The densest objects in the universe have a new heavyweight champion: a pulsar so small that it could nearly fit onto the island of Manhattan but weighs in at 2.04 times the mass of the sun. The pulsar could be used to help test Einstein鈥檚 theory of general relativity 鈥� but its very existence could put that theory in jeopardy.
Pulsars are rapidly spinning stellar corpses that sweep the sky with a lighthouse-like beam of radio waves as they spin. The fastest pulsars are in binary systems with another object, like a star or a white dwarf. The pulsar quickens its spin by stealing material from its companion. This arrangement can continue for billions of years before the two objects collide and merge.
According to Einstein鈥檚 theory of general relativity, which describes how gravity works, the two bodies excite strong ripples in spacetime 鈥� gravitational waves 鈥� as they spiral towards each other. Although gravitational waves haven鈥檛 been directly observed yet, we have strong evidence that they exist. Studying pulsar binary systems could be a good way to observe them and check that the predictions of general relativity are true.
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Enter J0348+0432, newly crowned as the heaviest pulsar yet recorded. The pulsar showed up in a blind survey conducted at the in West Virginia, when the telescope was partially shut down for repairs. For months, the 100-metre wide dish couldn鈥檛 be steered.
鈥淏ut the telescope could just point straight up and let the sky drift overhead, so we thought, why not?鈥� says of McGill University in Montreal, Canada, who presented the new discovery at the in Cambridge, Massachusetts, this week.
Heavy weight
She and her colleagues identified the pulsar by the radio pulses it emitted every 39 milliseconds. It is in a tight binary orbit with a white dwarf, a less-dense type of stellar corpse, which weighs 0.172 times the mass of the sun. Comparing the way both objects wobbled on their orbits let the team calculate the mass of the pulsar itself: an unprecedented 2.04 times the mass of the sun. The previous record holder was a relatively svelte 1.97 solar masses.
The pair is a particularly good lab for testing general relativity because of the disparity between the pulsar and white dwarf鈥檚 masses, Kaspi says. In some alternate theories of gravity that could take over if general relativity turns out to be wrong, the neutron star could have gravitational effects going on inside it that the relatively willowy white dwarf would not. Those effects could warp gravity and give off extra gravitational waves, making the distance between the two objects shrink faster 鈥� an effect that astronomers could observe even without seeing the gravitational waves directly.
鈥淵ou can start to constrain gravity in a totally new regime,鈥� Kaspi said.
But the pulsar鈥檚 extra bulk could be a problem for general relativity all by itself. Pulsars pack their sun-like masses into a ball no more than 20 or 24 kilometres across. Theories of how atoms collapse in such tight quarters predict that neutron stars can鈥檛 grow much heavier than twice the mass of the sun, or they鈥檒l be forced to collapse into a black hole.
鈥淚f the next record holder turns out to be significantly above 2 solar masses, then we鈥檙e going to have to go back to the drawing board鈥� possibly thinking of modifications to general relativity,鈥� says of the University of Arizona, Tucson. Ozel is waiting for the uncertainty about the new pulsar鈥檚 mass to be nailed down more before she gets worried, but 鈥�2.04 solar masses is getting to the point that we鈥檇 have to check everything鈥�.
