Radio Follow-Up Observations of a Weak-Line Quasar Exhibiting Remarkable X-ray Variability

TL;DR

SDSSJ1539+3954 displays extreme X-ray variability without a corresponding radio flare, challenging simple disk–jet coupling in WLQs. By combining archival Chandra data with multi-epoch GMRT/VLA radio measurements and optical spectra, the study finds a compact, steep-spectrum radio source that remains radio-quiet and non-variable over years. The results support the Thick-Disk plus Outflow (TDO) model for WLQs, attributing X-ray variability to geometric obscuration by a thick inner disk/outflow rather than jet activity, and suggesting that radio emission arises from AGN winds or star formation rather than coronal processes. High-resolution VLBI observations are proposed to pinpoint the radio-emitting regions and disentangle core/jet contributions from diffuse host emission.

Abstract

SDSSJ1539+3954 ($z\approx 1.935$), a radio-quiet weak-line quasar (WLQ), exhibited exceptional X-ray variability in 2019$-$2020, with its X-ray flux increasing by over 20 times from 2013 to 2019 and subsequently dropping by at least a factor of nine in 2020. Motivated by the empirical correlations between X-ray and radio emission in AGN cores, we carried out a follow-up radio study in the 0.3$-$10 GHz range using GMRT (2020, 2022, 2024) and VLA (2022), and analyzed archival VLASS 3 GHz data (2017-2023) to investigate the source's radio properties and potential connection with the X-ray behavior. Our observations reveal a compact radio source with a spectral index of -0.65$\pm$0.15 in the frequency range of 0.3$-$1.4 GHz and -1.09$\pm$0.16 in 3$-$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 longer timescale variability 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.

Figures (5)

• Figure 1: Flux density images with overplotted contours (dashed lines are used for negative contours) for the GMRT observations conducted in years 2020 (top row), 2022 (middle row), and 2024 (bottom row). The contour levels scale as: (image rms noise) $\times$ [-3, 3, 7, 9, 13, 15]. All the images are arranged in increasing order of frequency from left to right columns at 340 MHz, 750 MHz, and 1250 MHz, respectively. The solid ellipse at the bottom of every image represents the beam shape. The approximate beam sizes obtained for Band-3, Band-4, and Band-5 images are 6.5" $\times$ 5.3", 3.7" $\times$ 3.4" and 2.9" $\times$ 1.9" respectively. Respective flux density values for the source SDSSJ1539+3954 are mentioned within the image panels along with the corresponding frequency band and observation date.
• Figure 2: Flux density images with overplotted contours for the VLA observations conducted in 2022. flux density image of the source at frequency 2999 MHz (left panel), 5999 MHz (middle panel), and 9999 MHz (right panel). The contour levels scale as: (image rms noise) $\times$ [-3, 3, 7, 13, 15]. The solid ellipse at the bottom of every image plane represents the beam shape. The approximate beam sizes obtained for Band-S, Band-C, and Band-X images are 1.1" $\times$ 0.54", 0.51" $\times$ 0.3", and 0.33" $\times$ 0.23" respectively. Respective flux density values for the source SDSSJ1539+3954 are mentioned within the image planes along with the corresponding frequency band and observation date.
• Figure 3: Left panel: power-law fits for the radio flux densities obtained from GMRT (300 MHz--1.26 GHz) and VLA (3--10 GHz) observations. The cyan dashed line represents the GMRT epoch 2020 fit with spectral index $-0.58$, the yellow dot-dashed line for the GMRT epoch 2022 with spectral index $-0.65$, the solid brown line for the VLA epoch 2022 with spectral index $-1.09$ and the dotted magenta line for the GMRT epoch 2024 with spectral index $-0.61$. GMRT epoch 2020 flux densities are shown as circles (blue), epoch 2022 flux densities are shown as squares (orange), and epoch 2024 flux densities as stars (magenta). The VLA epoch 2022 flux densities are shown as diamonds (brown). Right panel: broken power-law fit (blue solid line) and curved power-law fit (brown dot-dashed line) for the flux densities obtained from the GMRT observation epoch 2022 and sub-bands from the VLA observation epoch 2022. The spectral index changes from $\alpha_{low} \approx -0.64$ to $\alpha_{high} \approx -1.19$ at the break frequency $\nu_{break} \sim 1660.37$ MHz. The shaded region shows the 1-$\sigma$ deviation in the fit parameters over several bootstrap iterations to fit the broken power-law. Similarly, the curved power-law fit provides a spectral index of $\alpha = -0.78 \pm 0.08$ which steepens at a rate of $2\beta$ per decade in frequency near 1 GHz, where $\beta = -0.31 \pm 0.13$.
• Figure 4: Historical representation of multiwavelength observations for the source SDSSJ1539+3954. Row 1.) X-ray observations from Chandra ACIS. Row 2.) ZTF Band-g light curve. A maximum variability of only about $\sim$0.3 $\pm$ 0.03 magnitude is observed for the quasar from the ZTF light curve, a level of variability that is typical among quasars. Row 3.) Maroon data points: VLA-FIRST observation at 1.4 GHz, Red data points: GMRT observations at 1.26 GHz, Yellow data points: VLASS observations at 3 GHz, Indigo data point: VLA observation at 3 GHz. The Chandra epochs are marked with vertical lines (orange) that show concurrent variability in X-ray and optical wavebands. The GMRT Band-5 observation epochs are marked with dot-dashed vertical lines (cyan).
• Figure 5: SDSS and HCT spectra of the WLQ SDSSJ1539+3954 in the rest wavelength range 1400 $\mathring{\mathrm{A}}$ to 2700 $\mathring{\mathrm{A}}$. Spectra obtained from the SDSS-DR16 catalog include observations of the source in the years 2002 (blue), 2012 (red), and 2017 (green). The spectra obtained in follow-up observations in 2020 and 2021 using HCT HFOSC are shown in magenta and cyan colors, respectively. No change can be observed in the spectrum and the emission-line signatures of the source.