June 24, 2013
Credit: NASA/CXC/SAO/R.Barnard, Z.Lee et al.
While six human space travelers crank away at their research in the low-Earth-orbit laboratory spaces of the International Space Station, a pair of aging robotic “great observatories” designed to examine the Universe from beyond the obscuring screen of Earth's atmosphere continue to fine-tune mankind's understanding of the cosmos. The Chandra X-ray Observatory has reaped an unprecedented harvest of potential black holes in the nearby Andromeda Galaxy, and at the same time has added to the database scientists are using to work out how stellar mass black holes produce the high-energy light that signals their presence. Meanwhile, the Hubble Space Telescope is shaking up theories of how planets form around stars with observations of a protoplanetary disk around the red dwarf star TW Hydrae.
Over the past 13 years, astronomers using the Chandra to look for the X-ray signatures of stellar mass black holes, formed when massive stars collapse into themselves, have found 35 of them in Andromeda. The galaxy is the closest to our Milky Way, but has a different central bulge that holds room for more of the stellar mass black holes, including seven within 1,000 light years of Andromeda's center (illustrated by the dotted circle in this composite X-ray image of Andromeda and its potential stellar mass black holes).
“When it comes to finding black holes in the central region of a galaxy, it is indeed the case where bigger is better,” say Stephen Murray of Johns Hopkins University and the Harvard-Smithsonian Center for Astrophysics (CfA). “In the case of Andromeda, we have a bigger bulge and a bigger supermassive black hole than in the Milky Way, so we expect more smaller black holes are made there as well.”
Not that all of those black holes are visible, since most don't have the companion X-ray sources visible in the 150 Chandra observations that went into the 13-year search, according to Robin Barnard of CfA. “While we are excited to find so many black holes in Andromeda, we think it's just the tip of the iceberg,” Barnard says.
Data from Chandra and other X-ray space telescopes, such as the European Space Agency's XMM-Newton X-ray Observatory, help researchers at NASA, Johns Hopkins and the University of Rochester simulate the process that produces the bright X-rays associated with stellar mass black holes.
According to a paper published in The Astrophysical Journal, black holes generate X-rays when gas falling toward a black hole spirals like water going down a drain and heats up as it is compressed. Simulations run on the Ranger supercomputer at the Texas Advanced Computing Center validated equations describing the process, including the motion of the gas and its effect on associated magnetic fields as it spins almost as fast as the speed of light. The work modeled both “soft” X-rays, generated by gas heated to 20 million deg. F, and “hard” X-rays emitted when gas is heated 10 to hundreds of times hotter.
At the other end of the temperature spectrum, the Hubble has generated evidence that there may be an infant planet forming in the disk of dust and gas around TW Hydrae, at a distance twice as far from that star as Pluto is from the Sun. The system is only 176 light years from Earth and, at only 5-10 million years old, “in the final throes of planet formation before its disk dissipates,” says Alycia Weinberger of the Carnegie Institution, who led the team that discovered the evidence. The team believes a planet may be accreting in a partial gap in the star's disk material that was spotted about 80 astronomical units out from the star using Hubble observations across the spectrum from visible to near infrared.
The discovery may change theory about how planets form, since the planet—if there is one in the gap—is so distant from its star.