Long ago and far across the universe, an enormous burst of gamma rays unleashed more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. In May of 2020, light from the flash finally reached Earth and was first detected by NASA's Neil Gehrels Swift Observatory. Scientists quickly enlisted other telescopes — including NASA's Hubble Space Telescope, the Very Large Array radio observatory, the W. M. Keck Observatory, and the Las Cumbres Observatory Global Telescope network — to study the explosion's aftermath and the host galaxy. It was Hubble that provided the surprise.
This image shows the glow from a kilonova caused by the merger of two neutron stars. The kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova, appears as a bright spot (indicated by the arrow) to the upper left of the host galaxy. The merger of the neutron stars is believed to have produced a magnetar, which has an extremely powerful magnetic field. The energy from that magnetar brightened the material ejected from the explosion. Credits: NASA, ESA, W. Fong (Northwestern University), and T. Laskar (University of Bath, UK)
Based on X-ray and radio observations from the other observatories,
astronomers were baffled by what they saw with Hubble: the near-infrared
emission was 10 times brighter than predicted. These results challenge
conventional theories of what happens in the aftermath of a short gamma-ray
burst. One possibility is that the observations might point to the birth of a
massive, highly magnetized neutron star called a magnetar.
"These observations do not fit traditional explanations for short
gamma-ray bursts," said study leader Wen-fai Fong of Northwestern
University in Evanston, Illinois. "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. In terms of trying to fit the
puzzle pieces of this gamma-ray burst together, one puzzle piece is not fitting
correctly."
Without Hubble, the gamma-ray burst would have appeared like many others,
and Fong and her team would not have known about the bizarre infrared behavior.
"It's amazing to me that after 10 years of studying the same type of
phenomenon, we can discover unprecedented behavior like this," said Fong.
"It just reveals the diversity of explosions that the universe is capable
of producing, which is very exciting."
Light Fantastic
The intense flashes of gamma rays from these bursts appear to come from
jets of material that are moving extremely close to the speed of light. The
jets do not contain a lot of mass — maybe a millionth of the mass of the Sun —
but because they're moving so fast, they release a tremendous amount of energy
across all wavelengths of light. This particular gamma-ray burst was one of the
rare instances in which scientists were able to detect light across the entire
electromagnetic spectrum.
This illustration shows
the sequence for forming a magnetar-powered kilonova, whose peak brightness
reaches up to 10,000 times that of a classical nova. 1) Two orbiting neutron
stars spiral closer and closer together. 2) They collide and merge, triggering
an explosion that unleashes more energy in a half-second than the Sun will
produce over its entire 10-billion-year lifetime. 3) The merger forms an even
more massive neutron star called a magnetar, which has an extraordinarily
powerful magnetic field. 4) The magnetar deposits energy into the ejected
material, causing it to glow unexpectedly bright at infrared wavelengths.
Credits: NASA, ESA, and
D. Player (STScI)
"As the data were coming in, we were forming a picture of the
mechanism that was producing the light we were seeing," said the study's
co-investigator, Tanmoy Laskar of the University of Bath in the United Kingdom.
"As we got the Hubble observations, we had to completely change our
thought process, because the information that Hubble added made us realize that
we had to discard our conventional thinking, and that there was a new
phenomenon going on. Then we had to figure out what that meant for the physics
behind these extremely energetic explosions."
Gamma-ray bursts — the most energetic, explosive events known — live fast
and die hard. They are split into two classes based on the duration of their
gamma rays.
If the gamma-ray emission is greater than two seconds, it's called a long
gamma-ray burst. This event is known to result directly from the core collapse
of a massive star. Scientists expect a supernova to accompany this longer type
of burst.
If the gamma-ray emission lasts less than two seconds, it's considered a
short burst. This is thought to be caused by the merger of two neutron stars,
extremely dense objects about the mass of the Sun compressed into the volume of
a city. A neutron star is so dense that on Earth, one teaspoonful would weigh a
billion tons! A merger of two neutron stars is generally thought to produce a
black hole.
Neutron star mergers are very rare but are extremely important because
scientists think that they are one of the main sources of heavy elements in the
universe, such as gold and uranium.
Accompanying 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 with a small black hole.
Magnetic Monster?
Fong
and her team have discussed several possibilities to explain the unusual
brightness that Hubble saw. While most short gamma-ray bursts probably result
in a black hole, the two neutron stars that merged in this case may have
combined to form a magnetar, a supermassive neutron star with a very powerful
magnetic field.
"You
basically have these magnetic field lines that are anchored to the star that
are whipping around at about a thousand times a second, and this produces a
magnetized wind," explained Laskar. "These spinning field lines
extract the rotational energy of the neutron star formed in the merger, and
deposit that energy into the ejecta from the blast, causing the material to glow
even brighter."
These two images taken
on May 26 and July 16, 2020, show the fading light of a kilonova located in a
distant galaxy. The kilonova appears as a spot to the upper left of the host
galaxy. The glow is prominent in the May 26 image but fades in the July 16
image. The kilonova's peak brightness reaches up to 10,000 times that of a
classical nova. A merger of two neutron stars — the source of the kilonova — is
believed to have produced a magnetar, which has an extremely powerful magnetic
field. The energy from that magnetar brightened the material ejected from the
explosion, causing it to become unusually bright at infrared wavelengths of
light.
Credits: NASA, ESA, W.
Fong (Northwestern University), T. Laskar (University of Bath, UK), and A.
Pagan (STScI)
If the extra brightness came from a magnetar that deposited energy into the
kilonova material, then within a few years, the team expects the ejecta from
the burst to produce light that shows up at radio wavelengths. Follow-up radio
observations may ultimately prove that this was a magnetar, and this may
explain the origin of such objects.
"With its amazing sensitivity at near-infrared wavelengths, Hubble
really sealed the deal with this burst," explained Fong. "Amazingly,
Hubble was able to take an image only three days after the burst. Through a
series of later images, Hubble showed that a source faded in the aftermath of
the explosion. This is as opposed to being a static source that remains
unchanged. With these observations, we knew we had not only nabbed the source,
but we had also discovered something extremely bright and very unusual.
Hubble's angular resolution was also key in pinpointing the position of the
burst and precisely measuring the light coming from the merger."
NASA's upcoming James Webb Space Telescope is particularly
well-suited for this type of observation. "Webb will completely
revolutionize the study of similar events," said Edo Berger of Harvard
University in Cambridge, Massachusetts, and principal investigator of the
Hubble program. "With its incredible infrared sensitivity, it will not
only detect such emission at even larger distances, but it will also provide
detailed spectroscopic information that will resolve the nature of the infrared
emission."
The team's findings appear in an upcoming issue of The Astrophysical Journal.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space Flight
Center in Greenbelt, Maryland, manages the telescope. The Space Telescope
Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science
operations. STScI is operated for NASA by the Association of Universities for
Research in Astronomy, in Washington, D.C.
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