The XMM-Newton X-ray space observatory (also called X-ray Multi-Mirror) from ESA has found a pulsar - the swirling remains of a once-massive star - that is a thousand times brighter than was thought possible. The pulsar, identified as NGC 5907 X-1 is also the most distant of its kind ever detected, with its light travelling 50 million light years before being detected by XMM-Newton.
Pulsars are spinning magnetized neutron stars that sweep regular pulses of radiation in two symmetrical beams across the cosmos. If properly aligned with the Earth these beams are like a beacon beacon appearing to blink on and off as it spins. They were once massive stars that exploded in a powerful supernova at the natural end of their life, before they...become extraordinarily dense little stellar corpses...
This X-ray source is the most luminous of its kind detected so far: 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! Ultraluminous X-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our galaxy. ULXs are usuallydescribed by scientific models as stellar-mass black holes (BHs) that accrete at very high rates or BHs of intermediate mass.
The XMM-Newton probe has observed the object several times over the past 13 years, with this discovery being the result of a systematic search for pulsars in the data archive - its periodic 1.13 second pulses revealed it. The signal was also identified in NASA's NuSTAR data archive, providing additional information.
The record-breaking pulsar, identified as NGC 5907 X-1, is in the spiral galaxy NGC 5907, which is also known as the Knife Blade Galaxy or Shard Galaxy. The image comprises X-ray emission data (blue/white) from ESA's XMM-Newton Space Telescope and NASA's Chandra X-ray observatory, plus optical data from the Sloan Digital Sky Survey (galaxy and stars in firstThe inset shows the spinning neutron star's X-ray pulse, which has a period of 1.13s, as determined by the XMM-Newton European Photon Imaging Camera. Credits: ESA/XMM-Newton; NASA/Chandra and SDSS
"Ultraluminous X-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our galaxy. ULXs are usually modeled as stellar-mass black holes (BH's) that accrete at very high rates or BH's of intermediate mass. We present observations showing that the NGC 5907 ULX is instead an X-ray neutron star(NS) with accretion and a rotation period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014.It has a peak isotropic luminosity of 1000 times the Eddington limit For a NS at 17.1 megaparsec.Standard accretion models cannot explain its luminosity, even assuming beam emission, but a strong multipolar magnetic field can describe its properties.These findingssuggest that other extreme ULX's (X-ray luminosity ≥ 1041 erg second-1) may harbor NS's."
Previously, it was believed that only black holes at least 10 times more massive than the Sun feeding on 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, of INAF-Osservatorio Astronomica diRome, Italy, lead author of the paper describing the study was published in the Science this week.
The archival data also revealed that the pulsar's rotation rate changed over time, from 1.43 s per rotation in 2003 to 1.13 s in 2014. The same relative acceleration in Earth's rotation would shorten a day by five hours in the same time period. "Only a neutron star is compact enough to hold together while spinning so fast," Gian Luca adds.
Although it is not uncommon for the rotation rate of a neutron star to change, the high rate of change in this case is probably linked to the object rapidly consuming mass from a companion.
"This object is really a challenge to our current understanding of the "accretion" process for high luminosity stars," says Gian Luca. "It is 1000 times more luminous than what was thought to be the maximum possible for a mass accreting neutron star, so something more is needed in our models in order to explain the huge amount of energy released by the object."
Scientists think that this pulsar must possess a strong and complex magnetic field near its surface, such that accretion on 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 in its rotational frequency, sets a new record for XMM-Newton, and is changing our ideas of how these objects really 'work,'" says Norbert Schartel, ESA's XMM-Newton project scientist.Source G. L. Israel et al, "An accretingpulsar with extreme properties drives an ultraluminous X-ray source in NGC 5907" in the Science (February 20, 2017) DOI: 10.1126/science.aai8635;
ESA website; "The brightest furthest pulsar in the Universe" . Accessed 21 February 2017.