Blackbody Quasar and Radio Source (BBQSORS): A Candidate of Transitional Little Red Dots with a $T\sim10^4\ K$ Blackbody Spectrum

Yuxing Zhong, Xiaoyang Chen, Kohei Ichikawa, Youwen Kong, Kentaro Aoki, Satoshi Yamada, Tohru Nagao, Daisaburo Kido, Toshihiro Kawaguchi, Yoshiki Matsuoka, Toru Misawa, Shoichiro Mizukoshi, Masafusa Onoue, Ayumi Takahashi, Yoshiki Toba

First listed 2026-03-14 | Last updated 2026-04-20

Abstract

We report Subaru/PFS spectroscopic follow-up of a radio-loud quasar at $z=1.715$ from the UNVEIL radio AGN catalog and with X-ray detections. The PFS spectrum displays a broad MgII emission line with an $\mathrm{FWHM}\gtrsim4000\ km/s$, accompanied by a narrow absorption feature. The spectrum reveals a characteristic $Λ$-shape over the rest-frame wavelength ranging $\sim1500-3500\ Å$. This underlying UV continuum is too curved to be reproduced by simply applying dust extinction to the spectrum of typical unobscured quasars. Alternatively, it is well described by a blackbody spectrum with a temperature of $T\approx10000\ K$. This result is in good agreement with its UV to MIR photometry that can be well modeled by three blackbody components representing the SMBH envelope ($\mathit{T}\approx9700\ K$), dust torus ($T\approx1500\ K$), and host galaxy dust ($T\approx80\ K$). The source is marginally detected in the GALEX NUV, revealing a potential V-shaped spectral energy distribution around $1400\ Å$, reminiscent of the spectral feature reported for recently discussed LRDs whose V-shapes occur around $3000-4000\ Å$. This wavelength shift is broadly consistent with the temperature contrast between our blackbody component, with $T\sim10^4\ K$, and the lower effective temperature of $T\sim5000\ K$ expected for an optically thick photosphere surrounding the SMBH in LRDs. These properties suggest that this source might be caught in a transient evolutionary phase in which the dense gas envelope characteristic of LRD has begun to fragment, allowing us to witness the emergence of a quasar from an LRD-like state.

Short digest

Subaru/PFS follow-up of a radio-loud, X-ray–detected UNVEIL source at z=1.715 (BBQSORS) reveals a broad Mg II line (FWHM ≳ 4000 km s−1) with narrow absorbers and a strongly curved, Λ-shaped UV continuum across 1500–3500 Å. The continuum is incompatible with simple dust reddening and is well matched by a T≈10^4 K blackbody; UV–MIR photometry is consistently fit by three blackbodies at ≈9700, 1500, and 80 K. A marginal GALEX NUV detection suggests a V-shaped SED break near 1400 Å—bluer than the 3000–4000 Å breaks seen in canonical LRDs—implying a hotter, thinning envelope around the SMBH. Together with the emerging X-ray and radio emission, the data point to a transitional LRD-to-quasar phase as the cocoon fragments and the nucleus becomes exposed.

Key figures to inspect

• Figure 1: Use the HSC+Spitzer cutouts with VLASS 3 GHz contours to confirm the radio core’s association with BBQSORS and look for a close companion/merger; in the PFS spectrum panels, inspect the Λ-shaped continuum and quantify the Mg II width, the narrow absorber that dips to the continuum, and accompanying Al III ab…
• Figure 2: Check the EPIC/pn 0.4–7.2 keV (rest 1–20 keV) detection and the ≈3.4σ significance; compare the best-fit spectral model and residuals to assess whether emerging hard X-rays are consistent with a dispersing envelope rather than heavy, Compton-thick obscuration.
• Figure 3: Examine the UV–MIR SED decomposition—blackbodies at ≈9700 K (envelope), ≈1500 K (torus), and ≈80 K (host dust)—and locate the V-shaped break near 1400 Å; contrast with the plotted LRD-like ≈5000 K component to see how a hotter envelope shifts the turnover.
• Figure 4: Compare BBQSORS to blue and dust-reddened quasar templates after de-reddening the PFS spectrum; note the persistent blue-end mismatch (<~2200 Å) arguing against dust-only explanations, and use the PFS vs SDSS/DESI overlay to gauge spectral-shape stability across epochs/instruments.