Jet Variability Under the Microscope Eric Perlman - Florida Institute of Technology Collaborators: Mihai Cara, Sayali Avachat, Raymond Simons, Matt Bourque (FIT) Markos Georganopoulos (UMBC) Cade Adams (Georgia and Clemson) Daniel E. Harris (Harvard/Smithsonian CfA) Eric Clausen-Brown, Maxim Lyutikov (Purdue) Juan P. Madrid (Swinburne) Lukasz Stawarz (Jagiellonian University and JAXA) C. C. “Teddy” Cheung (NRL) Bill Sparks, John Biretta (STScI) The HESS, VERITAS and MAGIC TeV Observatory teams Eric Perlman - Florida Institute of Technology Collaborators: Mihai Cara, Sayali Avachat, Raymond Simons, Matt Bourque (FIT) Markos Georganopoulos (UMBC) Cade Adams (Georgia and Clemson) Daniel E. Harris (Harvard/Smithsonian CfA) Eric Clausen-Brown, Maxim Lyutikov (Purdue) Juan P. Madrid (Swinburne) Lukasz Stawarz (Jagiellonian University and JAXA) C. C. “Teddy” Cheung (NRL) Bill Sparks, John Biretta (STScI) The HESS, VERITAS and MAGIC TeV Observatory teams

Outline  Introduction: What do we know about jets? Basic Properties Radiation Mechanisms Relativistic Effects Variability (and why it is important)  Putting Variability Under the Microscope M87: the first RESOLVED variable region! Multiwavelength lightcurves The nucleus & HST-1: two variability modes Spectral and polarimetric variations  Conclusions Timescales and Particle Acceleration Jet Structure and other physics  Introduction: What do we know about jets? Basic Properties Radiation Mechanisms Relativistic Effects Variability (and why it is important)  Putting Variability Under the Microscope M87: the first RESOLVED variable region! Multiwavelength lightcurves The nucleus & HST-1: two variability modes Spectral and polarimetric variations  Conclusions Timescales and Particle Acceleration Jet Structure and other physics

Stellar Mass NS/BH Binaries: Double star system, neutron star or black hole + main sequence or giant star. Matter accreting from normal star. Flow speeds c. Centers of Galaxies: Matter accreting from interstellar medium onto supermassive black hole. Flow speeds c+. Part of “active galactic nucleus” phenomenon. Gamma-Ray Burst: Death throes of a very massive star (>30-50 solar masses); asymmetric explosion drives a relativistic outflow. Flow speeds >0.99c but probably very mass loaded. Stellar Mass NS/BH Binaries: Double star system, neutron star or black hole + main sequence or giant star. Matter accreting from normal star. Flow speeds c. Centers of Galaxies: Matter accreting from interstellar medium onto supermassive black hole. Flow speeds c+. Part of “active galactic nucleus” phenomenon. Gamma-Ray Burst: Death throes of a very massive star (>30-50 solar masses); asymmetric explosion drives a relativistic outflow. Flow speeds >0.99c but probably very mass loaded. Examples of Relativistic Jets

The Unified AGN Model Supermassive ( M  ) black hole. M = 10 8 M   R G ~2 AU Accretion disk – thermal UV/X-ray lines from highly ionized atoms (R~ R G ) High velocity (>1000 km/s) broad- line clouds (R~ R G ) Dusty torus, which orbits in/near plane of accretion disk (R~ R G ) Lower velocity (few hundred km/s) narrow-line clouds (R~ R G ) Relativistic jet ( Γ ~ 5-30) – may be collimated on ~50 R G scales, can extend for many kiloparsecs Observed properties vary with viewing angle Supermassive ( M  ) black hole. M = 10 8 M   R G ~2 AU Accretion disk – thermal UV/X-ray lines from highly ionized atoms (R~ R G ) High velocity (>1000 km/s) broad- line clouds (R~ R G ) Dusty torus, which orbits in/near plane of accretion disk (R~ R G ) Lower velocity (few hundred km/s) narrow-line clouds (R~ R G ) Relativistic jet ( Γ ~ 5-30) – may be collimated on ~50 R G scales, can extend for many kiloparsecs Observed properties vary with viewing angle Urry & Padovani 1995

