Understanding Pulsar Wind Nebulae: Expansion, Particle Flow, and Broadband Emission

1. Pulsar Wind Nebulae

2. + G + + + + F R Pulsar Wind Nebulae b • Expansion boundary condition at • forces wind termination shock at • - wind goes from v = c/31/2 inside Rw to • v ~ RN/t at outer boundary } Pulsar logarithmic radial scale • Pulsar wind is confined by pressure • in nebula • - wind termination shock Wind MHD Shock Blast Wave  Particle Flow Ha or ejecta shell • Pulsar accelerates • particle wind - spectral break at where synchrotron lifetime of particles equals SNR age - radial spectral variation from burn-off of high energy particles • wind inflates bubble • of particles and • magnetic flux • particle flow in B-field • creates synchrotron • nebula Slane et al. 2004

3. Broadband Emission from PWNe Zhang et al. 2008 • Spin-down power is injected into the PWN at a • time-dependent rate • Based on studies of Crab Nebula, there appear to be • two populations – relic radio-emitting electrons and electrons injected in wind (Atoyan & Aharonian 1996) • Get associated synchrotron and IC emission from electron population, and some assumed B field (e.g. Venter & dE Jager 2006)

4. Broadband Emission from PWNe Del Zanna et al. 2006 • More completely, assume wind injected at • termination shock, with radial particle distribution • and latitude-dependent magnetic component: • Evolve nebula considering radiative and adiabatic • losses to obtain time- and spatially-dependent • electron spectrum and B field (e.g. Volpi et al. 2008) • - integrate over synchrotron and IC emissivity to • get spectrum Volpi et al. 2008 See also talk by O.C. de Jager

5. Connecting the Synchrotron and IC Emission • Energetic electrons in PWNe produce both synchrotron and inverse-Compton emission • - for electrons with energy ETeV, • synchrotron • inverse-Compton • - comparing photon energies from given electron • population gives B (e.g. Atoyan & Aharonian 1999) • Similarly, relative fluxes fE = E2 f(E) = nSngive B: • For low B, synchrotron lifetime is long, and fic/fs is large • - can expect bright TeV emission from collection of long-lived electrons; could explain • (some of the?) “dark” sources

6. A Point About Injection: 3C 58 • 3C 58 is a bright, young PWN • - morphology similar to radio/x-ray; suggests • low magnetic field • - low-frequency spectral break suggests • possible injection break • PWN and torus region observed in • Spitzer/IRAC and CFHT observations • - jet structure not seen above diffuse emission Slane et al. 2004

7. A Point About Injection: 3C 58 • 3C 58 is a bright, young PWN • - morphology similar to radio/x-ray; suggests • low magnetic field • - low-frequency spectral break suggests • possible injection break • PWN and torus region observed in • Spitzer/IRAC and CFHT observations • - jet structure not seen above diffuse emission Nebula Synchrotron Break Flux Density Injection E

8. Spitzer Observations of 3C 58 • 3C 58 is a bright, young PWN • - morphology similar to radio/x-ray; suggests • low magnetic field • - low-frequency spectral break suggests • possible injection break • PWN and torus region observed in • Spitzer/IRAC and CFHT observations • - jet structure not seen above diffuse emission VLA IRAC 4.5m Bietenholz 2006 Chandra IRAC 3.6m Slane et al. 2004 Slane et al. 2008

9. Spitzer Observations of 3C 58 • 3C 58 is a bright, young PWN • - morphology similar to radio/x-ray; suggests • low magnetic field • - low-frequency spectral break suggests • possible injection break • PWN and torus region observed in • Spitzer/IRAC and CFHT observations • - jet structure not seen above diffuse emission • IR flux for entire nebula falls within • extrapolation of x-ray spectrum • - indicates single break just below IR • - sub-mm observations would be of interest • Torus spectrum requires change in • slope between IR and x-ray bands • - challenges assumptions of single power • law for injection into nebula; TeV observations • should provide constraints Slane et al. 2008 Slane et al. 2008

