# The radio emission in radio-quiet quasars: the VLBA perspective

**Authors:** Sina Chen, Ari Laor, Ehud Behar, Ranieri D. Baldi, Joseph D. Gelfand  
**arXiv:** [2307.13599](https://arxiv.org/abs/2307.13599)  
**Date:** 2023-07-25  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.13599v1/

## Key points

- Study focuses on the origin of radio emission in radio-quiet quasars (RQQ)
- New observations using VLBA at frequencies of 1.6 and 4.9 GHz for ten RQQ
- Representative sample of 18 RQQ from Palomar-Green sample with low redshift AGN
- Wide range of spectral slopes, with nine out of 14 objects having flat spectral slopes
- Most objects show an unresolved flat-spectrum core aligned with optical Gaia position
- Extended emission tends to be steep-spectrum and displaced from optical core by 5-100 parsecs
- Tight correlation between VLBA core flux and X-ray flux suggests a radio to X-ray luminosity relation for objects below certain black hole mass threshold
- Flatness observed in core emission suggests compact source size likely originating from accretion disk corona surrounding central black hole
- Mas-scale extended emission is optically thin and exhibits clumpy structure, suggesting it may be produced by an outflow from the center
- Future radio observations at higher frequencies recommended to investigate nature of core emission in RQQ
- Study provides insights into radio properties of RQQ and potential mechanisms responsible for their emissions such as accretion disk corona and outflows

## Summary

This study focuses on the origin of radio emission in radio-quiet quasars (RQQ), which is still not well understood. The researchers present new observations using the Very Long Baseline Array (VLBA) at frequencies of 1.6 and 4.9 GHz for ten RQQ, with nine of them being detected. These observations, combined with previously published data on eight RQQ (five detected), form a representative sample of 18 RQQ from the Palomar-Green sample of low redshift AGN (active galactic nuclei) with z (< 0.5). The integrated emission from these quasars exhibits a wide range of spectral slopes, extending from very steep to strongly inverted. Notably, nine out of the 14 objects have flat spectral slopes. Additionally, most objects show an unresolved flat-spectrum core that aligns with the optical Gaia position. The extended emission in these RQQ tends to be steep-spectrum and has a low brightness temperature. It is also displaced from the optical core by approximately 5-100 parsecs. The researchers find a tight correlation between the VLBA core flux and X-ray flux, indicating a radio to X-ray luminosity relation for objects with black hole masses below a certain threshold. The flatness observed in the core emission suggests a compact source size, likely originating from the accretion disk corona surrounding the central black hole. On larger scales, mas-scale extended emission is optically thin and exhibits clumpy structure, suggesting it may be produced by an outflow from the center. To further investigate the nature of core emission in RQQ, future radio observations at higher frequencies are recommended to test whether it can be attributed to accretion disk coronal emission. Overall, this study provides valuable insights into the radio properties of RQQ and highlights potential mechanisms responsible for their radio emissions such as accretion disk corona and outflows from their centers. Moreover, it reveals a possible connection between their X-ray luminosity and VLBA core fluxes for objects with black hole masses below some threshold value.