This
chart illustrates the relative masses of super-dense cosmic objects,
ranging from white dwarfs to the supermassive black holes encased in the
cores of most galaxies.Image Credit: NASA/JPL-Caltech. Full image and caption
The
galaxy Messier 82 (M82) is seen here in two different lights. A
visible-light view from NASA's Hubble Space Telescope is at left, and an
X-ray view from NASA's Chandra X-ray Observatory is at right. Image Credit: NASA/STScI/SAO. Full image and caption
Astronomers
have found a pulsating, dead star beaming with the energy of about 10
million suns. This is the brightest pulsar – a dense stellar remnant
left over from a supernova explosion – ever recorded. The discovery was
made with NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR.
"You might think of this pulsar as the 'Mighty Mouse' of stellar
remnants," said Fiona Harrison, the NuSTAR principal investigator at the
California Institute of Technology in Pasadena, California. "It has all
the power of a black hole, but with much less mass."
The discovery appears in a new report in the Thursday Oct. 9 issue of the journal Nature.
The surprising find is helping astronomers better understand
mysterious sources of blinding X-rays, called ultraluminous X-ray
sources (ULXs). Until now, all ULXs were thought to be black holes. The
new data from NuSTAR show at least one ULX, about 12 million light-years
away in the galaxy Messier 82 (M82), is actually a pulsar.
"The pulsar appears to be eating the equivalent of a black hole
diet," said Harrison. "This result will help us understand how black
holes gorge and grow so quickly, which is an important event in the
formation of galaxies and structures in the universe."
ULXs are generally thought to be black holes feeding off companion
stars -- a process called accretion. They also are suspected to be the
long-sought after "medium-size" black holes – missing links between
smaller, stellar-size black holes and the gargantuan ones that dominate
the hearts of most galaxies. But research into the true nature of ULXs
continues toward more definitive answers.
NuSTAR did not initially set out to study the two ULXs in M82.
Astronomers had been observing a recent supernova in the galaxy when
they serendipitously noticed pulses of bright X-rays coming from the ULX
known as M82 X-2. Black holes do not pulse, but pulsars do.
Pulsars belong to a class of stars called neutron stars. Like black
holes, neutron stars are the burnt-out cores of exploded stars, but puny
in mass by comparison. Pulsars send out beams of radiation ranging from
radio waves to ultra-high-energy gamma rays. As the star spins, these
beams intercept Earth like lighthouse beacons, producing a pulsed
signal.
"We took it for granted that the powerful ULXs must be massive black
holes," said lead study author Matteo Bachetti, of the University of
Toulouse in France. "When we first saw the pulsations in the data, we
thought they must be from another source."
NASA's Chandra X-ray Observatory and Swift satellite also have
monitored M82 to study the same supernova, and confirmed the intense
X-rays of M82 X-2 were coming from a pulsar.
"Having a diverse array of telescopes in space means that they can
help each other out," said Paul Hertz, director of NASA's astrophysics
division in Washington. "When one telescope makes a discovery, others
with complementary capabilities can be called in to investigate it at
different wavelengths."
The key to NuSTAR's discovery was its sensitivity to high-energy
X-rays, as well as its ability to precisely measure the timing of the
signals, which allowed astronomers to measure a pulse rate of 1.37
seconds. They also measured its energy output at the equivalent of 10
million suns, or 10 times more than that observed from other X-ray
pulsars. This is a big punch for something about the mass of our sun and
the size of Pasadena.
How is this puny, dead star radiating so fiercely? Astronomers are
not sure, but they say it is likely due to a lavish feast of the cosmic
kind. As is the case with black holes, the gravity of a neutron star can
pull matter off companion stars. As the matter is dragged onto the
neutron star, it heats up and glows with X-rays. If the pulsar is indeed
feeding off surrounding matter, it is doing so at such an extreme rate
to have theorists scratching their heads.
Astronomers are planning follow-up observations with NASA's NuSTAR,
Swift and Chandra spacecraft to find an explanation for the pulsar’s
bizarre behavior. The NuSTAR team also will look at more ULXs, meaning
they could turn up more pulsars. At this point, it is not clear whether
M82 X-2 is an oddball or if more ULXs beat with the pulse of dead stars.
NuSTAR, a relatively small telescope, has thrown a big loop into the
mystery of black holes.
“In the news recently, we have seen that another source of unusually
bright X-rays in the M82 galaxy seems to be a medium-sized black hole,"
said astronomer Jeanette Gladstone of the University of Alberta, Canada,
who is not affiliated with the study. "Now, we find that the second
source of bright X-rays in M82 isn’t a black hole at all. This is going
to challenge theorists and pave the way for a new understanding of the
diversity of these fascinating objects."