February 25, 2014
By observing a gamma ray source named PSR J1311-3430 with optical telescope, astronomers find a black widow binary system with number of interesting and record-setting features – the tightest binary pulsar-small star system, one of the heaviest pulsars and first millisecond pulsar detected without the use of radio telescopes.
Pulsar with both gamma rays (magenta) and radio waves (green) next to its companion star in black widow binary system PSR J1311-3430. Credit: NASA's Goddard Space Flight Center
Normally when detecting pulsars, a rotating neutron star that emits a beam of electromagnetic radiation, astronomers use both gamma ray and radio telescopes. First, gamma ray sources are targeted with telescopes like Fermi Gamma-ray Space Telescope. Pulsars really stand out in gamma ray part of the electromagnetic spectrum, but without additional data about the source, it is difficult to confirm that the radiation is caused by a pulsar. This is why ground-based radio telescopes are then used to confirm that the source really is a pulsar.
However, there are gamma ray sources that apparently don't emit radio signals. When looking at one such source, PSR J1311-3430, Roger Romani from Lavli Institute for Particle Astrophysics and Cosmology in Menlo Park, California, USA, decided to take a look at it in visible light. What he discovered was fascinating.
Very close to PSR J1311-3430 there is a stellar object that appears to change its colour from red to blue and red again in little over 90 minutes. Stars that shine blue are considered hot compared to their red counterparts. This would indicate that this particular star changes its temperature in very short period or more accurately – one side of the star is heated to 12,000 degrees Celsius while its other side is at 2,700 degrees Celsius and in 93 minutes we see them interchangeably.
This is because the small star, 12-17 times the mass of Jupiter and only 60% the size, revolves very fast around a pulsar in a very tight system known as black widow binary system. Its partner is a millisecond pulsar no bigger than a city, but with mass twice the mass of the Sun. For neutron stars, this is one of the highest masses observed and the distance between two stars is similar to that of Earth and the Moon, making it the tightest such orbit discovered.
The binary system where a small star orbits a millisecond pulsar at very small distances is called the black widow binary because this setting has catastrophic consequences for the small star, similar to the fate of male black widow spider. Millisecond pulsars are pulsars that rotate up to 43,000 times per minute. Normally, pulsars lose the momentum with time and slow down, but millisecond pulsars have extremely high rotation rates for very long time because they are fueled by their companion stars. You can check more about neutron stars, pulsars and mechanics behind their behaviour here.
"The high-energy emission and wind from the pulsar basically heats and blows off the normal star's material and, over millions to billions of years, can eat away the entire star," says Alice Harding, of NASA's Goddard Space Flight Center in Greenbelt, USA. "These systems can completely consume their companion stars, and that's how we think solitary millisecond pulsars form."
In this case, by blowing away its companion and using its material to keep momentum, the pulsar has managed to speed up to more than 23,000 rotations per minute. Black widow systems also give us the opportunity to observe the effects the pulsar has on its companion star. It was previously thought that, by bombarding the star with high-energy particles, pulsar heats it uniformly, but as the PSR J1311-3430 shows, one side actually gets heated almost 4 times as much as the other.
Helped by this optical observation, which made the narrowing of search parameters possible, an international team led by Holger Pletsch, Albert Einstein Institute, Hannover, Germany, checked huge amounts of Fermi data and found pulsed gamma rays and confirmed PSR J1311-3430 as a pulsar. It is the first millisecond pulsar found without radio observatories.
"This was the first time a millisecond pulsar has ever been detected solely by pulsed gamma rays," Pletsch said. "Seeing this signal was extremely exciting and satisfying for our team, and it's another triumph for Fermi."
The radio signal was finally picked up by the Green Bank Telescope, but its intervals are irregular. "The pulsar heating is ablating its companion, literally blowing it away, so ionized gas fills the system, and this scatters or absorbs the radio emission most of the time," said Paul Ray, Naval Research Laboratory, Washington, USA. Unlike high-energy gamma rays, radio waves aren't able to penetrate said ionized gas which made this discovery difficult.
Here's a 10-minute video with comments from the astronomers involved, credited to NASA's Goddard Space Flight Center: