Weak-emission-line quasar SDSS J101353.45+492758.1

I. Continuum fitting

¹ Faculty of Physics, University of Białystok, ul. Ciołkowskiego 1L, 15-245 Białystok, Poland
² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, Warsaw, Poland
³ National Centre for Nuclear Research, Astrophysics Division, ul. Pasteura 7, 02-093 Warsaw, Poland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 December 2025
Accepted: 2 March 2026

Abstract

Context. Weak-emission-line quasars (WLQs) are active galactic nuclei (AGNs) characterized by unusually faint or absent broad emission lines. A subset also exhibits pronounced X-ray weakness, offering keys insights into accretion flow structure and the physical state of the broad-line region.

Aims. We present a broadband study of the WLQ SDSS J101353.45+492758.1, which displays a nearly featureless UV–optical spectrum with only a weak Mg II line alongside an exceptionally low X-ray flux.

Methods. We modeled its spectral energy distribution using the relativistic thin-disk model kerrbb with a power law and the multicomponent AGN model relagn, a physically motivated extension of agnsed that incorporates warm and hot Comptonizing regions. Our fits constrain the black hole (BH) mass, accretion rate, X-ray loudness, and coronal energetics.

Results. Both approaches yield consistent BH masses of M_BH ≈ 2 × 10⁹ M_⊙ and an Eddington accretion rate of ṁ ≈ 0.1. The relagn fit, which includes a warm Comptonizing region, provides a significantly improved representation of the UV-soft X-ray continuum. The warm corona, characterized by kT_e ≃ 0.20 keV, Γ ≃ 3.8, and an optical depth τ ≃ 7.26, extends to ∼34 R_g. The hot corona appears compact and energetically suppressed, leading to an intrinsically weak X-ray output with log(L_X/L_bol)≃ − 4.29, among the lowest reported for WLQs. The α_OX ∼ 2.06 indicates the source is in a high (soft) AGN spectral state.

Conclusions. The combination of a luminous, standard disk and an extremely weak hot corona suggests that this quasar hosts a highly inefficient inner coronal region. This explains its X-ray faintness and extreme deficit of high-ionization emission lines. The source may represent an AGN analog in an “ultrasoft” accretion state, or a system in which the ionizing continuum is suppressed by a compact or quenched corona. Our study suggests that the source is not accreting at a high Eddington ratio, highlighting the physical diversity of WLQs, and supports the view that geometric and radiative effects jointly shape WLQs’ extreme spectral properties.