When the most massive stars die, they collapse under their own
gravity and leave behind black holes; when stars that are a bit less
massive die, they explode in a supernova and leave behind dense, dead
remnants of stars called neutron stars. For decades, astronomers have
been puzzled by a gap that lies between neutron stars and black holes:
the heaviest known neutron star is no more than 2.5 times the mass of
our sun, or 2.5 solar masses, and the lightest known black hole is about
5 solar masses. The question remained: does anything lie in this
so-called mass gap?
Now, in a new study from the National Science Foundation's Laser
Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo
detector in Europe, scientists have announced the discovery of an object
of 2.6 solar masses, placing it firmly in the mass gap. The object was
found on August 14, 2019, as it merged with a black hole of 23 solar
masses, generating a splash of gravitational waves detected back on
Earth by LIGO and Virgo. A paper about the detection has been accepted
for publication in The Astrophysical Journal Letters.
"We've been waiting decades to solve this mystery," says co-author
Vicky Kalogera, a professor at Northwestern University. "We don't know
if this object is the heaviest known neutron star, or the lightest known
black hole, but either way it breaks a record."
"This is going to change how scientists talk about neutron stars and
black holes," says co-author Patrick Brady, a professor at the
University of Wisconsin, Milwaukee, and the LIGO Scientific
Collaboration spokesperson. "The mass gap may in fact not exist at all
but may have been due to limitations in observational capabilities. Time
and more observations will tell."
The cosmic merger described in the study, an event dubbed GW190814,
resulted in a final black hole about 25 times the mass of the sun (some
of the merged mass was converted to a blast of energy in the form of
gravitational waves). The newly formed black hole lies about 800 million
light-years away from Earth.
Before the two objects merged, their masses differed by a factor of
9, making this the most extreme mass ratio known for a
gravitational-wave event. Another recently reported LIGO-Virgo event,
called GW190412, occurred between two black holes with a mass ratio of about 4:1.
"It's a challenge for current theoretical models to form merging
pairs of compact objects with such a large mass ratio in which the
low-mass partner resides in the mass gap. This discovery implies these
events occur much more often than we predicted, making this a really
intriguing low-mass object," explains Kalogera. "The mystery object may
be a neutron star merging with a black hole, an exciting possibility
expected theoretically but not yet confirmed observationally. However,
at 2.6 times the mass of our sun, it exceeds modern predictions for the
maximum mass of neutron stars, and may instead be the lightest black
hole ever detected."
When the LIGO and Virgo scientists spotted this merger, they
immediately sent out an alert to the astronomical community. Dozens of
ground- and space-based telescopes followed up in search of light waves
generated in the event, but none picked up any signals. So far, such
light counterparts to gravitational-wave signals have been seen only
once, in an event called GW170817.
The event, discovered by the LIGO-Virgo network in August of 2017,
involved a fiery collision between two neutron stars that was
subsequently witnessed by dozens of telescopes on Earth and in space.
Neutron star collisions are messy affairs with matter flung outward in
all directions and are thus expected to shine with light. Conversely,
black hole mergers, in most circumstances, are thought not to produce
light.
According to the LIGO and Virgo scientists, the August 2019 event was
not seen by light-based telescopes for a few possible reasons. First,
this event was six times farther away than the merger observed in 2017,
making it harder to pick up any light signals. Secondly, if the
collision involved two black holes, it likely would have not shone with
any light. Thirdly, if the object was in fact a neutron star, its 9-fold
more massive black-hole partner might have swallowed it whole; a
neutron star consumed whole by a black hole would not give off any
light.
"I think of Pac-Man eating a little dot," says Kalogera. "When the
masses are highly asymmetric, the smaller neutron star can be eaten in
one bite."
How will researchers ever know if the mystery object was a neutron
star or black hole? Future observations with LIGO, Virgo, and possibly
other telescopes may catch similar events that would help reveal whether
additional objects exist in the mass gap.
"This is the first glimpse of what could be a whole new population of
compact binary objects," says Charlie Hoy, a member of the LIGO
Scientific Collaboration and a graduate student at Cardiff University.
"What is really exciting is that this is just the start. As the
detectors get more and more sensitive, we will observe even more of
these signals, and we will be able to pinpoint the populations of
neutron stars and black holes in the universe."
"The mass gap has been an interesting puzzle for decades, and now
we've detected an object that fits just inside it," says Pedro
Marronetti, program director for gravitational physics at the National
Science Foundation (NSF). "That cannot be explained without defying our
understanding of extremely dense matter or what we know about the
evolution of stars. This observation is yet another example of the
transformative potential of the field of gravitational-wave astronomy,
which brings novel insights to light with every new detection."
Source: LIGO Caltech
Webinar Series
For those wishing for a deeper dive into these LIGO-Virgo results and
other research from the latest observing run, the team has scheduled a
webinar intended for a scientific audience. Called the LIGO-Virgo-KAGRA
Webinar Series, this will be the first in a series of webinars
discussing the gravitational-wave network’s results in-depth. The
one-hour Zoom webinar will be on June 25 at 14:00 Universal Time
Coordinated (7:00am Pacific Daylight Time; 10:00am Eastern Daylight
Time; 16:00 Central European Summer Time; 23:00 Japan Standard Time).
To register, visit: https://zoom.us/webinar/register/3315925939436/WN_rsJximZ8R36WqZnMH16IrA
The Zoom webinar will also be live streamed and a recording will be available upon request.
Additional information about the gravitational-wave observatories:
LIGO is funded by the NSF and operated by Caltech and MIT, which
conceived of LIGO and lead the project. Financial support for the
Advanced LIGO project was led by the NSF, with Germany (Max Planck
Society), the U.K. (Science and Technology Facilities Council) and
Australia (Australian Research Council-OzGrav) making significant
commitments and contributions to the project. Approximately 1,300
scientists from around the world participate in the effort through the
LIGO Scientific Collaboration, which includes the GEO Collaboration. A
list of additional partners is available at https://my.ligo.org/census.php.
The Virgo Collaboration is currently composed of approximately 550
members from 106 institutes in 12 different countries including Belgium,
France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain.
The European Gravitational Observatory (EGO) hosts the Virgo detector
near Pisa in Italy, and is funded by Centre National de la Recherche
Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare
(INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo
Collaboration groups can be found at http://public.virgo-gw.eu/the-virgo-collaboration/.
More information is available on the Virgo website at http://www.virgo-gw.eu.
