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Norita Kawanaka (Hakubi Center, Kyoto Univ.) Kazumi Kashiyama (Univ. of Tokyo)

This study explores the TeV gamma-ray emission from the Geminga pulsar wind nebula (PWN) and its implications for cosmic-ray electrons/positrons. It discusses the excess of positrons observed and the need for primary positron sources. It also examines various astrophysical origins of cosmic-ray electrons/positrons, such as pulsars, supernova remnants, microquasars, gamma-ray bursts, and white dwarfs. The study presents a model for the transport of electrons/positrons inside and outside the PWN, as well as the injection and escape mechanisms. The results show discrepancies between the observed and modeled emission spectra, indicating the presence of additional sources or different origins for cosmic-ray electrons/positrons. The study concludes with the potential of future observations with instruments like CTA and CALET to provide further insights into the escape-limited model for cosmic-ray accelerators.

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Norita Kawanaka (Hakubi Center, Kyoto Univ.) Kazumi Kashiyama (Univ. of Tokyo)

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  1. Model of TeV Gamma-Ray Emission from Geminga Pulsar Wind Nebula and Implication for Cosmic-Ray Electrons/Positrons Norita Kawanaka(Hakubi Center, Kyoto Univ.) Kazumi Kashiyama (Univ. of Tokyo) Kohta Murase (Penn State Univ.) CTA Japan Workshop12/15-16/2016

  2. “positron excess” Observed CR e+ flux seems to exceed that expected from the standard secondary production model, and rises with the energy. Some primary positron sources are needed! PAMELA (~1-100GeV) (Adriani et al. 2008) AMS-02 (~1-500GeV)  Drop above ~200 GeV?

  3. Electron+Positron Flux H.E.S.S. ATIC/PPB-BETS (Chang et al. 2008) (Aharonian et al. 2008) • Excess from the conventional model •  Primary CR e± sources? • Bumpy structure and sharp cutoff at ~500 GeV? • drop above ~TeV Fermi-LAT (Abdo et al. 2009)

  4. Positron flux (AMS-02) Peak & Cutoff at 300GeV?

  5. Astrophysical Origin • Pulsars • Aharonian+ 95; Atoyan et al. 95; Chi+ 96; Zhang & Cheng 01; Grimani 07; Yuksel+ 08; Buesching+ 08; Hooper+ 08; Profumo 08; Malyshev+09; Grasso+ 09; NK, Ioka & Nojiri 10; NK, Ioka, Ohira & Kashiyama 11; Kisaka & NK 12; NK, Kashiyama & Murase in prep. • Supernova Remnant • Pohl & Esposito 98; Kobayashi+ 04; Shaviv+ 09; Hu+ 09; Fujita, Kohri, Yamazaki & Ioka 09; Blasi 09; Blasi & Serpico 09; Mertsch&Sarkar 09; Biermann+ 09; Ahlers, Mertsch & Sarkar 09; NK 12 • Microquasar (Galactic BH) • Heinz & Sunyaev 02 • Gamma-Ray Burst Ioka 10 • White Dwarfs Kashiyama, Ioka & NK 11

  6. CR e±source = Pulsar wind nebula? Crab nebula

  7. Previous Studies (1) Single source (a) E=0.9x1050erg, age=2x105yr, a=2.5 (b) E=0.8x1050erg, age=5.6x105yr, a=1.8 (c) E=3x1050erg, age=3x106yr, a=1.8 e+ fraction e±spectrum cooling cutoff energy ⇔ Age of the source Ioka 2010

  8. Previous Studies (2) Multiple sources e+ fraction solid lines：fave(ee) (average) dashed lines：fave(ee) ±Dfave Pulsar birth rate ~ 0.7x10-5/yr/kpc2 Ee+=Ee-~ 1048erg a ~ 1.9 • Average spectra are consistent with PAMELA, Fermi & H.E.S.S. • Large dispersion in the TeV range due to the small N(ee)  possible explanation for the cutoff inferred by H.E.S.S. e±spectrum NK et al. 2010

  9. Geminga PWN • one of the brightest g-ray sources • age: 3.42 x 105 yr • distance: 250+230-80 pc • Are we observing the emission from escaping e±? g-ray SED (MAGIC; Ahnen+ 2016) Milagro observation (Abdo+ 2009)

  10. e±Transport inside the PWN e± acceleration at the termination shock diffusive/convective transport in the nebula diffusive escape from the nebula into the ISM

