Pulsars are some of the most extreme and fascinating objects in the universe, and NASA’s Fermi Space Telescope has just unlocked a new way to study them. Using the orbiting observatory, astronomers have identified the first gamma-ray eclipses in “spider systems,” consisting of a pulsar and a smaller main sequence star. These are so-named as a reference to the arachnid tendency to consume one’s companion, which is what happens in these solar systems, too.
Before Fermi came online in 2008, science knew of just a few pulsars that emitted gamma rays. Today, Fermi has identified more than 300 of them. An international team of experts combed through a decade of Fermi data in search of something specific: a gamma-ray eclipse. The end goal is to accurately calculate the mass and velocity of these extreme stellar remnants, and the eclipses help get us there.
In some alien solar systems, there are two stars that age at very different rates. That can lead to a situation in which the larger of the pair may go supernova while the smaller one is still fusing hydrogen like the sun. This can lead to a spider system — the pulsar feeds off its smaller companion while superheating one side of it. According to NASA, scientists even have sub-categories based on the relationship between the two. A “Black widow” system has a star with less than 5% of the sun’s mass. A “Redback” spider system has a stellar companion weighing between 10% and 50% of a solar mass.
We can characterize spider systems using visible light and radio frequency observations — most of the time. It gets tricky when the plane of the system is aligned with ours. That makes the subtle changes too difficult to detect, but a gamma ray eclipse can reveal the truth. Using Fermi, researchers found seven spider systems exhibiting this phenomenon. Since gamma rays only come from the pulsar, their disappearance in the data indicates the smaller companion has eclipsed the pulsar. Pulsars emit radiation like clockwork, so just a few missing photons is enough to reveal an eclipse. With this data in hand, scientists can calculate the system’s tilt, and from that, its mass and velocity.
Take PSR B1957+20, for example — earlier estimates suggested this Black window system was tilted 65 degrees with a pulsar 2.4 times as massive as the sun. That made it one of the largest known. However, the new study shows this system has a tilt of 84 degrees, meaning the pulsar’s mass is just 1.8 times that of the sun.
The team believes that once the models are fine-tuned, Fermi will be able to answer some nagging questions about spider systems. For example, does the mass stolen from the companions make them the most massive population of pulsars? B1957 ended up smaller than we thought, but it could go the other way just as easily.
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