Radiation Processes in Jets Radiation from jets emitted by two processes: synchrotron and inverse- Compton. For inverse-Compton, the ‘scattered’ photon can be either from within the jet (often called synchrotron self-Compton) or some external source (e.g, the cosmic microwave background or emission line regions). B Synchrotron radiation emitted by relativistic particles in magnetic field e-e- Inverse-Compton – scattering interaction between photon and a relativistic particle that results in a higher-energy photon. Jet “beam”

Biretta et al. 1999

Superluminal motion in quasar jets: an optical illusion Speed of knot ( close to the speed of light) Positions of knot when two pictures were taken, one year apart. Small angle: the knot’s motion is mostly along the line of sight. Light paths: B A Light path B is shorter than path A. If the knot’s speed is close to the speed of light, B is almost a light-year shorter than A. This “head start” makes the light arrive sooner than expected, giving the appearance that the knot is moving faster than light. (Nothing actually needs to move that fast for the knot to appear to move that fast.) Not drawn to scale!

Relativistic Effects Time dilation:  app  δ   Blueshifting :  app = δ  “Superluminal” Motion: v app = v sin θ  β  cos  θ  Time dilation:  app  δ   Blueshifting :  app = δ  “Superluminal” Motion: v app = v sin θ  β  cos  θ  Curves for  = 3,5,8,12 θ v=βc Γ=(1-β 2 ) -½ δ=[Γ(1  β cos θ)] -1 Geometrical distortion: d Ω =d Ω ’/  2 Beaming for Synchrotron and SSC: L obs =  4 L em For EC: L obs =  6 L em /  2

Variability in Jets  Jet emission is highly variable.  Usually in blazars -- jet seen at very small angles (our line of sight is nearly along beaming axis) Relativistic speed confirmed by apparent Superluminal motion  Typical example: 3C (at right) Variable on all timescales Slow variability & Large flares Also intraday variability Broadband spectrum can change drastically.  However… in most sources the varying region is unresolved.  Physics not well constrained.

10 Variability and Source Size Variability timescale implies maximum emission region size scale r b =r´ b Spherical blob in comoving frame Γ Doppler Factor Source size from direct observations: Source size from temporal variability:

11 Variability and Source Location Variability timescale implies engine size scale, comoving size scale factor Γ larger and emission location Γ 2 larger than values inferred for stationary region Rapid variability by energizing regions within the Doppler cone x  1/ 

Chandra and HST

The Best-Studied Jet: M87 Images at right show the M87 jet at radio (bottom) and then in optical and UV (wavelength decreasing as you go up) Jet is knottier at higher photon energies Narrowing trend continues all the way up through X-rays Images at right show the M87 jet at radio (bottom) and then in optical and UV (wavelength decreasing as you go up) Jet is knottier at higher photon energies Narrowing trend continues all the way up through X-rays 2 cm 340 nm 230 nm 140 nm Sparks, Biretta & Macchetto 1996

~fowen/M87_layout.html

The M87 Jet (Marshall et al. 2002)

Optical Polarization of the M87 Jet  Jet can be very highly polarized -- up to 60%+ in spots  Natural for synchrotron emission  Position angle of polarization (pictured vectors, rotated 90 degrees to show B)  direction of magnetic field in emitting region  High & low polarization regions correlated with the location of knots  Often different in different bands  Can give clues to some very interesting physics… Perlman et al. 1999

Stratified jet… High energy particles nearer jet axis Low energy particles evenly distributed B compressed, perp. to jet in shocks Shocks accelerate particles, so they brighten optical emission but not radio Further development of magnetic field depends on: Magnetic field coherence length l B Synchrotron cooling length l cool Stratified jet… High energy particles nearer jet axis Low energy particles evenly distributed B compressed, perp. to jet in shocks Shocks accelerate particles, so they brighten optical emission but not radio Further development of magnetic field depends on: Magnetic field coherence length l B Synchrotron cooling length l cool Perlman et al l B = l cool l B =2l cool

Perlman & Wilson 2005

Madrid 2009

Variability in the M87 Jet?  Yes – dramatic variability!  Giant flare in HST-1  Seen in all bands  Smaller variability in other regions  Only opt/X-ray jet where varying region is isolated MUCH BETTER CONSTRAINTS ON PHYSICS  Superluminal motion as well  6c in HST-1, decreasing to c at ~12” out  Variability of opt spectrum & polarization will give clues to physics: Compression/shock/expansion Acceleration/cooling timescales Tracing motion of components Harris et al. 2009

