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March 9, 2018

X-ray and light measurements reveal a new low-mass x-ray binary

In a newly discovered low-mass binary system that emits x-rays, a neutron star is feeding on its companion

small image of neutron star

Figure 1: In an x-ray binary, a neutron star (bluish white object in top right) draws a swirling disk of gas from a companion star.

© MARK GARLICK/SCIENCE PHOTO LIBRARY

RIKEN astronomers have spotted a neutron star that is sucking the life from its companion star1. Further observations of the pair, known as a low-mass x-ray binary (LMXB), could offer insights into the incredibly dense material in the neutron star.

When a neutron star draws gas away from a conventional star, it forms a swirling accretion disk of matter that emits visible light and x-rays (Fig. 1). Some LMXBs show dramatic variations in these emissions, brightening when more matter is stripped from the donor star and then dimming as the feeding frenzy dies down. “They enter an x-ray-dim period when the donor star provides only small amounts of gas to the accretor,” says Megumi Shidatsu, part of RIKEN’s MAXI (Monitor of All-sky X-ray Image) Team.

On 13 March 2017, the MAXI team spotted an object in our galaxy called MAXI J1807+132, which they suspected could be an LMXB. They confirmed that it was by using data from the orbiting Swift X-ray Telescope and two ground-based optical telescopes in Japan.

Between 27 March and early April, the intensity of x-rays coming from MAXI J1807+132 declined roughly 100 fold, and the x-ray spectrum became dominated by lower energy x-rays. Over the same period, the intensity of visible light from the object also decreased.

small photo of Megumi Shidatsu

Megumi Shidatsu and her co-workers have found an unusual low-mass x-ray binary that shows a big change in its x-ray energy spectrum during its dim period.

© 2018 RIKEN

The declining x-ray emissions from MAXI J1807+132 suggest that the observations caught the binary in a dim phase. The lower energy x-rays were probably generated at the surface of the neutron star, says Shidatsu, while the large reduction in optical brightness suggests that the donor is a low-mass star.

After considering various possible explanations for the data, the team concluded that MAXI J1807+132 consists of a neutron star that is about 1.4 times the mass of the Sun and a companion star with less than about 4 times the solar mass.

“More than 100 LMXBs have been found in our galaxy,” says Shidatsu. “But this source is one of the few neutron-star LMXBs that show a big change in their x-ray energy spectrum during their dim periods.” This makes it a valuable find since the spectral behavior of LMXBs in their dim periods is not well understood due to limited observation time and sensitivity of x-ray detectors, she adds. 

When the neutron star completely stops sucking gas from its donor, it will gradually cool and the x-ray emissions from its surface will die away. That process depends on the properties of the matter inside the neutron star. “We want to continue monitoring the source and extract valuable information about the states of neutron-rich, super-dense matter,” says Shidatsu. “This information will be useful to nuclear physics.”

References

  1. Shidatsu, M., Tachibana, Y., Yoshii, T., Negoro, H., Kawamuro, T., Iwakiri, W., Nakahira, S., Makishima, K., Ueda, Y., Kawai, N. et al. Discovery of the new X-ray transient MAXI J1807+132: A candidate of a neutron star low-mass X-ray binary. The Astrophysical Journal 850, 155 (2017). doi: 10.3847/1538-4357/aa93f0 (Link)