Optical spectroscopic characterization of Fermi blazar candidates of uncertain type with TNG and DOT: First Results

TL;DR

This study uses optical spectroscopy from TNG and DOT telescopes to classify 27 Fermi gamma-ray blazar candidates of uncertain type, identifying emission lines and redshifts to improve their classification.

Contribution

First optical spectroscopic characterization of 27 BCUs using 3-4m class telescopes, providing new redshift measurements and spectral classifications.

Findings

8 sources show broad emission lines, with 3 having redshifts >1

2 spectra dominated by host galaxy absorption features

16 sources have featureless spectra

Abstract

The classification of gamma-ray-detected blazar candidates of uncertain type (BCU) is a relevant problem in extragalactic gamma-ray astronomy. Here we report the optical spectroscopic characterization, using two 3-4~m class telescopes, Telescopio Nazionale Galileo and Devasthal Optical Telescope, of 27 BCUs detected with the Fermi Large Area Telescope. Since the identification of emission lines is easier in broad-line blazars, which usually exhibit low frequency peaked (synchrotron peak frequency $\leqslant10^{14}$ Hz) spectral energy distribution, we primarily target such BCUs. We found that 8 out of 27 sources exhibit broad emission lines in their optical spectra, 3 of them have redshifts $>$1 and the farthest one is at $z=2.55$. The optical spectra of 2 of the 19 remaining objects are dominated by the absorption spectra of the host galaxy, and there is a tentative detection of the Lyman-$\alpha$ absorption feature in one source. The spectra of the remaining 16 objects, on the other hand, are found to be featureless.