Massive explosion from unknown source billions of light years
away baffles astronomers
Gamma ray burst
detected from 12 billion light years away - but no one knows what caused it.
·
By
Updated April 3, 2017 14:58 BST
At 10:49pm Western
Australian time on February 2 this year, cosmic gamma rays hit the Nasa
satellite, Swift,
orbiting the Earth. Within seconds of the detection, an alert was automatically
sent to the University
of WA
What emerged from the blackness, where nothing was seen before,
was a rapidly brightening "optical transient", which is something
visible in the sky for a brief period of time.
The event, named
GRB170202, was a very energetic gamma ray burst (GRB). After less than a minute, the gamma rays
switched off, and the GRB appeared as a brightening and then fading optical
beacon.
The Zadko Telescope recorded the entire evolution of the optical
outburst. During its biggest outburst, GRB170202 was equivalent in brightness
to millions of stars shining together from the same location.
About 9 hours 42 mins
after the GRB, the Very Large Telescope in Chile
Zadko
Telescope light curve of GRB170202, showing the evolving explosion and
subsequent fading of the optical afterglow from seconds to hours after the
gamma ray emission.Alain Klotz Zadko collaboration
This enabled a distance to the burst to be measured: about 12
billion light years. The universe has expanded to four times the size it was
then, 12 billion years ago, the time it took the light to reach Earth.
GRB170202 was so far away, even its host galaxy was not visible,
just darkness. Because the GRB was a transient, never to be seen again, it is
like turning on a light in a dark room (the host galaxy) and trying to record
the detail in the room before the light goes out.
Mystery of gamma ray burst
The flash of gamma radiation and subsequent optical transient is
the telltale signature of a black hole birth from the cataclysmic collapse of a
star. Such events are rare and require some special circumstances, including a
very massive star up to tens of solar masses (the mass of our Sun) rotating
rapidly with a strong magnetic field.
These ingredients are
crucial to launch two jets that punch through the collapsing star to produce
the gamma ray burst (see animation). The closest analogue (and
better understood transient) to a GRB is a supernova explosion
from a collapsing star. In fact, some relatively nearby GRBs reveal evidence of
an energetic supernova linked to the event.
Simulations show that most collapsing stars don't have enough
energy to produce a GRB jet, a so-called "failure to launch"
scenario. Both observation and theory show that GRBs are extremely rare when
compared to the occurrence of supernovae.
The stars that produce GRBs are born and die within some tens to
hundreds of thousands of years, unlike our Sun which has been around for
billions of years. This is because very massive stars exhaust their fuel very
quickly, and undergo violent gravitational collapse leading to a black hole, on
the timescale of seconds.
A plethora of rogue black
holes
The rates of black hole
formation throughout the universe can be inferred from the GRB rate. Based on
the observed GRB rate, there must be thousands of black hole births occurring each daythroughout
the entire universe.
So what is the fate of these cosmic monsters? Most will be
lurking in their host galaxies, occasionally devouring stars and planets.
Others will be in a
gravitational death dance with other black holes until they merge into a single
black hole with a burst of gravitational waves (GWs), such as the first discovery of such an event by the Laser Interferometer
Gravitational-Wave Observatory (LIGO).
At the frontier of
understanding black hole formation is the search for a special kind of GRB that
marks the merger (collision) of two neutron stars. So called "short GRBs" are flashes of gamma
radiation that last less than a second and could be the "smoking gun"
for neutron star mergers.
Importantly, merging neutron stars should be detected from their
gravitational radiation by LIGO. Hence, a coincident detection in gamma rays,
optical and gravitational waves is a real possibility.
This would be a monumental discovery allowing unprecedented
insight into the physics of black hole formation. The revolution is like
listening to the radio on a 1920s receiver and then watching a modern high
definition surround sound movie.
Future challenges
Given the above rate of thousands of black holes created per
day, it seems that coincident detection of GRBs and gravitational waves is a no
brainer.
But in reality we must take into account the limited sensitivity
of all the telescopes (and detectors). This reduces the potential observation
rate to some tens per year. This is high enough to inspire a global scramble to
search for the first coincident gravitational wave sources with electromagnetic
counterparts.
The task is extremely difficult because the gravitational wave
observatories cannot pinpoint the location of the source very well. To counter
this, a strategy of searching for coincident gravitational wave and
electromagnetic detections in time may be the best bet.
The newly funded ARC
Centre of Excellence OzGrav mission is to understand the extreme
physics of black holes.
One of the goals is to search for optical, radio and high energy
counterparts coincident with gravitational waves from black hole creation. Australia
David Coward, Associate professor, University of Western Australia
This article was originally
published on The Conversation.
Read the original article.
.
