1. BL LAC OBJECTS Marco Bondi INAF-IRA, Bologna, Italy

2. A “beamed” review This review includes:  Recent (2-3 years) developments on a few selected topics. This review will NOT cover:  Polarization properties  Parent population of BL Lacs and unified schemes

3. Outline The Blazar family New samples of BL Lacs Pc-scale kinematics Broad-band correlations Future developments

4. The Blazar Family e.g. Urry & Padovani '95 SED dominated by sync and IC processes from radio to gamma-ray. Flat radio spectra, extreme variability, high polarization (radio & optical), different opt spectroscopic properties. Unified scheme: BL Lacs as low luminosity blazars.

5. New samples of BL Lacs: why we need them ? The starting point: the “blazar sequence” (Fossati et al.1998) HBL, LBL, FSRQ Warning !!! Based on 3 highly biased samples lack of : low power low-energy peaked obj high power high-energy peaked obj

6. New samples of BL Lacs: testing the blazar sequence with CLASS The CLASS Blazar Sample ( Caccianiga & Marcha '04 ): 300 flat spectrum, > 30 mJy, sources. Motivation: l ooking for weak X-ray emitter BL Lac, i.e. steep radio-to-X-ray spectral index. Results: a significant and large fraction of low luminosity blazars in the sample lie outside the blazar sequence See also H. Bignall's talk.

7. New samples of BL Lacs: HST and VLBI obs of nearby objects see M. Giroletti poster Nearby (z<0.2) BL Lacs from the HST snapshot image survey (Giroletti et al. '06). Core dominance, jet/counter-jet ratio, SSC models used to constrain beaming parameters: Г ~ 4 Common origin for radio and optical “core” emission

8. AGN Jets: the fundamental questions modified from Lister '03 AGN jet speed distribution:  fast or slow jets ? what is the maximum jet speed ? Velocity profile:  radial: decelerating jets ?  transverse: spine & layer ? Relation of jet speed to other AGN properties:  dependence on BH mass ? optical class ?  are more luminous jets faster ?  which AGN have fastest jets ?

9. Jet speed distribution Jorstad et al. 2005: 7mm, 17 epochs, Some of the most extreme values confirmed (e.g. Piner et al. '06)

10. Pc-scale kinematics: TeV sources Jet motions in TeV BL Lacs (Piner & Edwards '04): multi-epoch 15 Ghz observations

11. TeV BL Lacs: pc-scale kinematics Jet motions in TeV BL Lacs (Piner & Edwards '04): multi-epoch 15 Ghz observations Similar jet morphologies: collimated, bending, transition to diffuse emission (see also Rector, Gabuzda & Stocke '03) Components in TeV BL Lacs are predominatly stationary or subluminal: Г ~ 2- 4 (also Giroletti etal '04)

12. TeV BL Lacs: radio and gamma ray beaming factors Г(VLBI)~2-4 Г(SED,gamma-ray)~10-50 Jet velocity structure  Radial: strong deceleration ( Georganopoulos & Kazanas '03 )  Transverse: fast spine and slow layer ( Ghisellini et al '05 ) one component sees the beamed radiation produced from the other and this yields to enhanced gamma-ray emission.

13. TeV BL Lacs: radio and gamma ray beaming factors Conical jet model ( Gopal-Krishna et al. '06 ):  each element of the jet cross section is boosted by a different amount because having different misaglinement. The inferred Г are underestimated.

14. Coordinated broad-band campaigns: radio-optical Whole Earth Blazar Telescope (WEBT) campaigns provided an unprecedented time sampling for radio and optical light curves:  BL Lac (Bach et al. '06)  3C 66A (Bottcher et al. '05)  0235+164 (Raitieri et al. '05) Major optical bursts linked to flux density and polarisation variations in the VLBI core with no apparent changes in the VLBI jet. Confirmed in OQ530 ( Massaro et al. '04 )

15. WEBT Campaigns on BL LAC itself Bach et al. '06 10 years optical and VLBI monitoring. VLBI allows to disentangle the different components contributing to the single dish radio light curves. Results:  spectral variability in the single-dish data traces the variability of the VLBI core.  fair correlation (time delay between 50-150 days) between variations in the optical and radio band.

16. Coordinated broad-band campaigns: radio-gamma-ray Connection between gamma-ray outbursts and the ejection of VLBI components have been found ( Pohl et al. '95, Jorstad et al. '01, Lähtenmäki & Valtaoja '03 ). Correlated radio and TeV variability in Mkn 421 investigated by Charlot et al. 06:  variations on few weeks timescale in the VLBI core  no significant variations in the VLBI jet

17. Future: AGILE http://agile.rm.iasf.cnr.it Gamma-ray (30 MeV-30 GeV) and hard X-ray (15-45 keV) imaging Consortium of Italian partners Launch 2007

18. AGILE gamma ray detector Source location determination:  5-20 arcmin (about twice better than EGRET) Sensitivity:  comparable to EGRET on-axis, better for off-axis FOV:  3 sr, about 6 times EGRET's  sensitivity for a 1-year all sky survey 3 times better than EGRET Broad-band monitoring of AGNs