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 stars 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 black holes 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 of 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
neutron stars, 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 massive stars. 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.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for IC 10:
Category: Neutron Stars/X-ray Binaries, Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 00h 20m 23.2s | Dec 59° 17´ 34.7"
Constellation: Cassiopeia
Observation Date: 6 pointings between December 2009 and September 2010
Observation Time: 24 hours 19.5 min
Obs. ID: 11081-11086
Instrument: ACIS
References: Laycock S. et al., 2017, ApJ, 836, 50; arXiv:1611.08611. Laycock S. et al., 2017, ApJ [in press]; arXiv:1701.03803
Color Code: X-ray (Blue); Optical (Red, Green, Blue)Distance Estimate: About 2.2 billion light years
Source: NASA’s Chandra X-ray Observatory
