Marco Landoni

ESO VERY LARGE TELESCOPE OPTICAL SPECTROSCOPY OF BL LACERTAE OBJECTS. IV. NEW SPECTRA AND PROPERTIES OF THE FULL SAMPLE

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 obtained in 2008-2009 confirming the BL Lac nature of 25 sources and for 5 objects we obtained new redshifts. These new observations are combined with our previous data in order to construct a homogeneous sample of � 70 BL Lacs with high quality spectroscopy. All these spectra can be accessed at the website http://www.oapd.inaf.it/zbllac/. The average spectrum, beaming properties of the full sample, discussion on intervening systems and future perspectives are addressed. Subject headings: BL Lacertae objects: general, Galaxies: quasars: absorption lines. Instrumentation: spectrographs

On the redshift of the bright BL Lacertae object PKS 0048-097

Aims. We attempt to determine the spectroscopic redshifts of bright BL Lac objects, which have proven difficult to measure. Methods. We use the spectra acquired using the newly available spectrograph X-Shooter at the European Southern Observatory Very Large Telescope, which combines high resolution with a large wavelength range, to obtain ultraviolet to near-infrared spectra of BL Lacs. Results. Our observations of PKS 0048-097 detect three emission lines, which permit us to derive a redshift z = 0.635. Moreover, we discover a Mg II absorption system at z = 0.154 that is associated with a foreground spiral galaxy a 50 kpc of projected distance. Conclusions. The measured redshift allows us to discuss the optical beaming factor and the absorption of the high energy spectrum by the extragalactic background light.

Optical spectroscopic observations of γ-ray blazar candidates - II. The 2013 KPNO campaign in the northern hemisphere

Context. We recently started a systematic search of low-energy counterparts of the unidentified γ-ray sources (UGSs) listed in the Fermi-Large Area Telescope (LAT) First Source Catalog (1FGL) and the Fermi-LAT 2-Year Source Catalog (2FGL). Aims. The main goal of our investigation is to find active galaxies belonging to the blazar class that lie within the positional uncertainty region of the UGSs and thus could be their potential low-energy counterparts. Methods. To achieve our aims, we first adopted several procedures based on the peculiar observational properties of blazars in the radio and in the IR. Then we carried out a follow-up spectroscopic campaign in the optical band to verify the nature of the candidates selected as potential counterparts of the UGSs. Here we present the results of the observations carried out in 2013 in the northern hemisphere at Kitt Peak National Observatory (KPNO). Optical spectroscopy is crucial to confirm the nature of the sources and can be used to estimate their redshifts; it will also allow us to test the robustness of our methods when the whole campaign is completed. Results. Here we present the optical spectroscopic observations of 39 sources. Within our sample we found that 6 sources are blazars, candidates to be low-energy counterparts of the UGSs listed in the 2FGL. We confirm that an additional 8 sources, previously classified as active galaxies of uncertain type and associated in the 2FGL, are also all BL Lac objects. Moreover, we also present 20 new spectra for known blazars listed in the Multi-frequency Catalogue of Blazars as having an uncertain redshift and/or being classified as BL Lac candidates. Conclusions. We conclude that our methods for selecting γ-ray blazar candidates allows us to discover new blazars and increase the list of potential low-energy counterparts for the Fermi UGSs.

Spectroscopy of BL Lacertae objects of extraordinary luminosity

Aims. We aim to determine the redshift (or stringent lower limits) of a number of bright BL Lacs objects. Methods. We secured medium resolution optical and near-infrared spectra of 4 bright BL Lac objects of unknown redshift using the spectrograph X-Shooter at the ESO-VLT. Results. In spite of the high quality of the spectra and the extended spectral range of the observations we have not detected intrisic spectral features for these sources. However we are able to provide strigent lower limits to their redshift. In particular, for the two TeV sources PG 1553+113 and H 1722+119 we infer z > 0:30 and z > 0:35 respectively. We also detect an intervening Ca II absorption doublet in the spectrum of MH 2136-428 that is ascribed to the the halo of a nearby giant elliptical galaxy at 100 kpc of projected distance. Conclusions. Under the hypothesis that all BL Lacs are hosted by luminous bulge dominated galaxies, the present state of art spectroscopic observations of bright BL Lacs indicate that these objects are likely sources with extremely beamed nuclear emission . We present simulations to show under which circustances it will be possible to probe this hypothesis from the detection of very weak absorptions using the next generation of extremely large optical telescopes.

Optical spectroscopic observations of

Blazars, one of the most extreme class of active galaxies, constitute so far the largest known population of

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-ray blazar candidates VI. Further observations from TNG, WHT, OAN, SOAR and Magellan telescopes

-ray sources and their number is continuously growing in the Fermi catalogs. However in the latest release of the Fermi catalog there is still a large fraction of sources that are classified as blazar candidate of uncertain type (BCUs) for which optical spectroscopic observations are necessary to confirm their nature and their associations. In addition about 1/3 of the

WHAT IS THE REDSHIFT OF THE GAMMA-RAY BL LAC SOURCE S4 0954+65?

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OPTICAL SPECTROSCOPIC OBSERVATIONS OF GAMMA-RAY BLAZAR CANDIDATES. IV. RESULTS OF THE 2014 FOLLOW-UP CAMPAIGN

-ray point sources listed in the Third Fermi-LAT Source Catalog (3FGL) are still unassociated and lacking an assigned lower energy counterpart. Since 2012 we have been carrying out an optical spectroscopic campaign to observe blazar candidates to confirm their nature. In this paper, the sixth of the series, we present optical spectroscopic observations for 30

SPECTROSCOPY OF OPTICALLY SELECTED BL LAC OBJECTS AND THEIR γ-RAY EMISSION

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