Identifying the 3FHL Catalog. I. Archival Swift Observations and Source Classification

Kaur, A.; Ajello, M.; Marchesi, S.; Omodei, N.

doi:10.3847/1538-4357/aaf649

Cited by 20 publications

(24 citation statements)

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“…This identification campaign to classify BCUs in the 3FHL catalog and to measure their redshift began in 2017, when 36 unassociated sources (out of 52 chosen) from 3FHL catalog were classified as BL Lacs with the help of machine-learning algorithms (Kaur et al 2019). For the second part of this campaign, 28 sources were observed with the 4 m telescope at the Kitt Peak National Observatory (KPNO) in Arizona, in order to perform optical spectroscopy.…”

Section: Discussionmentioning

confidence: 99%

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Identifying the 3FHL catalog: V. Results of the CTIO-COSMOS optical spectroscopy campaign 2019

Rajagopal,

Marchesi,

Kaur

et al. 2021

Preprint

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As a follow-up of the optical spectroscopic campaign aimed at achieving completeness in the Third Catalog of Hard Fermi-LAT Sources (3FHL), we present here the results of a sample of 28 blazars of uncertain type observed using the 4 m telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. Out of these 28 sources, we find that 25 are BL Lacertae objects (BL Lacs) and 3 are Flat Spectrum Radio Quasars (FSRQs). We measure redshifts or lower limits for 16 of these blazars, whereas it is observed that the 12 remaining blazars have featureless optical spectra. These results are part of a more extended campaign of optical spectroscopy follow up of 3FHL blazars, where until now 51 blazars of uncertain type have been classified into BL Lac and FSRQ categories. Further, this campaign has resulted in redshift measurements and lower limits for 15 of these sources. Our results contribute towards attaining a complete sample of blazars above 10 GeV, which then will be crucial in extending our knowledge on blazar emission mechanisms and the extragalactic background light.

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Section: Discussionmentioning

confidence: 99%

“…Spectroscopic redshifts were also obtained for 15 of these sources (Marchesi et al 2018;. In addition, machine learning algorithms have provided tentative classification for an additional 51 sources (Kaur et al 2019;Silver et al 2020).…”

Section: Introductionmentioning

confidence: 99%

Identifying the 3FHL catalog: V. Results of the CTIO-COSMOS optical spectroscopy campaign 2019

Rajagopal,

Marchesi,

Kaur

et al. 2021

Preprint

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“…Since then new Fermi-LAT source catalogues have been published, the latest being the 4FGL-DR2 (Ballet et al 2020), and new associations with blazars have been produced (see e.g. Acero et al 2013;Arsioli et al 2015;Arsioli & Chang 2017;Kaur et al 2019). Several groups have performed spectroscopic campaigns often focused on these new Fermi-LAT BL Lacs and BCUs.…”

Section: Introductionmentioning

confidence: 99%

Optical spectroscopy of blazars for the Cherenkov Telescope Array

Goldoni

Pita

Boisson

et al. 2021

A&A

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Context. Blazars are the most numerous class of high-energy (HE; E ∼ 50 MeV−100 GeV) and very high-energy (VHE; E ∼ 100 GeV−10 TeV) gamma-ray emitters. Currently, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacertae objects (BL Lacs), mainly due to the difficulty in measuring reliable redshifts from their nearly featureless continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars. Aims. This paper is the first in a series of papers that aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground-based gamma-ray observatory. Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi-LAT telescope still lacking redshift measurements, but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium- to high-resolution spectroscopy of 19 blazar optical counterparts with the Keck II, SALT, and ESO NTT telescopes. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux. Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482, one tentative redshift, three redshift lower limits including one at z ≥ 0.449 and another at z ≥ 0.868. Four BL Lacs show featureless spectra.

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“…Nevertheless, optical spectroscopy is still the only method that can determine the real nature of the associated counterpart, and spectroscopic campaigns confirm that most of the UGSs are indeed blazars, in particular of the BL Lac kind (Paggi et al 2014;Landoni et al 2015Landoni et al , 2018Ricci et al 2015;Falomo et al 2017;Kaur et al 2019;Franceschini et al 2019).…”

Section: Introductionmentioning

confidence: 99%

The γ-ray sky seen at X-ray energies

Marchesini

Paggi

Massaro

et al. 2020

A&A

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Context. Nearly 50% of all sources detected by the Fermi Large Area Telescope are classified as blazars or blazar candidates, one of the most elusive classes of active galaxies. Additional blazars can also be hidden within the sample of unidentified or unassociated γ-ray sources (UGSs) that constitute about one-third of all gamma-ray sources detected to date. We recently confirmed that the large majority of Fermi blazars of the BL Lac subclass have an X-ray counterpart. Aims. Using the X-ray properties of a BL Lac training set and combining these with archival multifrequency information, we aim to search for UGSs that could have a BL Lac source within their γ-ray positional uncertainty regions. Methods. We reduced and analyzed the Swift X-ray observations of a selected sample of 327 UGSs. We then compared the X-ray fluxes and hardness ratios of all sources detected in the pointed fields with those of known Fermi BL Lacs. Results. We find at least one X-ray source, lying within the γ-ray positional uncertainty at 95% confidence level, for 223 UGSs and a total of 464 X-ray sources in all fields analyzed. The X-ray properties of a large fraction of them, eventually combined with radio, infrared, and optical information, exhibit BL Lac multi-frequency behavior, thus allowing us to select high-confidence BL Lac candidates; some of them were recently observed during our optical spectroscopic campaign which confirmed their nature. Conclusions. We find that out of 50 X-ray sources that were confirmed as BL Lacs through optical spectroscopy, 12 do not show canonical mid-infrared or radio BL Lac properties. This indicates that the selection of X-ray BL Lac candidates is a strong method to find new counterparts within Fermi UGSs. Finally, we pinpoint a sample of 32 Swift/XRT candidate counterparts to Fermi UGSs that are most likely BL Lac objects.

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Identifying the 3FHL Catalog. I. Archival Swift Observations and Source Classification

Cited by 20 publications

References 50 publications

Identifying the 3FHL catalog: V. Results of the CTIO-COSMOS optical spectroscopy campaign 2019

Identifying the 3FHL catalog: V. Results of the CTIO-COSMOS optical spectroscopy campaign 2019

Optical spectroscopy of blazars for the Cherenkov Telescope Array

The γ-ray sky seen at X-ray energies

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