A radio counterpart of the local little-red-dot J092359+415736

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Little red dots (LRDs) are compact red sources at z>~4. Broad lines suggest accreting black holes, yet radio and X-ray measurements are usually weak. Local LRDs allow multi-frequency, multi-epoch radio studies that are inaccessible at high redshift. We search archival radio images for a counterpart to the local LRD J092359+415736 at z~0.323, quantify its long-term radio variability, and identify the plausible origin of the radio emission. Flux densities at the optical position are measured in FIRST, LoTSS, RACS, VAST, and VLASS products spanning 0.144-3.0 GHz and 1994-2024. A compact 144 MHz counterpart is detected at 42sigma in LoTSS, coincident with the Gaia position, and is recovered in FIRST, RACS, VAST, and VLASS. LoTSS and ASKAP give a spectral index alpha ~ -0.30 +/- 0.11. The source varies on multiple timescales: FIRST at 1.4 GHz (1994) is a factor of ~2.6 below ASKAP (2021-2024) and below the extrapolated LoTSS-ASKAP spectrum, indicating long-term brightening over 26 yr; the 3 GHz flux density rises by 2.3sigma between VLASS epochs 2.2 and 3.2. A star-formation-only origin would require ~200 Msun/yr, against an unobscured H-alpha upper limit of 3.3 Msun/yr, implying an extreme obscured starburst. Ordinary radio supernovae and tidal disruption events are strongly disfavoured by the luminosity and 30-yr recovery, while refractive scintillation would itself require an ultracompact nuclear emitter. The persistent radio counterpart is best explained by accretion onto the central black hole. Among the three radio-detected local LRDs, this source is intermediate in luminosity and the only one with reported multi-year variability, adding a temporal dimension to the radio diversity of the class.

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The radio counterpart of J092359+415736 is established by the LoTSS DR3 detection and confirmed across four additional surveys spanning 0.144–3.0 GHz and three decades. The combination of radio luminosity, flat spectral index, and multi-timescale variability (a factor of ∼ 2.6 \sim 2.6 brightening over 26 yr and a tentative ∼ 3 × \sim 3\times rise at 3 GHz over 2.6 yr) identifies accretion-powered emission as the most plausible origin. With L R / L bol ≃ 2.6 × 10 − 5 L_{\rm R}/L_{\rm bol}\simeq 2.6\times 10^{-5} , the source falls in the radio-quiet regime, and the short variability timescale constrains the emitting region to light-year scales, consistent with a jet base or magnetised corona.

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J092359+415736 is intermediate in luminosity among the three radio-detected local LRDs and the only one with reported multi-year variability, adding a temporal dimension to the emerging radio diversity of the class. The three objects together span two orders of magnitude in L 6 ​ GHz L_{6\,\mathrm{GHz}} and cover a range of spectral indices and variability behaviours, demonstrating that local LRDs do not constitute a uniform radio population. Whether this diversity reflects intrinsic differences in jet power, accretion rate, or viewing geometry remains to be determined. Simultaneous broadband spectra, VLBI imaging, and dedicated monitoring of local LRDs can distinguish a weak jet from coronal or wind-driven emission and provide a physical template for interpreting the still largely undetected radio emission of high-redshift LRDs.

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Figure 2 shows the ultraviolet-near-infrared of J092359+415736. We fit the SEDs using a two-component blackbody model from 4000 Å to 1.6 μ ​ m \rm\mu m , motivated by the results of Wang et al. (2026) , who suggested that LRDs may consist of two components with different characteristic temperatures. Unlike the two-component photospheric model adopted by Wang et al. (2026) , however, we employ a simplified model using only blackbody spectra. This choice is motivated by the limited sensitivity of the current SDSS spectra and the modest spectral resolution of the SPHEREx data ( Bock et al. 2026 ) , which make it difficult to constrain a photospheric model through stellar absorption features. We note that our primary goal is to compare J092359+415736 with other LRDs in terms of their bolometric luminosities and peak wavelengths (which are directly related to the characteristic temperatures). The choice of model has only a minor impact on the determination of these two quantities. As shown in Figure 2 , J092359+415736 exhibits a bolometric luminosity and characteristic temperature comparable to those of the majority of known gas-reddened LRDs ( de Graaff et al. 2025 ) . The source is detected in all four WISE bands. Its mid-infrared luminosity and colours are also reproduced by the starburst model of Lin et al. (2025) . The two-blackbody fit is used here only to characterize the optical–near-infrared continuum and its characteristic temperatures. No torus decomposition is attempted. The integrated blackbody luminosity L BB L_{\rm BB} shown in the right panel of Fig. 2 is derived from the model fit and is distinct from the AGN bolometric luminosity L bol L_{\rm bol} derived from the broad H α \alpha calibration in Sect. 2 .

