Scientists from Yunnan Observatories of the Chinese Academy of Sciences, Huazhong University of Science and Technology, and Yunnan University  proposed a novel method to constrain the high-energy dissipation region in a blazar. Their findings were published in The Astrophysical Journal.

Blazars are one special class of active galactic nuclei (AGN) pointing their relativistic jets at Earth. Multi-wavelength radiations covering from radio to TeV gamma-ray energies have been observed from blazars, presenting various extreme physical characteristics.

High-energy radiations from blazars play a key role in investigating accretion of super-massive black hole (SMBH) and jet physics, etc. However, the location where the high-energy radiations are produced in the jet remains unknown.

Due to the capability limitation of the current detectors, the high-energy emission region, which is usually compact and near the SMBH, cannot be directly located. Current methods proposed to constrain the location of the high-energy emission region have some disadvantages, like depending on too many assumptions as well as giving poor constraints.

Radio telescopes have the capability of resolving the structure of blazar jets on an approximately parsec scale. Using multi-frequency very long baseline interferometry (VLBI) observations, the magnetic field along the jet (on an approximately parsec scale) has been obtained.

It is found that there is a tight anti-relation between the magnetic field strength and the distance from the SMBH along the jet. The novel method proposed takes advantage of the above relation, and only depends on two assumptions: the relation of magnetic field and distance can be extrapolated into the sub-parsec scale of the jet from the SMBH, and optical and high-energy emissions are produced in the same region.

Using optical observations, the magnetic field strength in high-energy emissions can be constrained, and then one can locate the high-energy emission region with the relation of magnetic field and distance. The method is applied to two blazars. The results show that this method is very effective for the blazars with fast optical variability.