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

11/1/2007Journal Club2 Outline  Poor man’s outline of diffusive shock acceleration (DSA)  Issue in DSA  Background of SNR RX J  Chandra observations of SNR RX J  Conclusions

11/1/2007Journal Club3 Diffusive Shock Acceleration Shock moving out

11/1/2007Journal Club4 Diffusive Shock Acceleration Shock moving out

11/1/2007Journal Club5 Diffusive Shock Acceleration Shock moving out

11/1/2007Journal Club6 Diffusive Shock Acceleration Shock moving out

11/1/2007Journal Club7 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)

11/1/2007Journal Club8 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

11/1/2007Journal Club9 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

11/1/2007Journal Club10 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 B 0,ISM = B || B = turbulent Alfvén waves generate turbulent B I helped him.

11/1/2007Journal Club11 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

11/1/2007Journal Club12 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

11/1/2007Journal Club13 SNR RX J  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  V shock ~ 3000 km s -1 XMM-Newton (Hiraga et. al., 2005)

11/1/2007Journal Club14 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

11/1/2007Journal Club15 Broadband X-ray Spectrum  Suzaku data agrees well with theoretical expectation for spatially integrated synchrotron spectrum Uchiyama et. al., 2007

11/1/2007Journal Club16 Broken Power-law γ–ray Spectrum  Gamma-ray spectra are consistent with a model of π 0 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)

11/1/2007Journal Club17 Evidence For SNR RX J  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

11/1/2007Journal Club18 Chandra Observations  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 Uchiyama et. al., 2007

11/1/2007Journal Club19 Chandra Observations  Top is 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 Uchiyama et. al., 2007

11/1/2007Journal Club20 Chandra Observations  Top arrow is a 10σ “hot spot”  Bottom arrow is a 6σ “hot spot”

11/1/2007Journal Club21 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

11/1/2007Journal Club22 Timescales  Synchrotron loss timescale for electrons given by  DSA acceleration timescale of electrons given by  Average energy of synchrotron photon

11/1/2007Journal Club23 Field Magnitude  To have seen the “hot spots”, t acc 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  1  B ~ 1mG Independent of the acceleration mechanism, t synch must also be on the order of one year  B ~ 1mG

11/1/2007Journal Club24 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 ~ mG  The variability seen by Uchiyama represents the strongest amplification

11/1/2007Journal Club25 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

11/1/2007Journal Club26 References  Aharonian, F. A., many others, 2005, arXiv:astro- ph/ v2  Aharonian, F. A., many others, 2006, arXiv:astro- ph/ v2  Berezhko, E. G., Völk, H. J., 2006, A&A 451, 981–990  Drury, L., 1983, Rep. Prog. Phys., Vol. 46, pp  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