MADRID, 14 (EUROPA PRESS)
Observations with NASA's Imaging X-ray Polarimetry Explorer (IXPE) have challenged our understanding of what happens to matter in the vicinity of a black hole.
With this space observatory, astronomers can study incident X-rays and measure polarization, a property of light that describes the direction of its electric field.
The degree of polarization measures the alignment of these vibrations with respect to each other. Scientists can use a black hole's degree of polarization to determine the location of its corona (a region of extremely hot, magnetized plasma surrounding the black hole) and how it generates X-rays.
In April, astronomers used the IXPE to measure a polarization degree of 9.1% in the black hole IGR J17091-3624, much larger than expected based on theoretical models.
“The black hole IGR J17091-3624 is an extraordinary source that dims and brightens with the intensity of a heartbeat, and NASA’s IXPE allowed us to measure this unique source in a completely new way,” said Melissa Ewing of Newcastle University and lead author of the study published in the Monthly Notices of the Royal Astronomical Society in a statement.
In X-ray binary systems, an extremely dense object, such as a black hole, draws in matter from a nearby source, usually a neighboring star. This matter may begin to spin, flattening into a swirling structure known as an accretion disk.
The corona, located in the inner region of this accretion disk, can reach extreme temperatures of up to 1 billion degrees Celsius and emit very bright X-rays. These ultra-hot coronas are responsible for some of the brightest X-ray sources in the sky.
Despite the brightness of the corona in IGR J17091-3624, about 28,000 light-years from Earth, it is still too small and distant for astronomers to capture an image of.
"Normally, a high degree of polarization corresponds to a very edge-on view of the corona. The corona would have to be perfectly shaped and observed from just the right angle to achieve such a measurement," said Giorgio Matt, a professor at Roma Tre University in Italy and co-author of this paper, in a statement. "The dimming pattern has not yet been explained by scientists and could hold the key to understanding this category of black holes."
The companion star to this black hole is not bright enough for astronomers to directly estimate the viewing angle of the system, but the unusual brightness changes observed by IXPE suggest that the edge of the accretion disk was facing directly toward Earth.
Researchers explored different avenues to explain the high degree of polarization.
WIND OF MATTER
In one model, astronomers included a "wind" of matter pulled from the accretion disk and flung out of the system, a rare phenomenon. If coronal X-rays encountered this matter on their way to IXPE, Compton scattering would occur, resulting in these measurements.
"These winds are one of the key missing pieces for understanding the growth of all types of black holes," said Maxime Parra, who led the observation and works on this topic at Ehime University in Matsuyama, Japan. "Astronomers can expect future observations to produce even more surprising measurements of the degree of polarization."
Another model assumed that the plasma in the corona could exhibit very rapid outflow. If the plasma flowed outward at speeds up to 20% of the speed of light, or approximately 200 million kilometers per hour, relativistic effects could increase the observed polarization.
In both cases, the simulations were able to recreate the observed polarization without a very specific edge view. The researchers will continue modeling and testing their predictions to better understand the high degree of polarization for future research efforts.
