Related papers: On the redshift of TeV BL Lac objects
PG 1553+113 is a known BL Lac object, newly detected in the GeV-TeV energy range by H.E.S.S and MAGIC. The redshift of this source is unknown and a lower limit of $z > 0.09$ was recently estimated. The very high energy (VHE) spectrum of PG…
Context: BL Lacertae objects have been the targets for numerous recent multiwavelength campaigns, continuum spectral variability studies, and theoretical spectral and variability modeling. A meaningful interpretation of the results of such…
We present Very Large Telescope optical spectroscopy of nine BL Lac objects of unknown redshift belonging to the list of optically selected radio loud BL Lacs candidates. We explore their spectroscopic properties and the possible link with…
We report the first detection of very-high-energy (VHE) gamma-ray emission above 140 GeV from PKS 1424+240, a BL Lac object with an unknown redshift. The photon spectrum above 140 GeV measured by VERITAS is well described by a power law…
We present optical spectroscopy secured at the 10m Gran Telescopio Canarias of the counterparts of 20 extragalactic gamma-ray sources detected by the Fermi satellite. The observations allow us to investigate the nature of these sources and…
We present new spectroscopic data of the BL Lac RGB 2243+203, and its surroundings, obtained with the OSIRIS Multi Object Spectrograph (MOS) mounted in the Gran Telescopio Canarias (GTC). The spectra of neither the BL Lac nor its host…
BL Lac objects (BLL) are defined by the presence of very weak (typically $<$ 5 {\AA}) or even absent spectral lines. This makes determining their distance particularly challenging, especially at high redshift, where the sources are fainter…
In this paper, we present results of TeV $\gamma$--ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (z=0.182) with the $TACTIC$ (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The observations are…
Blazars are active galactic nuclei, and the most numerous High Energy (HE) and Very High Energy (VHE) gamma-ray emitters. Their optical emission is often dominated by non-thermal, and, in the case of BL Lacs, featureless continuum…
Recently, a new method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes has been developed, with the underlying assumption that the Very High Energy (VHE) spectrum corrected…
We present the results of a multi-wavelength study of blazars selected from the 5th ROMABZCAT catalog. We selected from this sample a subsample of 2435 objects having at least one counterpart in one of the three main archival X-ray…
High signal-to-noise ratio spectroscopic observations of the BL Lac object S4 0954+65 at the alleged redshift z = 0.367 are presented. This source was detected at gamma frequencies by MAGIC (TeV) and FERMI (GeV) telescopes during a…
We present the last chapter of a spectroscopy program aimed at deriving the redshift or a lower limit to the redshift of BL Lac objects using medium resolution spectroscopy. Here we report new spectra for 33 BL Lac object candidates…
The bright BL Lac object TXS 0506+056 is a most likely counterpart of the IceCube neutrino event EHE 170922A. The lack of this redshift prevents a comprehensive understanding of the modeling of the source. We present high signal-to-noise…
We present the results of the optical spectroscopy follow-up of a sample of 28 unclassified blazars from the Third Fermi-LAT Catalog of High-Energy Sources (3FHL). All the spectra were taken with the 4m Mayall telescope at Kitt Peak. With…
Cosmic opacity for very high-energy gamma rays ($E>10$ TeV) due to the interaction with the extragalactic background light can be strongly reduced because of possible Lorentz-violating terms in the particle dispersion relations expected,…
Blazars constitute the most numerous source class in the known extragalactic population of very high energy (VHE) gamma-ray sources. However, determining their redshifts is often challenging due to weak or non-existent emission lines in…
The intrinsic BL Lac spectra above few hundreds GeV can be very different from the observed ones due to the absorption effects by the diffuse Extragalactic Background Light (EBL), at present poorly known. With the recent results, there are…
The number of discovered TeV sources populating the extragalactic sky in 2017 is nearly 70, mostly blazars located up to a redshift ~1. Ten years ago, in 2007, less than 20 TeV emitters were known, up to a maximum redshift of 0.2. This is a…
The multi-frequency Sedentary Survey is a flux limited, statistically well-defined sample of highly X-ray dominated BL Lacertae objects (HBLs) which includes 150 sources. In this paper, the third of the series, we report the results of a…