Our Universe shines bright with light across the electromagnetic spectrum.
While most of this light comes from stars like our Sun in galaxies like our
own, we are often treated with brief and bright flashes that outshine entire
galaxies themselves. Some of these brightest flashes are believed to be
produced in cataclysmic events, such as the death of massive stars or the
collision of two stellar corpses known as neutron stars. Researchers have
long studied these bright flashes or ‘transients’ to gain insight into the
deaths and afterlives of stars and the evolution of our Universe.
Astronomers are sometimes greeted with transients that defy expectations and
puzzle theorists who have long predicted how various transients should look.
In October 2014, a long-term monitoring program of the southern sky with the
Chandra telescope—NASA’s flagship X-Ray telescope—detected one such
enigmatic transient called CDF-S XT1: a bright transient lasting a few
thousands of seconds. The amount of energy CDF-S XT1 released in X-rays was
comparable to the amount of energy the Sun emits over a billion years. Ever
since the original discovery, astrophysicists have come up with many
hypotheses to explain this transient; however, none have been conclusive.
In a recent study,[1] a team of astrophysicists led by OzGrav postdoctoral
fellow Dr. Nikhil Sarin (Monash University) found that the observations of
CDF-S XT1 match predictions of radiation expected from a high-speed jet
traveling close to the speed of light. Such “outflows” can only be produced
in extreme astrophysical conditions, such as the disruption of a star as it
gets torn apart by a massive black hole, the collapse of a massive star, or
the collision of two neutron stars.
Sarin et al’s study found that the outflow from CDF-S XT1 was likely
produced by two neutron stars merging together. This insight makes CDF-S XT1
similar to the momentous 2017 discovery called GW170817—the first
observation of gravitational-waves, cosmic ripples in the fabric of space
and time—although CDF-S XT1 is 450 times further away from Earth. This huge
distance means that this merger happened very early in the history of the
Universe; it may also be one of the furthest neutron star mergers ever
observed.
Neutron star collisions are the main places in the Universe where heavy
elements such as gold, silver, and plutonium are created. Since CDF-S XT1
occurred early on in the history of the Universe, this discovery advances
our understanding of Earth’s chemical abundance and elements.
Recent observations of another transient AT2020blt in January 2020—primarily
with the Zwicky Transient Facility—have puzzled astronomers. This
transient’s light is like the radiation from high-speed outflows launched
during the collapse of a massive star. Such outflows typically produce
higher energy gamma-rays; however, they were missing from the data – they
were not observed. These gamma rays can only be missing due to one of three
possible reasons: 1) The gamma-rays were not produced. 2) The gamma rays
were directed away from Earth. 3) The gamma-rays were too weak to be seen.
In a separate study,[2] led again by OzGrav researcher Dr. Sarin, the Monash
University astrophysicists teamed up with researchers in Alabama, Louisiana,
Portsmouth and Leicester to show that AT2020blt probably did produce
gamma-rays pointed towards Earth, they were just really weak and missed by
our current instruments.
Dr. Sarin says: “Together with other similar transient observations, this
interpretation means that we are now starting to understand the enigmatic
problem of how gamma-rays are produced in cataclysmic explosions throughout
the Universe”.
The class of bright transients collectively known as gamma-ray bursts,
including CDF-S XT1, AT2020blt, and AT2021any, produce enough energy to
outshine entire galaxies in just one second.
“Despite this, the precise mechanism that produces the high-energy radiation
we detect from the other side of the Universe is not known,” explains Dr
Sarin. “These two studies have explored some of the most extreme gamma-ray
bursts ever detected. With further research, we’ll finally be able to answer
the question we’ve pondered for decades: How do gamma-ray bursts work?”
References:
“CDF-S XT1: The off-axis afterglow of a neutron star merger at
z=2.23” by Nikhil Sarin, Gregory Ashton, Paul D. Lasky, Kendall Ackley,
Yik-Lun Mong and Duncan K. Galloway, 21 May 2021, Astrophysics > High
Energy Astrophysical Phenomena.
arXiv:2105.10108
“Low-efficiency long gamma-ray bursts: A case study with
AT2020blt” by Nikhil Sarin, Rachel Hamburg, Eric Burns, Gregory Ashton, Paul
D. Lasky and Gavin P. Lamb, 3 June 2021, Astrophysics > High Energy
Astrophysical Phenomena.
Tags:
Space & Astrophysics