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.
Credit: NASA’s Goddard Space Flight Center
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Unprecedented observations of a nova outburst in 2018 by a trio of satellites, including NASA’s Fermi and NuSTAR space telescopes, 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.
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.
Fermi detected its first nova in 2010 and has observed 14 to date. Gamma rays — the highest-energy form of light — require processes that accelerate subatomic particles to extreme energies, which happens in shock waves. When these particles interact with each other and with other matter, they produce gamma rays. Because the gamma rays appear at about the same time as a nova's peak in visible light, astronomers concluded that shock waves play a more fundamental role in the explosion and its aftermath.
The Fermi and BRITE data show flares in both wavelengths at about the same time, so they must share the same source — shock waves in the fast-moving debris.
Observations of one flare using NASA’s NuSTAR space telescope showed a much lower level of X-rays compared to the higher-energy Fermi data, 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.
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. 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.
This animation illustrates the likely phases of V906 Carinae's 2018 nova eruption. The explosion created a thick, complex cloud that was swept into a doughnut shape by the orbital motion of the stars. From our perspective, we viewed the cloud roughly edge-on. 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 cloud. This created shock waves that emitted gamma rays and visible light. 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. Both of the nova outflows likely arose from residual nuclear fusion reactions on the white dwarf’s surface.
Credit: NASA's Goddard Space Flight Center/Chris Smith (USRA)
V906 Carinae (circled) shines near peak brightness in this image taken on March 23, 2018, three days after the nova was discovered. The beautiful cloud of gas and dust dominating the picture is part of the Carina Nebula.
Credits: Copyright 2018 by A. Maury and J. Fabrega, used with permission
Nova Carinae 2018, now designated V906 Carinae, was a thermonuclear explosion on a white dwarf in a star system about 13,000 light-years away. This image shows the nova on April 7, about 18 days after its discovery and near its peak brightness.
Credit: Copyright 2018 by W. Paech + F. Hofmann, Team Chamaeleon, Chamaeleon and Onjala Observatory, Namibia, used with permission.
The BRITE-Toronto satellite observed V906 Carinae for 16 minutes out of every orbit, returning about 600 measurements each day and capturing the nova’s changing brightness in unparalleled detail. The 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.