In the nearby
Whirlpool galaxy and its companion galaxy, M51b, two supermassive black holes
heat up and devour surrounding material. These two monsters should be the most
luminous X-ray sources in sight, but a new study using observations from NASA's NuSTAR
(Nuclear Spectroscopic Telescope Array) mission shows that a much smaller object
is competing with the two behemoths.
The most
stunning features of the Whirlpool galaxy - officially known as M51a - are the
two long, star-filled "arms" curling around the galactic center like
ribbons. The much smaller M51b clings like a barnacle to the edge of the
Whirlpool. Collectively known as M51, the two galaxies are merging.
At the center
of each galaxy is a supermassive black hole millions of times more massive than
the Sun. The galactic merger should push huge amounts of gas and dust into those
black holes and into orbit around them. In turn, the intense gravity of the
black holes should cause that orbiting material to heat up and radiate, forming
bright disks around each that can outshine all the stars in their galaxies.
But neither
black hole is radiating as brightly in the X-ray range as scientists would expect
during a merger. Based on earlier observations from satellites that detect
low-energy X-rays, such as NASA's Chandra X-ray
Observatory, scientists believed that layers of gas and dust around
the black hole in the larger galaxy were blocking extra emission. But the new
study, published in the Astrophysical Journal, used NuSTAR's high-energy X-ray
vision to peer below those layers and found that the black hole is still dimmer
than expected.
"I'm
still surprised by this finding," said study lead author Murray Brightman,
a researcher at Caltech in Pasadena, California. "Galactic mergers are
supposed to generate black hole growth, and the evidence of that would be strong
emission of high-energy X-rays. But we're not seeing that here."
Brightman thinks
the most likely explanation is that black holes "flicker" during
galactic mergers rather than radiate with a more or less constant brightness
throughout the process.
"The
flickering hypothesis is a new idea in the field," said Daniel Stern, a
research scientist at NASA's Jet Propulsion Laboratory in Pasadena and the project
scientist for NuSTAR. "We used to think that the black hole variability
occurred on timescales of millions of years, but now we're thinking those timescales
could be much shorter. Figuring out how short is an area of active study."
Along with
the two black holes radiating less than scientists anticipated in M51a and M51b,
the former also hosts an object that is millions of times smaller than either
black hole yet is shining with equal intensity. The two phenomena are not
connected, but they do create a surprising X-ray landscape in M51.
The small
X-ray source is a neutron star, an incredibly dense nugget of material left
over after a massive star explodes at the end of its life. A typical neutron
star is hundreds of thousands of times smaller in diameter than the Sun - only
as wide as a large city - yet has one to two times the mass. A teaspoon of
neutron star material would weigh more than 1 billion tons.
Despite their
size, neutron stars often make themselves known through intense light
emissions. The neutron star found in M51 is even brighter than average and belongs
to a newly discovered class known as ultraluminous neutron stars. Brightman
said some scientists have proposed that strong magnetic fields generated by the
neutron star could be responsible for the luminous emission; a previous paper
by Brightman and colleagues about this neutron star supports that hypothesis. Some
of the other bright, high-energy X-ray sources seen in these two galaxies could
also be neutron stars.
NuSTAR is a
Small Explorer mission led by Caltech and managed by JPL 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 Corporation in Dulles, Virginia (now part of
Northrop Grumman). 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. Caltech manages JPL for NASA
News Media Contact
Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov
Source: NuSTAR/News
