The “Spider” star system finally seen in the penultimate phase
Spiders in space? The real scary critters have made the journey beyond our atmosphere, but that’s what happens when a star turns into a spider.
Astronomers have discovered the first example of a binary star system called a “spider” in a phase that had never been observed before. This system is 2,600 light years away and is thought to include a millisecond pulsar (ridiculously fast spinning neutron star). Orbiting each other are the supposed pulsar and a dying star slowly transforming into a white dwarf. Like a spider spinning silk around prey and sucking out its life, the pulsar of this death spiral devours the gaseous outer layers of its trapped victim.
This system is also the source of gamma rays whose origin was until now a mystery. US Naval Research Laboratory astronomer Sam Swihart and his research team used the SOAR telescope in Chile to capture this phenomenon at a near-end phase. The star that is currently turning into a white dwarf will actually become one once it has contracted enough, now that the pulsar has eaten away the outside. Swihart conducted a recently published study on the arXiv preprint server.
“We found this binary system in kind of the penultimate stage of its evolution,” he told SYFY WIRE. “We have observed many end-stage systems but not in this penultimate stage, so finding this system is a missing link in our understanding of the binary evolution of millisecond pulsars.”
It was possible to tell that the proto-white dwarf was orbiting something huge without actually seeing what it was orbiting. As an object moves towards the observer, the wavelengths of light shorten, moving into the ultraviolet range (blue shift), while the wavelengths lengthen and shift towards the ultraviolet. infrared when this object moves away (redshift). This was visible in the absorption lines of the white dwarf. Absorption lines appear in a spectrum when there is an absorbing material, such as a cloud of gas, between the light source and the observer.
Further measurements of radial velocity – the speed from the observer’s line of sight – revealed that the white dwarf orbited an invisible object every 15 hours or so. The height of the radial velocity curve ultimately revealed the mass of this object. It also spit out gamma rays nonstop, which made it most likely a neutron star and even more likely to be a millisecond pulsar, since all millisecond pulsars emit gamma rays. Observations by NASA’s Swift and ESA’s XMM-Newton space telescopes revealed that it also emitted X-rays.
“We only know about 5 extremely low-mass (ELM) white dwarfs with these short orbital periods (about a day or less) around a confirmed millisecond pulsar,” Swihart said. “The main difference between those systems and this one is that the companion radius in this system is about 5 times larger.”
Swihart and his team were able to find out the white dwarf’s orbital period, surface gravity, temperature and radius to determine how long it took to evolve and ultimately how old it is. This is also how they realized that the white dwarf was on fire and still contracting. It will take two billion years to finish evolving and end up like the other ELM white dwarfs in “spider” systems. If the gamma rays are really coming from a millisecond pulsar, it could mean there are other undiscovered spiders lurking in the dark.
“This system is relatively close, so confirmation of a millisecond pulsar would provide strong evidence that the census of millisecond pulsars is far from complete, even relatively close to the Sun,” Swihart said.