ESA’s XMM-Newton mission has spotted a pulsar — a rapidly rotating, magnetized neutron star that sweeps regular pulses of radiation in two symmetrical beams across the cosmos — that is about 1,000 times brighter than previously thought possible. The discovery is reported Feb. 20, 2017 in the online edition of the journal Science.
NGC 5907 X-1 is in the spiral galaxy NGC 5907, which is also known as the Knife Edge Galaxy or Splinter Galaxy. This image comprises X-ray emission data (blue/white) from ESA’s XMM-Newton space telescope and NASA’s Chandra X-ray observatory, and optical data from the Sloan Digital Sky Survey. The inset shows the X-ray pulsation of the spinning neutron star, which has a period of 1.13 sec, as determined by XMM-Newton’s European Photon Imaging Camera. Image credit: ESA / XMM-Newton / NASA / Chandra / SDSS.
The pulsar in question is called NGC 5907 X-1.
This X-ray source is located in the spiral galaxy NGC 5907, which is also known as the Knife Edge Galaxy or Splinter Galaxy.
NGC 5907 X-1 is the most luminous of its type detected to date: it is 10 times brighter than the previous record holder.
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,” said Dr. Gian Luca Israel, from INAF-Osservatorio Astronomica di Roma in Italy.
At about 50 million light-years away, NGC 5907 X-1 is also the most distant of its kind ever detected.
XMM-Newton observed this source several times in the last 13 years, with the discovery a result of a systematic search for pulsars in the data archive — its 1.13 sec periodic pulses giving it away.
The signal was also identified in NASA’s NuSTAR archive data, providing additional information.
The NuSTAR data also revealed that the pulsar’s spin rate has changed over time, from 1.43 sec per rotation in 2003 to 1.13 sec in 2014.
The same relative acceleration in Earth’s rotation would shorten a day by 5 hours in the same time span.
“Only a neutron star is compact enough to keep itself together while rotating so fast,” Dr. Israel said.
Although it is not unusual for the rotation rate of a 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,” Dr. Israel said.
“It is 1,000 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.”
Dr. Israel and his colleagues 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 NGC 5907 X-1, 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,” said Dr. Norbert Schartel, ESA’s XMM-Newton project scientist.
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Gian Luca Israel et al. An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907. Science, published online February 20, 2017; doi: 10.1126/science.aai8635
