SPECTROSCOPY OF OPTICALLY SELECTED BL LAC OBJECTS AND THEIR Γ-Ray EMISSION

Abstract

We present Very Large Telescope optical spectroscopy of nine BL Lac objects of unknown redshift belonging to the list of optically selected radio-loud BL Lac candidates. We explore their spectroscopic properties and possible link with gamma-ray emission. From the new observations we determine the redshifts of four objects from faint emission lines or from absorption features of their host galaxies. In three cases we find narrow intervening absorptions from which a lower limit to the redshift is inferred. For the remaining two featureless sources, lower limits to the redshift are deduced from the absence of spectral lines. A search for γ counterpart emission shows that six out of the nine candidates are Fermi γ -ray emitters and we find two new detections. Our analysis suggests that most of the BL Lac objects still lacking redshift information are most likely located at high redshifts.

Gamma-Ray and Optical Oscillations in PKS 0537–441

“…Sandrinelli et al (2014a) examined the Fermi light curves of the prototypical BL Lac object PKS 2155-304 (redshift z=0.116, R magnitude ∼13) and discovered a significant periodicity of T ∼ 630 d, which is twice that obtained by Zhang et al (2014) making a collection of all the published optical photometry of the source in 35 years. The existence of the optical periodicity was confirmed by our independent Rapid Eye Mounting Telescope (REM, Zerbi et al 2004;Covino et al 2004) photometry, which covered the source from 2008 to 2015/03 (Sandrinelli et al 2014b(Sandrinelli et al , 2015. Recent papers (Ackermann et al 2015;Hughes et al 2015) considered the Fermi light curves of an-1 http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl l/ other bright BL Lac object PG 1553+113 (z∼0.4, R∼14).…”

“…(2013) γ-ray light curve. Both high and low γ-ray states have clear counterparts in the optical bands (see also Sandrinelli et al 2015). The historical optical light curves, in fact, show that high states of duration of years are not infrequent (Pian et al 2007).…”

“…We report also the R-band light curve obtained combining REM and Small & Moderate Aper-ture Research Telescope System data (SMARTS 2 , Bonning et al 2012). Details can be found in Sandrinelli et al (2015).…”

“…Our procedure for searching for regular oscillations follows closely that of Sandrinelli et al (2015) and is based on the scheme proposed by Schulz & Mudelsee (2002). Shortly, 1) it yields the Lomb and Scargle (Scargle 1982) periodograms also accounting for unevenly spaced photometry, when it is the case, 2) it models the red noise with a first-order autoregressive process as null hypothesis of stochastic events, 3) it provides the biascorrected periodograms and significance levels using Monte Carlo simulations.…”

“…A periodicity of 2.18 years is found with interesting significance S 99%, which shows up clearly also in the R-monitoring. In Sandrinelli et al (2015) we examined the γ-ray and optical light curves of PKS 0537-441, OJ 287, 3C 279, PKS 1510-089, PKS 2005-489 and PKS 2155-304. While the power spectra indicated several peaks in all sources, sometimes at related frequencies in γ-ray and optical filters, with the exception PKS 2155-304 the significance is not very high.…”