Properties of optically selected BL Lacertae candidates from the SDSS

Abstract

Context.Deep optical surveys open the avenue for finding large numbers of BL Lac objects that are hard to identify because they lack the unique properties classifying them as such. While radio or X-ray surveys typically reveal dozens of sources, recent compilations based on optical criteria alone have increased the number of BL Lac candidates considerably. However, these compilations are subject to biases and may contain a substantial number of contaminating sources. Aims. In this paper we extend our analysis of 182 optically selected BL Lac object candidates from the SDSS with respect to an earlier study. The main goal is to determine the number of bona fide BL Lac objects in this sample. Methods. We examine their variability characteristics, determine their broad-band radio-UV spectral energy distributions (SEDs), and search for the presence of a host galaxy. In addition we present new optical spectra for 27 targets with improved signal-to-noise ratio with respect to the SDSS spectra. Results. At least 59% of our targets have shown variability between SDSS DR2 and our observations by more than 0.1-0.27 mag depending on the telescope used. A host galaxy was detected in 36% of our targets. The host galaxy type and luminosities are consistent with earlier studies of BL Lac host galaxies. Simple fits to broad-band SEDs for 104 targets of our sample derived synchrotron peak frequencies between 13.5 ≤ log 10 (ν peak ) ≤ 16 with a peak at log 10 ∼ 14.5. Our new optical spectra do not reveal any new redshift for any of our objects. Thus the sample contains a large number of bona fide BL Lac objects and seems to contain a substantial fraction of intermediate-frequency peaked BL Lacs.

Intra-night optical monitoring of three γ-ray detected narrow-line Seyfert 1 galaxies

“…In the optical band, this NLSy1 has been under regular monitoring since 2008 in the CRTS (Drake et al 2009). Until 2016 a total variability amplitude of 0.7 mag has thus been recorded (DOF16), which is blazar-like (see, e.g., Kügler et al 2014). On the intranight scale, prior to our observations, Paliya et al (2013) had monitored this NLSy1 on 4 nights during April-May (2012) which coincided with its moderately active γ-ray phase (DOF16).…”

Multi-epoch intra-night optical monitoring of 8 radio-quiet BL Lac candidates

“…Here it is pertinent to note that although a photometric redshift (z ∼ 1.54) has been reported for this source ) its extragalactic nature remains uncertain. This is, firstly, in view of a hint of proper motion (Table 1), but more so because of the lack of any features in its optical spectrum (e.g., see Plotkin et al 2010a;Kügler et al 2014a). For optical polarization of this source, Smith et al (2007) have given an upper limit of 2.33%.…”

“…This is consistent with the restframe value of ∼ 1% estimated for intra-day optical variabil- 3 WLQs, namely J104833.5+620305.0, J123743.1+630144.9 and J232428.4+144324.4 were found to vary prominently over a rest-frame time interval of ∼ 4, 2 and 4 yr respectively (Table 4). In particular, the variability of 1.655±0.009 mag exhibited by J104833.5+620305.0 appears blazar-like, since on such time scales, optical variability is mostly limited to ∼ 0.4-mag even for BL Lacs belonging to optically selected samples; variability amplitudes above 1.5-mag are extremely rare (e.g., Kügler et al 2014b). This makes it all the more important to ascertain the extragalactic as well as radio-quiet classification for this object through more sensitive optical spectroscopy and deeper radio measurements reaching a detection threshold of ∼ 0.1 mJy (Sect.…”

“…For instance, a remarkable ramification would be that in some rare relativistic jets, either the synchrotron radio emission is abnormally weak intrinsically, or the relativistic Doppler boost somehow is uncharacteristically ineffective in the radio domain than at optical wavelengths. It is, however, clear that if at all such exotic objects exist, they must be extremely rare, as suggested by the afore-mentioned studies of WLQs employing a number of basic diagnostics, such as the multi-wavelength spectral/polarimetric measurements, and optical flux variability, all of which seem to favor the notion that at least an overwhelming majority of RQWLQs is different from BL Lacs, whether radio or X-ray selected (e.g., Diamond-Stanic et al 2009;Shemmer et al 2009;Lane et al 2011;Kügler et al 2014a;Meusinger & Balafkan 2014;Plotkin et al 2015). This inference about rarity of jet emission in RQWLQs is further supported by the frequently observed deficit of Xray emission, though it is somewhat unclear how much of it is caused due to absorption (see, e.g., Luo et al 2015).…”

Semi-empirical AGN detection threshold in spectral synthesis studies of Lyman-continuum-leaking early-type galaxies

“…In the limiting case of a dominant AGN and high LyC-photon escape fraction, the optical SED of a galaxy is therefore expected to entirely lack both nebular (continuum and line) emission and stellar absorption features and superficially somewhat resemble that of a BL Lac (e.g. Oke & Gunn 1974;Marchã et al 1996;Kügler et al 2014). …”