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Results. A compact 144 MHz counterpart is detected at 42 ​ σ 42\sigma in LoTSS, coincident with the Gaia position, and is recovered in FIRST, RACS, VAST, and VLASS. The LoTSS and ASKAP measurements give α ≃ − 0.30 ± 0.11 \alpha\simeq-0.30\pm 0.11 . The source exhibits multi-timescale radio variability: FIRST at 1.4 GHz (1994) is a factor of ∼ 2.6 \sim 2.6 below the ASKAP measurements at similar frequencies (2021–2024) and below the extrapolated LoTSS–ASKAP spectrum, indicating long-term brightening over 26 yr, while the 3 GHz flux density rises by 2.3 ​ σ 2.3\sigma between VLASS epochs 2.2 and 3.2, suggesting shorter-term variability. A star-formation-only origin would require a rate of ≃ 200 ​ M ⊙ ​ yr − 1 \simeq 200\,M_{\odot}\,\mathrm{yr}^{-1} , compared with an unobscured H α \alpha upper limit of 3.3 ​ M ⊙ ​ yr − 1 3.3\,M_{\odot}\,\mathrm{yr}^{-1} , and hence an extreme, heavily obscured starburst. Ordinary single radio supernovae and tidal disruption events are strongly disfavoured by the luminosity and 30-yr recovery history, while refractive interstellar scintillation, if responsible, itself requires an ultracompact nuclear emitter.

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Conclusions. The persistent radio counterpart of J092359+415736 is best explained by accretion onto its central black hole. Among the three radio-detected local LRDs, it is intermediate in luminosity and the only one with reported multi-year variability, adding a temporal dimension to the emerging radio diversity of the class.

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At each epoch we fitted an elliptical Gaussian matched to the restoring beam and fixed at the Gaia DR3 position, together with a local planar background. The Gaussian was normalised to unity at its centre, so its fitted scale gives the peak intensity. This peak intensity equals the source flux density when the source is unresolved. At z = 0.323 z=0.323 , the major axes of the LoTSS, FIRST, VLASS, and RACS beams correspond to ∼ 28 \sim 28 , ∼ 25 \sim 25 , ∼ 13 \sim 13 , and up to ∼ 180 \sim 180 kpc, respectively 1 1 1 We adopt a flat Λ \Lambda CDM cosmology with H 0 = 70 ​ km ​ s − 1 ​ Mpc − 1 H_{0}=70\penalty\ \mathrm{km\,s^{-1}\,Mpc^{-1}} and Ω m = 0.3 \Omega_{\rm m}=0.3 , matching Lin et al. (2025) . At the source redshift, 1 ​ ″ = 4.69 1\arcsec=4.69 kpc. Spectral indices are defined by S ν ∝ ν α S_{\nu}\propto\nu^{\alpha} . . Positional coincidence at these resolutions does not establish nuclear compactness. The baseline fit used a circular aperture with radius 1.6 times the beam major axis. Repeating the fit with radii of 1.2 and 2.0 times the major axis and with constant or planar backgrounds produced amplitudes consistent within the adopted errors.

