A GIF cycles between an image of V906 Carinae taken on April 7, 2018, about 18 days after the nova's discovery and near its peak brightness, and one showing its faded appearance on May 4, 2019. Credit: Copyright 2018 by W. Paech + F. Hofmann, Team Chamaeleon, Chamaeleon and Onjala Observatory, Namibia, used with permission.
Unprecedented observations of a nova outburst in 2018 by a trio of
satellites, including two NASA missions, have captured the first direct
evidence that most of the explosion’s visible light arose from shock
waves — abrupt changes of pressure and temperature formed in the
explosion debris.
A nova is a sudden, short-lived brightening of an otherwise inconspicuous star. It occurs when a stream of hydrogen from a companion star flows onto the surface of a white dwarf, a compact stellar cinder not much larger than Earth. NASA’s Fermi and NuSTAR space telescopes, together with the Canadian BRITE-Toronto satellite and several ground-based facilities, studied the nova.
NASA’s Fermi and NuSTAR space telescopes, together
with another satellite named BRITE-Toronto, are providing new insights
into a nova explosion that erupted in 2018. Detailed measurements of
bright flares in the explosion clearly show that shock waves power most
of the nova's visible light. Credits: NASA’s Goddard Space Flight Center. Download high-resolution video and images from NASA’s Scientific Visualization Studio
“Thanks to an especially bright nova and a lucky break, we were able
to gather the best-ever visible and gamma-ray observations of a nova to
date,” said Elias Aydi, an astronomer at Michigan State University
in East Lansing who led an international team from 40 institutions.
“The exceptional quality of our data allowed us to distinguish
simultaneous flares in both optical and gamma-ray light, which provides
smoking-gun evidence that shock waves play a major role in powering some
stellar explosions.”
The 2018 outburst originated from a star system later dubbed V906
Carinae, which lies about 13,000 light-years away in the constellation
Carina. Over time — perhaps tens of thousands of years for a so-called
classical nova like V906 Carinae — the white dwarf’s deepening hydrogen
layer reaches critical temperatures and pressures. It then erupts in a
runaway reaction that blows off all of the accumulated material.
Each nova explosion releases a total of 10,000 to 100,000 times the annual energy output of our Sun. Astronomers discover about 10 novae each year in our galaxy.
Fermi detected its first nova in 2010 and has observed 14 to date. Although X-ray and radio
studies had shown the presence of shock waves in nova debris in the
weeks after the explosions reached peak brightness, the Fermi discovery
came as a surprise.
Gamma rays — the highest-energy form of light — require processes
that accelerate subatomic particles to extreme energies. When these
particles interact with each other and with other matter, they produce
gamma rays. But astronomers hadn’t expected novae to be powerful enough
to produce the required degree of acceleration.
Because the gamma rays appear at about the same time as the peak in
visible light, astronomers concluded that shock waves play a more
fundamental role in the explosion and its aftermath.
In 2015, a paper
led by Brian Metzger at Columbia University in New York showed how
comparing Fermi gamma-ray data with optical observations would allow
scientists to learn more about nova shock waves. In 2017, a study
led by Kwon-Lok Li at Michigan State found that the overall gamma-ray
and visible emissions rose and fell in step in a nova known as V5856
Sagittarii. This implied shock waves produced more of the eruption’s
light than the white dwarf itself.
The new observations from V906 Carinae, presented in a paper led by Aydi and published on Monday, April 13, in Nature Astronomy, spectacularly confirm this conclusion.
On March 20, 2018, the All-Sky Automated Survey for Supernovae,
a set of two dozen robotic telescopes distributed around the globe and
operated by Ohio State University, discovered the nova.
By month’s end,
V906 Carinae was dimly visible to the naked eye.
Fortuitously, a satellite called BRITE-Toronto was already studying
the nova’s patch of sky. This miniature spacecraft is one of five
7.9-inch (20 centimeter) cubic nanosatellites comprising the Bright Target Explorer (BRITE) Constellation.
Operated by a consortium of universities from Canada, Austria and
Poland, the BRITE satellites study the structure and evolution of bright
stars and observe how they interact with their environments.
BRITE-Toronto was monitoring a red giant star called HD 92063, whose
image overlapped the nova’s location. The satellite observed the star
for 16 minutes out of every 98-minute orbit, returning about 600
measurements each day and capturing the nova’s changing brightness in
unparalleled detail.
