1. Relativistic outflows and GLAST
Greg Madejski
Stanford Linear Accelerator Center
and
Kavli Institute for Particle Astrophysics and Cosmology
Outline:
GLAST and sources of celestial g-ray emission
How do we infer structure and physics operating in celestial g-ray sources?
Relativistic outflows: active galaxies as a case study
Nature of the relativistic jet: observed spectra/variability ->
emission processes -> content of the jet->
connection of the jet to the “central engine”
Other examples of relativistic outflows
Future observational prospects: GLAST

2. EGRET All Sky Map (>100 MeV)
3C279
Cygnus
Region
Vela
Geminga
Crab
PKS
LMC
Cosmic Ray
Interactions
With ISM
PSR B
PKS

3. Celestial gamma-ray sources
* Gamma-ray emission implies very energetic
processes – close connection to particle and high energy physics
* Our approach here is like “peeling of an onion”:
* Study the properties observed via “messengers” – photons (g-ray and other bands)
* Understand the photon emission mechanism
* Infer the structure from the properties observed in all other accessible bands
* Form a model consistent with the known physics

4. Most prevalent point-like g-ray sources on the sky:
active galaxies dominated by emission from relativistic outflows
Two examples of broad-band spectra :
g-ray emission dominates the E x F(E) spectrum – energetically important
Bright EGRET-detected GeV emitting blazar: 3C279 (data from Wehrle et al. 1998)
First TeV-emitting blazar: Mkn 421
(data from Macomb et al. 1995)

5. Active galaxies: thumbnail sketch
Presumably all active galaxies have the same basic ingredients: they are all powered by a flow of galaxian material onto a supermassive black hole
The accreting structure (disk) radiates quasi-thermally, releasing most of the gravitational energy of the flow – giving us some clues…
Some (but not all!) active galaxies possess relativistic jet that must be coupled to the disk
When the relativistically boosted jet points close to the line of sight, it is so bright that its emission masks the isotropically emitting “central engine”
Importantly, the radiating particles must be accelerated to multi-GeV energies to provide the “radiating agent”
This energy must be tapped from the bulk motion of the jet, but ultimately – from the accretion onto the black hole
Diagram from Padovani and Urry

6. EGRET - detected active galaxies always show relativistic jets
* How do we know a jet is relativistic?  Mainly because the structures change with time, showing apparent “superluminal” expansion
* This is easily explained as just a geometrical effect, but the velocity has to be close to c (Lorentz factor Gj ~ 10) and the angle to the line of sight – small
* Very strong Doppler-boost!
Credit: Very Large Array Image Archive

7. More evidence for relativistic outflows…
* In other cases, we infer the presence of the jet, using arguments based on the derived physical conditions
Gamma-ray and X-ray flux variability of EGRET-detected
active galaxy 3C279:
Short time scales and enormous brightness require the entire emission to originate in a Doppler-boosted relativistic outflow

8. Let’s summarize… BURNING QUESTIONS: WE KNOW THAT…
Relativistic jets (bulk Lorentz factors Gj~ 10) are common ingredients of active galaxies
Jets are powered by release of gravitational energy via accretion of galaxian matter onto a supermassive black hole
When pointed at us, jet radiation dominates because of Doppler boost
Synchrotron + Compton models work well for radiation processes
Radiating particles must have multi-GeV energies (gel ~ 103 – 105)
BURNING QUESTIONS:
* How is the jet formed? What is the connection of the jet to the accretion process? (electromagnetic processes - unipolar inductor?)
* How is the jet so precisely collimated, over many decades of distance?
* What is the jet content? e+/e- pairs, or e-/p+ ?
* How are radiating particles accelerated to multi-GeV energies? (=conversion of “bulk” to “random”)
* Role of hadronic processes?

9. Relativistic outflows are not unique to active galaxies
Invoking relativistic outflows solves many problems associated with source energetics in manly classes of sources
Good examples are gamma-ray bursts – now that we know they are at cosmological distances, luminosity would be v. large
-> invoking collimated relativistic motion solves many problems
If the g-ray emission were isotropic, the total energy emitted during the (short!) burst would be unreasonable
Examples of time-series of soft (MeV) g-ray flux measured from gamma-ray bursts (data from BATSE)
UNDERSTANDING THE PHYSICAL PROCESSES – AND THUS STRUCTURE –
REQUIRES MEASUEMENT OF TIME-RESOLVED
SPECTRUM TO GEV GAMMA-RAY ENERGIES (GLAST!)

10. Relativistic jets / outflows in Galactic binary systems
A very interesting class of astrophysical jets are galactic sources, associated with collapsed stars
Those have been discovered by radio VLBI imaging of galactic binary X-ray sources
Significant new insight has been gained by multiple “visits” – those also suggest superluminal expansion and thus relativistic speeds (“micro-quasars”)
Time ->
Time sequence of radio maps of a relativistic jet
in the galactic binary system (BH + a star)

11. GLAST LAT has much higher sensitivity to weak sources, with much better angular resolution
EGRET
GLAST will detect ~ 10,000 jet-dominated active galaxies – population studies possible!
GLAST

12. AGN: Extragalactic Background Light (From Eduardo’s talk)
High Energy photons (e.g. from AGN) can be absorbed via pair production
GeV (TeV) photons interact with intergalactic low energy photons UV(IR)
strong dependence on the distance from the source (inferred from redshift)
GLAST will see thousands of AGN
look for systematic effects vs redshift
key energy range for cosmological distances
A dominant factor in EBL models is the era of galaxy formation
AGN spectral roll-offs may help distinguish models of galaxy formation
Intrinsic
Salomon & Stecker
Primack & Bullock

13. GLAST will usher new era of studies of relativistic outflows
* Better sensitivity of GLAST
will allow more detailed
cross-correlations of variability with
other bands – location of the g-ray
producing region, connection
to the black hole, accretion disk, …
* Many more sources will be
detected – “parent population”
of jet-dominated active
galaxies (radio galaxies?)
* Detailed studies of properties of
gamma-ray bursts via broad band
spectra
* Galactic analogues to jets in
active galaxies – are micro-
quasars emitters of g-rays?
* Needed will be observations in
other bands
GLAST LAT’s ability to measure the flux and spectrum of 3C279 for a flare similar to that seen by EGRET in 1996 (from Seth Digel)

14. EGRET vs. GLAST…
Gamma-ray and X-ray flux variability of EGRET-detected
active galaxy 3C279

15. Backup slide: pulsars
* Jet-like structures are seen in many different astronomical objects: 
- Isolated neutron stars (Crab),
- Accreting binaries with a black hole as the compact object
- Accreting neutron stars
* Early work suggested that relativistic jets are always associated with flow of material onto a black hole
* This is now being challenged by
observations indicating that even neutron stars can have jets associated with them
X-ray image of the Crab Nebula

16. Jets in active galaxies radiate over a many decades of distance
Time-resolved radio map of quasar 3C345, revealing superluminal expansion
X-ray image of radio galaxy Cyg A (Chandra Observatory)
Jets in active galaxies radiate over a many decades of distance
from the central source: light-days to millions of light-years