The best opportunity to study black holes is when they’re actively accreting matter.
During these times, matter gathers in an accretion disk around the black hole, where it heats up and emits electromagnetic radiation.
At other times, there’s simply no light.
Some stellar mass black holes, black holes with only a few solar masses, are in what astronomers call “x-ray binaries.” These are binary stars where one is called the donor and is often a main sequence stars.
The other in the pair is called the accretor, and can be a neutron star, a white, dwarf, or a stellar mass black hole.
New research published in The Astrophysical Journal Letters presents observations of an x-ray binary named 4U 1630-472.
The observations come from the Japan Aerospace Exploration Agency’s (JAXA) X-Ray Imaging and Spectroscopy Mission (XRISM) satellite and its Resolve instrument. The research is titled “XRISM Spectroscopy of the Stellar-mass Black Hole 4U 1630-472 in Outburst,” and the lead author is Jon Miller from the Department of Astronomy at the University of Michigan.
“We report on XRISM/Resolve spectroscopy of the recurrent transient and well-known black hole candidate 4U 1630−472 during its 2024 outburst,” the researchers write. Rather than finding the simple disk winds detected by other telescopes like Chandra and XMM-Newton, these observations with the XRISM satellite found something much messier.
“Being surprised is good. Seeing expectations proven naive means progress.” – Jon Miller, lead author, University of Michigan.
Astrophysicists are eager to understand when disk winds are launched in stellar mass black holes and what factors affect them. Disk winds act to remove material from a black hole’s accretion disk. While that makes less material available for their growth, the winds and the lost material also remove angular momentum from the disk, which can end up helping remaining matter in the disk spiral more easily into the black hole.
Disk winds also create a negative feedback. The higher the accretion rate, the hotter the disk becomes, which drives stronger winds. Those winds can drive more material away from the disk, helping regulate accretion. Black hole feedback is an important concept in astrophysics, and researchers think that whatever they can learn from stellar mass black holes may also apply to supermassive black holes.
Scientists observe these winds in certain states. “Disk winds are preferentially observed in spectrally soft, disk-dominated states and in systems wherein the line of sight passes close to the plane of the accretion disk,” the authors write in their paper. This is because in this state and orientation, the x-ray spectrum they’re observing is from the accretion disk rather than being contaminated by coronal emissions. The corona is above the accretion disk and can’t tell scientists as much about disk winds.
Different factors can influence the disk winds, including “changes in the gas ionization, column density, or magnetic field configuration,” the authors explain. For this reason, astrophysicists have worked hard at recognizing the exact moment when winds are launched or quenched and what conditions are present at those times. That’s why these observations of the end of the x-ray binary’s 2024 outburst are important.
“The first XRISM observation of a transient stellar-mass black hole caught 4U 1630−472 at the tail end of a soft, disk-dominated state,” the authors write in their research. Instead of the simple disk winds found at other times, these findings were a surprise. “Resolve obtained a very complicated and highly variable ionized absorption spectrum,” the authors explain in their research. “The data likely reflect a combination of failed winds, anisotropy that may be consistent with an overflowing accretion stream, and potential fast outflows with high mass transfer rates.”
Though surprising, these findings give scientists plenty to sink their teeth into. Untangling what they’re seeing into a coherent understanding would represent real progress.
“Being surprised is good. Seeing expectations proven naive means progress,” said lead author Miller in a press release. “Having entirely new avenues to chase down is tremendous. Astronomy is full of surprises and never boring.”
XRISM and its exceptional capabilities are responsible for these findings. While NASA’s Chandra and the ESA’s XMM-Newton are powerful x-ray telescopes, they’re 25 years old. That means that XRISM has improved performance compared to its older peers. The satellite, which is a joint mission between JAXA, NASA, and the ESA, is able to turn what can appear to be noise into useful data. “XRISM is at least 10 times more sensitive than any prior X-ray instrument,” said lead author Miller. “For that reason, we’re suddenly able to see really dramatic spectral lines that would have just looked like noise in the data that we’ve had for the last two decades.”
It’s impossible to observe these same disk wind features around supermassive black holes (SMBH) because it would take hundreds of millions of years. But for stellar mass black holes like this one, the changes are much more rapid. This one flares every two years or so, providing lots of observational windows. During these outbursts, its brightness can increase by 10,000 times in a single week.
“We got to see a range of gas flow rates that we never get to observe with massive black holes at the centers of galaxies,” Miller said. “The time scale to see something like that from the black hole in the Milky Way would be hundreds of millions of years. So one of the reasons we study these smaller ones is to get a clue as to how gas flows onto very massive black holes could change over the evolution of a galaxy.”
What the researchers expected to see is not what they found. It makes sense that as the gas flow slowed, the matter moving into the black hole’s accretion disk would be slow and measured. It’s like pouring water, according to lead author Miller. “You expect to spill a lot if you try to pour a bucket of water into a cup, but not when you are pouring a cup of water into a bucket,” he said. “Black holes seem to spill in both extremes.”
Instead of an orderly flow of gas there was a chaotic mess. “There was still mass being thrown around instead of accreted directly into the black hole,” Miller said.
In their research, the authors explain that their analysis has divided the black hole’s spectrum into just two parts. “The sensitivity of the resultant spectra suggests that the data could be divided into smaller time segments and that the evolution of the absorption spectrum could be traced with higher fidelity.
These investigations are beyond the scope of our current analysis, but they hold real potential,” they write. This is XRISM’s first look at 4U 1630−472, and it’s likely that the satellite will examine this black hole again in the future, but in greater detail.
“We’re not done with what NASA calls the mission’s prime phase, which is a two-year period where you find examples of groundbreaking science that you can do,” Miller said. “Then you spend the rest of the mission diving into those areas once you’ve unearthed them.”
SMBH lie at the center of large galaxies like the Milky Way, and their feedback has a powerful effect on galaxy growth and star formation.
Their AGN feedback is the primary growth regulation mechanism. While the winds in SMBH are difficult to observe in this way, these and future XRISM observations of active black holes will hopefully lead to more progress in understanding SMBHs and their profound effect on galaxies.
Written by Evan Gough/Universe Today.
