A Czech team of astrophysicists from the Academy of Sciences has achieved a major breakthrough in understanding the behaviour of black holes. In an international collaboration, the scientists have published two studies that provide new insights into the behaviour of X-ray black hole binary systems. A stellar-mass black hole in such a system attracts mass from its stellar counterpart, forming a so-called accretion disk around it. At the poles of the black hole, matter can then escape outwards from the system in the form of high-energy outbursts.
A new discovery
The existence of the X-ray binary system Swift J1727.8-1613 was unknown until August last year. The dark spot in the sky began to glow brightly at all wavelengths and caught the attention of astronomers worldwide, especially in the X-ray field, where for several months the object was brighter than another known source of X-rays inside our galaxy - the Crab Nebula. Sudden flares are typical of binary star systems, but rarely are they so bright and relatively close.
That's why all the major X-ray observatories, including the IXPE, a joint project between NASA and the Italian Space Agency. On board IXPE is a special telescope that allows you to measure the polarization of X-rays. Polarization determines the amount of light waves oscillating in a given direction. This is proving to be a key tool for understanding the processes taking place, for example, near black holes, which have strong gravitational effects on the surrounding material and radiation. Polarisation helps us to elucidate the structure of matter near a black hole.
Changes in spectra accompanied by changes in X-ray polarization
The first study, led by Jiří Svoboda from the Institute of Astronomy of the Czech Academy of Sciences, focused on the dramatic changes in X-ray polarization observed during different phases of the system's flare Swift J1727.8-1613 from August 2023 to February this year. "Our research has shown that during the transition between hard and soft spectral states of X-rays, significant changes in polarization also occur," Svoboda explains.
The initial phase of the flare was dominated by the corona, which is the main source of hard X-rays in the system. In the subsequent soft state, when the observed radiation is less energetic, more X-ray photons come directly from the accretion disk. Astronomers first proposed new observations, which then revealed a dramatic drop in polarisation. The fact that changes in the energy spectrum, i.e. its colour, are accompanied by changes in polarisation confirms that the structure of the accretion disk and corona changes significantly during this phase. "It is the polarization that demonstrates the geometric changes in the mass distribution of black holes," Svoboda points out.
Restoring the polarisation state: a surprising finding
Second study of the building Swift J1727.8-1613which was led by Jakub Podgorny again from the Institute of Astronomy of the CAS, dealt with the recovery of X-ray polarization after the transition back from the soft to the hard state. The hard state means that the radiation from the distant source is much more energetic. The observed binary returned to this state in April this year after several months, but with roughly 100 times the brightness of the initial flare in August 2023."For the first time we have observed X-ray polarization in a binary system at this transition," says Podgorný, adding: "We find that the polarization has returned to the values observed during the initial phase of the outburst for the same source, which means that the coronal geometry does not change significantly for this hard state, although the X-ray brightness drops significantly."
Although the decrease in X-ray brightness was predicted, the current full recovery of the polarization state was unexpected. This is the last observation of a source with IXPE suggests that the structure of the corona and disk is much more stable in binary systems after the return from the soft state than previously thought. Since the dominant direction of light oscillations in such a case Swift J1727.8-1613 identical to the previously observed mass ejection, it can be assumed that the hot plasma in the corona is once again expanding in the direction of the disk plane, similar to the original high-brightness hard state.
The importance of discoveries
These results represent a significant step forward in our understanding of how black holes interact with the surrounding material and demonstrate that X-ray polarization analysis is an ideal tool. "Further observations with IXPE for multiple sources are necessary to fully determine the configuration near black holes in binary systems. The successful first observing campaign of Swift J1727.8-1613 in different states is thus the most important start of a new chapter," he concludes Michal Dovčiak from the Institute of Astronomy of the Czech Academy of Sciences, who also contributed to the publications and is the leader of the working group IXPEthat focuses specifically on stellar-mass black holes.
The studies were published in peer-reviewed journals The Astrophysical Journal Letters a Astronomy & Astrophysics and are available to view for further information here:
https://iopscience.iop.org/article/10.3847/2041-8213/ad402e
https://www.aanda.org/articles/aa/full_html/2024/06/aa50566-24/aa50566-24.html
On the NASA website: https://www.nasa.gov/missions/ixpe/nasas-ixpe-details-shapes-of-structures-at-newly-discovered-black-hole/
Institute of Astronomy of the CAS/ gnews - RoZ
ILLUSTRATIVE PHOTO - pixabay
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