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We define the empirical image-plane error σ stat \sigma_{\rm stat} as the largest of the local background rms , the robust scatter of identical fits at blank positions where available, and the scatter among the six fitting variants. These estimates probe overlapping noise contributions, so we take their maximum rather than add them in quadrature. The total error is σ tot 2 = σ stat 2 + ( f cal ​ S ν ) 2 \sigma_{\rm tot}^{2}=\sigma_{\rm stat}^{2}+(f_{\rm cal}S_{\nu})^{2} , with f cal = 0.05 f_{\rm cal}=0.05 , 0.10, and 0.15 for FIRST, LoTSS/RACS/VAST, and VLASS, respectively. The conservative 15% term accounts for the lower calibration fidelity of the VLASS images relative to the ≲ 3 % \lesssim 3\% quoted for VLASS Single Epoch products. Free-centroid fits were used only as an astrometric check, not to select positive noise peaks. Their offsets from the Gaia position are 0.25 ​ ″ 0.25\arcsec for LoTSS, < 0.8 ​ ″ <0.8\arcsec for FIRST and VLASS, and 4 ​ ″ 4\arcsec – 6 ​ ″ 6\arcsec for the elongated RACS beams, the latter being ≲ 0.5 \lesssim 0.5 of the beam minor axis. Fixing the centroid changes the VLASS amplitudes by < 20 % <20\% , within the adopted errors. Allowing small positional offsets changes the ASKAP amplitudes by ≲ 8 % \lesssim 8\% for detections above 3 ​ σ 3\sigma , and FIRST/VLASS residual maps show no recurring compact component at a common offset (Appendix E ). The survey resolutions do not separate nuclear emission from unresolved host-galaxy emission. For FIRST we use the raw image-plane amplitude, 428 ± 139 ​ μ ​ Jy 428\pm 139\,\mu\mathrm{Jy} , and source-injection tests confirm no measurable bias from the fixed-position fit (Appendix E ). The FIRST measurement is a factor of ∼ 2.6 \sim 2.6 below the LoTSS–ASKAP spectrum (Appendix E ).

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We assess whether refractive interstellar scintillation (RISS) could contribute to the tentative 3-GHz variability. For the sightline to J092359+415736, NE2001 gives SM = 6.5 × 10 − 5 kpc m − 20 / 3 \mathrm{SM}=6.5\times 10^{-5}\,\mathrm{kpc\,m^{-20/3}} ( Cordes & Lazio 2002 ) . With a representative 1 1 -kpc scattering screen, the prescription of Walker (1998) gives a transition frequency of ν 0 ≃ 6.16 \nu_{0}\simeq 6.16 GHz. The 3-GHz observations are therefore in the strong-scattering regime. Their refractive timescale is ≃ 0.55 \simeq 0.55 d for v ⟂ = 50 ​ km ​ s − 1 v_{\perp}=50\,\mathrm{km\,s^{-1}} , so survey snapshots separated by years sample the process stochastically and cannot exclude a RISS contribution.

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We fitted the radio emission both at the Gaia DR3 position and at the local maximum within 0.5 0.5 of the beam minor axis. For ASKAP detections above 3 ​ σ 3\sigma , the fitted amplitudes differ by ≲ 8 % \lesssim 8\% . At ∼ 2 ​ σ \sim 2\sigma , the local maximum becomes sensitive to nearby noise fluctuations, and we use the flux density measured at the Gaia position. FIRST and VLASS residual maps after subtraction of Gaia -centred models show no recurring compact component at a common offset.

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anck ΛCDM cosmology with H0 = 67.4 km s−1 Mpc−1, Ωm = 0.315, and ΩΛ = 0.685 ( Planck Collaboration et al. 2020), we find that, for a source at z = 0.4332, the angular diameter distance is DA = 1202 Mpc and the luminosity distance is DL = 2468 Mpc. The proper size of the source is given by l = DAθS, which results in an upper limit to the proper size of the source of ≤4.7 kpc. This upper limit is not stringent, but it will be greatly improved with the future VLBA observations.

