Browsing by Author "Chavushyan, V."

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    Accurate classification of 75 counterparts of objects detected in the 54-month Palermo Swift/BAT hard X-ray catalogue
    (2014) Parisi, P.; Masetti, N.; Rojas, A. F.; Jiménez Bailón, E.; Chavushyan, V.; Palazzi, E.; Bassani, L.; Bazzano, A.; Bird, A. J.; Galaz, Gaspar; Minniti, D.; Morelli, L.; Ubertini, P.; Parisi, P.; Masetti, N.; Rojas, A. F.; Jiménez Bailón, E.; Chavushyan, V.; Palazzi, E.; Bassani, L.; Bazzano, A.; Bird, A. J.; Galaz, Gaspar; Minniti, D.; Morelli, L.; Ubertini, P.
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    AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system
    (EDP Sciences, 2024) Hernández-García, L.; Muñoz-Arancibia, A. M.; Lira, P.; Bruni, G.; Cuadra, J.; Arévalo, P.; Sánchez-Sáez, P.; Bernal, S.; Bauer, Franz Erik; Catelan, Márcio; Panessa, F.; Pávez-Herrera, M.; Ricci, C.; Reyes-Jainaga, I.; Ailawadhi, B.; Chavushyan, V.; Dastidar, R.; Deconto-Machado, A.; Forster, F.; Gangopadhyay, A.; García-Pérez, A.; Márquez, I.; Masegosa, J.; Misra, K.; Patiño-Alvarez, V. M.; Puig-Subira, M.; Rodi, J.; Singh, M.
    With a growing number of facilities able to monitor the entire sky and produce light curves with a cadence of days, in recent years there has been an increased rate of detection of sources whose variability deviates from standard behavior, revealing a variety of exotic nuclear transients. The aim of the present study is to disentangle the nature of the transient AT 2021hdr, whose optical light curve used to be consistent with a classic Seyfert 1 nucleus, which was also confirmed by its optical spectrum and high-energy properties. From late 2021, AT 2021hdr started to present sudden brightening episodes in the form of oscillating peaks in the Zwicky Transient Facility (ZTF) alert stream, and the same shape is observed in X-rays and UV from Swift data. The oscillations occur every ≈60-90 days with amplitudes of ≈0.2 mag in the g and r bands. Very Long Baseline Array (VLBA) observations show no radio emission at milliarcseconds scale. It is argued that these findings are inconsistent with a standard tidal disruption event (TDE), a binary supermassive black hole (BSMBH), or a changing-look active galactic nucleus (AGN); neither does this object resemble previous observed AGN flares, and disk or jet instabilities are an unlikely scenario. Here, we propose that the behavior of AT 2021hdr might be due to the tidal disruption of a gas cloud by a BSMBH. In this scenario, we estimate that the putative binary has a separation of ≈0.83 mpc and would merge in ≈7 × 104 years. This galaxy is located at 9 kpc from a companion galaxy, and in this work we report this merger for the first time. The oscillations are not related to the companion galaxy.
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    BL Lacertae identifications in a ROSAT-selected sample of Fermi unidentified objects
    (2013) Masetti, N.; Sbarufatti, B.; Parisi, P.; Jimenez-Bailon, E.; Chavushyan, V.; Vogt, F. P. A.; Sguera, V.; Stephen, J. B.; Palazzi, E.; Bassani, L.; Bazzano, A.; Fiocchi, M.; Galaz, G.; Landi, R.; Malizia, A.; Minniti, D.; Morelli, L.; Ubertini, P.
    The optical spectroscopic followup of 27 sources belonging to a sample of 30 high-energy objects selected by positionally cross correlating the first Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is presented here. It has been found or confirmed that 25 of them are BL Lacertae objects (BL Lacs), while the remaining two are Galactic cataclysmic variables (CVs). This strongly suggests that the sources in the first group are responsible for the GeV emission detected with Fermi, while the two CVs most likely represent spurious associations. We thus find an 80% a posteriori probability that the sources selected by matching GeV and X-ray catalogs belong to the BL Lac class. We also show suggestions that the BL Lacs selected with this approach are probably high-synchrotron-peaked sources and in turn good candidates for the detection of ultra-high-energy (TeV) photons from them.
