The LIGO and Virgo Scientific Collaboration have just announced a brand new detection,
GW190814. This event is a bit special and requires us to delve into some of the science
behind the observation. So I’m Geraint Pratten, a Post-Doctoral Researcher here at the University
of Birmingham and a Gravitational Wave Researcher in the LIGO Collaboration. The puzzle really
does begin with the Stellar Graveyard. So, when stars reach the end of their lives, they
can undergo cataclysmic explosions called supernova and the star is blown apart. All
of this happens within a matter of seconds, these really are quite violent phenomena.
Now, most of the stars will burn leaving behind a core, or a remnant. This remnant
can be several times heavier than our own Sun, so they really are quite massive objects
in their own right. However, a number of distinct astrophysical scenarios can play out, depending
on the exact mass and composition of this remnant. If, after the supernova, the remnant
has enough mass, then the gravitational field is sufficiently strong that gravitational
collapse will continue and the star will collapse in on itself, and a Black Hole will be born.
Now, Black Holes that are produced this way will have masses that are five to several
ten times the size of our own Sun - these are quite heavy Black Holes. For stars that
are initially less massive, so their gravitational fields are weaker, then the remnant of the
massive star can collapse into a super dense object called a neutron star. Neutron stars
are only a little bit heavier than our own Sun, but are so incredibly dense that the
entire star has a radius of 10 or 11km - this would be like taking our very own Sun and
compressing it into an object the size of Birmingham itself. These really are quite
fascinating and exotic astrophysical objects. Now, the heaviest known neutron stars are
about 2.5 times the mass of our own Sun, or about 2.5 solar masses, whereas the lightest
known Black Holes are about 5 solar masses. So, what can we say about all of the masses
in-between? This is actually known as the Mass Gap and is one of the current outstanding
questions understanding the origin and evolution of neutron stars and Black Holes. Why do we
observe an apparent gap between the heaviest neutron stars and the lightest Black Holes?
On the 14th August 2019, the LIGO and Virgo gravitational wave detectors observed a rather
special event: GW190814. And what we observed was the merger of two compact objects, where
the smallest object only had a mass of 2.6 solar masses and the heaviest object 23 solar
masses. So, the largest object is clearly a Black Hole, but the smaller object is a
real peculiarity. We don’t know if this object was the heaviest known neutron star or
the lightest known Black Hole. If it was a Black Hole, then it exists in the Mass Gap,
right where we would not expect to find such Black Holes. Now, it could be that this Mass
Gap doesn’t really exist, and our limitations and abilities to observe such light Black
Holes have prevented us from seeing them before. This is something we really hope to understand
using gravitational wave observations. Now, when we do observe a gravitational wave signal,
such as GW190814, an alert is sent to astronomers around the globe, enabling them to point their
telescopes in the direction of the signal. Now, the key goal here is to hunt for any
electromagnetic radiation. This would be a key indicator that one of the compact objects
was indeed a neutron star. For GW190814, dozens of ground and space based telescopes participated
in dedicated campaigns, but so far no such electromagnetic signal has been observed.
So what could this tell us about GW190814? Firstly, if the signal was produced by the
merge of two Black Holes, then Einstein’s theory of General Relativity predicts that
we wouldn’t see any electromagnetic signal. If, however, the merger was of a neutron star
and a Black Hole, we could hope that the neutron star is torn apart by the extreme gravitational
fields producing an electromagnetic signal. However, for GW190814, the larger Black Hole
was so much bigger than the small object and if this was a neutron star, it’s highly
likely that it would have been swallowed whole by the Black Hole, before it had a chance
to be torn apart - so we wouldn’t have seen any electromagnetic signal anyway. Finally,
GW190814 is more than six times further away than the binary neutron star merger that we
observed in 2017. This means that any electromagnetic signal could have been just too faint to observe.
So unfortunately, this question is still open, that we were not able to see any electromagnetic
radiation is in good agreement with our current understanding and theories. It is really only
by observing many more of these events with gravitational wave detectors, and working
closely with international teams of astronomers, that we can start to find enough pieces of
the puzzle to solve the mystery. I hope you’ve found this useful and thank you for listening.
If you have any comments, please leave them below and we’ll try to respond to as many
as we can. Thank you!