The classification of BL Lacertae objects: the Ca H&amp;K break

Landt, Hermine; Padovani, P.; Giommi, P.

doi:10.1046/j.1365-8711.2002.05832.x

Cited by 83 publications

(132 citation statements)

References 56 publications

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“…We also estimated a lower limit for the jet-to-host ratio assuming that the host galaxy is a bright elliptical and found a jet-to-galaxy ratio > ∼ 20−40. For this high ratio, a featureless spectrum is expected (Landt et al 2002). Therefore, the observed spectrum of PKS 0447-439 is also consistent with the proposed redshift.…”

Section: Discussionsupporting

confidence: 85%

“…In other words, the probability that the host galaxy of PKS 0447-439 is a member of the reported group is > ∼ 97%. Landt et al (2002) suggested that BL-Lac spectra become featureless for jet-to-galaxy luminosity ratios > ∼ 10. In the case of PKS 0447-439, where the spectrum was taken with a high signal-to-noise ratio, a jet-to-galaxy ratio > ∼ 20−40 might be more appropriate (compare the spectrum in Fig.…”

Section: Pks 0447-439 As a Group Membermentioning

confidence: 99%

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The BL-Lac gamma-ray blazar PKS 0447-439 as a probable member of a group of galaxies atz= 0.343

Muriel

Donzelli

Rovero

et al. 2015

A&A

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Context. The BL-Lac blazar PKS 0447-439 is one of the brightest HE gamma-ray sources that were first detected by Fermi-LAT. It was also detected by H.E.S.S. at VHE gamma-rays, which allowed constraining the redshift of PKS 0447-439 by considering the attenuation caused by gamma-ray interactions with ambient photons in the extragalactic background light (EBL). This constraint agreed with color-magnitude and spectroscopic redshift constraints (0.179 < z < 0.56). Recently, however, a much higher redshift was proposed for this blazar (z > 1.2). This value was debated because if true, it would imply either that the relevant absorption processes of gamma-rays are not well understood or that the EBL is dramatically different from what is believed today. This high redshift was not confirmed by three independent new spectroscopic observations at high signal-to-noise ratios. The scenario is clear evidence of the difficulties in estimating the redshift of BL-Lac objects, whose optical spectra are typically featureless. Neither of the estimated redshifts for PKS 0447-439 are confirmed as yet. Aims. Given that BL-Lac are typically hosted by elliptical galaxies, which in turn are associated with groups, we aim to find the host group of galaxies of PKS 0447-439. The ultimate goal is to estimate a redshift for this blazar. Methods. Spectra of twenty-one objects in the field of view of PKS 0447-439 were obtained with the Gemini Multi-Object Spectrograph. Based on the redshifts and coordinates of these galaxies, we searched for groups of galaxies. Using a deep catalog of groups, we studied the probability of finding by chance a group of galaxies in the line of sight of PKS 0447-439. Results. We identified a group of galaxies that was not previously cataloged at z = 0.343 with seven members, a virial radius of 0.42 Mpc, and a velocity dispersion of 622 km s −1 . We found that the probability of the host galaxy of PKS 0447-439 to be a member of the new group is > ∼ 97%. Therefore, we propose to adopt z = 0.343 ± 0.002 as the most likely redshift for PKS 0447-439.

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Section: Discussionsupporting

confidence: 85%

Section: Pks 0447-439 As a Group Membermentioning

confidence: 99%

The BL-Lac gamma-ray blazar PKS 0447-439 as a probable member of a group of galaxies atz= 0.343

Muriel

Donzelli

Rovero

et al. 2015

A&A

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“…Giommi et al 2012a). Following the work of Landt et al (2002) and Piranomonte et al (2007), we assume a giant elliptical host galaxy to be present in every blazar that would leave no imprint on the optical band if the total observed flux f obs exceeds the galaxy flux f ell by at least a factor of 10. Therefore, a lack of redshift allows us to set a lower limit based on the apparent magnitude m in the optical band.…”