A Tale of two components Two different regions, two different variability behaviors:  HST-1: Giant Flare Flux increased by >100X Dominant timescale ~1 year  Nucleus: Numerous smaller flares Largest variation is a factor 4 Timescales ~few months or less Not resolved Two different regions, two different variability behaviors:  HST-1: Giant Flare Flux increased by >100X Dominant timescale ~1 year  Nucleus: Numerous smaller flares Largest variation is a factor 4 Timescales ~few months or less Not resolved

Optical Lightcurves Optical and X-ray Variability Closely track one another Madrid 2009

Where are the Flaring Regions?  Nucleus: innermost few parsecs or smaller Very smooth structure, no obvious “flaring” or motions  HST-1: 0.86” (62 parsecs projected) from nucleus Knotty structure, superluminal motions

Analyzing the lightcurve  Can look for many things, e.g., Does one band flare first? If so, is there a consistent lag? Does one band increase faster or slower?  However, the lightcurve contains interesting hints: X-rays increase, decrease more rapidly to main peak X-rays also contain more month-timescale variability  Can look for many things, e.g., Does one band flare first? If so, is there a consistent lag? Does one band increase faster or slower?  However, the lightcurve contains interesting hints: X-rays increase, decrease more rapidly to main peak X-rays also contain more month-timescale variability Harris et al. 2009

Analyzing the lightcurve  Quantifying properties in the X- rays. Use first derivative – as fractional change per year This is 1/region size for light- travel time Constrain varying region to <45 light-days from largest derivative.  Quantifying properties in the X- rays. Use first derivative – as fractional change per year This is 1/region size for light- travel time Constrain varying region to <45 light-days from largest derivative. Harris et al. 2009

Quasi-periodic behavior in HST-1  Found during the increasing phase of the flare  No single period, but an increase and decrease was observed every 6-10 months  “Impulsive” acceleration?  Found during the increasing phase of the flare  No single period, but an increase and decrease was observed every 6-10 months  “Impulsive” acceleration? Harris et al. 2009

Close-up on the flare  Optical, X-ray behavior similar  Behavior not monolithic  First derivative changes sign ~  Secondary flare in 2006  Optical, X-ray behavior similar  Behavior not monolithic  First derivative changes sign ~  Secondary flare in 2006 Harris et al. 2009

Velocity Structure of HST-1  Four moving components, motions tracked for over three years  One knot – C – split in 2005, coinciding with main flare. Also location of flux peak Birth of a new component in jet during flare.  Four moving components, motions tracked for over three years  One knot – C – split in 2005, coinciding with main flare. Also location of flux peak Birth of a new component in jet during flare.

Velocity Structure of HST-1  Four moving components, motions tracked for over three years  One knot – C – split in 2005, coinciding with main flare. Also location of flux peak Birth of a new component in jet during flare.  Fastest components moved at 4.3 c  Four moving components, motions tracked for over three years  One knot – C – split in 2005, coinciding with main flare. Also location of flux peak Birth of a new component in jet during flare.  Fastest components moved at 4.3 c

Gamma-ray flaring Abramowski et al. 2012

Polarization, Spectral Variability in HST-1  Strong correlation between flux, polarization  Very little change in PA  Only one region involved in variability -- fully resolved  Strong correlation between flux, polarization  Very little change in PA  Only one region involved in variability -- fully resolved Perlman et al. 2011

 Strong correlation of polarization with flux magnetic field involved in particle accel  Complicated relationship between flux and spectral index Epochs 4-9 – “hard lagging” Epochs – “soft lagging”  The latter is more common; implies shorter acceleration timescales than cooling timescales.  The latter normally requires the opposite relationship … but X-ray is also synchrotron  Possibility: most energy losses are actually in inverse-Comptonizing external photons near Klein- Nishina limit.  Strong correlation of polarization with flux magnetic field involved in particle accel  Complicated relationship between flux and spectral index Epochs 4-9 – “hard lagging” Epochs – “soft lagging”  The latter is more common; implies shorter acceleration timescales than cooling timescales.  The latter normally requires the opposite relationship … but X-ray is also synchrotron  Possibility: most energy losses are actually in inverse-Comptonizing external photons near Klein- Nishina limit. Polarization & Spectral Behavior Perlman et al. 2011

Close-up on the flare  Optical, X-ray behavior similar  Behavior not monolithic  First derivative changes sign ~  Secondary flare in 2006  Switch in fpy, in both X-ray & optical, corresponded with switch in direction of “looping”  Optical, X-ray behavior similar  Behavior not monolithic  First derivative changes sign ~  Secondary flare in 2006  Switch in fpy, in both X-ray & optical, corresponded with switch in direction of “looping” Harris et al. 2009

