Studied by the SOAR telescope operated by NOIRLab, the binary system is the first to be found at penultimate stage of its evolution.
Using the 4.1-meter SOAR telescope in Chile, astronomers have discovered the first example of a binary system where a star about to become a white dwarf orbits a neutron star that has just finished transforming into a rapidly spinning pulsar. The pair, originally detected by Fermi Gamma Ray Space Telescope, is a “missing link” in evolution of these binary systems.
A bright, mysterious source of gamma rays has been discovered to be a rapidly spinning neutron star – dubbed a millisecond pulsar – that orbiting a star in process of evolving into extremely low mass white dwarf. These types of binary systems are called “spider” by astronomers because the pulsar tends to “eat” the outer parts of companion star as it transforms into a white dwarf.
The duo was detected by astronomers using 4.1-meter SOAR telescope on Cerro Pachón in Chile, part of the Cerro Tololo Inter-American Observatory, a program of NSF’s NOIR Lab as published in The Astrophysical Journal.
NASA’s Fermi Gammaray Space Telescope has listed objects in the Universe that produce copious gamma rays since its launch in 2008, but not all of the sources of gamma rays it detects have been classified. One of these sources, named 4FGL J1120.0-2204 by astronomers, was the second-brightest gamma-ray source in the entire sky that had not been identified until now.
Astronomers from the US & Canada, led by Samuel Swihart of US Naval Research Laboratory in Washington, D.C, used the Goodman spectrograph on the SOAR telescope to determine the true identity of 4FGL J1120.0-2204. The gamma ray source, which also emits x-rays, as observed by NASA’s Swift & ESA’s XMM Newton space telescopes, was found to be a binary system consisting of a “millisecond pulsar” that spins hundreds of times per sec & precursor to a white dwarf of extremely low mass. The pair are located about 2,600 light years.
“Michigan State University’s time on SOAR telescope, its southern hemisphere location, and the accuracy & stability of the Goodman spectrograph were important aspects of this discovery,” says Swihart.
“This is a prime example of how mid size telescopes in general & SOARs in particular, can be used to help characterize unusual discoveries made with other ground & space based facilities,” notes Chris Davis, program director of NOIR Lab in United States National Science Foundation. “We anticipate that SOAR will play a critical role in follow up many other time-varying & multi-message sources over the next decade.
The optical spectrum of the binary system measured by the Goodman spectrograph showed that the light of the proto-white dwarf companion is Doppler shifted – shifted alternately to red & blue – indicating that it orbits compact, big neutron star every 15hrs.
“The spectra also allowed us to limit the approximate temperature & surface gravity of companion star” says Swihart, whose team was able to take these properties & apply them to models that describe how binary star system evolve. This allowed them to determine that companion is the precursor of an extremely low mass white dwarf with a surface temperature of 8,200 ° C and a mass of only 17% that of the Sun.
When a star with a mass similar to that of the Sun or less reaches end of its life, it will run out of hydrogen used to fuel nuclear fusion processes in its core. For a time, helium takes over & powers the star, causing it to contract & heat up and causing it to expand & evolve into a red giant hundreds of millions of kilometers in size.
Eventually, the outer layers of this swollen star may be accreted onto a binary companion & nuclear fusion is disrupted, leaving behind an Earth-sized white dwarf and sizzling at temperatures above 100,000°C (180,000 °F).
The proto-white dwarf in 4FGL J1120.0-2204 system has not yet finished evolving. “It is currently bloated and has a radius about 5 times the size of normal white dwarfs with similar masses,” explains Swihart. “It will continue to cool & contract, and in about 2 billion years it will look like many of extremely low mass white dwarfs that we already know.
Millisecond pulsars spin hundreds of times every second. They are spun by accerting matter from a companion, in this case from star-turned-white-dwarf star. Most millisecond pulsars emit gamma rays & X-rays, often when pulsar wind, which is a stream of charged particles emanating from spinning neutron star, collides with material emitted by a companion star.
About 80 extremely low-mass white dwarfs are known, but “this is the first precursor of an extremely low-mass white dwarf found that is orbiting a neutron star,” says Swihart. Therefore, 4FGL J1120.0-2204 is a unique look at tail end of this spinning process. All of the other white dwarf-pulsar binaries that have been discovered are well past spinning up stage.
“Follow-up spectroscopy with the SOAR telescope, aiming at unassociated Fermi gamma sources, allowed us to see that the companion was orbiting something,” says Swihart. “Without these observations, we could not have found this exciting system.