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    Observations of the First Electromagnetic Counterpart to a Gravitational-wave Source by the TOROS Collaboration
    (2017) Díaz, Marco A.; Macri, L.; Lambas, D.; Oliveira, C.; Castellón, J.; Ribeiro, T.; Sánchez, B.; Schoenell, W.; Abramo, L.; Padilla, Nelson; Akras, S.; Alcaniz, J.; Artola, R.; Beroiz, M.; Bonoli, S.; Cabral, J.; Camuccio, R.; Castillo, M.; Chavushyan, V.; Coelho, P.; Colazo, C.
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    Optical spectroscopic observations of gamma-ray blazar candidates VIII : the 2016-2017 follow up campaign carried out at SPM, NOT, KPNO and SOAR telescopes
    (2019) Marchesini, E.J.; Pena Herazo, H.A.; Crespo, N. Alvarez; Ricci, Claudio; Negro, M.; Milisav, D.; Massaro, F.; Masetti, N.; Landoni, M.; Chavushyan, V.
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    Optical spectroscopic observations of gamma-ray blazar candidates. IX. Optical archival spectra and further observations from SOAR and OAGH
    (2019) Pena-Herazo, H.A.; Massaro, F.; Chavushyan, V.; Marchesini, E.J.; Paggi, A.; Landoni, M.; Masetti, N.; Ricci, Claudio; D'Abrusco, R.; Milisavljevic, D.; Jiménez-Bailón, E.; La Franca, F.; Smith, Howard A.; Tosti, G.
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    Optical spectroscopic observations of low-energy counterparts of Fermi-LAT ∼-ray sources
    (2020) Pena-Herazo, H. A.; Amaya-Almazan, R. A.; Massaro, F.; de Menezes, R.; Marchesini, E. J.; Chavushyan, V.; Paggi, A.; Landoni, M.; Masetti, N.; Ricci, F.; D'Abrusco, R.; Cheung, C. C.; La Franca, F.; Smith, H. A.; Milisavljevic, D.; Jimenez-Bailon, E.; Patino-Alvarez, V. M.; Tosti, G.
    Context. A significant fraction of all gamma-ray sources detected by the Large Area Telescope aboard the Fermi satellite is still lacking a low-energy counterpart. In addition, there is still a large population of gamma-ray sources with associated low-energy counterparts that lack firm classifications. In the last 10 years we have undertaken an optical spectroscopic campaign to address the problem of unassociated or unidentified gamma-ray sources (UGSs), mainly devoted to observing blazars and blazar candidates because they are the largest population of gamma-ray sources associated to date.Aims. Here we describe the overall impact of our optical spectroscopic campaign on sources associated in Fermi-LAT catalogs, coupled with objects found in the literature. In the literature search we kept track of efforts by different teams that presented optical spectra of counterparts or potential counterparts of Fermi-LAT catalog sources. Our summary includes an analysis of additional 30 newly collected optical spectra of counterparts or potential counterparts of Fermi-LAT sources of a previously unknown nature.Methods. New spectra were acquired at the Blanco 4 m and OAN-SPM 2.1 m telescopes, and those available in the Sloan Digital Sky Survey (data release 15) archive.Results. All new sources with optical spectra analyzed here are classified as blazars. Thanks to our campaign, altogether we discovered and classified 394 targets with an additional 123 objects collected from a literature search. We began our optical spectroscopic campaign between the release of the second and third Fermi-LAT source catalogs (2FGL and 3FGL, respectively), classified about 25% of the sources that had uncertain nature and discovered a blazar-like potential counterpart for similar to 10% of UGSs listed therein. In the 4FGL catalog, about 350 Fermi-LAT sources have been classified to date thanks to our campaign.Conclusions. The most elusive class of blazars are found to be BL Lacs since the largest fraction of Fermi-LAT sources targeted in our observations showed a featureless optical spectrum. The same conclusion applied to the literature spectra. Finally, we confirm the high reliability of mid-IR color-based methods to select blazar-like candidate counterparts of unassociated or unidentified gamma-ray sources.
