# Origin of X-rays in the low state of the FSRQ 3C 273: Evidence of inverse Compton emission

## TL;DR

This study analyzes X-ray and UV data of quasar 3C 273, revealing that its X-ray emission during low states is likely due to inverse Compton scattering in a thermal corona, with spectral variations explained by changing optical depth.

## Contribution

It provides new insights into the origin of X-ray emission in 3C 273, especially during low flux states, and links spectral behavior to inverse Compton processes and corona properties.

## Key findings

- Detection of a harder-when-brighter X-ray spectrum trend.
- Observation of an anti-correlation between X-ray and UV emissions during low states.
- Identification of inverse Compton scattering as the primary X-ray emission mechanism.

## Abstract

We analyze the 2.5--10 keV X-ray spectra of the luminous quasar 3C 273 and simultaneous observations in UV wavelengths from XMM--Newton between 2000 and 2015. The lowest flux level ever was observed in 2015. The continuum emission from 3C 273 is generally best described by an absorbed power-law but during extremely low states the addition of fluorescence from the K-shell iron line improves the fit. We study the spectral evolution of the source during its extended quiescent state and also examine connections between the X-ray and ultraviolet emissions, which have been seen in some, but not all, previous work. We detect a possible anti-correlation between these two bands during the low state that characterized 3C 273 for most of this period; however, this was not present during a flaring state. A harder-when-brighter trend for the X-ray spectrum was observed in these long-term observations of 3C 273 for the first time. We suggest that the X-ray emission in 3C 273 is the result of inverse Compton scattering of soft UV seed photons (emitted from the local environment of the AGN), most likely in a thermal corona. We can explain the significant temporal variation of the spectral continuum as an outcome of changing optical depth of the comptonizing medium, along the lines of the wind-shock model proposed by Courvoisier and Camenzind (1989).