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Draft version August 18, 2026 Typeset using LATEX default style in AASTeX7.0.1 A Radio-Bright Local Little Red Dot Analog Luis F. Rodr´ıguez ,1 I. F´elix Mirabel ,2 Rosa A. Gonz´alez-L´opezlira ,1 and Laurent Loinard 1 1Instituto de Radioastronom´ıa y Astrof´ısica Universidad Nacional Aut´onoma de M´exico, Apdo. Postal 3-72, Morelia, Michoac´an 58089, Mexico 2Instituto de Astronom´ıa y F´ısica del Espacio (IAFE) CONICET-Universidad de Buenos Aires, C1428 Buenos Aires, Argentina ABSTRACT The James Webb Space Telescope revealed the existence in the early Universe (z > 4) of large populations of little red dots (LRDs), compact red luminous sources that host rapidly growing super- massive black holes (SMBHs) surrounded by coeval nuclear starbursts. LRDs have defining properties, one of which is the absence of radio detections. LRDs could be radio-undetected because of their large distances. A way to investigate the radio properties of LRDs is to search for radio emission in their local analogs, the Local Little Red Dot (LLRD) galaxies at z < 1 with analogous properties to LRDs. We report the radio continuum detection of the LLRD analog J204837.26−002437.2 (z = 0.4332, hereinafter J2048). This adds to the previous radio detection of two other LLRDs. However, with a flux density of 3.0±0.2 mJy at 3.0 GHz, J2048 is two orders of magnitude more radio luminous than the previous detections. The spectral index of α = -0.39±0.04 is consistent with moderately optically-thick synchrotron emission. The radio luminosity of J2048 is 1.2 × 1041 erg s−1, in the lower end of the range defined by radio-loud giant elliptical galaxies and quasars of LR = 1041−46 erg s−1. The expected radio luminosity of the SMBH associated with J2048 is estimated to be about an order of magnitude larger, suggesting that its radio luminosity could potentially be strongly dimmed. A cosmological (z > 4) LRD with radio luminosity similar to that of J2048 would be detectable using the VLA with moderately long integrations. Keywords: Galaxy formation (595) — Massive stars (732) — Radio continuum emission (1340) — Supermassive black holes (1663) 1. INTRODUCTION Several James Webb Space Telescope (JWST) surve

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where A int A_{\mathrm{int}} is the integration area in beam units, and σ cal \sigma_{\mathrm{cal}} is the calibration error conservatively assumed to be 20% for the 144 MHz images and 10% for the GHz-range archival images when they are used ( Perley and Butler, 2013 ; Sweijen et al., 2022 ) . J0930+4614 and J1120+6711 are well fitted by a single Gaussian component, while for the extended irregular emission of J1643+3156 we found that a 6-Gaussian components fitting provided the best results (the total flux density is 1 σ \sigma in agreement with that measured manually within the 5 ​ σ 5\sigma contours). The major ( θ maj \theta_{\rm maj} ) and minor ( θ min \theta_{\rm min} ) axes are calculated as the deconvolved full-width-half-maximum (FWHM) of the Gaussian fit for J0930+4614 and J1120+6711, while for J1643+3156 complex extended emission we manually measured a size of ∼ 1.9 ′′ × 2.3 ′′ \sim 1.9^{\prime\prime}\times 2.3^{\prime\prime} within its 5 ​ σ 5\sigma contours. In Table 2 we report flux densities in column 1, and θ maj \theta_{\rm maj} and θ min \theta_{\rm min} in columns 4 and 5, respectively.

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Results. We successfully produced high-quality ILT images for the three DAGN in the pilot sample. In one source at z = 1.49 z=1.49 , we detect both AGN components at > 5 ​ σ >5\sigma , making it one of the highest- z z and faintest DAGN detected at radio frequencies. Brightness temperature measurements confirm that ILT 144 MHz observations can efficiently distinguish AGN-powered emission from star formation even in sub-mJy sources. In the other two sources we detect only one AGN. In one of them, the new ILT imaging constrains the low-frequency radio spectrum of the compact steep spectrum source, indicating that its complex morphology is shaped by jet-ISM interaction rather than by the companion galaxy. In addition, for the full 92 DAGN sample, we found a detection rate of ∼ \sim 24% in both LoTSS DR2 and DR3, significantly larger than that of single AGN matched in selection ( ∼ \sim 9–10%), suggesting an enhanced detectability of DAGN in the LoTSS.

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A morphological analysis on a subset of LoRDs with HSC coverage shows that the majority of sources ( > 80 % >80\% ) are compact regardless of their spectral shape (see Section IV.1 ). We find that LoRDs with a V-shaped continuum are more likely to have a resolved morphology compared to sources without the V-shape. This suggests that some of the V-shaped spectra ( ∼ 40 % \sim 40\% ) are the result of an underlying stellar population and thus are unlikely to serve as proper LRD analogs. The remaining ∼ 60 % \sim 60\% of V-shaped LoRDs are proper LRD analog candidates, but space-based imaging is necessary to quantify their compactness.

