BL Lacertae objects beyond redshift 1.3 – UV-to-NIR photometry and photometric redshift forFermi/LAT blazars

Abstract

Context. Observations of the γ-ray sky with Fermi led to significant advances towards understanding blazars, the most extreme class of active galactic nuclei. A large fraction of the population detected by Fermi is formed by BL Lacertae (BL Lac) objects, whose sample has always suffered from a severe redshift incompleteness due to the quasi-featureless optical spectra. Aims. Our goal is to provide a significant increase of the number of confirmed high-redshift BL Lac objects contained in the 2 LAC Fermi/LAT cataloge. Methods. For 103 Fermi/LAT blazars, photometric redshifts using spectral energy distribution fitting have been obtained. The photometry includes 13 broad-band filters from the far ultraviolet to the near-IR observed with Swift/UVOT and the multi-channel imager GROND at the MPG/ESO 2.2 m telescope. Data have been taken quasi-simultaneously and the remaining source-intrinsic variability has been corrected for. Results. We release the UV-to-near-IR 13-band photometry for all 103 sources and provide redshift constraints for 75 sources without previously known redshift. Out of those, eight have reliable photometric redshifts at z > ∼ 1.3, while for the other 67 sources we provide upper limits. Six of the former eight are BL Lac objects, which quadruples the sample of confirmed high-redshift BL Lac. This includes three sources with redshifts higher than the previous record for BL Lac, including CRATES J0402-2615, with the best-fit solution at z ≈ 1.9.

NEW HIGH-z FERMI BL LACS WITH THE PHOTOMETRIC DROPOUT TECHNIQUE

“…However, since Swift-UVOT cycles through every filter, in addition to the non-simultaneous GROND-Swift observations, these variations in the emission could add to the existing uncertainties. Rau et al (2012) established that the inherent blazar variability introduces a systematic uncertainty of Δm=0.1 mag for each UVOT filter. The other important factor to consider is that the total SED is obtained by combining the UVOT and GROND data, which are two different instruments, which thus need to be calibrated against each other.…”

“…Estimating redshifts of BL Lacs in the traditional spectroscopic way is yet another challenge, because of the weakness, or even absence of lines (e.g., Shaw et al 2013;Massaro et al 2015;Álvarez-Crespo et al 2016). Rau et al (2012) initiated a program to determine redshifts for BL Lac sources via a photometric technique. The underlying principle for this photometric redshift (photo-z) determination is based on the absorption of UV photons by neutral hydrogen along the line of sight, which absorbs photons bluewards of the Lyman limit.…”

“…This leads to a dropout in the flux at the Lyman limit whose position in the SED can be used to determine the photo-z. This approach was applied by Rau et al (2012) for a sample of 103 blazars.…”

“…Only 19 BL Lacs with > z 1.3 are known, of which 13 are reported in the third catalog of Fermi-detected AGNs (3LAC, Ackermann et al 2015). A total of 9BL Lacs were reported in Rau et al (2012), of which 3sources are provided among the 13 in the 3LAC catalog. Therefore, Rau et al (2012) provided sixnew high-z BL Lacs with the photometric technique.…”

“…A total of 9BL Lacs were reported in Rau et al (2012), of which 3sources are provided among the 13 in the 3LAC catalog. Therefore, Rau et al (2012) provided sixnew high-z BL Lacs with the photometric technique. The outline of this paper is as follows.Section 2 explains the details of observations.…”