Did rapid spin delay 2017 collapse of merged neutron stars into black
hole?
Excess X-ray emissions from remnant four years after merger hint at
bounce from delayed collapse

Date:
March 1, 2022
Source:
University of California - Berkeley
Summary:
Continuing X-ray observations by Chandra of the kilonova from
the merger of two neutron stars to form a black hole hint at
new processes.

Initially, a gamma-ray burst and subsequent X-ray emissions told
of a jet of material produced by the merger, but X-rays from this
jet should be dimming. They're not, suggesting that ejecta from
the merger, given an extra bounce from the merged neutron stars
a second before collapse, is also generating X-rays.



FULL STORY ==========================================================================
When two neutron stars spiral into one another and merge to form a black
hole - - an event recorded in 2017 by gravitational wave detectors and telescopes worldwide -- does it immediately become a black hole? Or does
it take a while to spin down before gravitationally collapsing past the
event horizon into a black hole?

========================================================================== Ongoing observations of that 2017 merger by the Chandra X-ray Observatory,
an orbiting telescope, suggests the latter: that the merged object stuck around, likely for a mere second, before undergoing ultimate collapse.

The evidence is in the form of an X-ray afterglow from the merger,
dubbed GW170817, that would not be expected if the merged neutron stars collapsed immediately to a black hole. The afterglow can be explained
as a rebound of material off the merged neutron stars, which plowed
through and heated the material around the binary neutron stars. This
hot material has now kept the remnant glowing steadily more than four
years after the merger threw material outward in what's referred to as
a kilonova. X-ray emissions from a jet of material that was detected by
Chandra shortly after the merger would otherwise be dimming by now.

While the excess X-ray emissions observed by Chandra could come from
debris in an accretion disk swirling around and eventually falling into
the black hole, astrophysicist Raffaella Margutti of the University of California, Berkeley, favors the delayed collapse hypothesis, which is predicted theoretically.

"If the merged neutron stars were to collapse directly to a black hole
with no intermediate stage, it would be very hard to explain this X-ray
excess that we see right now, because there would be no hard surface
for stuff to bounce off and fly out at high velocities to create this afterglow," said Margutti, UC Berkeley associate professor of astronomy
and of physics. "It would just fall in. Done. The true reason why I'm
excited scientifically is the possibility that we are seeing something
more than the jet. We might finally get some information about the
new compact object." Margutti and her colleagues, including first
author Aprajita Hajela, who was Margutti's graduate student when she
was at Northwestern University before moving to UC Berkeley, report
their analysis of the X-ray afterglow in a paper recently accepted for publication in The Astrophysical Journal Letters.



==========================================================================
The radioactive glow of a kilonova Gravitational waves from the
merger were first detected on Aug. 17, 2017, by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo collaboration. Satellite- and ground-based telescopes quickly followed up
to record a burst of gamma rays and visible and infrared emissions that together confirmed the theory that many heavy elements are produced in
the aftermath of such mergers inside hot ejecta that produces a bright kilonova.

The kilonova glows because of light emitted during the decay of
radioactive elements, like platinum and gold, that are produced in the
merger debris.

Chandra, too, pivoted to observe GW170817, but saw no X-rays until nine
days later, suggesting that the merger also produced a narrow jet of
material that, upon colliding with the material around the neutron stars, emitted a cone of X- rays that initially missed Earth. Only later did
the head of the jet expand and begin emitting X-rays in a broader jet
visible from Earth.

The X-ray emissions from the jet increased for 160 days after the
merger, after which they steadily grew fainter as the jet slowed down and expanded. But Hajela and her team noticed that from March 2020 -- about
900 days after the merger -- until the end of 2020, the decline stopped,
and the X-ray emissions remained approximately constant in brightness.

"The fact that the X-rays stopped fading quickly was our best evidence yet
that something in addition to a jet is being detected in X-rays in this source," Margutti said. "A completely different source of X-rays appears
to be needed to explain what we're seeing." The researchers suggest that
the excess X-rays are produced by a shock wave distinct from the jets
produced by the merger. This shock was a result of the delayed collapse
of the merged neutron stars, likely because its rapid spin very briefly counteracted the gravitational collapse. By sticking around for an extra second, the material around the neutron stars got an extra bounce that
produced a very fast tail of kilonova ejecta that created the shock.



==========================================================================
"We think the kilonova afterglow emission is produced by shocked material
in the circumbinary medium," Margutti said. "It is material that was in
the environment of the two neutron stars that was shocked and heated up by
the fastest edge of the kilonova ejecta, which is driving the shock wave."
The radiation is reaching us only now because it took time for the heavy kilonova ejecta to be decelerated in the low-density environment and for
the kinetic energy of the ejecta to be converted into heat by shocks,
she said.

This is the same process that produces radio and X-rays for the jet,
but because the jet is much, much lighter, it is immediately decelerated
by the environment and shines in the X-ray and radio from the very
earliest times.

An alternative explanation, the researchers note, is that the X-rays
come from material falling towards the black hole that formed after the
neutron stars merged.

"This would either be the first time we've seen a kilonova afterglow
or the first time we've seen material falling onto a black hole after
a neutron star merger," said co-author Joe Bright, a UC Berkeley
postdoctoral researcher.

"Either outcome would be extremely exciting." Chandra is now
the only observatory still able to detect light from this cosmic
collision. Follow-up observations by Chandra and radio telescopes could distinguish between the alternative explanations, however. If it is a
kilonova afterglow, radio emission is expected to be detected again in
the next few months or years. If the X-rays are being produced by matter falling onto a newly formed black hole, then the X-ray output should
stay steady or decline rapidly, and no radio emission will be detected
over time.

Margutti hopes that LIGO, Virgo and other telescopes will capture
gravitational waves and electromagnetic waves from more neutron star
mergers so that the series of events preceding and following the merger
can be pinned down more precisely and help reveal the physics of black
hole formation. Until then, GW170817 is the only example available
for study.

"Further study of GW170817 could have far-reaching implications,"
said co- author Kate Alexander, a postdoctoral researcher who also is
from Northwestern University. "The detection of a kilonova afterglow
would imply that the merger did not immediately produce a black
hole. Alternatively, this object may offer astronomers a chance to
study how matter falls onto a black hole a few years after its birth."
Margutti and her team recently announced that the Chandra telescope had detected X-rays in observations of GW170817 performed in December 2021.

Analysis of that data is ongoing. No radio detection associated with
the X-rays has been reported.

========================================================================== Story Source: Materials provided by
University_of_California_-_Berkeley. Original written by Robert
Sanders. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* The_merger_of_two_neutron_stars_produced_a_black_hole ========================================================================== Journal Reference:
1. A. Hajela, R. Margutti, J. S. Bright, K. D. Alexander,
B. D. Metzger, V.

Nedora, A. Kathirgamaraju, B. Margalit, D. Radice, E. Berger, A.

MacFadyen, D. Giannios, R. Chornock, I. Heywood, L. Sironi,
O. Gottlieb, D. Coppejans, T. Laskar, Y. Cendes, R. Barniol Duran,
T. Eftekhari, W.

Fong, A. McDowell, M. Nicholl, X. Xie, J. Zrake, S. Bernuzzi, F. S.

Broekgaarden, C. D. Kilpatrick, G. Terreran, V. A. Villar, P. K.

Blanchard, S. Gomez, G. Hosseinzadeh, D. J. Matthews,
J. C. Rastinejad.

The emergence of a new source of X-rays from the binary neutron
star merger GW170817. The Astrophysical Journal Letters (accepted),
2022 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220301162014.htm

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