A bizarre pulsar alternates between two modes of brightness. Astronomers have finally figured out why.

Pulsars are the lighthouses of the universe.

These rotating dead stars shoot out two beams of radiation from their poles, usually at a predictable rhythm. But sometimes pulsars behave strangely, and one pulsar in particular has puzzled astronomers for years.

Called PSR J1023+0038, a decade ago it shut down its jets and began oscillating between two levels of brightness in an unpredictable pattern.

Now scientists think they understand why: it’s busy eating a neighboring star.

When a supergiant nears the end of its life, it explodes and collapses into a back hole if it has enough mass, or into a neutron star if it doesn’t have enough mass.

Neutron stars are the remaining, ultradense cores of old stars.

They often spin very quickly, and a subset of them become pulsars. PSR J1023+0038 began living this way, and when it was discovered in 2007 it behaved like a normal pulsar.

But it didn’t stop there.

In 2013 something changed. The radio pulses – evidence of the two lighthouse beams – were turned off.

There was a sudden explosion of energy at multiple wavelengths: gamma-rays and X-rays increased by a factor of five, and in visible light the star became brighter by a factor of one to two.

Astronomers also discovered that an accretion disk appeared to have formed: a hot, swirling mass of material surrounding the star.

Perhaps strangest, the star began to cycle between two intensities in the X-ray wavelength range: a high mode and a low mode, and has done so for the full decade since then.

It spends about 70% of its time in high mode, the rest in low mode, switching between the two every few seconds or minutes on an unpredictable schedule.

Recently, astronomers have come up with an ambitious plan to find out what’s going on.

“Our unprecedented campaign of observations to understand the behavior of this pulsar involved a dozen state-of-the-art ground-based and space-based telescopes,” says Francesco Coti Zelati of the Institute of Space Sciences in Barcelona.

Covering a wide spectrum of electromagnetic wavelengths, these telescopes allowed astronomers to reconstruct what was happening.

Here’s what they found. The accretion disk consists of matter pulled away from the pulsar’s neighboring star. As this matter approaches the pulsar and begins to accumulate, it is heated by the solar wind.

The matter begins to glow in X-ray, UV, and visible light, and the hot, glowing material is what astronomers have seen as the pulsar’s “high mode.” Eventually, however, a process occurs in which the matter is ejected at high energy, escaping perpendicular to the accretion disk in the direction of the pulsar’s jets.

“Enormous amounts of matter, similar to cosmic cannonballs, are thrown into space in a very short span of tens of seconds,” says Maria Cristina Baglio of New York University Abu Dhabi and Italy’s National Institute for Astrophysics.

This violent ejection causes the pulsar to return to its “low mode” after removing the heated material from its surroundings.

The cycle then repeats itself.

Through an incredible collaborative effort by astronomers around the world, and using the finest instruments known to mankind, the mystery of PSR J1023+0038 has been solved.

Lessons learned from this strange pulsar have taught us more about the physics of accretion, and this knowledge can now be applied in studying other unexplained variable phenomena, including the accretion disks of some black holes.

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Written by Scott Alan Johnston/ Universe Today.