GW170817
Credit: NASA/CXC/Trinity University/D. Pooley et al.
The spectacular merger of two neutron stars that generated gravitational waves announced last fall likely did something else: birthed a black hole. This newly spawned black hole would be the lowest mass black hole ever found, as described in our latest press release.
After two separate stars underwent supernova
explosions, two ultra-dense cores (that is, neutron stars) were left
behind. These two neutron stars were so close that gravitational wave
radiation pulled them together until they merged and collapsed into a
black hole. The artist's illustration
shows a key part of the process that created this new black hole, as
the two neutron stars spin around each other while merging. The purple
material depicts debris from the merger. An additional illustration
shows the black hole that resulted from the merger, along with a disk of
infalling matter and a jet of high-energy particles.
GW170817
Illustration Credit: NASA/CXC/M.Weiss
A new study analyzed data from NASA's Chandra X-ray Observatory
taken in the days, weeks, and months after the detection of
gravitational waves by the Laser Interferometer Gravitational Wave
Observatory (LIGO) and gamma rays by NASA's Fermi mission on August 17,
2017.
X-rays
from Chandra are critical for understanding what happened after the two
neutron stars collided. The question is: did the merged neutron star
form a larger, heavier neutron star or a black hole?
Chandra observed GW170817 multiple times. An observation two to three
days after the event failed to detect a source, but subsequent
observations 9, 15 and 16 days after the event, resulted in detections
(bottom left). The source went behind the Sun soon after, but further
brightening was seen in Chandra observations about 110 days after the
event (bottom right), followed by comparable X-ray intensity after about
160 days.
If the neutron stars merged and formed a heavier neutron star, then
astronomers would expect it to spin rapidly and generate a very strong
magnetic field. This, in turn, would have created an expanding bubble of
high-energy particles that would result in bright X-ray emission.
Instead, the Chandra data show levels of X-rays that are a factor of a
few to several hundred times lower than expected for a rapidly spinning,
merged neutron star and the associated bubble of high-energy particles,
implying a black hole likely formed instead.
By comparing the Chandra observations with those by the NSF's Karl G.
Jansky Very Large Array (VLA), researchers explain the observed X-ray
emission as being due entirely to the shock wave
— akin to a sonic boom from a supersonic plane — from the merger
smashing into surrounding gas. There is no sign of X-rays resulting
from a neutron star. Thus, the researchers in this study claim this is a
strong case for the merger of two neutron stars merging to then produce
bursts of radiation and form a black hole.
A paper describing this result appears in the latest issue of The Astrophysical Journal Letters and is available online.
The authors of this paper are David Pooley (Trinity University, San
Antonio, Texas), Pawan Kumar (University of Texas at Austin), J. Craig
Wheeler (University of Texas) and Bruce Grossan (University of
California, Berkeley). NASA's Marshall Space Flight Center in
Huntsville, Alabama, manages the Chandra program for NASA's Science
Mission Directorate in Washington. The Smithsonian Astrophysical
Observatory in Cambridge, Massachusetts, controls Chandra's science and
flight operations.
Fast Facts for GW170817:
Scale: About 0.5 arcmin across (about 205,000 light years)
Category: Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 13h 09m 48.1s | Dec -23° 22´ 53.4"
Constellation: Hydra
Observation Date: August 26, 2017, September 1, 2017, September 2, 2017, December 3, 2017, December 6, 2017
Observation Time: (first image) 3 days 3 hours 39 minutes; (second image) 1 day 3 hours 32 minutes
Obs. ID: 19294, 20728, 18988, 20860, 20861
Instrument: ACIS
References: Pooley et al, ApJ Letters, 2018. arXiv:1712.03240
Color Code: Intensity: X-ray (purple)
Distance Estimate: About 130 million light years
Source: NASA’s Chandra X-ray Observatory
