Located a few thousand light years from Earth, in the constellation Scorpio, this little gluttonous monster, like all other black holes, devours everything around it. Nothing unusual, you would say, but it stands out from its peers by two other unique characteristics : its luminosity is cyclical; like a heart, it beats regularly. A phenomenon of absolute rarity that was previously only known in one other star system, GRS 1915+105, a microquasar also including a black hole orbiting around its star.
Second characteristic which classifies it among the celestial objects out of the ordinary, he has just shattered the world record for polarization of X-rays. Polarization is, in a way, the indicator of the geometric organization of a light source. In an environment as chaotic as that of a black hole, light waves oscillate in all directions randomly. On the other hand, when a source is highly polarized, this means that the electric field of the photons aligns. Consequently, IGR J17091-3624 does not expel matter in the form of a vertical jet, like the overwhelming majority of known black holes, but forms a flattened structure called ” in sandwich ».
Its corona of superheated gas spreads parallel to the accretion disk, modifying the trajectory of the radiation it emits. A particularity which has been the subject of a study published in August 2025 in the journal Monthly Notices of the Royal Astronomical Society. IGR J17091-3624 joins the list of black holes “ exotic », like that of the galaxy LRG 3-757, weighing 36 billion times the mass of the Sun.
IGR J17091-3624: order or nothing
It was thanks to the IXPE (Imaging X-ray Polarimetry Explorer) satellite that the research team was able to measure the polarization rate of IGR J17091-3624. This is quite simply stratospheric since it reaches 9.1% ; that is, out of 100 photons (particles of light) emitted by the black hole’s environment, almost 10 oscillate in exactly the same direction. To properly illustrate this value, we must keep in mind that the polarization rate of most other black holes barely reaches 4% and only very rarely exceeds it.
A phenomenon which can be explained by Compton scattering: the X-rays bounce off the electrons of the burning plasma (the corona) which surrounds the black hole. If this plasma formed a disorganized sphere or cloud around the abyss, it would bounce in all directions, canceling the alignment of the light. But such a high rate proves that photons bounce on an extremely flat and extensive surfacea thin layer of burning gas that covers the disk of material.
Since the energy escaping from IGR J17091-3624 does not escape in vertical jets, it becomes trapped and its sandwich-shaped structure causes a monumental build-up of heat and pressure. It is this mechanism which is at the origin of light oscillations (the famous “heartbeat”).
Since the energy cannot flow upwards, it saturates the plasma corona and the black hole expels this excess in a sudden burst of X-rays, before gravity takes over and crushes the gas into a flat layer again. This cycle of compression and release repeats itself cyclically, and seen from our instruments, IGR J17091-3624 flickers, each light flash corresponding to the energy it releases.
Towards a new typology of black holes?
If IGR J17091-3624 behaves like this, it could mean that a high degree of polarization is actually the norm for black holes when they reach the ” Hard State » (very energetic X-ray emissions at high frequency). This is in any case the conclusion of the authors, who explain that this indicator could be a new sign to look for to identify the true shape of the black hole environment.
The interest is to determine the angle from which we observe these cosmic monsters. If the latter is oblique, it is because the black hole is surrounded by a corona so flattened that it is only visible at the edge, forcing the light to ricochet horizontally towards us. Conversely, if the angle is facing us (view from above), the black hole returns photons whose vibrations go in all directions, masking its internal organization.
A new methodology that would allow classify these celestial objects more precisely no longer only by their mass or their brilliance, but by their geometry. A paradigm shift in astrophysics, since, until the understanding of the luminous oscillation phenomenon of IGR J17091-3624, polarization was only a secondary factor while she is finally the discriminating criterion for classifying black holes according to their morphology.
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