  11. Transport equation inside the PWN (one-dimensional) fint: distribution function DPWN: diffusion coefficient inside the PWN V: convection velocity inside the PWN P: cooling rate (synch./IC/adiabatic) Q: injection spectrum Assumptions nebula radius: rout =30 pc inner radius: rTS = 0.0025 pc contact discontinuity: rCD = 0.01 pc V = V0r -1/2, B = B0r 3/2 :inside the CD V = 0, B = BISM = 3 mG: outside the CD

  12. e±Transport outside the PWN time dependent diffusion equation in the ISM Q: injection luminosity at the PWN outer boundary = 4 p rout2 ・(outgoing flux at the boundary) solution r = 30 – 100 pc (around the PWN) & 250 pc (observed CR e±)

  13. Result: observed CR spectrum e± spectrum total e± energy: Etot ~ 1047 erg injection spectrum: broken power-law a1=1.9, a2=3.0, break @ gb=6x105 e+ spectrum very difficult to reproduce both of the spectra with a single source… • different origins? • softer spectrum? • no cutoff?

  14. Results: emission around the PWN • synchrotron and IC scattering • SED: inconsistent with MAGIC/Milagro data • harder e± spectrum? higher break energy? • brightness distribution: extending outside the nebula 10-12 erg cm-2 s-1 (250pc) Spectral energy distribution ~7° brightness distribution MAGIC/Milagro

  15. e± A young PSR/PWN is surrounded by a SNR.  CR e±from a PWN should go through the SNR shock without trapped by the magnetic field around the shock Kennel & Coroniti 93 r shock front Escape condition: Lesc eesc(t): given by models LE CR HE CR e± spectrum from a young pulsar should have a low energy cutoff  Probe of the energy-dependent escape scenario (Ohira+ 2010; Ohira, Yamazaki, NK & Ioka 2012; Ohira, NK & Ioka 2016) Models of eesc(t) x

  16. TeV e± spectrum can prove the CR escape! Without energy-dependent escape • Electron spectrum from Vela SNR/PSR (d=290pc, tage~104yr, Etot=1048erg) • Only e± s with ee>eesc(tage) can run away from the SNR. •  Low Energy Cutoff • 5yr obs. by CALET (SWT=220m2sr days; see next slide) may detect it. eesc(t) from Ptuskin & Zirakashvili 03 NK+ 2011 Direct Evidence of Escape-Limited Model for CR accelerators (=SNR)!

  17. TeV Gamma-Ray Sky HESS sources ~40 … e± emitting PWN? CTA: larger number of sources with better statistics e± ~ 1048erg TeV emission ~ 5mCrab @20kpc

  18. Summary • pulsar wind nebulae = cosmic-ray e± sources? • e± escaping thePWN … spatially extended emission from e±is expected • We solve the transport of high energy e± inside/outside the PWN (e.g. Geminga), and compare the CR electron/positron spectra with those observed by Fermi/AMS-02, and compare the g-ray emission feature around the nebula with that observed by Milagro and MAGIC (in prep.) • High energy (>~ TeV) e± spectral feature may tell us the properties of CR sources. • PWNe might account for the significant fraction of un-identified TeV sources.

  19. Backup Slides

  20. Why are the PAMELA/AMS-02 results “anomaly”? • Positrons are generated from CR protons (secondary origin) • Higher energy protons can escape the Galaxy earlier • Higher energy positrons are less produced:① • Observed positron spectrum becomes softer because of the escape and energy loss:② • Electrons: primary origin •  Spectral change should be only from ② primary ① secondary ② Observed e+ spectrum CR e+ spectrum should be softer than that of e-

  21. CR escape from SNRs (Ptuskin & Zirakashvili 05; Caprioli+ 09; Gabici+ 09; Ohira+ 10 etc.) Lesc r shock front e>eesc(t) e<eesc(t) • The particles with highest energy can escape the SNR shock at the beginning of the Sedov phase • As the shock decelerates, lower energy particles become able to escape the shock x Nesc observed CR spectrum If the CR injection rate increases with time, the observed CR spectrum would become softer  consistent with observations (CR spectrum ∝ e-2.7) eesc(t) g-ray spectra of SNRs (Ohira+ 10), CR helium hardening (Ohira, NK+ 16) e

  22. Constraints on pulsar-type decay time * A significant fraction of observed electrons are emitted recently. pulsar type: t0=105yr H.E.S.S. pulsar type: t0=104yr

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