A neutron star in the Scutum-Crux Arm of the Milky Way recently flared to 10,000 times its original brightness after being bombarded by gas from its companion star. The outburst was seen in X-rays during an observation on March 15, 2010, by the European Space Agency's XMM-Newton telescope, currently in orbit around Earth.
The 4-hour-long flaring occurred as the neutron star absorbed a blob of gas that blew out of its companion star, an enormous blue supergiant. (Together the stars form a system known as IGR J18410-0535.) As the hot gas settled onto the neutron star, its surface became hotter and hotter, eventually emitting high-energy radiation. Astronomers had seen flare-ups in this system before — but never anything so dramatic.
"This was a huge bullet of gas that the star shot out, and it hit the neutron star, allowing us to see it," says Enrico Bozzo (University of Geneva), the team leader for this project. As he and his team report in a forthcoming article in Astronomy & Astrophysics, the dramatic outburst likely resulted from a blob of matter some 10 million miles (16 million km) across and having 1016 tons of mass.
Few objects in the universe are as exotic as neutron stars. Formed by powerful gravitational forces in the aftermath of a supernova, a neutron star packs the mass of a star like our Sun into a sphere smaller than many cities. For example, the neutron star observed by XMM-Newton has a diameter of approximately 6 miles (10 km).
This observation helps answer an outstanding question about binary systems consisting of supergiants and neutron stars. Astronomers have long noted that some binaries emit X-rays at an almost constant rate. In such cases the orbits of neutron stars around their companions tend to be circular, allowing them to accrete gas from the supergiants at a steady rate. So the emission of X-rays also tends to be constant.
But some binaries emit X-rays in short bursts, even though they have the same components — neutron stars and supergiants — as the more regular systems. Astronomers have theorized that this irregularity stems from variations in the stellar wind blowing from the supergiants. If this wind were "clumpy," containing big, dense bubbles of material, then the irregular flashes would make sense.
The flashes captured by XMM-Newton provide the "first clear evidence of the existence of these massive clumps," says Bozzo. Launched in 1999, XMM-Newton has three X-ray telescopes, each consisting of 58 nested, concentric mirrors. (XMM stands for "X-ray Multi-Mirror".) To make sure its observations would not be affected by the ions in Earth's magnetosphere, ESA flight controllers put XMM-Newton into an orbit that takes it one-third of the distance to the Moon.
The sighting of the flaring neutron star "highlights XMM-Newton's unique capabilities," says Norbert Schartel, the mission's project scientist. Although XMM-Newton alone captured IGR J18410-0535's 2010 outburst, it's not the only X-ray eye in space. Others include the Chandra X-ray Observatory, launched by NASA in 1999; Swift, another NASA craft launched in 2004; and Suzaku, launched in 2005 by the Japan Aerospace Exploration Agency (JAXA).
Raphael Rosen is interning at Sky & Telescope for the summer.