10. Spitzer Observations of 3C 58 • 3C 58 is a bright, young PWN • - morphology similar to radio/x-ray; suggests • low magnetic field • - low-frequency spectral break suggests • possible injection break • PWN and torus region observed in • Spitzer/IRAC and CFHT observations • - jet structure not seen above diffuse emission • IR flux for entire nebula falls within • extrapolation of x-ray spectrum • - indicates single break just below IR • - sub-mm observations would be of interest • Torus spectrum requires change in • slope between IR and x-ray bands • - challenges assumptions of single power • law for injection into nebula; TeV observations • should provide constraints PRELIMINARY

11. MSH 15-52 • Contains PWN powered by energetic • pulsar B1509-58 (Edot = 1037.3erg s-1) • - complex nebula, with arcs, time-varying • clumps, and bright jet extending to SE • HESS observations reveal emission with • extended morphology following jet • - similar morphology to x-ray synchrotron • emission suggests leptonic origin • - IC model requires B~17 mG and an • anomalously high energy density • in dust IR emission, possibly from • SNR (Aharonian et al. 2005) • - Zhang et al. (2008) find similar • results from broad-band evolutionary • model (slightly higher B, lower Udust)

12. MSH 15-52 Aharonian et al. 2005 • Contains PWN powered by energetic • pulsar B1509-58 (Edot = 1037.3erg s-1) • - complex nebula, with arcs, time-varying • clumps, and bright jet extending to SE • HESS observations reveal emission with • extended morphology following jet • - similar morphology to x-ray synchrotron • emission suggests leptonic origin • - IC model requires B~17 mG and an • anomalously high energy density • in dust IR emission, possibly from • SNR (Aharonian et al. 2005) • - Zhang et al. (2008) find similar • results from broad-band evolutionary • model (slightly higher B, lower Udust) 6 arcmin

13. MSH 15-52 • Contains PWN powered by energetic • pulsar B1509-58 (Edot = 1037.3erg s-1) • - complex nebula, with arcs, time-varying • clumps, and bright jet extending to SE • HESS observations reveal emission with • extended morphology following jet • - similar morphology to x-ray synchrotron • emission suggests leptonic origin • - IC model requires B~17 mG and an • anomalously high energy density • in dust IR emission, possibly from • SNR (Aharonian et al. 2005) • - Zhang et al. (2008) find similar • results from broad-band evolutionary • model (slightly higher B, lower Udust) 6 arcmin Zhang et al. 2008

14. Reverse Shock PWN Shock Forward Shock Pulsar Termination Shock Pulsar Wind Unshocked Ejecta Shocked Ejecta Shocked ISM PWN ISM PWNe and Their SNRs • Pulsar Wind • - sweeps up ejecta; shock decelerates • flow, accelerates particles; PWN forms • Supernova Remnant • - sweeps up ISM; reverse shock heats • ejecta; ultimately compresses PWN; particles accelerated at forward shock generate • magnetic turbulence; other particles scatter off this and receive additional acceleration Gaensler & Slane 2006

15. HESS J1640-465 Lemiere et al. 2008 5 arcmin • Extended source identified in HESS GPS • - no known pulsar associated with source • - may be associated with SNR G338.3-0.0 • XMM observations (Funk et al. 2007) identify extended • X-ray emission, securing an associated X-ray PWN • Chandra observations (Lemiere et al. 2008) reveal point source within extended nebula, • apparently identifying associated neutron star • - HI absorption indicates a distance d ~ 8 – 13 kpc • - Lx ~ 1033.1erg s-1 Edot ~ 1036.7erg s-1 • - X-ray and TeV spectrum well-described by leptonic model with B ~ 6 mG and t ~ 15 kyr

16. HESS J1825-137 1 deg G18.0-0.7 XMM de Jager & Djannati-Atai 2008 Gaensler et al. 2003 • Discovered as part of HESS Galactic Plane survey • - large diameter (~1 deg) which makes this an exceptionally large PWN (R = 8.7 dkpc pc) • unless it is very nearby (which, it apparently is not because…) • Apparently associated with PSR B1823-13, whose DM distance is 4.12 kpc • - G18.0-0.7 is known PWN associated with pulsar; asymmetric morphology suggests • reverse shock interaction (Gaensler et al. 2003) • - TeV emission shows same basic asymmetry on much larger scale; TeV-emitting electrons • have lower energy than X-ray electrons  very low magnetic field See talk by M. Renaud

17. HESS J1825-137 • TeV spectral index steepens w/ increasing radius • - effects of IC/synchrotron cooling • HESS J1825-137 appears to be a very extended PWN • living in a large, unseen, old SNR that has evolved in a • non-uniform ISM (causing reverse shock to displace PWN from position of pulsar) • - may indicate a class of PWNe with low-energy electrons observable only (pardoxically) • in TeV g-rays!

