High Energy Studies of Evolved Pulsar Wind Nebulae

1. High Energy Studies of Evolved Pulsar Wind Nebulae Collaborators: D. Castro S. Funk J. Gelfand T. Temim D. Foight J.P. Hughes M. Lemoine-Goumard R. Rousseau B. Gaensler and others…

2. Evolution of PWN Structure • PWN expands within SNR as it • sweeps up ejecta • - eventually, SNR reverse shock • reaches PWN and halts expansion • X-ray observations provide • ambient density estimates • - connect to age, energetics • Magnetic field in PWN decreases • dramatically during adiabatic • expansion • Upon RS interaction, PWN is • compressed and magnetic field • is increased • - high energy particles burn off • rapidly • - PWN is disrupted, often dramatically CMB inverse Compton synchrotron

3. Evolution of PWN Emission Spitkovsky 2008 1000 yr 2000 yr 5000 yr • Injected spectrum is expected to be • Maxwellian w/ nonthermal tail • - note that Maxwellian has never been • definitively detected • Emax and fraction of energy in PL • likely to vary within PWN • Energetic electrons produce synchrotron • emission in X-ray band, and IC emission • in g-ray band • Note that X-ray emission decreases • with time, while g-ray emission increases CMB inverse Compton synchrotron

4. From Young to Old

5. HESS J1640-465 Slane et al. 2010 • Composite SNR in late evolution • PWN model with evolved power • law electron spectrum fits X-ray • and TeV emission, but not GeV

6. HESS J1640-465 Slane et al. 2010 • Composite SNR in late evolution • PWN model with evolved power • law electron spectrum fits X-ray • and TeV emission, but not GeV • Modifying low-energy electron • spectrum by adding Maxwellian • produces GeV IC emission • similar to results from Vela X • possible evidence of long-sought • Maxwellian component expected from • shock acceleration models

7. MSH 11-62 • Radio observations reveal shell with • bright, flat-spectrum nebula in center • - no pulsar known, but surely a PWN • Distance not well known, but ≈5 kpc • - R ≈ 10.6 pc

8. MSH 11-62 CXO • X-ray spectrum gives n0 ≈ 0.6 cm-3 • SNR/PWN modeling gives t ≈ 5 kyr • - SNR reverse shock has begun to interact • with PWN • X-ray studies show thermal shell with a • central PWN • - pulsar candidate seen as hard point source • in center of PWN (offset from radio center)

9. MSH 11-62 Fermi LAT • Spectrum well-described by cut-off PL • - Ecut ≈ 5 GeV, consistent w/ pulsar spectra • Pion model gives acceptable fit • - requires n0 = 7 cm-3 and Ecut = 70 GeV • - these values are too high and too low, • respectively, for a typical SNR scenario • 1FGL J1112.1-6041 is spatially associated • with MSH 11-62 • - F(>100 MeV) ≈ 1 x 10-10 erg cm-2 s-1 • Two nearby pulsars w/ Ė > 1033 erg/s • - neither can yield more than 3% of the flux

10. MSH 11-62 • PWN model with PL injection cannot fit • broadband spectrum • - either over-predicts TeV flux or under- • predicts GeV flux • - similar to Vela X and HESS J1640-465 • Model w/ Maxwellian + PL gives good • approximation to data • - ge ≈ 106, G ≈ 2.7; EPL = 0.01 EMaxwellian • - B ≈ 2 mG, typical of evolved PWN • Fermi observations of MSH 11-62 are • consistent with emission arising from • an evolved PWN • - if correct, broadband modeling appears to • provide additional support for presence of • Maxwellian electron component • - cannot rule out pulsar as origin of GeV • emission • - timing and sub-mm observations important

11. MSH 15-56 MOST • Prototypical composite SNR • Radio observations reveal shell with • bright, flat-spectrum nebula in center • - no pulsar known, but surely a PWN • - nebula significantly displaced from SNR • center • Kinematic distance ≈4 kpc based on • Ha radial velocity measurements • - R ≈ 20 pc

12. MSH 15-56 ROSAT CXO • X-ray spectrum gives n0 ≈ 0.1 cm-3 • SNR/PWN modeling gives t ≈ 12 kyr • - SNR reverse shock has completely • disrupted PWN • X-ray studies show thermal shell w/ • very faint hard emission near PWN • - pulsar candidate seen as hard point source • w/ faint X-ray trail extending to PWN

13. MSH 15-56 Fermi LAT • PL spectrum provides reasonable fit • - any cutoff much harder than for pulsars • Pion model gives acceptable fit • - requires n0 = 1.5 cm-3 and Ecut = 300 GeV • - these values are too high and a bit low, • respectively, for a typical SNR scenario • 1FGL J1552.4-5609 is spatially associated • with MSH 15-56 • - F(> 100 MeV) ≈ 3.3 x 10-10 erg cm-2 s-1 • One nearby pulsar w/ Ė > 1033 erg/s • - can not yield more than 5% of the flux

14. MSH 15-56 • PWN model with PL injection cannot fit • broadband spectrum • - either over-predicts TeV flux or under- • predicts GeV flux • Broken PL reproduced general behavior, • but misses radio index, X-ray flux, and • TeV upper limit • - low X-ray to radio flux ratio consistent • with post-compression PWN phase • Fermi observations of MSH 15-56 may • be consistent with emission from an • evolved PWN • - if correct, pulsar has essentially departed • relic PWN and is injecting particles into • newly-forming nebula • - additional modeling required to consider • possible evidence for low-energy electron • component

15. Missing Cousins? G272.2-3.2 • Do other SNRs with ages similar to the • detected composite SNRs also show GeV • emission? • - Certainly not all. Little or no evidence from • G272.2-3.2, Kes 73, or G299.2-2.9 • - no strong evidence of emission from magnetar • in Kes 73 either… • Not exactly a “control sample” • - distance from Galactic Plane is large for • G272.2-3.2 and G299.2-2.9 (density low, • but confusion from Plane also low…) • - small number statistics… • Need to carry out further Fermi studies • to establish whether or not composite • SNRs are more prevalent GeV emitters • (which would point to either PWN or • pulsar as origin of emission d ~ 5 kpc, R ~ 12 pc, t ~ 6-15 kyr Kes 73 d ~ 7.5 kpc, R ~ 4 pc, t ~ 1 kyr G299.2-2.9 d ~ 5 kpc, R ~ 7 pc, t ~ 4.5 kyr

16. Conclusions • Both X-ray and g-ray emission are produced in composite SNRs • - GeV emission is expected to increase as PWN evolves; X-ray • emission decreases • Broadband modeling places strong constraints on evolution of • composite SNRs • - provide probe of underlying electron spectrum • MSH 11-62 may provide additional evidence of a Maxwellian electron • component accompanying the power law tail • - spectrum could also be produced by pulsar; pulsation searches • and studies of spatial extent needed • - sub-mm observations to confirm or refute presence of Maxwellian • electron distribution are crucial for this and other systems • MSH 15-56 appears to be in late phase of evolution • - GeV emission provides unique probe of evolved electron population • Continued X-ray and g-ray studies of composite SNRs hold promise