Discovering High-$z$ BL Lacs Using Swift/UVOT and SARA Observations with the Dropout Technique

TL;DR

This work tackles the challenge of obtaining redshifts for featureless BL Lacs by applying the photometric dropout technique to a 64-blazar sample using multi-band data from Swift/UVOT and SARA, analyzed with LePHARE. The authors report four new high-$z$ BL Lacs ($z>1.3$) and provide upper limits for 50 sources, increasing the known high-$z$ BL Lacs to 23. They validate redshift estimates with template degeneracies and cross-checks, and interpret the results in the context of the CGRH, blazar sequence, and Fermi blazar divide, highlighting the potential for masquerading BL Lacs. These findings bolster constraints on Extragalactic Background Light models at high redshift and suggest that redshift incompleteness may bias population studies, while also identifying promising masquerading BL Lac candidates for further follow-up.

Abstract

Measuring spectroscopic redshifts for BL Lacertae (BL Lac) objects, a class of blazar, is challenging because their optical spectrum lacks, or has weak, emission lines ( equivalent width $\leqslant5Å$). In this situation, alternative techniques are necessary for the estimation of distances to these sources. In this paper, we estimate the redshift by the photometric dropout technique for a sample of 64 blazars (59 BL Lacs and five blazar candidates of uncertain type). Two telescopes are utilized to observe the sample. The Ultraviolet/Optical Telescope (UVOT) on board {\it Swift} ({\it Swift}/UVOT) observes sources in $uvw2,\ uvm2,\ uvw1,\ u,\ b,\ v$ filters, while the ground-based telescopes SARA-CT/RM observed sources in $g',\ r,' \ i',\ z'$ filters. We fit the photometric data with the LePHARE package and report four new high-$z$ ($z>1.3$) BL Lacs at $2.03^{+0.07}_{-0.05}$, $1.84^{+0.10}_{-0.03}$, $2.04^{+0.16}_{-0.14}$, and $2.93^{+0.01}_{-0.04}$ as well as upper limits for 50 sources. This work increased the number of high-$z$ BL Lacs found by this method up to 23. The high-$z$ sources are discussed in the context of the cosmic gamma-ray horizon, blazar sequence, Fermi blazar divide, and masquerading BL Lacs.

Figures (5)

• Figure 1: The illustration of the aperture and background regions. The source aperture and background annulus for SARA data. The FWHM is measured in pixel scale. The aperture is centered at the target source with a radius of $3\times\text{FWHM}$. The background region is an annulus with an inner radius of $5\times\text{FWHM}$ and an outer radius of $8\times\text{FWHM}$, which ensures the background region area is $\sim4$ times the aperture one.
• Figure 2: The SEDs of the four high-redshift BL Lacs. The $x$ axis is the wavelength while the y axis is the magnitudes. The solid circles with black edges are photometric data from Table \ref{['tab:result']}. The magnitudes are ordered in the following the central wavelength of the filters: $uw2,\ um2,\ uw1,\ uuu,\ ubb,\ g',\ uvv,\ r',\ i',\ z'$. The solid line represents power-law model while dashed line represents a galaxy model.
• Figure 3: The cosmic gamma-ray horizon plot. The grey circles are from the 4LAC blazars. The blue and orange stars are the high-$z$ BL Lacs found by the Photo-$z$ campaign Rau2012kaur2017kaur2018Rajagopal_2020sheng2024revealing including this work. Note that only high-$z$ BL Lac reported with highest energy photon (HEP) are plotted here. The green stars are BL Lacs with spectroscopic redshift determined by marchesi2018identifyingdesai2019identifyingpaiano2020opticalrajagopal2021identifyinggoldoni2021opticalgarcia2023opticalkasai2023optical.
• Figure 4: The powerlaw fit to the SED data. The data are obtained from the 4FGL and 3FHL catalogs and plotted using green and purple dots, respectively. The 3FHL data for 4FGL J1823.5$+$6858 is not available. Therefore, only the 4FGL data is shown in the plot. The green bands are the uncertainties from the Fermi-LAT data reduction and include statistical uncertainties. The black dotted curve is the best joint power-law fit, which is attenuated by $e^{-\tau(E,z)}$ due to the EBL absorption saldana2021observationalfinke2022modelingdominguez2024new_ebl_model, showing the energy at where the cutoff is. The redshift uncertainty has a small effect on the SED fit, indicated by the gray-shaded uncertainty region of the SED curve.
• Figure 5: The blazar sequence and Fermi blazar divide plot. The colored dots are calculated from the 4LAC catalog Ajello_2020. The triangles are the previous high-$z$ BL Lacs Rau2012kaur2017kaur2018Rajagopal_2020, while the squares are the ones found in this work. The vertical dotted lines divides the blazars to LSP, ISP, and HSP groups. The orange crosses are the masquerading BL Lacs from mbl_lacspaiano2023spectrasahakyan2023multimessenger. The indices are not corrected for the EBL absorption. Note that in (d), 4FGL J1125.1$-$2101 overlaps with 4FGL J1823.5$+$6858.