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Register a new userFilling the Infrared Gap: ISO Observations of 1 Jy BL Lacertae Objects
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P. Padovani
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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 IR 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 IR gap by presenting new, simultaneous ISOCAM and ISOPHOT observations over the 7 - 200 micron range for half the sample. A measurement of the position of the synchrotron peak frequency, nu_peak, can provide information about particle acceleration mechanisms and constrain the inverse Compton radiation that will be detected by up-coming new gamma-ray missions. We have observed 17 1 Jy BL Lacertae objects with the camera and the photometer on board the Infrared Space Observatory (ISO) satellite. The ISOPHOT data reduction was done employing a novel correction, which mitigates the effect of chopping for faint sources. Using our new ISO data, complemented by nearly-simultaneous radio and optical observations for 10 and 4 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 nu_peak. Its distribution is centered at 10^13 Hz (30 micron) 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 IR data, may indicate strong nu_peak variations in a number of sources, possibly associated with large flares as observed in the high-energy peaked BL Lac MKN 501. (abridged)
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We present new BeppoSAX LECS and MECS observations, covering the energy range 0.1 - 10 keV (observers frame), of four BL Lacertae objects selected from the 1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which we interpret as inverse Compton emission. One object shows evidence for a low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the synchrotron component merging into the inverse Compton one around ~ 2 keV. A variable synchrotron tail would explain why the ROSAT spectra of our sources are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The broad-band spectral energy distributions fully confirm this picture and model fits using a synchrotron inverse Compton model allow us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. By combining the results of this paper with those previously obtained on other sources we present a detailed study of the BeppoSAX properties of a well-defined sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s) 1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous radio and X-ray powers for the 1-Jy sources in which the X-ray band is dominated by inverse Compton emission, which points to a strong link between X-ray and radio emission components in these objects.
We review the main results from several radio, X-ray and multi-frequency surveys on the topic of cosmological evolution of BL Lacertae objects. Updated findings on BL Lac evolution following the recent identification of many sources in the ``Sedentary Multi-Frequency survey are also discussed. By means of extensive Monte Carlo simulations we test some possible explanations for the peculiar cosmological evolution of BL Lacs. We find that a dependence of the relativistic Doppler factor on radio luminosity (as expected within the beaming scenario) may induce low values of V/V_max and that both edge effects at the low luminosity end of the BL Lacs radio luminosity function, and incompleteness at faint optical magnitudes may be the cause of the low V/V_max found for extreme HBL sources in X-ray selected samples.
We present 37 GHz data obtained at Metsahovi Radio Observatory in 2001 December - 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5 - 37 GHz and 37 - 90 GHz. Approximately 34 % of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lacs (LBLs). The variability index values of the sample were diverse, the mean fractional variability of the sample being Delta S_2 = 0.31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the high-energy BL Lacs (HBL), which are usually considered radio-quiet, can at times be detected at 37 GHz.
BL Lac objects are active nuclei, hosted in massive elliptical galaxies, the emission of which is dominated by a relativistic jet closely aligned with the line of sight. This implies the existence of a parent population of sources with a misaligned jet, that have been identified with low-power radiogalaxies. The spectrum of BL Lacs, dominated by non-thermal emission over the whole electromagnetic range, together with bright compact radio cores, high luminosities, rapid and large amplitude flux variability at all frequencies and strong polarization make these sources an optimal laboratory for high energy astrophysics. A most distinctive characteristic of the class is the weakness or absence of spectral lines, that historically hindered the identification of their nature and ever thereafter proved to be a hurdle in the determination of their distance. In this paper we review the main observational facts that contribute to the present basic interpretation of this class of active galaxies. We overview the history of the BL Lac objects research field and their population as it emerged from multi-wavelength surveys. The properties of the flux variability and polarization, compared with those at radio, X-ray and gamma-ray frequencies, are summarized together with the present knowledge of the host galaxies, their environments, and central black hole masses. We focus this review on the optical observations, that played a crucial role in the early phase of BL Lacs studies, and, in spite of extensive radio, X-ray, and recently gamma-ray observations, could represent the future major contribution to the unveiling of the origin of these sources. In particular they could provide a firm conclusion on the long debated issue of the cosmic evolution of this class of active galactic nuclei and on the connection between formation of supermassive black holes and relativistic jets.
Observations of candidate BL Lacertae objects spectroscopically selected from the Sloan Digital Sky Survey (SDSS) reveal a large fraction with high polarization (P > 3%). This result confirms that synchrotron radiation makes an important contribution to the observed optical continuum for most objects in the sample. The SDSS sample can be divided into separate categories, with objects of undetermined redshift generally having the highest optical polarization. Polarization as high as 23% and the lack of spectral features suggests that the synchrotron continuum completely dominates the spectra of these sources. The mean polarization levels observed for objects having measured redshifts is much lower, with the maximum observed polarization for this group being ~10%. The lower polarizations of these objects are reminiscent of the less spectacular polarization levels shown by BL Lac objects discovered in X-ray surveys. We find no SDSS BL Lac candidates at z > 1 with P > 3%, calling their classification as BL Lac objects into question. In addition, the existence of radio-quiet BL Lac objects is not verified since none of 10 potentially radio-weak BL Lac candidates observed are highly polarized. Regardless of whether the high-redshift and radio-weak objects are included in this optical sample, the overall levels of polarization observed are intermediate between those seen for X-ray and radio-selected BL Lac objects.
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