A starburst with the prospect of gravitational waves

Credit: X-ray: NASA/CXC/UMass Lowell/S. Laycock et
al.; Optical: Bill Snyder Astrophotography

In 1887, American astronomer Lewis Swift discovered a glowing
cloud, or nebula, that turned out to be a small galaxy about
2.2 billion light years from Earth. Today, it is known as the
“starburst” galaxy IC 10, referring to the intense star
formation activity occurring there.

More than a hundred years after Swift’s discovery, astronomers
are studying IC 10 with the most powerful telescopes of the
21st century. New observations with NASA’s Chandra X-ray
Observatory reveal many pairs of that may one day become sources of perhaps
the most exciting cosmic phenomenon observed in recent years:
gravitational waves.

By analyzing Chandra observations of IC 10 spanning a decade,
astronomers found over a dozen and neutron stars feeding off gas
from young, massive stellar companions. Such double star
systems are known as “X-ray binaries” because they emit large
amounts of X-ray light. As a massive star orbits around its
compact companion, either a black hole or neutron star,
material can be pulled away from the giant star to form a disk
of material around the compact object. Frictional forces heat
the infalling material to millions of degrees, producing a
bright X-ray source.

When the massive companion star runs out fuel, it will undergo
a catastrophic collapse that will produce a supernova
explosion, and leave behind a black hole or neutron star. The
end result is two compact objects: either a pair of black
holes, a pair of , or a black hole
and neutron star. If the separation between the compact objects
becomes small enough as time passes, they will produce
gravitational waves. Over time, the size of their orbit will
shrink until they merge. LIGO has found three examples of black
hole pairs merging in this way in the past two years.

Starburst galaxies like IC 10 are excellent places to search
for X-ray binaries because they are churning out stars rapidly.
Many of these newly born stars will be pairs of young and
. The most massive of the pair will
evolve more quickly and leave behind a black hole or a neutron
star partnered with the remaining massive star. If the
separation of the stars is small enough, an X-ray binary system
will be produced.

This new composite image of IC 10 combines X-ray data from
Chandra (blue) with an optical image (red, green, blue) taken
by amateur astronomer Bill Snyder from the Heavens Mirror
Observatory in Sierra Nevada, California. The X-ray sources
detected by Chandra appear as a darker blue than the stars
detected in optical light.

The young stars in IC 10 appear to be just the right age to
give a maximum amount of interaction between the massive stars
and their compact companions, producing the most X-ray sources.
If the systems were younger, then the massive stars would not
have had time to go supernova and produce a neutron star or
black hole, or the orbit of the massive star and the compact
object would not have had time to shrink enough for mass
transfer to begin. If the star system were much older, then
both compact objects would probably have already formed. In
this case transfer of matter between the compact objects is
unlikely, preventing the formation of an X-ray emitting disk.



Chandra detected 110 X-ray sources in IC 10. Of these, over
forty are also seen in optical light and 16 of these contain
“blue supergiants”, which are the type of young, massive, hot
stars described earlier. Most of the other sources are X-ray
binaries containing less massive stars. Several of the objects
show strong variability in their X-ray output, indicative of
violent interactions between the compact stars and their
companions.

A pair of papers describing these results were published in the
February 10th, 2017 issue of The Astrophysical Journal and is
available online here and here. The authors of the
study are Silas Laycock from the UMass Lowell’s Center for
Space Science and Technology (UML); Rigel Capallo, a graduate
student at UML; Dimitris Christodoulou from UML; Benjamin
Williams from the University of Washington in Seattle; Breanna
Binder from the California State Polytechnic University in
Pomona; and, Andrea Prestwich from the Harvard-Smithsonian
Center for Astrophysics in Cambridge, Mass.

Explore further:
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299: Galactic Goulash

More information: The X-ray Binary Population of the
Nearby Dwarf Starburst Galaxy IC 10: Variable and Transient
X-ray Sources. arXiv. arxiv.org/abs/1611.08611

Blue Supergiant X-ray Binaries in the Nearby Dwarf Galaxy IC
10. arXiv. arxiv.org/abs/1701.03803

Journal reference: arXiv

Provided by: NASA