Shocks Compression ratio k

Behavior of Polarization in shock  If shock is localized, planar and perpendicular to jet (as pictured in last plot), MHD predicts a polarization  Our data require a jet bulk Lorentz factor of ~4-5 – consistent with speeds observed in VLBI observations.  Beaming factor can range over a wider range of values  If shock is localized, planar and perpendicular to jet (as pictured in last plot), MHD predicts a polarization  Our data require a jet bulk Lorentz factor of ~4-5 – consistent with speeds observed in VLBI observations.  Beaming factor can range over a wider range of values Perlman et al. 2011

A Tale of two components Two different regions, two different variability behaviors:  HST-1: Giant Flare Flux increased by >100X Dominant timescale ~1 year  Nucleus: Numerous smaller flares Largest variation is a factor 4 Timescales ~few months or less Not resolved Two different regions, two different variability behaviors:  HST-1: Giant Flare Flux increased by >100X Dominant timescale ~1 year  Nucleus: Numerous smaller flares Largest variation is a factor 4 Timescales ~few months or less Not resolved Harris et al. 2009

Analyzing the lightcurve Harris et al  Larger values of fpy than HST-1  Largest fpy~20 => region is smaller than ~20 light-days.  Goes along with faster variability in general  Larger values of fpy than HST-1  Largest fpy~20 => region is smaller than ~20 light-days.  Goes along with faster variability in general

Patterns in the nuclear variability?  None seen  But … are we sampling often enough to see them? Maybe not, given small region size.  None seen  But … are we sampling often enough to see them? Maybe not, given small region size. Harris et al. 2009

Gamma-ray flaring Abramowski et al. 2012

 Faster Variability  Complex polarization behavior  No obvious correlation between flux, polarization … unless you look more carefully  No evidence for spectral index variability  Faster Variability  Complex polarization behavior  No obvious correlation between flux, polarization … unless you look more carefully  No evidence for spectral index variability Polarization and Spectral Variability in M87 Jet: Nucleus Perlman et al. 2011

Polarization behavior  “Loop” in I-P plane – very different from HST-1!  No correlation with EVPA (varies all over the place) or spectral index (near constant)  “Loop” in I-P plane – very different from HST-1!  No correlation with EVPA (varies all over the place) or spectral index (near constant) Perlman et al. 2011

Dynamics and helical jets  Helical jets introduce additional complexity into your jet model  Can introduce a helical distortion  Or velocity variations  Helical jets introduce additional complexity into your jet model  Can introduce a helical distortion  Or velocity variations Perlman et al Not shown: Mag. Pitch angle ψ Shock compression (ratio k)

 Nuclear X-ray also synchrotron but more complicated  No correlation with EVPA (varies all over the place) or spectral index (near constant)  Consistent with either:  a helical distortion  Shock compression in a helical jet  We favor the latter because of the morphology, known helical morphology of nuclear B field.  Neither one is 100% satisfactory for reproducing EVPA behavior.  Nuclear X-ray also synchrotron but more complicated  No correlation with EVPA (varies all over the place) or spectral index (near constant)  Consistent with either:  a helical distortion  Shock compression in a helical jet  We favor the latter because of the morphology, known helical morphology of nuclear B field.  Neither one is 100% satisfactory for reproducing EVPA behavior. Perlman et al. 2011

Marshall et al. 2010

Conclusions  Variability can reveal unique information about jets  We have for the first time isolated a variable region in a jet  HST-1’s Variability was similar to that observed in blazars X-ray variability was fastest … but spectral behavior complex Quasi-periodic acceleration during increasing phase Complex flare shape Polarization characteristics consistent with a simple shock  Nucleus was also variable Faster variability timescale, smaller region size No pattern to variability (but are we observing often enough?) “Loop” in (I,P) plane suggests helical morphology to varying region.  Variability can reveal unique information about jets  We have for the first time isolated a variable region in a jet  HST-1’s Variability was similar to that observed in blazars X-ray variability was fastest … but spectral behavior complex Quasi-periodic acceleration during increasing phase Complex flare shape Polarization characteristics consistent with a simple shock  Nucleus was also variable Faster variability timescale, smaller region size No pattern to variability (but are we observing often enough?) “Loop” in (I,P) plane suggests helical morphology to varying region.