The UV-Optical-Near-IR Emission of BL Lacertae Objects

The Astrophysical …, 2000

The HST NICMOS Camera 2 was used for H-band imaging of 12 BL Lacertae objects taken from the larger sample observed with the WFPC2 in the R band . Ten of the 12 BL Lacs are clearly resolved, and the detected host galaxies are large, bright ellipticals with average absolute magnitude M H = −26.2 ± 0.45 mag and effective radius r e = 10 ± 5 kpc. The rest-frame integrated color of the host galaxies is on average R-H = 2.3 ± 0.3, consistent with the value for both radio galaxies and normal, non-active elliptical galaxies, and indicating the dominant stellar population is old. The host galaxies tend to be bluer in their outer regions than in their cores, with average color gradient ∆(R − H)/∆ log r = −0.2 mag, again consistent with results for normal non-active elliptical galaxies. The infrared Kormendy relation, derived for the first time for BL Lac host galaxies, is µ e = 3.8 log r e + 14.8, fully in agreement with the relation for normal ellipticals. The close similarity between BL Lac host galaxies and normal ellipticals suggests the active nucleus has surprisingly little effect on the host galaxy. This supports a picture in which all elliptical galaxies harbor black holes which can be actively accreting for some fraction of their lifetime.

New Astronomy Reviews, 2002

We analyze images of BL Lacertae objects obtained with the Hubble Space Telescope WFPC2 and the F814W filter. The nine objects cover a redshift range of 0.19 to 0.997. The relatively deep images are sufficient to detect galaxies at least one magnitude below M * I (-21.4) and in most cases to three magnitudes below M *. Galaxy enhancement over the average background is found around four out of the nine objects. Results for some cases are confirmed by ground-based imaging. In the other cases, the redshifts of the target BL Lac objects may be incorrect or they are truly isolated. These findings reinforce the idea that on average, BL Lac objects are found in regions of above average galaxy density. However, isolated objects apparently can host BL Lac nuclei too, a result that has implications for the processes that trigger/fuel the nuclear activity.

The Astrophysical Journal, 2018

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 literature and found that they lie at z < 1. Moreover, two of them exhibit broad emission lines that are not characteristic of BL Lacertae object. On the other hand, for eight targets we improve the tentative value of z, previously based on only one feature, by detecting a number emission lines. Finally, in three cases we detect onset of Ly-α 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-fine BL Lacs.

Publications of the Astronomical Society of Australia, 2002

We present the relation between optical luminosity and spectral slope for eight BL Lac objects (3C66A, PKS 0422+00, S5 0716+71, OJ 287, ON 231, OQ 530, S5 1803+78 and BL Lacertae), derived from B, V, R, and I observations spanning a time interval of about 5 years. Four objects show a marked correlation between spectral slope and luminosity, being bluer when brighter, while for the other four the correlation is weaker or absent. Possible explanations are briefly discussed in the framework of current models of the BL Lac phenomenon.

Astronomy &amp; Astrophysics, 2010

Aims. In a previous study we suggested that the broad-band emission and variability properties of BL Lacertae can be accounted for by a double synchrotron emission component with related inverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range. Methods. The GLAST-AGILE Support Program (GASP) of the whole earth blazar telescope (WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical frequencies to follow its flux behaviour. During this period, high-energy observations were performed by XMM-Newton, Swift, and Fermi. We analyse these data with particular attention to the calibration of Swift UV data, and apply a helical jet model to interpret the source broad-band variability. Results. The GASP-WEBT observations show an optical flare in 2008 February-March, and oscillations of several tenths of mag on a few-day time scale afterwards. The radio flux is only mildly variable. The UV data from both XMM-Newton and Swift seem to confirm a UV excess that is likely caused by thermal emission from the accretion disc. The X-ray data from XMM-Newton indicate a strongly concave spectrum, as well as moderate (∼4-7%) flux variability on an hour time scale. The Swift X-ray data reveal fast (interday) flux changes, not correlated with those observed at lower energies. We compare the spectral energy distribution (SED) corresponding to the 2008 low-brightness state, which was characterised by a synchrotron dominance, to the 1997 outburst state, where the inverse-Compton emission was prevailing. A fit with an inhomogeneous helical jet model suggests that two synchrotron components are at work with their self inverse-Compton emission. Most likely, they represent the radiation from two distinct emitting regions in the jet. We show that the difference between the source SEDs in 2008 and 1997 can be explained in terms of pure geometrical variations. The outburst state occurred when the jet-emitting regions were better aligned with the line of sight, producing an increase of the Doppler beaming factor. Conclusions. Our analysis demonstrates that the jet geometry can play an extremely important role in the BL Lacertae flux and spectral variability. Indeed, the emitting jet is probably a bent and dynamic structure, and hence changes in the emitting regions viewing angles are likely to happen, with strong consequences on the source multiwavelength behaviour.

