# Presentation: 900245

## GLAST & Multiwaveband Monitoring as Probes of Bright Blazars

- Alan Marscher
- Boston University
- Research Web Page: www.bu.edu/blazars

## Sketch of Physical Structure of Jet, AGNBased on Current Observations & Theory

- Goals: 1. Determine where & how gamma-ray emission originates
-             2. Use multiwaveband light curves + VLBI images to probe compact jet

## Jets of EGRET Blazars: Very Fast

- Superluminal motion as high as almost 50*c*
- Bulk* *Lorentz factor up to 50
-  Doppler factor can approach 100

**3C 279: apparent speeds range from ~ 5*****c***** to >20*****c***

## Changes in Direction

- Change in apparent speed possible from change in direction even if  constant
- Nonthermal luminosity seems to be related to direction of jet
- Changes amplified greatly by projection effects
- Velocity seems ballistic in some jets but seems to follow twisting jet in many others

- Changes in direction tend can be abrupt, unlike precession (more like an unstable firehose)

## The Core of Blazar Jets

- a. Frequencies below ~ 40 GHz:  ~ 1 surface (deeper at higher frequencies)
- b. At higher frequencies, perhaps some lower frequencies:
- Conical standing shock (Daly & Marscher 1988)
-  Reproduces polarization pattern if randomly oriented B field is compressed by conical shock (Cawthorne 2006; e.g.; non-EGRET BL Lac object 1803+784 shown below)
- c. Even higher frequencies (~300 GHz): End of zone of flow acceleration
- - Where Doppler factor reaches asymptotic value

- 1803+784

## Connection between γ-ray Flares and Superluminal Ejections

- Jorstad et al. (2001): In 10 out of 23 cases the ejection of a superluminal component coincided to within 1σ with time of  high state of the γ-ray flux. Flare in 15 GHz polarized flux occurs prior to time of high γ-ray flux
- Lähteenmäki & Valtaoja (2003): high γ-ray flux occurs during rising phase of 37 GHz outbursts
- Conclusion: mm-wave outburst + superluminal knot ejection occur before peak in γ-ray flux
- γ-rays are produced in the knots, well beyond broad emission-line region
- Need to test with better timing data: GLAST + well-sampled VLBI data

## EGRET/VLBI: Likely Site(s) of Gamma-ray Emission

**Moving shock waves = superluminal knots**
-      Signature: outburst lasting weeks/months with persistent polarization direction
**Standing (oblique/conical) shocks = core & other stationary features**
-      Signature: steady but typically low polarization, usually parallel to jet axis
**3. Combination of the two** (as in M87; Cheung et al. 2007, ApJL, submitted)
- - Method for testing: (1) Well-sampled (monthly) time sequences of 43 GHz VLBI images, both total & polarized intensity; (2) polarization light curves from radio to optical; (3) total flux light curves from radio to gamma-ray

## Variations from Turbulence (Not all flares are new shocks!)

**Shock waves moving through turbulent jet plasma**
-      Signature: flux & polarization % and  fluctuate about values that change more smoothly over weeks-months
**Turbulence passing through standing conical shock system** (e.g., core of    0420-014; D'Arcangelo et al. 2007, ApJL)
-      Signature: polarization % and  fluctuate about low base polarization
- - Method for observing: (1) 10-20 day campaigns of daily polarization monitoring in optical & as frequently as possible (at least 3 times per week) with VLBA  at 43 GHz; (2) polarization monitoring at intermediate frequencies if possible

- Dark: optical
- Light diamonds: core at 43 GHz
- Light crosses: knot in jet for comparison

- 

- %

- MJD

## Cryptic Guide to VLBI Observations

- Frequencies lower than ~ 15 GHz:
- - Sensitive but relatively coarse resolution (>0.5 mas), inner jet opaque
- Good for surveys but need to wait to see moving knots
- - Images of > 100 sources at 15 GHz available at MOJAVE website (~100 sources): www.physics.purdue.edu/astro/MOJAVE and ~1000 sources at 5 GHz from VIPS (G. Taylor, UNM) in the future
- 43 & 22 GHz (VLBA, add Effelsberg et al. for better resolution):
**+** Fine resolution (0.1-0.2 mas), can see deeper into jet
-  see knots shortly after ejection, get apparent speeds & dates of ejection after several months
**** Sensitive enough only for > 1 Jy blazars, 0.1 Jy with phase referencing
**+** Need to observe at least once per month to follow knots
- - Images of 29 blazars will be available at www.bu.edu
- 86 GHz (Global Mm-VLBI Array):
**+** Highest non-experimental frequency + Finest resolution (40 as)
**** Lowest sensitivity  probes core region, not much else
**** Poor time coverage - twice per year (unless core brighter than a few Jy so possible with stand-alone VLBA)

## Number of Objects in a VLBA Session

- Basic principles:
- Don't usually care about faint outer structure  can observe a given object for only 5-10 min per hour, cycle through 5-10 sources + calibrators
- 24 hours: can observe ~25 sources with flux > 1 Jy with dynamic range better than 100:1 (usually adequate to follow knots) even at 43 GHz
- Observe at as high a frequency as possible given brightness of source & dynamic range needs
- Surveys of basic properties (as opposed to monitoring changes): can observe >100 objects at lower frequencies in a single 24-hour session
- Difficult objects (e.g., TeV BL Lacs with weak knots): Observe more frequently during session to get better image fidelity
- Weak objects: sandwich program sources between scans of phase calibrator

## The Desired Observational Result

- Densely sampled light curves from radio to gamma-ray for correlation analysis
- Times of superluminal ejections and flux history of core & knots
- ID of features on VLBI images responsible for higher-frequency outbursts
- Cross-correlate light curves at different   connections & time lags

## A Taste of the Gamma-Ray/Lower Frequency Connection: BLLac

- 0.2-1 TeV detection most significant during late 2005 X-ray/optical/radio outburst associated with the ejection of a bright superluminal radio knot

## Recipe for Definitive Results: Don't Cheat on the Ingredients (Time & Frequency Coverage, Images)

- Need both long-term monitoring to observe major outbursts/gradual trends + campaigns to sample short-term fluctuations
- Gaps in coverage leave doubt about correlations & non-correlations
- Polarization can be useful for identifying turbulence & connecting features on VLBI images to variable optical component
- Comprehensive multiwaveband observations in concert with GLAST and the VLBA can locate where high-E emission occurs in jet
- VLBA images available on web if groups are adequately supported
- GRAB A TELESCOPE (OR 10) & JOIN THE FUN!

- SAVE THE VLBA & ITS USERS!!!
- Tell the NSF and the NRAO director how crucial the VLBA is!
**& Support the theorists - we need more sophisticated models!**