Cosmic Ray Electrons with CTA. R.D. Parsons

Transcription

1 Cosmic Ray Electrons with CTA R.D. Parsons

2 Cosmic Ray Electrons In addi:on to the well known hadronic component of cosmic rays there is a more poorly understood electronic component Has a lower flux than the hadronic component and a steeper energy spectrum (~E - 3 )

3 Electron Origins Primary Origin: Electrons are accelerated by shock accelera:on, within the same sources as hadrons Expected to be produced from several sources (e.g. SNR, PWN ) These sources should produce few positrons, as the electrons are accelerated from the area surrounding the source Sources are accompanied by gamma- ray emission (Inverse Compton + Synchrotron)

4 Electron Origins Secondary Origin: Secondary Electron Electrons are also produced by secondary interac:ons of hadronic cosmic rays with the ISM These hadronic interac:ons produce a series of pions, which eventually decay to produce high energy electrons and positrons in equal numbers p p p p π - π + ν µ µ + ν e e +

5 Electron Diffusion Secondary Electron Primary Hadron Primary Electron Electrons are scavered in the randomly oriented component of the galac:c B- field Most of the source informa:on lost at detec:on point

6 Energy Loss Synchrotron: Energy lost as the electron interacts with the galac:c magne:c fields Inverse Compton: Energy lost as the electron interacts with the ambient radia:on fields (op:cal, IR & CMB) Synchrotron photons High energy electron High energy electron Lower energy electron Direc:on of B- field Op:cal photon Gamma- ray

7 Energy Loss Synchrotron: Inverse Compton: What life:me of electron does this lead to? Energy lost as the electron interacts Energy lost as the electron interacts For with a the 1 TeV galac:c electron magne:c life:me fields is only ~ 1 x 10 5 years with the ambient radia:on fields (op:cal, IR & CMB) Compare with the ~ 10 7 year life:me of hadronic cosmic rays, where only ionisa:on and hadronic interac:ons are the only important loss mechanisms Synchrotron photons What does this tell us? Direc:on of B- field High energy electron High energy electron Cosmic ray electron spectrum is steeper than that of hadrons (~ E - 3 ) Very High Energy cosmic ray electrons can only originate from nearby sources Lower energy electron The cosmic ray energy spectrum is very dependent on the local source distribu@on Op:cal photon Gamma- ray

8 Electron Spectrum (ATIC) Close agreement with other data + model at low energies Bump in spectrum, centred at ~ 500 GeV

9 Positron frac:on (PAMELA) Difficult to produce this increase if positrons only come from interac:ng hadrons Suggests another source of high energy positrons

10 Fermi measurements The Fermi satellite is a pair conversion telescope, used primarily for observing high energy gamma- rays However the instrument is also sensi:ve to electrons which interact similarly in the detector As it is in orbit it is possible to obtain very high event sta:s:cs There is however a large background from hadronic cosmic rays Must be rejected based of the width of the par:cle cascade produced in the detector

11 Electron spectrum (Fermi) Collec:on area of IACTs needed

12 HESS measurements Electrons interact with the atmosphere to produce a cascade of high energy par:cles This electromagne:c shower is very similar to that produced by a primary gamma- ray IACTs, such as HESS, can help greatly in their observa:on due to their very high effec:ve detec:on area (>10 5 m 2 ) Allows detec:on of electrons into the mul:- TeV range

13 HESS measurements Electrons are a diffuse source (like protons), so angular cut can t help in hadron rejec:on Random forest method was used to increase background rejec:on power Even poin:ng off the galac:c plane there may be some contamina:on by diffuse gamma- rays Some gamma rejec:on can be made by looking at the X max distribu:on

14 Electron Spectrum (HESS) Electron flux falls of rapidly, fit well by a broken power law Spectrum agrees with Fermi within systema:c errors Systema:cs are large due to uncertain:es in hadronic interac:on models

15 What does the spectrum look like?

16 Explana:ons: Dark MaVer Dark MaVer is expected to accumulate at the centre of large poten:al wells (massive objects) If these par:cles build up to high enough densi:es it becomes likely that dark maver annihila:ons will occur These annihila:ons will produce a number of par:cles, ojen resul:ng in electrons ajer a series of decays Electrons and positrons produced in equal amounts This provides an extra source of electrons and positrons from massive objects, producing a bump in the energy spectrum

17 Explana:ons: Nearby Source Bump in the spectrum can be adequately explained by a single conventional source Addi@onal electron component added Source would have to be local (a few hundred pc) to explain high energy electrons Klein-Nishina effects may also be important

18 Explana:ons: Nearby Source It may be possible that a local pulsar could be producing electron positron pairs This would also explain the PAMELA electron spectrum Possible candidates: Geminga Vela Pato et al. 2010

19 Cosmic Electrons with CTA CTA should be able to greatly improve on the results of HESS It can push down to lower energies, with its large telescopes As well reach higher energies do to is increase effec:ve area There should also be an improvement in the background rejec:on capabili:es, due to the higher precision of measurements

20 Cosmic Electrons with CTA Sub- array I used: 3 Large Telescopes (23m) 18 Medium Telescopes (12m) 38 Large Telescopes (6.7m) I was chosen for its good performance at high energies (large effec:ve area)

21 Cosmic Electrons with CTA Example Spectrum HESS broken power- law fit Fermi HESS CTA CTA is able to make measurements up to ~20 TeV

22 Cosmic Electrons with CTA Fermi HESS HESS exponen:al cut- off + young source CTA If a local source is present this may significantly modify the spectrum at high energies

23 Summary Study of the cosmic electron spectrum is able to provide a great deal of informa:on about local cosmic ray sources Recent missions have shown important features in both the spectrum and positron frac:on Nearby pulsars and dark maver annihila:on (and many other theories) have been put forward to explain the features in the spectrum CTA measurements will have a significant overlap with the Fermi spectrum measurements CTA should be able to extend measurements to > 10 TeV In measuring these high energy par:cles CTA may be able to differen:ate between some of the models put forward