Examinando por Autor "Milisavljevic, D."
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Ítem Optical archival spectra of blazar candidates of uncertain type in the 3rd fermi large area telescope catalog(Springer Netherlands, 2016-09) Álvarez Crespo, N.; Massaro, F.; D’Abrusco, R.; Landoni, M.; Masetti, N.; Chavushyan, V.; Jiménez-Bailón, E.; La Franca, F.; Milisavljevic, D.; Paggi, A.; Patiño-Álvarez, V.; Ricci, F.; Smith, Howard A.Despite the fact that blazars constitute the rarest class among active galactic nuclei (AGNs) they are the largest known population of associated γ -ray sources. Many of the γ -ray objects listed in the Fermi-Large Area Telescope Third Source catalog (3FGL) are classified as blazar candidates of uncertain type (BCUs), either because they show multifrequency behavior similar to blazars but lacking optical spectra in the literature, or because the quality of such spectra is too low to confirm their nature. Here we select, out of 585 BCUs in the 3FGL, 42 BCUs which we identify as probable blazars by their WISE infrared colors and which also have optical spectra that are available in the Sloan Digital Sky Survey (SDSS) and/or Six-Degree Field Galaxy Survey Database (6dFGS). We confirm the blazar nature of all of the sources. We furthermore conclude that 28 of them are BL Lacs, 8 are radio-loud quasars with flat radio spectrum and 6 are BL Lac whose emission is dominated by their host galaxy.Ítem Optical spectroscopic observations of gamma-ray blazar candidates VIII: the 2016–2017 follow up campaign carried out at SPM, NOT, KPNO and SOAR telescopes(Astrophysics and Space Science, 2019-01-01) Marchesini, E.J.; Peña-Herazo, H.A.; Álvarez Crespo, N.; Ricci, F.; Negro, M.; Milisavljevic, D.; Massaro, F.; Masetti, N.; Landoni, M.; Chavushyan, V.; D’Abrusco, R.; Jiménez-Bailón, E.; La Franca, F.; Paggi, A.; Smith, H.A.; Tosti, G.The third Fermi source catalog lists 3033 γ-ray sources above 4 σ significance. More than 30% are classified as either unidentified/unassociated Gamma-ray sources (UGSs), with about 20% classified as Blazar candidates of uncertain types (BCUs). To confirm the blazar-like nature of candidate counterparts of UGSs and BCUs, we started in 2012 an optical spectroscopic follow up campaign. We report here the spectra of 36 targets with observations from the Observatorio Astronómico Nacional San Pedro Mártir, the Southern Astrophysical Research Observatory, the Kitt Peak National Observatory and the Northern Optical Telescope, between 2016 and 2017. We confirm the BL Lac nature of 23 sources, and the flat spectrum radio quasar nature of other 7 ones. We also provide redshift estimates for 19 out of these 30 confirmations, with only one being a lower limit due to spectral features ascribable to intervening systems along the line of sight. As in previous analyses, the largest fraction of now-classified BCUs belong to the class of BL Lac objects, that appear to be the most elusive class of active galactic nuclei. One of the BL Lacs identified in this work, associated with 3FGL J2213.6-4755, lies at a redshift of z> 1.529 , making it one of the few distant gamma-ray BL Lac objects.Ítem Optical spectroscopic observations of gamma-ray blazar candidates. VII. Follow-up campaign in the southern hemisphere(Springer Netherlands, 2017-12) Peña-Herazo, H.A.; Marchesini, E.J.; Álvarez Crespo, N.; Ricci, F.; Massaro, F.; Chavushyan, V.; Landoni, M.; Strader, J.; Chomiuk, L.; Cheung, C.C.; Masetti, N.; Jiménez-Bailón, E.; D’Abrusco, R.; Paggi, A.; Milisavljevic, D.; La Franca, F.; Smith, H.A.; Tosti, G.Searching for low energy counterparts of γ-rays sources is one of the major challenges in modern γ-ray astronomy. In the third Fermi source catalog about 30% of detected sources are unidentified/unassociated Gamma-ray Sources (UGSs). We recently started an optical spectroscopic follow up campaign to confirm the blazar-like nature of candidates counterparts of UGSs. Here we report the spectra of 61 targets collected with the Southern Astrophysical Research Telescope (SOAR) between 2014 and the 2017. Our sample includes 33 potential counterparts of UGSs, selected on the basis of WISE colors, and 27 blazar candidates of uncertain type associated with gamma-ray sources of the last release of the Fermi catalog. We confirm the BZB nature of 20 sources lying within the positional uncertainty region of the UGSs. All the observed BCUs show blazar-like spectra, classified as 2 BZQs and 25 BZBs, for which we obtained 6 redshift estimates. Within the BCUs observations we report the redshift estimate for the BZB associated with, 3FGL J1106.4-3643 that is the second most distant BL Lac known to date, at z≥ 1.084. © 2017, Springer Science+Business Media B.V., part of Springer Nature.Ítem The gamma-ray blazar quest: new optical spectra, state of art and future perspectives(Springer Netherlands, 2016-10) Massaro, F.; Álvarez Crespo, N.; D’Abrusco, R.; Landoni, M.; Masetti, N.; Ricci, F.; Milisavljevic, D.; Paggi, A.; Chavushyan, V.; Jiménez-Bailón, E.; Patiño-Álvarez, V.; Strader, J.; Chomiuk, L.; La Franca, F.; Smith, Howard A.; Tosti, G.We recently developed a procedure to recognize γ -ray blazar candidates within the positional uncertainty regions of the unidentified/unassociated γ -ray sources (UGSs). Such procedure was based on the discovery that Fermi blazars show peculiar infrared colors. However, to confirm the real nature of the selected candidates, optical spectroscopic data are necessary. Thus, we performed an extensive archival search for spectra available in the literature in parallel with an optical spectroscopic campaign aimed to reveal and confirm the nature of the selected γ -ray blazar candidates. Here, we first search for optical spectra of a selected sample of γ -ray blazar candidates that can be potential counterparts of UGSs using the Sloan Digital Sky Survey (SDSS DR12). This search enables us to update the archival search carried out to date. We also describe the state-of-art and the future perspectives of our campaign to discover previously unknown γ -ray blazars.