We present a sample of 386 BL Lacertae candidates identified from 2860deg^2^ of the Sloan Digital Sky Survey (SDSS) spectroscopic database. The candidates are primarily selected to have quasi-featureless optical spectra and low proper motions as measured from SDSS and USNO-B positions; however, our ability to separate Galactic from extragalactic quasi-featureless objects (QFOs) on the basis of proper motion alone is limited by the lack of reliable proper motion measurements for faint objects. Based largely on this remarkably clean color separation, we subdivide the sample into 240 probable candidates and 146 additional less probable (likely stellar) candidates.
Controversial studies on the jet collimation profile of BL Lacertae (BL Lac), the eponymous blazar of the BL Lac objects class, complicate the scenario in this already puzzling class of objects. Understanding the jet geometry in connection with the jet kinematics and the physical conditions in the surrounding medium is fundamental for better constraining the formation, acceleration, and collimation mechanisms in extragalactic jets. With the aim of investigating the jet geometry in the innermost regions of the BL Lac jet, and resolving the controversy, we explore the radio jet in this source using high-resolution millimeter-wave VLBI data. We collect 86GHz GMVA and 43 GHz VLBA data to obtain stacked images that we use to infer the jet collimation profile by means of two comparable methods. We analyze the kinematics at 86GHz, and we discuss it in the context of the jet expansion. Finally, we consider a possible implication of the Bondi sphere in shaping the jet of BL Lac. The jet in BL Lac expands with an overall conical geometry. A higher expanding rate region is observed between ~5 and 10pc (de-projected) from the black hole. Such a region is associated with the decrease in brightness usually observed in high frequency VLBI images of BL Lac. The jet retrieves the original jet expansion around 17pc, where the presence of a recollimation shock is supported by both the jet profile and the 15GHz kinematics (MOJAVE survey). The change in the jet expansion profile occurring at ~5pc could be associated with a change in the external pressure at the location of the Bondi radius (~3.3x10^5^R_S_)/
Based on the idea that a group of extragalactic objects which can be regarded as a category with similar physical properties must fit the Hubble relation, we present a new, independent evidence that BL Lacertae objects are different from Optically Violently Variable (OVV) quasars. The unified model of BL Lac objects, FR-I, and FR-II radio galaxies is also discussed.
The most elusive and extreme subclass of active galactic nuclei (AGNs), known as BL Lac objects, shows features that can only be explained as the result of relativistic effects occurring in jets pointing at a small angle with respect to the line of sight. A longstanding issue is the identification of the BL Lac parent population with jets oriented at larger angles. According to the "unification scenario" of AGNs, radio galaxies with low luminosity and an edge-darkened radio morphology are the most promising candidate parent population of BL Lacs. Here we compare the large-scale environment, an orientation-independent property, of well-defined samples of BL Lacs with samples of radio galaxies all lying in the local universe. Our study reveals that BL Lacs and radio galaxies live in significantly different environments, challenging predictions of the unification scenario. We propose a solution to this problem proving that large-scale environments of BL Lacs are statistically consistent with those of compact radio sources, known as FR 0s, and share similar properties. This implies that highly relativistic jets are ubiquitous and are the natural outcome of the accretion of gas into the deep gravitational potential well produced by supermassive black holes.
Observations of the {gamma}-ray sky with Fermi led to significant advances towards understanding blazars, the most extreme class of active galactic nuclei. A large fraction of the population detected by Fermi is formed by BL Lacertae (BL Lac) objects, whose sample has always suffered from a severe redshift incompleteness due to the quasi-featureless optical spectra. Our goal is to provide a significant increase of the number of confirmed high-redshift BL Lac objects contained in the 2 LAC Fermi/LAT catalog.
We report on 16 BL Lacertae objects that were proposed to be at z>1. We present spectroscopic observations secured at the 10.4m GTC that allowed us to assess the redshift of these sources. In particular, for five objects, we disprove the previous value of the redshift reported in the literature and found that they lie at z<1. Moreover, two of them exhibit broad emission lines that are not characteristic of BL Lacertae objects. On the other hand, for eight targets, we improve the tentative value of z, previously based on only one feature, by detecting a number of emission lines. Finally, in three cases, we detect the onset of the Ly{alpha} forest at z>2.50. Based on the new high quality spectra, we found that only half of the observed objects can be classified as bona-fide BL Lacs.
