first it’s hot & then it’s not extremely fast acceleration of cosmic rays in a supernova...

First It’s Hot & Then It’s Not Extremely Fast Acceleration of

Cosmic Rays In A Supernova Remnant

Peter Mendygral Journal Club

November 1, 2007

11/1/2007 Journal Club 2

Outline Poor man’s outline of diffusive shock

acceleration (DSA) Issue in DSA Background of SNR RX J1713.7-3946 Chandra observations of SNR RX

J1713.7-3946 Conclusions


Diffusive Shock Acceleration

Shock moving out



Shock moving out


Diffusive Shock Acceleration Original mechanism

proposed by Fermi in 1949 as an attempt to explain the power-law nature of the cosmic ray spectrum

Particles accelerated in some region by successive scattering events where the recoil of the scatterer is negligible (i.e. particle hits a wall)


Diffusive Shock Acceleration In the presence of a shock

Particle scatters off of B┴ on either side of shock

In particle’s frame, B┴ on either side of shock appears to be approaching (walls moving at it)

A resonance forms and particle gains lots of energy

Particle has energy-independent escape probability


Diffusive Shock Acceleration B┴ is first generated by plasma instabilities due

to the high energy thermal particles passing through the shock

For these systems a spectrum of Alfvén waves are produced yielding B┴

Shock will amplify B┴ produced upstream Particles will scatter approximately over the

gyroradius of the interaction

Bqmvr


DSA Outline

Shock moving out

High energy thermal proton/electron encounters shock

Bounces off previously made Alfvén wave and gains some energy

Gyroradius increases with increased energy

Higher energy particle escapes as CR

B0,ISM = B||

B = turbulent

Alfvén waves generate turbulent B

I helped him.


Shock Amplification Collisionless shocks can produce a compression

ratio (post-shocked to pre-shocked) given by

For γ = 5/3, as M→∞ r→4 B┴ can be amplified by a factor of 4 Amplifications beyond this are not well

understood

211

M

r


Field Amplification Observations of some SNRs suggest

amplifications beyond 4 Tycho Cassiopeia A

> 4 amplification is predicted by non-linear DSA Bell & Lucek can get ~100

An independent measurement of the field strength in an SNR would verify if amplifications of this order are real


SNR RX J1713.7-3946 Discovered in the

ROSAT All-Sky Survey

Brightest source of non-thermal X-rays among shell-type SNRs

Core collapse of type II/Ib of massive progenitor

Age is ~1600 yr Distance is ~1 kpc

Vshock ~ 3000 km s-1

XMM-Newton (Hiraga et. al., 2005)


Power-law X-ray Spectrum XMM-Newton spectra of the rim are consistent

for power-law with Γ ranging from 2.1−2.6

Hiraga et. al., 2005


Broadband X-ray Spectrum Suzaku data agrees well with theoretical expectation for

spatially integrated synchrotron spectrum

Uchiyama et. al., 2007


Broken Power-law γ–ray Spectrum Gamma-ray spectra are consistent with a model of π0 decay following

inelastic proton-proton interactions Imply proton acceleration in the shell up to 200 TeV Could be consistent with IC scattering by 100 TeV electrons if B ~ 10μG ~

ISM value Difficult to reconcile weak field with prediction that DSA will greatly amplify B

2004, 2005 gamma-ray excess HESS images (counts / smoothed region) (Aharonian et. al., 2007)


Evidence For SNR RX J1713.7-3946 We have significant evidence that the

system is a CR accelerator X-ray data is a non-thermal power-law

spectrum consistent with synchrotron spectrum

γ-ray data suggests presence of 200 TeV protons

Those regions are coincident Fits description of candidate accelerator

through DSA process


Chandra Observations 1-2.5 keV Chandra ACIS

image Color scale is (0-1.2)x10-7

photons cm-2 s-1 pixel-1 TeV γ-ray HESS contours

overlaid γ-ray contours coincident

with x-ray



Chandra Observations Top is 1-2.5 keV

observations made in July 2000, July 2005, July 2006 (region b)

Bottom is hard-band (3.5-6 keV) observations (region c)

Color scale same as last image



Chandra Observations

Top arrow is a 10σ “hot spot”

Bottom arrow is a 6σ “hot spot”


Chandra Observations Any arbitrary x-ray variation over the course

of one year must take place in a compact region of angular size cΔt (θ < 1 arcmin) Doesn’t alone rule out thermal processes

Also occur from a process where losses happen sufficiently fast over one year Rules out any thermal processes Thermal Bremsstrahlung and Free-Free emission

ruled out


Timescales Synchrotron loss timescale for electrons given

by

DSA acceleration timescale of electrons given by

Average energy of synchrotron photon

yearskeVmG

Btsynch5.05.1

5.1

yearss

kmv

mGB

keVt shockacc

25.15.0

000,3

2

016.0

TeVE

mGB


Field Magnitude To have seen the “hot spots”, tacc can’t

significantly exceed the x-ray variability Spots appeared within a few years

Assuming particle acceleration proceeds at maximum effective (Bohm-diffusion) regime with η 1B ~ 1mG

• Independent of the acceleration mechanism, tsynch must also be on the order of one yearB ~ 1mG


Field Magnitude Lower limits on the magnitude of B were

estimated indirectly by measuring the width of x-ray filaments

Interpretation of these structures in terms of diffusion and synchrotron cooling gives B ~ 0.07-0.25 mG The variability seen by Uchiyama represents the

strongest amplification


Implications Interpretation of γ-ray data as hadronic proton-

proton interactions is most likely IC is ruled out by B field measurement Protons and nuclei are accelerated to PeV energies

(electrons are short-lived at that energy) Confirms that field amplifications over several

orders of magnitude are possible Non-linear DSA produces observed amplification

but many microscopic process remain unexplored


References Aharonian, F. A., many others, 2005,

arXiv:astro-ph/0511678v2 Aharonian, F. A., many others, 2006,

arXiv:astro-ph/0511678v2 Berezhko, E. G., Völk, H. J., 2006, A&A 451, 981–990 Drury, L., 1983, Rep. Prog. Phys., Vol. 46, pp. 973-1027 Hiraga, J. S., Uchiyama, Y., Aharonian, F. A., 2005, A&A

431, 953–961 Uchiyama, Y., Aharonian, F. A., Tanaka, T., Takahashi,

T., Maeda, Y., 2007, Nature, Volume 449, Issue 7162, pp. 576-578

first it’s hot & then it’s not extremely fast acceleration of cosmic rays in a supernova...

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