Abstract We report the discovery of Balmer broad absorption lines (BALs) in the quasar LBQS 1206+1052 and present a detailed analysis of the peculiar absorption line spectrum. Besides the Mg II λλ2796, 2803 doublet, BALs are also detected in the He I* multiplet at λλ2946, 3189, 3889 Å arising from the metastable helium 23S level, and in Hα and Hβ from the excited hydrogen H I* n = 2 level, which are rarely seen in quasar spectra. We identify two components in the BAL troughs of ∆v ~2000 km s−1 width: One component shows an identical profile in H I*, He I* and Mg II with its centroid blueshifted by −vc ≈ 726 km s−1 . The other component is detected in He I* and Mg II with −vc ≈ 1412 km s−1. We estimate the column densities of H I*, He I*, and Mg II, and compare them with possible level population mechanisms. Our results favor the scenario that the Balmer BALs originate in a partially ionized region with a column density of NH ~ 1021 − 1022 cm−2 for an electron density of ne ~ 106 − 108 cm−3 via Lyα resonant scattering pumping. The harsh conditions needed may help to explain the rarity of Balmer absorption line systems in quasar spectra. With an i-band PSF magnitude of 16.50, LBQS 1206+1052 is the brightest Balmer-BAL quasar ever reported. Its high brightness and unique spectral properties make LBQS 1206+1052 a promising candidate for follow-up high-resolution spectroscopy, multi-band observations, and long-term monitoring.

Keywords galaxies: active ;quasar: absorption lines ; quasar: individual (LBQS 1206+1052)

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