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 Workshop　12/15-16/2016

“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?

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)

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

Astrophysical Origin Pulsars Supernova Remnant 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

CR e± source = Pulsar wind nebula? Crab nebula

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

Previous Studies (2) Multiple sources Ee+=Ee-~ 1048erg a ~ 1.9 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

Geminga PWN g-ray SED 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)

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

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 = V0 r -1/2, B = B0 r 3/2 : inside the CD V = 0, B = BISM = 3 mG : outside the CD

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±)

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?

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° MAGIC/Milagro brightness distribution

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

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)!

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

Summary pulsar wind nebulae = cosmic-ray e± sources? e± escaping the PWN … 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.

Backup Slides

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-

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

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