Lunch discussion on motivations for studying blazar variability Greg Madejski, SLAC Parts of this presentation use slides by Benoit Lott and Jun Kataoka * Introduction and general comments: * “Blazar” is a phenomenological term, defined by observational characteristics * Variability over ALL energy bands is a defining property of blazars * Other characteristics include: * compact radio source (not always resolved, even via radio interferometry) * polarization in radio through optical * appearance of a large-scale jet in radio, opt., X-ray * many blazars show that they are hosted in galaxies (but galaxy is not always detected) * Variability holds a promise to understand the blazar phenomenon * Best model (“paradigm”) has the blazar emission originating in relativistic jet, pointing close to our line of sight * Relativistic motion of the jet Doppler-boosts the emission in the direction of motion * Within this, “misdirected blazars” are probably radio galaxies (generally, much more numerous in the Universe) * For the purpose of this discussion: blazars are strong and variable  -ray emitters, and correlation of variability patterns should shed light on the origin and structure of the jet

Radio, optical and X-ray images of the jet in M 87 * Jets are common in AGN – and radiate in radio, optical and X-ray wavelengths * Blazars are the objects where jet is pointing close to the line of sight * In many (but not all) blazars, the jet emission dominates the observed spectrum

Unified picture of active galaxies Diagram from Padovani and Urry

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

Source “compactness” (old radio astronomy arguments) if the source is as small as variability scales indicate, particle and photon energy are v. high -> the radiative losses due to Compton emission would be prohibitive - violation ⇒beaming Elliot Shapiro relation assume stationary emission, Eddington-limited flow  t > R s /c ~ 10 3 M 8 s L Edd = M 8 erg s -1 L /  t < erg s -2 violation ⇒beaming Gamma-ray transparency (to e+/e-pair production) R < c  t/(1+z) if X-rays are produced in the same region as  -rays   >> 1 ⇒ beaming Magnetic field limits (Catanese 1997) (somewhat model dependent) Correlated variability between optical/X-ray and GeV/TeV (E syn   t) -1/3 < B < E syn  /E c 2 violation ⇒beaming Evidence for beaming Simple light travel argument relates the emission size scale to the variability time scale

Blazars are variable in all observable bands

Standard (leptonic) model Photons are produced by energetic electrons low energy peak is produced by synchrotron emission, high energy peak is due to Compton emission both due to non-thermal population of relativistic electrons, synchrotron peak – particle interaction with B field, Compton peak – particle interaction with ambient photon field Competing (hadronic) model Protons are accelerated, lose energy mainly due to p-p or p-  interaction, produce pions, … Both models require acceleration of particles to very high energies (we now little about it!) BUT ALL MODELS INVOKE RELATIVISTIC JET – INDEPENDENTLY, “BULK” ACCELERATION IS MOST LIKELY REQUIRED Blazar models There is such a thing as a “standard model” for blazars (well, one “standard” and one “competing” model)

Two examples of blazar spectra

blazar variability: what can we learn?  Variability time scale: origin of flares constraints on source size ⇒ beaming tests, bulk motion identification of source as a blazar  Correlated variability - time lags: acceleration/ deceleration processes source geometry (one zone…) importance of external fields: disk, BLR, torus jet matter content (e+/e- vs p+/e-)  Loop diagrams (flux vs index): acceleration/ deceleration models, SSC vs ERC models  “Orphan” flares - anomalous components: test of SSC models jet matter content (e + /e - vs p + /e - ) UHECR acceleration?  Radio knot ejection after GeV flares?: jet launching sites, jet acceleration/deceleration  X-ray precursor: jet matter content (e + /e - vs Poynting flux, p + /e - ), jet environment  Correlated variability in different bands: counterpart association  Steady component: distinction between inner jet and extended Chandra jets

What makes a rapid variability ? BLR cloud BLR cloud       cm (sub-pc) X-ray/  -rays Assume that the central BH mass is M and 10 r g = cm. Modulation of relativistic flows - faster shell    catches up with the slower one    at D  ~ 10  r g ~ [cm] e - e + (and possibly smaller fraction of p ) are accelerated in the shock, and emit Sync/ inv Comp radiation. Similar to the GRB prompt emission, but t acc ~ t cool ≲ R/  v shock ~ 1 day. from Jun Kataoka

Flare and Quiescent ? flare duration : internal E: t crs ~ c  D 0  1+2  2 22 E m ~ Mc 2 (  1 +   1+2 ) Max electron E:  max  v s /c  D -1 22 11  1+2 =        If      is large, collision takes place at small distances, with large/short variability.  If  is small, collision at large distance, with only small variability. significant increase in  max Mrk 421 Mrk 501 1day JK+ 2001; Tanihata daily flares - only visible at the LE/HE peak. - changes in acceleration eff. “ steady” component - commonly observed in all freq. - changes in the mass acc rate ? from Jun Kataoka

“Seed” for the ERC process is UV photons reflected by the BLR. Soft X-ray “precursor” before the GeV flare?  -ray flare (int. shock) Soft-Xray flare (bulk Compton)  slow  fast Broad Line region  E diff ~ 10 eV, L diff ~ erg/s Before the collision, both the fast and slow shells upscatter UV photons via the “bulk-Comptonization” to E BC ~  BLK 2 E diff ~ 1 keV. After the collision,  -rays are emitted via the ERC process, peaking at E ERC ~  p 2 E diff ~ 1GeV  p ~ 10 4 for shock accelerated electrons  soft X (slow) soft-X (fast)  -ray flare Moderski from Jun Kataoka

Such scenario, however, assumes cold pair plasma (e - e + ) as a matter content of jets. Matter content of the jet? If significant protons are involved, such precursor will not be observed. Also, “no precursor” may imply that  -ray flares are produced by reconnection events rather than by internal shocks.  Absence of “bump” feature? → N e /N p < 50 ?  jets at pc-scale are still dominated by B-field??? Collaboration with SWIFT and Suzaku will Clarify this further ! pure e - e + Sikora & Madejski 2000 Sikora et al from Jun Kataoka

Jorstad et al. 2001a, b  -ray flares were followed by the appearance of new radio-knots. EGRET radio t=0 of radio -knot ejection Radio-knots in QHBs often shows super-luminal motion; v app /c ~ 10. Radio knot ejection after GeV flare (QHBs) ?  Jet is highly relativistic even on pc-scale, at least in QHBs   -ray events “trigger” the ejection of radio-knots? key to understanding launching site of the jets ! from Jun Kataoka

PKS X-ray (Chandra) +optical Radio map + fractional polarization Tests of the Compton-scattered CMBR interpretation of extended X-ray jets

GLAST LAT’s ability to measure the flux and spectrum of 3C279 for a flare similar to that seen in 1996 (from Seth Digel) * In summary, to learn about the structure of blazars, origin of relativistic jets, acceleration and radiation processes,  -ray variability must be studied in the Context of as many bands as possible! * Most other bands study objects “ one at a time ” – we will need lots of resources The picture on the left leads to Benoit ’ s presentation: GLAST ’ s improvement in variability studies over EGRET goes only as the ratio of effective areas