Arxiv preprint astro-ph/ …, 2000

The HST NICMOS Camera 2 was used for H-band imaging of 12 BL Lacertae objects taken from the larger sample observed with the WFPC2 in the R band . Ten of the 12 BL Lacs are clearly resolved, and the detected host galaxies are large, bright ellipticals with average absolute magnitude M H = −26.2 ± 0.45 mag and effective radius r e = 10 ± 5 kpc. The rest-frame integrated color of the host galaxies is on average R-H = 2.3 ± 0.3, consistent with the value for both radio galaxies and normal, non-active elliptical galaxies, and indicating the dominant stellar population is old. The host galaxies tend to be bluer in their outer regions than in their cores, with average color gradient ∆(R − H)/∆ log r = −0.2 mag, again consistent with results for normal non-active elliptical galaxies. The infrared Kormendy relation, derived for the first time for BL Lac host galaxies, is µ e = 3.8 log r e + 14.8, fully in agreement with the relation for normal ellipticals. The close similarity between BL Lac host galaxies and normal ellipticals suggests the active nucleus has surprisingly little effect on the host galaxy. This supports a picture in which all elliptical galaxies harbor black holes which can be actively accreting for some fraction of their lifetime.

2005

The status of Very High Energy (VHE, E > 50 GeV) gamma-ray observations of BL Lacertae objects is presented. The catalogue of well-established BL Lacertae objects detected at VHE energies contains seven members, and there have been recent reports of the detection of another four. All are nearby, X-ray bright sources. The temporal, spectral and broadband multiwavelength properties of the sources are reviewed and possible implications for the gamma-ray production mechanism discussed. The most recent detections provide more stringent constraints on the cosmic extragalactic background light level and imply that the Universe is more transparent to VHE gamma radiation than previously thought.

The Astrophysical …, 2004

We present results from observations of 29 BL Lacertae objects, taken with the Whipple Observatory 10 m Gamma-Ray Telescope between 1995 and 2000. The observed objects are mostly at low redshift (z < 0.2) but observations of objects of z up to 0.444 are also reported. Five of the objects are EGRET sources and two are unconfirmed TeV sources. Three of the confirmed sources of extragalactic TeV gamma rays were originally observed as part of this survey and have been reported elsewhere. No significant excesses are detected from any of the other objects observed, on time scales of days, months or years. We report 99.9% confidence level flux upper limits for the objects for each observing season. The flux upper limits are typically 20% of the Crab flux although, for some sources, limits as sensitive as 6% of the Crab flux were derived. The results are consistent with the synchrotron-self-Compton (SSC) model predictions considered in this

Arxiv preprint astro-ph/ …, 2006

Aims. The large majority of BL Lacertae objects belonging to the 1 Jy sample, the class prototype for radio-selected sources, are thought to emit most of their synchrotron power in the far infrared band. Ironically, this spectral region is very sparsely sampled, with only a minority of the objects having IRAS data (most of them being upper limits or low-quality detections). We aim at filling this infrared gap by presenting new, simultaneous ISOCAM and ISOPHOT observations over the 7 − 200µm range (observer's frame) for half the sample. A precise measurement of the position of the synchrotron peak frequency, ν peak , can provide direct information about particle acceleration mechanisms and constrain the inverse Compton radiation that will be detected by up-coming new γ-ray missions. Methods. We have observed seventeen 1 Jy BL Lacertae objects with the camera and the photometer on board the Infrared Space Observatory (ISO) satellite. Given the intrinsic variability of these sources, the data were taken by concatenating the pointings to ensure simultaneity. The ISOPHOT data reduction was done employing a novel correction, which mitigates the effect of chopping for faint sources. Results. Using our new ISO data, complemented by nearly-simultaneous radio and optical observations for ten and four objects respectively, and other multi-frequency data, we have built the spectral energy distributions of our sources (plus a previously published one) and derived the rest-frame ν peak . Its distribution is centered at ∼ 10 13 Hz (∼ 30µm) and is very narrow, with ∼ 60% of the BL Lacs in the 1 − 3 × 10 13 Hz range. Given our set of simultaneous infrared data, these represent the best determinations available of the synchrotron peak frequencies for low-energy peaked BL Lacs. A comparison with previous such estimates, based on non-simultaneous optical and near infrared data, may indicate strong ν peak variations in a number of sources, possibly associated with large flares as observed in the high-energy peaked BL Lac MKN 501.

Astronomy & Astrophysics

Context. BL Lac objects detected in γ-rays and, particularly, those detected at very high energies (E > 100 GeV) by Cherenkov telescopes are extreme sources with most having redshifts lower than 0.2. Their study gives insights on the acceleration mechanisms in play in such systems and is also a valuable tool for putting constraints on the density of extragalactic background light, especially if several objects are detected at different redshifts. As their spectra are dominated by the non-thermal emission of the jet and the spectral features are weak and narrow in the optical domain, measuring their redshift is challenging. However, such a measure is fundamental as it allows a firm determination of the distance and luminosity of the source, and, therefore, a consistent model of its emission. Aims. Measurement of the redshift of BL Lac objects detected in γ-rays and determination of global properties of their host galaxies is the aim of this study. Methods. We observed a sample of eight BL Lac objects with the X-shooter spectrograph installed at the ESO Very Large Telescope (VLT) to take advantage of its unprecedented wavelength coverage and its resolution, which is about five times higher than generally used in such studies. We extracted UVB to NIR spectra that we then corrected for telluric absorption and calibrated in flux. We systematically searched for spectral features. When possible, we determined the contribution of the host galaxy to the overall emission. Results. Of the eight BL Lac sources, we measured the redshift of five of them and determined lower limits for two through the detection of intervening systems. All seven of these objects have redshifts greater than 0.2. For the remaining one, we estimated, using an indirect method, that its redshift is greater than 0.175. In two cases, we refuted redshift values reported in other publications. Through careful modelling, we determined the magnitude of the host galaxies. In two cases, the detection of emission lines allowed to provide hints on the overall properties of the gas in the host galaxies. Even though we warn that we are dealing with a very small sample, we remark that the redshift determination efficiency of our campaign is higher than for previous campaigns. We argue that it is mainly the result of the comparatively higher resolution of X-shooter.

