A Luminous Pulsar

Combing through 13 years of data from two separate missions, scientists have found a pulsar 50 million light-years away. Not only is NGC 5907 X-1 (astronomers are so creative at naming objects) the furthest pulsar ever detected, it’s also the brightest. In fact, the newly-discovered pulsar is a thousand times brighter than previously thought possible!

Pulsars are rapidly spinning, magnetized neutron stars. Once upon a time, they were massive stars that ended their lives by exploding as powerful supernovae, leaving behind small and extraordinarily dense stellar corpses—neutron stars! Because angular momentum is conserved, the resulting star spins very fast, its magnetic field creating two symmetrical beams of radiation sweeping across the cosmos. If suitably aligned with our planet, these beams are like a lighthouse beacon appearing to pulse on and off as it rotates (hence the name “pulsar,” for “pulsating star”).

ESA’s XMM-Newton observed the object several times since 2003, and the discovery a result of a systematic search for pulsars in the data archive. The pulsar’s 1.13 per second periodic pulses gave it away and the signal was also identified in NASA’s more recent NuSTAR archive data. This X-ray source is the most luminous of its type detected to date—in one second it emits the same amount of energy released by our Sun in 3.5 years.

“Before, it was believed that only black holes at least 10 times more massive than our sun feeding off their stellar companions could achieve such extraordinary luminosities, but the rapid and regular pulsations of this source are the fingerprints of neutron stars and clearly distinguish them from black holes,” says Gian Luca Israel, from INAF-Osservatorio Astronomico di Roma, Italy, lead author of the study, published in Science this week.

The archival data also show that the pulsar’s spin rate has changed over time, from 1.43 seconds per rotation in 2003 to 1.13 seconds in 2014. (If Earth slowed down that much, our day would by a full five hours in the same time span.) “Only a neutron star is compact enough to keep itself together while rotating so fast,” explains Gian Luca.

Although it is not unusual for the rotation rate of a pulsar or neutron star to change, the high rate of change in this case is likely linked to the object rapidly consuming mass from a companion. “This object is really challenging our current understanding of the accretion process for high-luminosity stars,” says Gian Luca. “It is one thousand times more luminous than the maximum thought possible for an accreting neutron star, so something else is needed in our models in order to account for the enormous amount of energy released by the object.”

The scientists think there must be a strong, complex magnetic field close to its surface, such that accretion onto the neutron star surface is still possible while still generating the high luminosity.

“The discovery of this very unusual object, by far the most extreme ever discovered in terms of distance, luminosity and rate of increase of its rotation frequency, sets a new record for XMM-Newton, and is changing our ideas of how such objects really work,” says Norbert Schartel, ESA’s XMM-Newton project scientist.