Astronomers have identified the first example of a binary system in which a star in the process of becoming a white dwarf orbits a neutron star that has just finished changing into a rapidly spinning pulsar using the 4.1-meter SOAR Telescope in Chile. The pair represents a “missing link” in the evolution of binary systems, having been discovered by the Fermi Gamma-ray Space Telescope.
A fast-spinning neutron star known as a millisecond pulsar circling a star in the process of transitioning into an extremely low-mass white dwarf has been discovered as a bright and puzzling source of gamma rays. Astronomers call these binary systems “spiders” because the pulsar tends to “eat” the companion star’s outer parts as it transforms into a white dwarf.
The duo was detected by astronomers using the 4.1-meter SOAR Telescope on Cerro Pachón in Chile, part of Cerro Tololo Inter-American Observatory (CTIO), a Program of NSF’s NOIRLab.
NASA’s Fermi Gamma-ray Space Telescope Since its launch in 2008, the Space Telescope has been collecting objects in the Universe that emit copious gamma rays, although not all of the sources of gamma rays that it finds have been categorized. Astronomers named one of these sources 4FGL J1120.0-2204, the second brightest gamma-ray source in the entire sky that had remained unnamed until today.
The Goodman Spectrograph on the SOAR Telescope was utilized by astronomers from the United States and Canada, led by Samuel Swihart of the US Naval Research Laboratory in Washington, D.C., to identify the true identity of 4FGL J1120.0-2204.
This is a great example of how mid-sized telescopes in general, and SOAR in particular, can be used to help characterize unusual discoveries made with other ground and space-based facilities. We anticipate that SOAR will play a crucial role in the follow-up of many other time-variable and multi-messenger sources over the coming decade.
Chris Davis
The gamma-ray source, which also emits X-rays, was discovered to be a binary system consisting of a “millisecond pulsar” that spins hundreds of times per second and the precursor of a very low-mass white dwarf, as detected by NASA’s Swift and ESA’s XMM-Newton space observatories. The two are more than 2600 light-years apart.
“Michigan State University’s dedicated time on the SOAR Telescope, its location in the southern hemisphere, and the precision and stability of the Goodman spectrograph, were all important aspects of this discovery,” says Swihart.
“This is a great example of how mid-sized telescopes in general, and SOAR in particular, can be used to help characterize unusual discoveries made with other ground and space-based facilities,” notes Chris Davis, NOIRLab Program Director at US National Science Foundation. “We anticipate that SOAR will play a crucial role in the follow-up of many other time-variable and multi-messenger sources over the coming decade.”
The light from the proto-white dwarf partner is Doppler shifted alternately to the red and blue, indicating that it orbits a compact, massive neutron star every 15 hours, according to the binary system’s optical spectrum observed by the Goodman spectrograph.
“The spectra also allowed us to constrain the approximate temperature and surface gravity of the companion star,” says Swihart, whose team was able to take these properties and apply them to models describing how binary star systems evolve.
They were able to calculate that the companion is the forerunner to an exceptionally low-mass white dwarf, with a surface temperature of 8200 °C (15,000 °F) and a mass of only 17% of the Sun’s.
When a star with a mass equal to or less than that of the Sun reaches the end of its life, it will run out of hydrogen, which is used to power the nuclear fusion processes at its core. Helium takes over and fuels the star for a period, forcing it to constrict and heat up, leading it to expand and evolve into a red giant with a diameter of hundreds of millions of kilometers.
The outer layers of this bloated star can eventually be accreted onto a binary partner, and nuclear fusion ceases, leaving a white dwarf the size of Earth that sizzles at temperatures surpassing 100,000 °C (180,000 °F).
The proto-white dwarf in the 4FGL J1120.0-2204 system hasn’t finished evolving yet.
“Currently it’s bloated, and is about five times larger in radius than normal white dwarfs with similar masses,” says Swihart. “It will continue cooling and contracting and, in about two billion years, it will look identical to many of the extremely low mass white dwarfs that we already know about.”
Every second, millisecond pulsars whirl hundreds of times. They are an accreting matter from a companion, in this case, the star that became a white dwarf, to spin them up.
When the pulsar wind, which is a stream of charged particles emerging from the rotating neutron star, collides with material ejected from a partner star, most millisecond pulsars emit gamma rays and X-rays.
There are about 80 extremely low-mass white dwarfs known, but Swihart claims that “this is the first precursor to an extremely low-mass white dwarf found that is likely orbiting a neutron star.”
As a result, 4FGL J1120.0-2204 offers a unique perspective on the last stages of the spin-up process. All of the other discovered white dwarf-pulsar binaries are well past the spinning-up stage.
“Follow-up spectroscopy with the SOAR Telescope, targeting unassociated Fermi gamma-ray sources, allowed us to see that the companion was orbiting something,” says Swihart. “Without those observations, we couldn’t have found this exciting system.”