The Astrophysical Journal, 2003

Light curves of eight BL Lac objects in the BVRI bands have been analyzed. All of the objects tend to be bluer when brighter. However spectral slope changes differ quantitatively from those of a sample of QSOs analyzed in a previous paper (Trevese & Vagnetti 2002) and appear consistent with a different nature of the optical continuum. A simple model representing the variability of a synchrotron component can explain the spectral changes. Constraints on a possible thermal accretion disk component contributing to the optical luminosity are discussed.

The Astrophysical Journal, 2011

We present far-UV HST/COS spectra of four nearby BL Lac Objects. BL Lac spectra are dominated by a smooth, power-law continuum which arises in a relativistic jet. However, the spectra are not necessarily featureless; weak, broad-and/or narrow-line emission is sometimes seen in high-quality optical spectra. We present detections of Lyα emission in HST/COS spectra of Mrk 421 (z = 0.030) and PKS 2005−489 (z = 0.071) as well as an archival HST/GHRS observation of Mrk 501 (z = 0.0337). Archival HST/STIS observations of PKS 2155−304 (z = 0.116) show no Lyα emission to a very low upper limit. Using the assumption that the broad-line region (BLR) clouds are symmetrically placed around the AGN, we use these measured Lyα emission features to constrain either the relativistic Γ values for the ionizing continuum produced by the jet (in the ionization-bounded case) or the mass of warm gas (in the density-bounded case). While realistic Γ values can be obtained for all four cases, the values for Mrk 421 and PKS 2155−304 are high enough to suggest that covering factors of broadline-region clouds of ∼1-2% might be required to provide consistency with earlier values of Doppler boosting and viewing angles suggested for this class of BL Lacs. This discrepancy also exists in the case of M 87, where the amount of Doppler boosting in our direction is expected to be minimal, again suggestive of a small covering factor of broad-line-region clouds. If, as these small covering factors might suggest, the assumptions of a density-bounded model could be more correct, then the observed Lya luminosities require that BL Lac/FR I nuclei possess very little warm gas (10 −4 to 10 −5 M ⊙) as suggested by Guilbert, Fabian & McCray (1983). If these clouds are in pressure balance with a hotter (∼ 10 6 K) gas, the BLR contains too little mass to power the AGN by accretion alone.

We present a sample of 386 BL Lacertae (BL Lac) candidates identified from 2860 deg 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. Fortunately, high proper motion QFOs, mostly DC white dwarfs, populate a well defined region of color space, approximately corresponding to blackbodies with temperatures in the range 7000-12000 K. QFOs with measurable redshifts or X-ray or radio counterparts (i.e., evidence of an extragalactic/AGN nature) loosely follow a track in color space that corresponds to power-law continua plus host galaxy starlight, with typical power-law slopes in the range 1 < α opt < 2 (f ν ∝ ν −α ). Based largely on this remarkably clean color separation, we subdivide the sample into 240 probable

The Astrophysical Journal, 1999

We present ASCA observations of the radio-selected BL Lacertae objects 1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 September and November, respectively. The ASCA spectra of both sources can be described as a first approximation by a power law with photon index Γ ≈ 2. This is flatter than for most X-ray-selected BL Lacs observed with ASCA, in agreement with the predictions of current blazar unification models. While 1749+096 exhibits tentative evidence for spectral flattening at low energies, a concave continuum is detected for 2200+420: the steep low-energy component is consistent the high-energy tail of the synchrotron emission responsible for the longer wavelengths, while the harder tail at higher energies is the onset of the Compton component. The two BL Lacs were observed with ground-based telescopes from radio to TeV energies contemporaneously with ASCA. The spectral energy distributions are consistent with synchrotron-self Compton emission from a single homogeneous region shortward of the IR/optical wavelengths, with a second component in the radio domain related to a more extended emission region. For 2200+420, comparing the 1995 November state with the optical/GeV flare of 1997 July, we find that models requiring inverse Compton scattering of external photons provide a viable mechanism for the production of the highest (GeV) energies during the flare. In particular, an increase of the external radiation density and of the power injected in the jet can reproduce the flat γ-ray continuum observed in 1997 July. A directly testable prediction of this model is that the line luminosity in 2200+420 should vary shortly after (∼ 1 month) a non-thermal synchrotron flare.

Astronomy & Astrophysics, 2013