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Quasar projected pairs (QPPs) can be used for investigating the circumgalactic medium of quasars through the study of intervening absorption lines in the spectrum of the background quasar (QSOB) that are at the same redshift of the foreground quasar (QSOF). Here we report on optical spectroscopy, gathered at Gran Telescopio Canarias, of 14 QPPs. In 7 cases we find MgII absorption lines associated with the foreground quasar. Only for two cases line-of-sight absorptions (LOS) are revealed. These new observations complement our previous study performed on other 30 QPPs. A brief discussion of the properties of the intervening absorption lines associated with the foreground quasar for the full dataset is reported. [ABSTRACT FROM AUTHOR]

We investigate the environments of galaxies around BL Lacertae (BL Lac) objects and Fanaroff–Riley class I (FR I) radio galaxies, the alleged parent populations of misaligned sources. We compare the environment of a sample of 50 BL Lac objects at 0.1 <  z  < 0.33 with that of a sample of 90 FR I galaxies at 0.1 <  z  < 0.15. The galaxy environment is studied by using Sloan Digital Sky Survey Data Release 14 images in the i band. We find that the galaxy excess density within 0.5 Mpc around FR I radio galaxies is a factor of ∼2 larger than that around BL Lacs. This implies a reconsideration of the parent population of BL Lac objects. [ABSTRACT FROM AUTHOR]

The availability of about a decade of uninterrupted sky monitoring by the Fermi satellite has made it possible to study long-term quasi-periodicities for high-energy sources. It is therefore not a surprise that for several blazars in the recent literature claims for such periodicities, with various level of confidence, have been reported. The confirmation of these findings could be of tremendous importance for the physical description of this category of sources and have consequences for the gravitational wave background interpretation. In this work, we carry out a temporal analysis of the Fermi light curves for several of the sources mentioned in recent literature by means of a homogeneous procedure and find that, globally, no strong cases for blazar year-long quasi-periodicities can be confirmed. The computed power spectral densities are all essentially consistent with being generated by red-noise only. We further discuss the meaning and the limitations of the present analysis. [ABSTRACT FROM AUTHOR]

Blazars are a subpopulation of quasars whose jets are nearly aligned with the line of sight, which tend to exhibit multiwavelength variability on a variety of time-scales. Quasi-periodic variability on year-like time-scales has been detected in a number of bright sources, and has been connected to the orbital motion of a putative massive black hole binary. If this were indeed the case, those blazar binaries would contribute to the nanohertz gravitational-wave stochastic background. We test the binary hypothesis for the blazar population observed by the Fermi Gamma-Ray Space Telescope, which consists of BL Lacertae objects and flat-spectrum radio quasars. Using mock populations informed by the luminosity functions for BL Lacertae objects and flat-spectrum radio quasars with redshifts z ≤ 2, we calculate the expected gravitational-wave background and compare it to recent pulsar timing array upper limits. The two are consistent only if a fraction ≲10−3 of blazars hosts a binary with orbital periods <5 yr. We therefore conclude that binarity cannot significantly explain year-like quasi-periodicity in blazars. [ABSTRACT FROM AUTHOR]

We report on a study of the galaxy environments of low redshift physical quasars pairs. We selected 20 pairs having projected separation < 0.5 Mpc and difference of systemic velocity < 800 km s−1. Using Sloan Digital Sky Survey images, we evaluated the galaxy overdensity around these quasars in pairs and then compare it with that of a sample of isolated quasars with same redshift and luminosity. It is found that on average there is a systematic larger overdensity of galaxies around quasars in pairs with respect to that of isolated quasars. This may represent a significant link between nuclear activity and galaxy environment. However, at odds with that, the closest quasar pairs seem to inhabit poorer environments. Implications of present results and perspectives for future work are briefly discussed. [ABSTRACT FROM AUTHOR]

We examine the 2008-2016 γ-ray and optical light curves of three bright BL Lac objects, 0716+714, MRK421, BL Lac, which exhibit large structured variability. We searched for periodicities by using a fully Bayesian approach. For two out of three sources investigated, no significant periodic variability was found. In the case of BL Lac, we detected a periodicity of ~680 days. Although the signal related to this is modest, the coincidence of the periods in both gamma and optical bands is indicative of a physical relevance. Taking into consideration previous literature results, possibly related γ-ray and optical periodicities of about one year time scale are proposed in four bright γ-ray blazars out of the ten examined in detail. Compared with results from periodicity search of optical archives of quasars, the presence of quasi-periodicities in blazars may be more frequent by a large factor. This suggests the intriguing possibility that the basic conditions for their observability are related to the relativistic jet in the observer direction, but the overall picture remains uncertain. [ABSTRACT FROM AUTHOR]

We examine the 2008-2016 gamma-ray and optical light curves of a number of bright Fermi blazars. In a fraction of them, the periodograms show possible evidence of quasi-periodicities related in the two bands. This coincidence strengthens their physical meaning. Comparing with results from the periodicity search of quasars, the presence of quasi-periodicities in blazars suggests that the basic condition for its observability is related to the relativistic jet in the observer direction, but the overall picture remains uncertain. [ABSTRACT FROM PUBLISHER]