Section: Lower Limits On Redshiftsmentioning

confidence: 99%

1WHSP: An IR-based sample of ~1000 VHEγ-ray blazar candidates

Arsioli

Fraga

Giommi

et al. 2015

A&A

Self Cite

103

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Context. Blazars are the dominant type of extragalactic sources at microwave and at γ-ray energies. In the most energetic part of the electromagnetic spectrum (E > ∼ 100 GeV) a high fraction of high Galactic latitude sources are blazars of the high synchrotron peaked (HSP) type, that is BL Lac objects with synchrotron power peaking in the UV or in the X-ray band. Building new large samples of HSP blazars is key to understand the properties of jets under extreme conditions, and to study the demographics and the peculiar cosmological evolution of these sources. Aims. High synchrotron peaked blazars are remarkably rare, with only a few hundreds of them expected to be above the sensitivity limits of currently available surveys, some of which include hundreds of millions of sources. To find these very uncommon objects, we have devised a method that combines ALLWISE survey data with multi-frequency selection criteria. Methods. The sample was defined starting from a primary list of infrared colour-colour selected sources from the ALLWISE all sky survey database, and applying further restrictions on IR-radio and IR-X-ray flux ratios. Using a polynomial fit to the multi-frequency data (radio to X-ray), we estimated synchrotron peak frequencies and fluxes of each object. Results. We assembled a sample including 992 sources, which is currently the largest existing list of confirmed and candidates HSP blazars. All objects are expected to radiate up to the highest γ-ray photon energies. In fact, 299 of these are confirmed emitters of GeV γ-ray photons (based on Fermi-LAT catalogues), and 36 have already been detected in the TeV band. The majority of sources in the sample are within reach of the upcoming Cherenkov Telescope Array (CTA), and many may be detectable even by the current generation of Cherenkov telescopes during flaring episodes. The sample includes 425 previously known blazars, 151 new identifications, and 416 HSP candidates (mostly faint sources) for which no optical spectra is available yet. The full 1WHSP catalogue is online at http://www.asdc.asi.it/1whsp/, providing a direct link to the SED building tool where multi-frequency data for each source can be easily visualised.

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“…Historically, large and fast optical variability was one of the parameters used to define the BL Lac objects (see e.g., Landt et al 2002). There is more than one way to define a numerical index to quantify the variability of a source, like those used by Abdo et al (2010b).…”

Section: Nir and Optical Observationsmentioning

confidence: 99%

Near-infrared and gamma-ray monitoring of TANAMI gamma-ray bright sources

Nesci

Tosti

Pursimo

et al. 2013

A&A

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Context. Spectral energy distribution and its variability are basic tools for understanding the physical processes operating in active galactic nuclei (AGN). Aims. In this paper we report the results of a one-year near-infrared (NIR) and optical monitoring of a sample of 22 AGN known to be gamma-ray emitters, aimed at discovering correlations between optical and gamma-ray emission. Methods. We observed our objects with the Rapid Eye Mount (REM) telescope in J, H, K, and R bands nearly twice every month during their visibility window and derived light curves and spectral indexes. We also analyzed the gamma-ray data from the Fermi gamma-ray Space Telescope, making weekly averages. Results. Six sources were never detected during our monitoring, proving to be fainter than their historical Two micron all sky survey (2MASS) level. All of the sixteen detected sources showed marked flux density variability, while the spectral indexes remained unchanged within our sensitivity limits. Steeper sources showed, on average, a larger variability. From the NIR light curves we also computed a variability speed index for each detected source. Only one source (PKS 0208-512) underwent an NIR flare during our monitoring. Half of the sources showed a regular flux density trend on a one-year time scale, but do not show any other peculiar characteristic. The broadband spectral index α ro appears to be a good proxy of the NIR spectral index only for BL Lac objects. No clear correlation between NIR and gamma-ray data is evident in our data, save for PKS 0537−441, PKS 0521−360, PKS 2155−304, and PKS 1424−418. The gamma-ray/NIR flux ratio showed a large spread, QSO being generally gamma-louder than BL Lac, with a marked correlation with the estimated peak frequency (ν peak ) of the synchrotron emission.

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The classification of BL Lacertae objects: the Ca H&K break

Cited by 83 publications

References 56 publications

The BL-Lac gamma-ray blazar PKS 0447-439 as a probable member of a group of galaxies atz= 0.343

The BL-Lac gamma-ray blazar PKS 0447-439 as a probable member of a group of galaxies atz= 0.343

1WHSP: An IR-based sample of ~1000 VHEγ-ray blazar candidates

Near-infrared and gamma-ray monitoring of TANAMI gamma-ray bright sources

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