18. Vela X t = 10,000 yr t = 20,000 yr t = 30,000 yr t = 56,000 yr Blondin et al. 2001 van der Swaluw, Downes, & Keegan 2003 • Vela X is the PWN produced by the Vela pulsar • - located primarily south of pulsar • - apparently the result of relic PWN being disturbed by asymmetric passage of the • SNR reverse shock • Elongated “cocoon-like” hard X-ray structure extends southward of pulsar • - clearly identified by HESS as an extended VHE structure • - this is not the pulsar jet (which is known to be directed to NW); presumably the • result of reverse shock interaction

19. Vela X LaMassa et al. 2008 • XMM spectrum shows nonthermal and ejecta-rich thermal emission from end of cocoon • - reverse-shock crushed PWN and mixed in ejecta? • Radio, X-ray, and -ray measurements appear consistent with synchrotron and I-C • emission from power law particle spectrum w/ two spectral breaks • - density derived from thermal emission 10x lower than needed for pion-production to • provide observed g-ray flux • - much larger X-ray coverage of Vela X is required to fully understand structure

20. Vela X de Jager et al. 2008 • Radio and VHE spectrum for entire PWN suggests presence of two distinct electron populations • - radio-emitting particles may be relic population; higher energy electrons injected by pulsar • Maximum energy of radio-emitting electrons not well-constrained • - this population will generate IC emission in GLAST band; spectral features will identify • indentify emission from dintinct up-scattered photon populations • - upcoming observations will provide strong constraints on this electron population

21. G327.1-1.1: Another Reverse-Shock Interaction • G327.1-1.1 is a composite SNR • with a bright central nebula • - nebula is offset from SNR center • - “finger” of emission extends toward • northwest • X-ray observations reveal compact • source at tip of radio finger • - trail of emission extends into nebula • - Lx suggests Edot ~ 1037.3 erg s-1 Temim et al. 2008 • Compact X-ray emission is extended; presumably pulsar torus • - PWN has apparently been disturbed by SNR reverse shock, • and is now re-forming around pulsar, much like Vela X et al. • Curious prong-like structures extend in direction opposite the • relic PWN • - these prongs appear to connect to a bubble blown by the • pulsar in the SNR interior, apparently in the region recently • crossed by the reverse shock; ANY TEV EMISSION?

22. G327.1-1.1: Another Reverse-Shock Interaction Temim et al. 2008 • G327.1-1.1 is a composite SNR • with a bright central nebula • - nebula is offset from SNR center • - “finger” of emission extends toward • northwest • X-ray observations reveal compact • source at tip of radio finger • - trail of emission extends into nebula • - Lx suggests Edot ~ 1037.3 erg s-1 • Compact X-ray emission is extended; presumably pulsar torus • - PWN has apparently been disturbed by SNR reverse shock, • and is now re-forming around pulsar, much like Vela X et al. • Curious prong-like structures extend in direction opposite the • relic PWN • - these prongs appear to connect to a bubble blown by the • pulsar in the SNR interior, apparently in the region recently • crossed by the reverse shock; ANY TEV EMISSION?

23. Conclusions • PWNe are reservoirs of energetic particles injected from pulsar • - synchrotron and inverse-Compton emission places strong constraints • on the underlying particle spectrum and magnetic field • Modeling of broadband emission constrains evolution of particles and B field • - modeling form of injection spectrum and full evolution of particles still • in its infancy • Reverse-shock interactions between SNR and PWNe distort nebula and • may explain TeV sources offset from pulsars • - multiwavelength observations needed to secure this scenario (e.g. Vela X. • HESS J1825-137, and others) • Low-field, old PWNe may fade from X-ray view, but still be detectable sources • of TeV emission • - VHE g-ray surveys are likely to continue uncovering new members of this class