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    Searching for active galactic nuclei among unidentified INTEGRAL sources
    (2011) Maiorano, E.; Landi, R.; Stephen, J. B.; Bassani, L.; Masetti, N.; Parisi, P.; Palazzi, E.; Parma, P.; Bird, A. J.; Bazzano, A.; Ubertini, P.; Jimenez-Bailon, E.; Chavushyan, V.; Galaz, G.; Minniti, D.; Morelli, L.
    We report on a new method to identify active galactic nuclei (AGNs) among unidentified INTEGRAL sources. This method consists of cross-correlating unidentified sources listed in the fourth Imager on Board the INTEGRAL Satellite (IBIS) Survey Catalogue first with infrared and then with radio catalogues and a posteriori verifying, by means of X-ray and optical follow-up observations, the likelihood of these associations. In order to test this method, a sample of eight sources has been extracted from the fourth IBIS catalogue. For seven sources of the sample, we obtained an identification, whereas the last one (IGR J03103+5706) has insufficient information for a clear classification and deserves more in-depth study. We identified three objects (IGR J08190-3835, IGR J17520-6018 and IGR J21441+4640) as AGNs and suggest that three more (IGR J00556+7708, IGRJ17219-1509 and IGR J21268+6203) are likely active galaxies on the basis of their radio spectra, near-infrared photometry and location above the Galaxy plane. One source (IGR J05583-1257) has been classified as a starburst galaxy, but it might have been spuriously associated with the INTEGRAL detection.
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    The nature of 50 Palermo Swift-BAT hard X-ray objects through optical spectroscopy
    (2017) Rojas, A. F.; Masetti, N.; Minniti, D.; Jiménez Bailón, E.; Chavushyan, V.; Hau, G.; McBride, V. A.; Bassani, L.; Bazzano, A.; Galaz, Gaspar; Rojas, A. F.; Masetti, N.; Minniti, D.; Jiménez Bailón, E.; Chavushyan, V.; Hau, G.; McBride, V. A.; Bassani, L.; Bazzano, A.; Galaz, Gaspar
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    TOROS optical follow-up of the advanced LIGO–VIRGO O2 second observational campaign
    (2020) Artola, R.; Beroiz, M.; Cabral, J.; Camuccio, R.; Castillo, M.; Chavushyan, V.; Colazo, C.; Cuevas, H.; DePoy, D. L.; Díaz, M. C.; Domínguez, M.; Dultzin, D.; Fernández, D.; Ferreyra, A. C.; Fonrouge, A.; Franco, J.; Graña, D.; Girardini, C.; Gurovich, S.; Kanaan, A.; Lambas, D. G.; Lares, M.; Hinojosa, A. F.; Hinojosa, A.; Hinojosa, A. F.; López-Cruz, O.; Macri, L. M.; Marshall, J. L.; Melia, R.; Mendoza, W.; Castelló, J. L. N.; Padilla, N.; Perez, V.; Peñuela, T.; Rattray, W.; Renzi, V.; Ríos-López, E.; Rivera, A. R.; Ribeiro, T.; Rodriguez, H.; Sánchez, B.; Schneiter, M.; Schoenell, W.; Starck, M.; Vrech, R.; Quiñones, C.; Tapia, L.; Tornatore, M.; Torres-Flores, S.; Vilchis, E.; Zadrożn, A.
    We present the methods and results of the optical follow-up, conducted by the Transient Optical Robotic Observatory of the South Collaboration, of gravitational wave events detected during the Advanced LIGO–Virgo second observing run (2016 November–2017 August). Given the limited field of view (∼100 arcmin) of our observational instrumentation, we targeted galaxies within the area of high localization probability that were observable from our sites. We analysed the observations using difference imaging, followed by a random forest algorithm to discriminate between real and spurious transients. Our observations were conducted using telescopes at Estación Astrofísica de Bosque Alegre, Cerro Tololo Inter-American Observatory, the Dr. Cristina V. Torres Memorial Astronomical Observatory, and an observing station in Salta, Argentina.