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

EGRET All Sky Map (>100 MeV) 3C279 Cygnus Region Vela Geminga Crab PKS 0528+134 LMC Cosmic Ray Interactions With ISM PSR B1706-44 PKS 0208-512

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

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)

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

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

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

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?

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!)

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 1915+105 (BH + a star)

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

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

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)

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

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

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