Optical Imaging and Spectroscopy of BL Lac Objects

Monthly Notices of the Royal Astronomical Society

Bachev

et al. 2011

We present the results of our study of spectral energy distributions (SEDs) of a sample of 10 low‐ to intermediate‐synchrotron‐peaked blazars. We investigate some of the physical parameters most likely responsible for the observed short‐term variations in blazars. To do so, we focus on the study of changes in the SEDs of blazars corresponding to changes in their respective optical fluxes. We model the observed spectra of blazars from radio to optical frequencies using a synchrotron model that entails a log‐parabolic distribution of electron energies. A significant correlation among the two fitted spectral parameters (a, b) of log‐parabolic curves and a negative trend among the peak frequency and spectral curvature parameter, b, emphasize that the SEDs of blazars are fitted well by log‐parabolic curves. On considering each model parameter that could be responsible for changes in the observed SEDs of these blazars, we find that changes in the jet Doppler factors are most important.

Section: Resultsmentioning

confidence: 93%

Spectral energy distribution variation in BL Lacs and flat spectrum radio quasars

Monthly Notices of the Royal Astronomical Society

Bachev

et al. 2011

“…PKS 0735+178 : The blazar PKS 0735+718 has been classified as a BL Lac object (Carswell et al 1974). Papers concerning its redshift determination (Carswell et al 1974; Falomo & Ulrich 2000) had set a lower limit of z > 0.4 and z = 0.424 was reported for the source using a HST snapshot image (Sbarufatti, Treves & Falomo 2005). Since its optical identification, the source has been extensively observed across the whole EM spectrum (Teräsranta et al 2004; Gu et al 2006; Gupta et al 2008b; Ciprini et al 2007).…”

Section: Analysis and Resultsmentioning

confidence: 99%

Optical intraday variability studies of 10 low energy peaked blazars

Monthly Notices of the Royal Astronomical Society

Joshi

et al. 2011

We have carried out optical (R band) intraday variability (IDV) monitoring of a sample of 10 bright low energy peaked blazars (LBLs). 40 photometric observations, of an average of ∼4 h each, were made between 2008 September and 2009 June using two telescopes in India. Measurements with good signal‐to‐noise ratios were typically obtained within 1–3 min, allowing the detection of weak, fast variations using N‐star differential photometry. We employed both structure function and discrete correlation function analysis methods to estimate any dominant time‐scales of variability and found that in most of the cases any such time‐scales were longer than the duration of the observation. The calculated duty cycle of IDV in LBLs during our observing run is ∼52 per cent, which is low compared to many earlier studies; however, the relatively short periods for which each source was observed can probably explain this difference. We briefly discuss possible emission mechanisms for the observed variability.

“…There have been several papers concerning its redshift determination (e.g. Carswell et al 1974; Falomo & Ulrich 2000, and references therein) with the most recent result of z = 0.424 for PKS 0735+718 found using a Hubble Space Telescope ( HST ) snapshot image (Sbarufatti, Treves & Falomo 2005). Since it was optically identified, this source has been extensively observed across the whole EM spectrum (Fan & Lin 2000; Teräsranta et al 2004; Gu et al 2006; Gupta et al 2008c; Ciprini et al 2007, and references therein).…”

Section: Resultsmentioning

confidence: 99%

Short-term flux and colour variations in low-energy peaked blazars

Monthly Notices of the Royal Astronomical Society

Strigachev

et al. 2010

We have measured multiband optical flux and colour variations for a sample of 12 low-energy peaked blazars (LBLs) on short, day-to-month, time-scales. Our sample contains six BL Lacertae objects (BL Lacs) and six flat spectrum radio quasars (FSRQs). These photometric observations, made during 2008 September to 2009 June, used five optical telescopes, one in India and four in Bulgaria. We detected short-term flux variations in 11 of these blazars and colour variability in eight of them. Our data indicate that six blazars (3C 66A, AO 0235+164, S5 0716+714, PKS 0735+178, OJ 287 and 3C 454.3) were observed in pre-or post-outburst states, five (PKS 0420−014, 4C 29.45, 3C 279, PKS 1510−089 and BL Lac) were in a low state, while one (3C 273) was in an essentially steady state. The duty cycles for flux and colour variations on short time-scales in these LBLs are ∼92 and ∼33 per cent, respectively. The colour versus magnitude correlations seen here support the hypothesis that BL Lac objects tend to become bluer with increase in brightness; however, FSRQs may show the opposite trend, and there are exceptions to these trends in both categories of blazar. We briefly discuss emission models for active galactic nuclei that might explain our results.