Posted in | Quantum Physics

Discovery of Nebula Wind Surrounding Magnetrar Opens Avenues for Understanding Magnetrars

A massive cloud of high-energy particles known as a wind nebula was discovered for the first time by astronomers, around a rare ultra-magnetic neutron star called magnetar. This discovery gives an insight into the environment, properties and outburst history of magnetars - the strongest magnets in the world.

This X-ray image shows extended emission around a source known as Swift J1834.9-0846, a rare ultra-magnetic neutron star called a magnetar. The glow arises from a cloud of fast-moving particles produced by the neutron star and corralled around it. Color indicates X-ray energies, with 2,000-3,000 electron volts (eV) in red, 3,000-4,500 eV in green, and 5,000 to 10,000 eV in blue. The image combines observations by the European Space Agency's XMM-Newton spacecraft taken on March 16 and Oct. 16, 2014. (Credits- ESA/XMM-Newton/Younes et al. 2016)

The crushed core of a huge star that ran short of fuel, collapsed due to its own weight, and burst as a supernova is a neutron star. Each neutron star compresses the equivalent weight of half a million Earths into a ball 20 kms across, approximately the length of New York's Manhattan Island. Neutron stars are often found as pulsars, and produce visible light, gamma rays, radio, and X-rays at different locations in their neighboring magnetic fields. Astronomers identify emission pulses when a pulsar spins these regions in our direction; hence it derived its name.

Conventional pulsar magnetic fields are capable of being 100 billion to 10 trillion times stronger than Earth's magnetic field. Magnetar fields achieve strengths that are a thousand times stronger, and scientists still find it difficult to decipher how these fields are developed. Only 29 of the known 2,600 neutron stars are classified as magnetars to date.

A magnetar called Swift J1834.9-0846 (J1834.9 for short) is surrounded by a newfound nebula. Swift J1834.9 was discovered by NASA's Swift satellite during a brief X-ray outburst on August 7, 2011. Astronomers suggest that the object is related to the W41 supernova remnant, positioned around 13,000 light-years away in the Scutum constellation toward the centre of the galaxy.

Right now, we don't know how J1834.9 developed and continues to maintain a wind nebula, which until now was a structure only seen around young pulsars. If the process here is similar, then about 10 percent of the magnetar's rotational energy loss is powering the nebula’s glow, which would be the highest efficiency ever measured in such a system.

George Younes, Postdoctoral Researcher at George Washington University

A research group headed by Younes viewed J1834.9 again using the European Space Agency's (ESA) XMM-Newton X-ray observatory after a month of the Swift discovery. This revealed a strange lopsided glow around 15 light-years across at the center of the magnetar. New XMM-Newton observations in March and October 2014, along with archival data from XMM-Newton and Swift, prove that this extended glow is the first wind nebula ever discovered surrounding a magnetar. The Astrophysical Journal will publish a report on the analysis.

"For me the most interesting question is, why is this the only magnetar with a nebula? Once we know the answer, we might be able to understand what makes a magnetar and what makes an ordinary pulsar," said Chryssa Kouveliotou, co-author of the study, Professor in the Department of Physics at George Washington University’s Columbian College of Arts and Sciences.

At the center of the Crab Nebula supernova remnant within the Taurus constellation, lies the well-known wind nebula, that is powered by a pulsar which is less than a thousand years old. Young pulsars such as this rotate fast, often a dozen times per second. The pulsar's rapid rotation and strong magnetic field together increase the speed of electrons and other particles to extremely high energies. This results in an outflow which the astronomers refer to as a pulsar wind that acts as the source of particles in a wind nebula.

Making a wind nebula requires large particle fluxes, as well as some way to bottle up the outflow so it doesn't just stream into space. We think the expanding shell of the supernova remnant serves as the bottle, confining the outflow for a few thousand years. When the shell has expanded enough, it becomes too weak to hold back the particles, which then leak out and the nebula fades away.

Alice Harding, Astrophysicist, NASA's Goddard Space Flight Center

This elucidates the reason behind the absence of wind nebulae among older pulsars, including those driving strong outflows.

To generate light and increase the speed of its pulsar wind, a pulsar taps its rotational energy. In contrast, energy stored within the super-strong magnetic field powers a magnetar outburst. This energy is released as an outburst of gamma rays and X-rays when the field suddenly turns into a lower-energy state. Even though magnetars may not develop the constant breeze of a typical pulsar wind; they can generate brief gales of accelerated particles during outbursts.

"The nebula around J1834.9 stores the magnetar's energetic outflows over its whole active history, starting many thousands of years ago," said Jonathan Granot, team member and Associate Professor in the Department of Natural Sciences at the Open University in Ra'anana, Israel. "It represents a unique opportunity to study the magnetar's historical activity, opening a whole new playground for theorists like me."

ESA's XMM-Newton satellite, launched on December 10, 1999, from Kourou, French Guiana, continues to record observations. XMM-Newton instrument package elements were funded by NASA and it provides the NASA Guest Observer Facility in Goddard, which facilitates U.S. astronomers to use the observatory.

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