From Keck Observatory, Maunakea, Hawaii
November
12, 2020 -- Astronomers have discovered the brightest infrared light from a
short gamma-ray burst ever seen, with a bizarre glow that is more luminous than
previously thought was possible.
Its half-second flash of light, detected
in May of this year, came from a violent explosion of gamma rays billons of
light-years away that unleashed more energy in a blink of an eye than the Sun
will produce over its entire 10-billion-year lifetime.
The study has been accepted in The
Astrophysical Journal and will be published online later this year. A pre-print is available on arXiv.org.
“It’s amazing to me that after 10 years
of studying the same type of phenomenon, we can discover unprecedented behavior
like this,” said Wen-fai Fong, assistant professor of physics and astronomy at
Northwestern University and lead author of the study. “It just reveals the
diversity of explosions that the universe is capable of producing, which is
very exciting.”
NASA’s Hubble Space Telescope quickly captured
the glow within just three days after the burst and determined its
near-infrared emission was 10 times brighter than predicted, defying
conventional models.
“These observations do not fit
traditional explanations for short gamma-ray bursts,” said Fong. “Given what we
know about the radio and X-rays from this blast, it just doesn’t match up. The
near-infrared emission that we’re finding with Hubble is way too bright.”
To zero in on this new phenomenon’s
exact brightness, the team used W. M. Keck Observatory on Maunakea in Hawaii to
pinpoint the precise distance of its host galaxy.
“Distances are important in calculating
the burst’s true brightness as opposed to its apparent brightness as seen from
Earth,” said Fong. “Just as the brightness of a light bulb when it reaches your
eye depends on both its luminosity and its distance from you, a burst could be
really bright because either it is intrinsically luminous and distant, or not
as luminous but much closer to us. With Keck, we were able to determine the
true brightness of the burst and thus the energy scale. We found it was to be
much more energetic than we originally thought.”
Using Keck Observatory’s Low Resolution
Imaging Spectrometer (LRIS) and DEep Imaging and Multi-Object Spectrograph
(DEIMOS), the team determined the burst came from a galaxy located at a
redshift of z = 0.55 – quite a bit farther than the initial calculated
distance.
Lasting less than two seconds, short
gamma-ray bursts are among the most energetic, explosive events known; they live
fast and die hard. Scientists think they’re caused by the merger of two neutron
stars, extremely dense objects about the mass of the Sun compressed into the
volume of a small city. A neutron star is so dense that on Earth, one
teaspoonful would weigh a billion tons!
Neutron star mergers are very rare and
extremely important because scientists think they are one of the main sources
of heavy elements in the universe, such as gold and uranium.
Along with a short gamma-ray burst,
scientists expect to see a “kilonova” whose peak brightness typically reaches
1,000 times that of a classical nova. Kilonovae are an optical and infrared
glow from the radioactive decay of heavy elements and are unique to the merger
of two neutron stars, or the merger of a neutron star and a black hole.
What Fong and her team saw was too
bright to be explained even by a traditional kilonova. They provide one
possible explanation for the unusually bright blast. While most short gamma-ray
bursts probably result in a black hole, the neutron star merger in this case
may have instead formed a magnetar, a supermassive neutron star with a very
powerful magnetic field. The magnetar deposited a large amount of energy into
the ejected material of the kilonova, causing it to glow even brighter.
“What we detected even outshines the one
confirmed kilonova discovered in 2017,” said co-author Jillian Rastinejad, a
graduate student with Fong’s team at Northwestern University. “As a first-year
graduate student working with real-time data for the first time when this burst
happened, it’s remarkable to see our discovery motivate a new and exciting
magnetar-boosted model.”
With such an event, the team expects the
ejecta from the burst to produce light at radio wavelengths in the next few
years. Follow-up radio observations may ultimately prove the origin of the
burst was indeed a magnetar. The birth of a magnetar from a neutron star merger
has never definitively been seen before, as they are expected to be rare
outcomes.
The short gamma-ray burst was first detected
with NASA’s Neil Gehrels Swift Observatory. Once the alert went out, the team
quickly enlisted other telescopes to conduct multi-wavelength observations.
They analyzed the afterglow in X-ray with Swift Observatory, optical and
near-infrared with Las Cumbres Observatory Global Telescope, Hubble, and Keck
Observatory, and in radio wavelengths with the Very Large Array. This
particular gamma-ray burst was one of the rare instances in which scientists
were able to detect light across the entire electromagnetic spectrum.
NASA’s upcoming James Webb Space
Telescope is particularly well-suited for this type of observation.
“We can’t wait to combine the power of
Keck and JWST along with other facilities as a team to go after even more
enigmatic events like these,” said Keck Observatory Chief Scientist John
O’Meara. “This study shows that we have much left to learn.”
No comments:
Post a Comment