# Variable jet Lorentz factors can explain soft self-absorbed radio spectra of accreting black-holes

## TL;DR

This paper demonstrates that variable jet Lorentz factors, through acceleration or deceleration, can produce soft radio spectra in accreting black holes without requiring large energy deposits at large distances, aligning with observations of certain quasars and BL Lac objects.

## Contribution

It introduces a model where changing jet Lorentz factors explain soft radio spectra, eliminating the need for high energy deposition at large distances.

## Key findings

- Variable jet Lorentz factors can produce soft spectra.
- The model explains spectra of Mrk 421 and Mrk 501.
- Soft spectra do not require large energy at large distances.

## Abstract

We study the effect of variable jet bulk Lorentz factors, i.e., either jet acceleration or deceleration, on partially synchrotron self-absorbed radio spectra from cores of radio-loud active galactic nuclei and black-hole binaries in the hard state. In about a half of quasars and radio galaxies, their core radio spectra are observed to be soft, i.e., have the spectral index of $\alpha<0$. If they are emitted by jets with constant Lorentz factors, that softness implies deposition of large amounts of energy at large distances from the centre. We show here that such soft spectra can be explained without that energetic requirement by emission of jets with the Doppler factor increasing with the distance. This can happen for either jet acceleration or deceleration, depending on the jet viewing angle. We find our model can explain the quiescent radio to X-ray spectra of the BL Lac objects Mrk 421 and Mrk 501.