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In this paper we have shown that the JWST filters used to select LRDs at z ∼ 5 z\sim 5 align well with the SDSS filters at both z = 0.38 z=0.38 and z = 0.78 z=0.78 . We leverage this mapping to photometrically select Local Red Dots (LoRDs) from SDSS using selection criteria similar to the red1/red2 criteria from Greene et al. [45] ; our selection is given in Equations 2 and 3 . In total we find ∼ 1300 \sim 1300 QSOs which share similar photometric colors to the LRDs (Section II.1 ). While not every LoRD we identify serves as a proper analog to LRDs, this is the first step towards studying LRD-like systems in the local Universe at the population level where archival data is plentiful.

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Figure 1: Visual representation of the series of failed attempts to detect the radio continuum emission in AGN discovered by JWST via stacking experiments. Blue points represent the results from stacking experiments of photometrically selected LRDs, while red markers indicate samples of spectroscopically confirmed Type I AGN. The average redshift of the samples are 5 < z < 6 5<z<6 .

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Table 1: Summary table of the stacking attempts performed so far to detect JWST-discovered AGN and LRDs in radio images. The first column refers to the literature work in which the stacking was performed, the second is ’S’ if the sample was spectroscopic and ’P’ if photometric, the third refers to the number of objects in the stack, the fourth to the frequency of the radio image, the fifth the the 3 ​ σ 3\sigma upper limit obtained from the stack.

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The radio non-detections are still consistent with expectations from empirical L X − L H ​ α L_{X}-L_{\text{H}\alpha} and L X − L R L_{X}-L_{R} correlations. From their BH masses and H α \alpha luminosities, we determined an expected radio luminosity of ∼ \sim 10 39 erg s -1 at 1.4 GHz of the population if these empirical relations hold. Note, however, that previous X-ray observations suggest that these are not regular type I AGN. We conclude that our current constraints from radio observations are not deep enough to determine if these JWST discovered AGN candidates are radio-weak.

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This work focuses on five samples of AGN candidates in COSMOS, GOODS-N, and GOODS-S discovered by [ 32 ] , [ 34 ] , [ 26 ] , [ 1 ] , and [ 28 ] . A summary of the fields and samples is given in Table 1 . The sample of [ 32 ] contains 10 sources in GOODS-N and 2 sources in GOODS-S, which have been obtained from the JADES survey [ 10 ] with spectroscopic redshifts ranging from 4.1-6.8. We note that only ∼ \sim 20% of these sources would be photometrically selected as LRD from their colors. The samples of [ 34 ] have been obtained from the EIGER [ 24 ] and FRESCO [ 42 ] surveys in multiple fields including GOODS-N and -S. In their work, they present the discovery of 20 new AGN candidates at z = 4.2 − 5.5 z=4.2-5.5 , including 7 in GOODS-N and 2 in GOODS-S. The AGN candidate samples of [ 26 ] have been obtained from the PRIMER NIRCam survey in COSMOS and the JADES and NGDEEP survey [ 5 ] in GOODS-S. The sample of [ 26 ] contains 81 sources in COSMOS and 56 in GOODS-S, however, only 1 of these sources is spectroscopically confirmed in each field. The vast majority of sources in their sample therefore only have photometric redshifts, ranging from 2.4-11.9 (with a median z phot = 6.1 z_{\text{phot}}=6.1 ). The sample of [ 1 ] contains 434 candidate AGN that have been photometrically selected from the COSMOS-Web survey [ 7 ] with photometric redshift ranging from 4.5-9.4 (median z phot = z_{\text{phot}}= 6.3). Finally, we include 27 and 78 AGN candidates from [ 28 ] in the GOODS-S and COSMOS fields, respectively. These have also been photometrically selected using data from multiple JWST programs: PRIMER (in COSMOS), FRESCO, JADES, and JEMS ( Williams et al. 57 , in GOODS-S).

