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Abstract We report deep Karl G. Jansky Very Large Array (VLA) observations of the optically ultraluminous and radio-quiet quasar SDSS J010013.02+280225.8 (hereafter J0100+2802) at redshift z = 6.3. We detected the radio continuum emission at 1.5 GHz, 6 GHz, and 10 GHz. This leads to a radio power-law spectral index of α = −0.52 ± 0.18 ( S ∝ ν α ). The radio source is unresolved in all VLA bands with an upper limit to the size of 0.″2 (i.e., ∼1.1 kpc) at 10 GHz. We find variability in the flux density (increase by ∼33%) and the spectral index (steepened) between observations in 2016 and 2017. We also find that the VLA 1.5 GHz flux density observed in the same year is 1.5 times that detected with the Very Long Baseline Array (VLBA) in 2016 at the same frequency. This difference suggests that half of the radio emission from J0100+2802 comes from a compact core within 40 pc, and the rest comes from the surrounding few-kiloparsec area, which is diffuse and resolved out in the VLBA observations. The diffuse emission is 4 times brighter than what would be expected if driven by star formation. We conclude that the central active galactic nucleus is the dominant power engine of the radio emission in J0100+2802.
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This content will become publicly available on December 23, 2026
Radio Follow-up Observations of a Weak-line Quasar Exhibiting Remarkable X-Ray Variability
Abstract SDSS J1539+3954 (z ≈ 1.935), a radio-quiet weak-line quasar (WLQ), exhibited exceptional X-ray variability in 2019–2020, with its X-ray flux increasing by over 20 times from 2013 to 2019 and subsequently dropping by at least a factor of 9 in 2020. Motivated by the empirical correlations between X-ray and radio emission in the cores of active galactic nuclei (AGNs), we carried out a follow-up radio study in the 0.3–10 GHz range using the Giant Metrewave Radio Telescope (GMRT; 2020, 2022, 2024) and Very Large Array (VLA; 2022), and analyzed archival Very Large Array Sky Survey (VLASS) 3 GHz data (2017–2023) to investigate the source’s radio properties and potential connection with the X-ray behavior. Our observations reveal a compact radio source with a spectral index of −0.65 ± 0.15 in the frequency range 0.3–1.4 GHz and −1.09 ± 0.16 in 3–10 GHz. While the source was undetected in VLA-FIRST (1994) and VLASS epochs, the GMRT and VLA observations show no statistically significant variability over the monitored period. The absence of detectable changes in the radio flux, despite strong X-ray variability, suggests no direct connection between the X-ray variability and the radio emission, consistent with the thick-disk-plus-outflow (TDO) model for WLQs. However, the sensitivity limit of the surveys prevents us from drawing definitive conclusions regarding variability on longer timescales between the VLA-FIRST and GMRT epochs. We further explore possible mechanisms driving the radio emission from this source. Our analysis rules out small-scale jets and coronal emission as the primary drivers of the radio emission, suggesting that extended emission from AGN winds and star formation is the more plausible mechanism.
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- Award ID(s):
- 2407089
- PAR ID:
- 10688971
- Publisher / Repository:
- The Astrophysical Journal
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 996
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 23
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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