It has become evident that one-zone synchrotron self-Compton models are not always adequate for very high-energy (VHE) gamma-ray-emitting blazars. While two-component models perform better, they are difficult to constrain due to the large number of free parameters. In this work, we make a first attempt at taking into account the observational constraints from very long baseline interferometry (VLBI) data, long-term light curves (radio, optical, and X-rays), and optical polarisation to limit the parameter space for a two-component model and test whether or not it can still reproduce the observed spectral energy distribution (SED) of the blazars. We selected five TeV BL Lac objects based on the availability of VHE gamma-ray and optical polarisation data. We collected constraints for the jet parameters from VLBI observations. We evaluated the contributions of the two components to the optical flux by means of decomposition of long-term radio and optical light curves as well as modelling of the optical polarisation variability of the objects. We selected eight epochs for these five objects based on the variability observed at VHE gamma rays, for which we constructed the SEDs that we then modelled with a two-component model. We found parameter sets which can reproduce the broadband SED of the sources in the framework of two-component models considering all available observational constraints from VLBI observations. Moreover, the constraints obtained from the long-term behaviour of the sources in the lower energy bands could be used to determine the region where the emission in each band originates. Finally, we attempt to use optical polarisation data to shed new light on the behaviour of the two components in the optical band. Our observationally constrained two-component model allows explanation of the entire SED from radio to VHE with two co-located emission regions.
The mechanisms producing fast variability of the {gamma}-ray emission in active galactic nuclei (AGNs) are under debate. The MAGIC telescopes detected a fast, very-high-energy (VHE, E>100GeV) {gamma}-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5+/-0.3)x10^-10^photons/cm^2^/s and halving time of 26+/-8min. The MAGIC observations were triggered by a high state in the optical and high-energy (HE, E>100MeV) {gamma}-ray bands. In this paper we present the MAGIC VHE {gamma}-ray data together with multi-wavelength data from radio, optical, X-rays, and HE {gamma} rays from 2015 May 1 to July 31. Well-sampled multi-wavelength data allow us to study the variability in detail and compare it to the other epochs when fast, VHE {gamma}-ray flares have been detected from this source. Interestingly, we find that the behaviour in radio, optical, X-rays, and HE {gamma}-rays is very similar to two other observed VHE {gamma}-ray flares. In particular, also during this flare there was an indication of rotation of the optical polarization angle and of activity at the 43GHz core. These repeating patterns indicate a connection between the three events. We also test modelling of the spectral energy distribution based on constraints from the light curves and VLBA observations, with two different geometrical setups of two-zone inverse Compton models. In addition we model the {gamma}-ray data with the star-jet interaction model. We find that all of the tested emission models are compatible with the fast VHE {gamma}-ray flare, but all have some tension with the multi-wavelength observations.
The Z Cam-type dwarf nova AT Cancri (AT Cnc) displays a classical nova (CN) shell, demonstrating that mass transfer in cataclysmic binaries decreases substantially after a CN eruption. The hibernation scenario of cataclysmic binaries predicts such a decrease, on a time-scale of a few centuries. In order to measure the time since AT Cnc's last CN eruption, we have measured the radial velocities of a hundred clumps in its ejecta with SITELLE, Canada-France-Hawaii Telescope's recently commissioned imaging Fourier transform spectrometer. These range from -455 to +490km/s. Coupled with the known distance to AT Cnc of 460pc, the size of AT Cnc's shell, and a simple model of nova ejecta deceleration, we determine that the last CN eruption of this system occurred 330^+135^_-90_yr ago. This is the most rapid transition from a high mass-transfer rate, nova-like variable to a low mass-transfer rate, dwarf nova yet measured, and in accord with the hibernation scenario of cataclysmic binaries. We conclude by noting the similarity in the deduced outburst date (within a century of 1686 CE) of AT Cnc to a 'guest star' reported in the constellation Cancer by Korean observers in 1645 CE.
Using LOFAR (LOw Frequency ARray), Westerbork telescope and survey data, we map the spectral index and age of the remnant radio galaxy by using different ageing models.
