NGC 5907 ULX is the brightest pulsar ever observed. 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 (galaxy and foreground stars). The inset shows the
X-ray pulsation of the spinning neutron star.Credit: ESA/XMM-Newton;
NASA/Chandra and SDSS
There's a new record holder for brightest pulsar ever found -- and
astronomers are still trying to figure out how it can shine so brightly.
It's now part of a small group of mysterious bright pulsars that are
challenging astronomers to rethink how pulsars accumulate, or accrete,
material.
A pulsar is a spinning, magnetized neutron star that sweeps regular
pulses of radiation in two symmetrical beams across the cosmos. If
aligned well enough with Earth, these beams act like a lighthouse beacon
-- appearing to flash on and off as the pulsar rotates. Pulsars were
previously massive stars that exploded in powerful supernovae, leaving
behind these small, dense stellar corpses.
The brightest pulsar, as reported in the journal Science,
is called NGC 5907 ULX. In one second, it emits the same amount of
energy as our sun does in three-and-a-half years. The European Space
Agency's XMM-Newton satellite found the pulsar and, independently,
NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) mission also
detected the signal. This pulsar is 50 million light years away, which
means its light dates back to a time before humans roamed Earth. It is
also the farthest known neutron star.
"This object is really challenging our current understanding of the
accretion process for high-luminosity pulsars," said Gian Luca Israel,
from INAF-Osservatorio Astronomica di Roma, Italy, lead author of the
Science paper. "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."
The previous record holder for brightest pulsar was reported in October 2014.
NuSTAR had identified M82 X-2, located about 12 million light-years
away in the "Cigar Galaxy" galaxy Messier 82 (M82), as a pulsar rather
than a black hole. The pulsar reported in Science, NGC 5907 ULX, is 10
times brighter.
Another extremely bright pulsar, the third brightest known, is called
NGC 7793 P13. Using a combination of XMM-Newton and NuSTAR, one group of
scientists reported the discovery in the Astrophysical Journal Letters, while another used XMM-Newton to report it in the Monthly Notices of the Royal Astronomical Society.
Both studies were published in October 2016. Scientists call three
extremely bright pulsars "ultraluminous X-ray sources" (ULXs). Before
the 2014 discovery, many scientists thought that the brightest ULXs were
black holes.
"They are brighter than what you would expect from an accreting black
hole of 10 solar masses," said Felix Fuerst, lead author of the
Astrophysical Journal Letters study based at the European Space
Astronomy Center in Madrid. Fuerst did this work while at Caltech in
Pasadena, California.
How these objects are able to shine so brightly is a mystery. The
leading theory is that these pulsars have strong, complex magnetic
fields closer to their surfaces. A magnetic field would distort the flow
of incoming material close to the neutron star. This would allow the
neutron star to continue accreting material while still generating high
levels of brightness.
It could be that many more ULXs are neutron stars, scientists say.
"These discoveries of 'light,' compact objects that shine so brightly, is revolutionizing the field," Israel said.
NuSTAR is a Small Explorer mission led by Caltech and managed by NASA's
Jet Propulsion Laboratory, Pasadena, California, for NASA's Science
Mission Directorate in Washington. NuSTAR was developed in partnership
with the Danish Technical University and the Italian Space Agency (ASI).
The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia.
NuSTAR's mission operations center is at UC Berkeley, and the official
data archive is at NASA's High Energy Astrophysics Science Archive
Research Center. ASI provides the mission's ground station and a mirror
archive. JPL is managed by Caltech for NASA.
Source: NuSTAR/Caltech/News
