Blazars are some of the most enigmatic and fascinating objects in the universe. These celestial bodies are powered by supermassive black holes at their centers and produce some of the most intense energy emissions known to man. Blazars are characterized by powerful astrophysical jets that produce extremely bright x-rays and gamma rays. Despite decades of research, much of the underlying mechanisms that produce these emissions remain a mystery. However, recent discoveries, including those from the Imaging X-ray Polarimetry Explorer (IXPE) telescope, are helping scientists unravel the mysteries of blazars.
Blazars and their Cousins: Equasars and Quasars
Blazars belong to a class of celestial objects called active galactic nuclei (AGNs), which also includes quasars and equasars. Quasars are the most distant and luminous members of this family, with some of them emitting light that has traveled over 13 billion years to reach us. Equasars, on the other hand, are AGNs whose jets are pointed directly at Earth, making them brighter than normal AGNs. Blazars, like equasars, have jets that are pointed towards Earth, but they are more powerful than those of equasars, making them the most powerful sources of gamma rays in the sky.
Black Holes and Jets
All blazars have supermassive black holes at their centers, which are millions or billions of times more massive than the sun. These black holes are surrounded by an accretion disk, a flat structure composed of gas and dust that spirals towards the black hole’s event horizon. As matter falls towards the black hole, it becomes heated and emits energy, including x-rays and gamma rays. However, not all AGNs produce jets. Scientists believe that the magnetic fields generated by the black hole’s accretion disk and the rotation of the black hole itself are responsible for launching the jets. These jets are made up of highly energized particles, including electrons and positrons, which are accelerated to nearly the speed of light.
X-Ray Polarization and Blazar Mysteries
The recent discovery of x-ray polarization in blazars using the IXPE telescope is providing new insights into the underlying mechanisms that produce the powerful emissions associated with these celestial objects. X-ray polarization refers to the twisting of x-ray light in a particular direction due to its interaction with magnetic fields or electric fields. Until recently, measuring x-ray polarization has been challenging due to the difficulty of detecting x-rays from space and the absorption of x-rays by the Earth’s atmosphere.
Using the IXPE telescope, scientists were able to observe x-ray polarization from a nearby blazar called Mrk 501, which is approximately half a billion light-years away. They found that the polarization of the x-rays differed depending on their frequency, with higher frequency x-rays being more polarized than lower frequency x-rays. This finding implies that the x-rays are being polarized at different points along the jet, with high-energy particles becoming polarized much sooner and possessing more polarization than low-energy particles.
The researchers concluded that the polarization of the x-rays is likely caused by shockwaves within the jet. When high-energy particles collide with these shockwaves, they become accelerated to even higher energies, which causes them to emit x-rays that are highly polarized. The origin of these shockwaves is still a mystery, although they may be related to the activity of the black hole and the accretion disk surrounding it.