“BRITE-Toronto revealed eight brief flares that fired up around the
time the nova reached its peak, each one nearly doubling the nova’s
brightness,” said Kirill Sokolovsky at Michigan State. “We’ve seen hints
of this behavior in ground-based measurements, but never so clearly.
Usually we monitor novae from the ground with many fewer observations
and often with large gaps, which has the effect of hiding short-term
changes.”
Fermi, on the other hand, almost missed the show. Normally its Large
Area Telescope maps gamma rays across the entire sky every three hours.
But when the nova appeared, the Fermi team was busy troubleshooting the
spacecraft’s first hardware problem in nearly 10 years of orbital
operations — a drive on one of its solar panels stopped moving in one direction. Fermi returned to work just in time to catch the nova’s last three flares.
In fact, V906 Carinae was at least twice as bright at
billion-electron-volt, or GeV, energies as any other nova Fermi has
observed. For comparison, the energy of visible light ranges from about 2
to 3 electron volts.
“When we compare the Fermi and BRITE data, we see flares in both at
about the same time, so they must share the same source — shock waves in
the fast-moving debris,” said Koji Mukai, an astrophysicist at the University of Maryland Baltimore County
and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “When we
look more closely, there is an indication that the flares in gamma rays
may lead the flares in the visible. The natural interpretation is that
the gamma-ray flares drove the optical changes.”
The team also observed the eruption’s final flare using NASA’s NuSTAR space telescope, which is only the second time the spacecraft has detected X-rays during a nova’s optical and gamma-ray emission. The nova’s GeV gamma-ray output far exceeded the NuSTAR X-ray emission, likely because the nova ejecta absorbed most of the X-rays. High-energy light from the shock waves was repeatedly absorbed and reradiated at lower energies within the nova debris, ultimately only escaping at visible wavelengths.
Putting all of the observations together, Aydi and his colleagues
describe what they think happened when V906 Carinae erupted. During the
outburst’s first few days, the orbital motion of the stars swept a thick
debris cloud made of multiple shells of gas into a doughnut shape that
appeared roughly edge-on from our perspective. The cloud expanded
outward at less than about 1.3 million mph (2.2 million kph), comparable
to the average speed of the solar wind flowing out from the Sun.
Next, an outflow moving about twice as fast slammed into denser
structures within the doughnut, creating shock waves that emitted gamma
rays and visible light, including the first four optical flares.
Finally, about 20 days after the explosion, an even faster outflow
crashed into all of the slower debris at around 5.6 million mph (9
million kph). This collision created new shock waves and another round
of gamma-ray and optical flares. The nova outflows likely arose from
residual nuclear fusion reactions on the white dwarf’s surface.
Astronomers have proposed shock waves as a way to explain the power
radiated by various kinds of short-lived events, such as stellar
mergers, supernovae — the much bigger blasts associated with the
destruction of stars — and tidal disruption events, where black holes
shred passing stars. The BRITE, Fermi and NuSTAR observations of V906
Carinae provide a dramatic record of such a process. Further studies of
nearby novae will serve as laboratories for better understanding the
roles shock waves play in other more powerful and more distant events.
The Fermi Gamma-ray Space Telescope is an astrophysics and particle
physics partnership managed by NASA's Goddard Space Flight Center in
Greenbelt, Maryland. Fermi was developed in collaboration with the U.S.
Department of Energy, with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.
NuSTAR is a Small Explorer mission led by Caltech and managed by JPL
for NASA's Science Mission Directorate in Washington. NuSTAR was
developed in partnership with the Danish Technical University and the
Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences
Corp. in Dulles, Virginia. NuSTAR's mission operations center is at the
University of California Berkeley, and the official data archive is at
NASA's High Energy Astrophysics Science Archive Research Center. ASI
provides the mission's ground station and a mirror archive. Caltech
manages JPL for NASA.
By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Media contact:
Claire Andreoli
NASA’s Goddard Space Flight Center, Greenbelt, Md.
(301) 286-1940
Editor: Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Media contact:
Claire Andreoli
NASA’s Goddard Space Flight Center, Greenbelt, Md.
(301) 286-1940
Editor: Francis Reddy
Source: NASA/Fermi Space Telescope
