Shown are the known masses for neutron stars
and stellar-mass black holes, with uncertainty
ranges (lines without dots are mass ranges,
while arrows point up from a lower limit).
Most neutron stars lie below 2 solar masses,
whereas known black holes generally lie above
5 solar masses. The object produced by the
neutron-star merger lies in the gap in between.
See page 11 for the recently announced ﬁfth
black hole merger - although detected before
GW170817, it was not vetted and announced
until November 2017.

http://www.STELLARCOLLAPSE.ORG

Sky and Telescope - February 2018

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