Optical spectroscopy of SDSS J004054.65-0915268: three possible scenarios for the classification. A z \sim 5 BL Lacertae, blue FSRQ or a weak emission line quasar

TL;DR

This study investigates the nature of a high-redshift object, SDSS J004054.65-0915268, using optical-near IR spectroscopy and SED modeling to determine if it is a BL Lac, blue FSRQ, or weak emission line quasar.

Contribution

The paper provides new optical-near IR spectra and comprehensive SED modeling to explore the classification of a distant BL Lac candidate, considering three possible scenarios.

Findings

No emission lines detected, setting upper limits on C IV luminosity.

Spectral energy distribution fits are consistent with a BL Lac or other scenarios.

Optical variability and radio flux data suggest a thermalized accretion disk.

Abstract

The haunt of high redshift BL Lacerate objects is day by day more compelling, to firmly understand their intrinsic nature and evolution. SDSS J004054.65-0915268 is, at the moment, one of the most distant BL Lac candidate at z \sim 5 (Plotkin et al 2010). We present a new optical-near IR spectrum obtained with ALFOSC-NOT with a new, custom designed dispersive grating aimed to detect broad emission lines that could disprove this classification. In the obtained spectra we do not detect any emission features and we provide an upper limit to the luminosity of the C IV broad emission line. Therefore, the nature of the object is then discussed, building the overall spectral energy distribution and fitting it with three different models. Our fits, based on the SED modeling with different possible scenarios, cannot rule out the possibility that this source is indeed a BL Lac object although, the absence of optical variability and lack of strong radio flux, they seems to suggest that the observed optical emission originate from a thermalized accretion disk.