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A&A proofs: manuscript no. main Table 3. JWST/NIRSpec emission line results for each image of A2744- QSO1 in each epoch of available spectroscopy (see Tab. 2). The line fluxes and underlying continuum flux densities are de-magnified, but the EWs are independent of magnification. Line Image A Image B Image C Epoch 7 Epoch 8 EW0 [Å] Hβ 48+2 −2 53+4 −4 50+2 −2 62+5 −5 Hα 285+7 −6 289+9 −8 307+4 −5 346+11 −11 Fline [10−19 erg cm−2 s−1] Hβ 2.7 ± 0.2 2.9 ± 0.3 2.4 ± 0.3 2.9 ± 0.3 Hα 20.2 ± 0.8 24.7 ± 1.1 18.3 ± 1.4 24.7 ± 1.1 fλ,continuum [10−22 erg cm−2 s−1 Å−1] Hβ 6.9 ± 0.3 6.7 ± 0.3 5.6 ± 0.2 4.8 ± 0.4 Hα 8.8 ± 0.3 10.6 ± 0.4 6.7 ± 0.2 6.6 ± 0.5 −800 −600 −400 −200 0 200 ∆t0 [d] 45 50 55 60 65 70 EW0 [˚A] Image A – Hβ Image B – Hβ Image C – Hβ −6000 −4000 −2000 0 2000 ∆tobs [d] 280 300 320 340 360 EW0 [˚A] Image A – Hα Image B – Hα Image C – Hα Fig. 2. Spectroscopic variability of A2744-QSO1 as a function of time, with image A shown in blue, image B in red, and image C in green as in Fig. 3. –Top: Hβ rest-frame EW, presenting a drop of 22 ± 8 % between images C and A. –Bottom: Hα rest-frame EW, presenting a drop of 18± 3 % in the time between images C and A. We note that the variability is consistent in both broad Balmer lines, as can be expected since they both originate from the broad-line region of A2744-QSO1. The EWs represent excellent indicators of variability since they are insensitive to most systematics such as lensing, slit-losses, calibration, etc. absolute line fluxes, the EWs are neither prone to lensing nor instrumental systematics (e.g. slit-losses), which is why we use them as the primary and most robust indicators of variability (see also Ji et al. 2025). As can be seen in Fig. 2, both Hα and Hβ show a consistent variability in their EW over time2. More quan- titatively, we note a drop in EW of 18 ± 3 % in Hα and 22 ± 8 % in Hβ between the times probed by image C and image A (in both its epochs). The absolute calibration of the red edge of the 2 Here and throughout this letter, we define the time axis in days rel- ative to the UNCOVER imaging (epoch 2 at MJD = 59890.23, see Tab. 2). The relative time is then obtained by adding the difference be- tw

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tics estimated for these magnification values can reach ∼40 % (Zitrin et al. 2015). However, the exact level does not affect the conclusions of this work. Article number, page 3 of 11

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Balmer-absorption AGNs are exceedingly rare in the overall low- z z type 1 population: we identify seven sources among 14,584 objects ( ∼ 0.05 % \sim 0.05\% ). The incidence increases substantially in the low- R Fe ​ II {R}_{\mathrm{Fe\,II}} and high-Eddington ratio locus, reaching ∼ 10 % \sim 10\% when restricting to this rare region of parameter space.

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The absorption lines are intrinsically narrow ( σ ≈ 20 − 200 \sigma\approx 20-200 km ​ s − 1 \mathrm{km\,s^{-1}} ) and often unresolved in SDSS-quality spectra, requiring convolution with the instrumental line-spread function. Detectability also depends on velocity offset and on blending with narrow Balmer emission and the [N II ] λ ​ λ \lambda\lambda 6548, 6584 doublet, leading to strong selection effects at moderate resolution. In line with this, degrading our SDSS/DESI spectra to JWST prism-like resolution would cause some absorbers to be missed, implying that current JWST detections may be biased toward broader and/or more optically thick systems.

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The spectral index of − - 0.85 is typical of optically-thin synchrotron emission and favors a non-thermal mechanism to produce the radio emission and possibly energize the LRDs. The radio source is unresolved at a scale of 0 ​ . ​ 2 ′′ 0\hbox to0.0pt{.\hss}{{}^{\prime\prime}}2 (< 600 pc at d = 700 Mpc).

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Another important result is the evidence of significant time variability in the radio emission. Correcting the 5.0 GHz flux density of 2010 by a factor ( 5.0 / 6.0 ) − 0.85 (5.0/6.0)^{-0.85} to compare with the 2018 flux density at 6.0 GHz, we find that the flux density has increased by a factor of 2.3 ± \pm 0.2 over this 7.5 yr time period. A detectable variability over a timescale of years is not uncommon in AGNs and again points to a SMBH, but as we will see, another explanation is favored. This important change in radio flux density contrasts with the lack of variability at r-band reported for J1047+0739 at the 3-4% level by Burke et al. (2025).