Context. Blazars are astrophysical sources whose emission is dominated by non-thermal processes, i.e. synchrotron and inverse Compton emission. Although the general picture is rather robust and consistent with observations, many aspects are still unexplored. Aims. Polarimetric monitoring can offer a wealth of information about the physical processes in blazars. Models with largely different physical ingredients can provide almost indistinguishable predictions for the total flux, but usually are characterized by different polarization properties. We explore the possibility to derive structural information about the emitting regions of blazars by means of a joint analysis of rapid variability of the total and polarized flux at optical wavelengths. Methods. Short timescale (from tens of seconds to a couple of minutes) optical linear polarimetry and photometry for two blazars, BL Lacertae and PKS 1424+240, was carried out with the PAOLO polarimeter at the 3.6 m Telescopio Nazionale Galileo. Several hours of almost continuous observations were obtained for both sources. Results. Our intense monitoring allowed us to draw different scenarios for BL Lacertae and PKS 1424+240, with the former characterized by intense variability and the latter practically constant in total flux. Essentially the same behavior is observed for the polarized flux and the position angle. The variability time-scales turned out to be as short as a few minutes, although involving only a few percent variation of the flux. The polarization variability time-scale is generally consistent with the total flux variability. Total and polarized flux appear to be essentially uncorrelated. However, even during our relatively short monitoring, different regimes can be singled out. Conclusions. No simple scenario is able to satisfactorily model the very rich phenomenology exhibited in our data. Detailed numerical simulations show that the emitting region should be characterized by some symmetry, and the inclusion of turbulence for the magnetic field may constitute the missing ingredient for a more complete interpretation of the data. [ABSTRACT FROM AUTHOR]

We investigate the properties of the galaxy environment of a sample of 14 low-redshift (z < 0.85) quasar (QSO) physical pairs extracted from Sloan Digital Sky Survey Data Release 10 archives. The pairs have a systemic radial velocity difference ΔV∥ ≤ 600 km s−1 (based on [O iii]5007 Å line) and projected distance R⊥ ≤ 600 kpc. The physical association of the pairs is statistically confirmed at a level of ∼90 per cent. For most of the images of these QSOs we are able to resolve their host galaxies that turn out to be on average similar to those of QSOs not in pairs. We also found that QSOs in a pair are on average in region of modest galaxy overdensity extending up 0.5 Mpc from the QSO. This galaxy overdensity is indistinguishable from that of a homogeneous sample of isolated QSOs at the same redshift and with similar host galaxy luminosity. These results, albeit derived from a small (but homogeneous) sample of objects, suggest that the rare activation of two QSOs with small physical separation does not require any extraordinary environment. [ABSTRACT FROM PUBLISHER]

Context. We present the light curves of six BL Lac objects, PKS 0537-441, PKS 0735+17, OJ 287, PKS 2005-489, PKS 2155-304, and W Comae, and of the flat spectrum radio quasar PKS 1510-089, as a part of a photometric monitoring program in the nearinfrared/ optical bands started in 2004. All sources are Fermi blazars. Aims. Our purpose is to investigate flux and spectral variability on short and long time scales. Systematic monitoring, independent of the activity of the source, guarantees large sample size statistics, and allows an unbiased view of different activity states on weekly or daily time scales for the whole timeframe and on nightly time scales for some epochs. Methods. Data were obtained with the REM telescope located at the ESO premises of La Silla (Chile). Light curves were gathered in the optical/near-infrared VRIJHK bands from April 2005 to June 2012. Results. Variability ≳3 mag is observed in PKS 0537-441, PKS 1510-089 and PKS 2155-304, the largest ranges spanned in the near-infrared. The color intensity plots show rather different morphologies. The spectral energy distributions in general are well fitted by a power law, with some deviations that are more apparent in low states. Some variability episodes during a night interval are well documented for PKS 0537-441 and PKS 2155-304. For the latter source the variability time scale implies a large relativistic beaming factor. [ABSTRACT FROM AUTHOR]

The electrical activity of grain boundaries (GB’s) in polycrystalline silicon is strongly affected by heat treatments, which are known to induce the segregation of oxygen. The large, quantitative variations of the electrical activity of GB’s observed experimentally, which are a function not only of the heat treatments but also of the carbon and oxygen content of the specific sample examined, could not be explained however, without assuming that carbon and oxygen play a synergistic role. We demonstrate in this work, by using secondary ion mass spectrometry, that oxygen and carbon segregate simultaneously, albeit spatially resolved, at grain boundaries. This result appears to be of major importance when interpreting electron or light beam induced current profiles at grain boundaries in polycrystalline silicon. [ABSTRACT FROM AUTHOR]

We have considered the Fermi γ-ray light curve of the blazar PKS 0537–441 during a high state extending from 2008 August 10 to 2011 August 27. The periodogram exhibits a peak at T ∼ 280 days, with a significance of ∼99.7%. A peak of similar relevance at is found in the optical light curves. Considering the entire duration of the Fermi light curve 2008–2015, no significant peak is revealed, while the optical one remains meaningful. Comparing with recent observations of PKS 2155–304 and PG 1553+113 it seems that month–year oscillations can characterize some blazars. Month-scale oscillations can also show up only during phases of enhanced or bursting emission, such as in the case of PKS 0537–441. [ABSTRACT FROM AUTHOR]