Gamma-ray emission from middle-aged supernova remnants interacting with molecular clouds: the challenge for current models

We compare the $\gamma$-ray spectra from 10 middle-aged supernova remnants (SNRs), which are interacting with molecular clouds (MCs), with the model prediction from widely used escaping scenario and direct interaction scenario. It is found that the $\gamma$-ray data is inconsistent with the escaping scenario statistically, as it predicts a diversity of spectral shape which is not observed. The inconsistency suggests that the free escape boundary adopted in the escaping model is not a good approximation, which challenges our understanding of cosmic ray (CR) escaping in SNRs. In addition, we show that ambient CRs is potentially important for the $\gamma$-ray emission of illuminated MCs external to W28 and W44. In direct interaction scenario, the model involving re-acceleration of pre-existing CRs and adiabatic compression is able to explain the emission from most SNRs. The dispersion shown in the TeV data is naturally explained by different acceleration time of CR particles in SNRs. Re-acceleration of pre-existing CRs suggests a transition of seed particles, which is from thermal injected seed particle in young SNRs to ambient CRs in old SNRs. The transition needs to be tested by future multi-wavelength observation. In the end, we propose that radiative SNR without MC interaction is also able to produce a significant amount of $\gamma$-ray emission. A good candidate is S147. With accumulated Fermi data and CTA in future we expect to detect more remnants like S147.Comment: 15 pages and 7 figures. Accepted by MNRA

Sound wave generation by a spherically symmetric outburst and AGN Feedback in Galaxy Clusters

We consider the evolution of an outburst in a uniform medium under spherical symmetry, having in mind AGN feedback in the intra cluster medium (ICM). For a given density and pressure of the medium, the spatial structure and energy partition at a given time $t_{age}$ (since the onset of the outburst) are fully determined by the total injected energy $E_{inj}$ and the duration $t_b$ of the outburst. We are particularly interested in the late phase evolution when the strong shock transforms into a sound wave. We studied the energy partition during such transition with different combinations of $E_{inj}$ and $t_b$. For an instantaneous outburst with $t_b\rightarrow 0$, which corresponds to the extension of classic Sedov-Taylor solution with counter-pressure, the fraction of energy that can be carried away by sound waves is $\lesssim$12% of $E_{inj}$. As $t_b$ increases, the solution approaches the "slow piston" limit, with the fraction of energy in sound waves approaching zero. We then repeat the simulations using radial density and temperature profiles measured in Perseus and M87/Virgo clusters. We find that the results with a uniform medium broadly reproduce an outburst in more realistic conditions once proper scaling is applied. We also develop techniques to map intrinsic properties of an outburst $(E_{inj}, t_b$ and $t_{age})$ to the observables like the Mach number of the shock and radii of the shock and ejecta. For the Perseus cluster and M87, the estimated $(E_{inj}, t_b$ and $t_{age})$ agree with numerical simulations tailored for these objects with $20-30\%$ accuracy.Comment: Accepted by MNRAS, add one figure in appendix and minor changes in text based on referee's commen

Sound wave generation by a spherically symmetric outburst and AGN feedback in galaxy clusters II: impact of thermal conduction

We analyze the impact of thermal conduction on the appearance of a shock-heated gas shell which is produced when a spherically symmetric outburst of a supermassive black hole inflates bubbles of relativistic plasma at the center of a galaxy cluster. The presence of the hot and low-density shell can be used as an ancillary indicator for a high rate of energy release during the outburst, which is required to drive strong shocks into the gas. Here we show that conduction can effectively erase such shell, unless the diffusion of electrons is heavily suppressed. We conclude that a more robust proxy to the energy release rate is the ratio between the shock radius and bubble radius. We also revisited the issue of sound waves dissipation induced by thermal conduction in a scenario, where characteristic wavelength of the sound wave is set by the total energy of the outburst. For a fiducial short outburst model, the dissipation length does not exceed the cooling radius in a typical cluster, provided that the conduction is suppressed by a factor not larger than $\sim$100. For quasi-continuous energy injection neither the shock-heated shell nor the outgoing sound wave are important and the role of conduction is subdominant.Comment: 12 pages, 10 figures, MNRAS in pres

Particle Transport in Young Pulsar Wind Nebulae

The model for pulsar wind nebulae (PWNe) as the result of the magnetohydrodynamic (MHD) downstream flow from a shocked, relativistic pulsar wind has been successful in reproducing many features of the nebulae observed close to the central pulsars. However, observations of well-studied young nebulae like the Crab Nebula, 3C 58, and G21.5--0.9 do not show the toroidal magnetic field on a larger scale that might be expected in the MHD flow model; in addition, the radial variation of spectral index due to synchrotron losses is smoother than expected in the MHD flow model. We find that pure diffusion models can reproduce the basic data on nebular size and spectral index variation for the Crab, 3C 58, and G21.5--0.9. Most of our models use an energy independent diffusion coefficient; power law variations of the coefficient with energy are degenerate with variation in the input particle energy distribution index in the steady state, transmitting boundary case. Energy dependent diffusion is a possible reason for the smaller diffusion coefficient inferred for the Crab. Monte Carlo simulations of the particle transport allowing for advection and diffusion of particles suggest that diffusion dominates over much of the total nebular volume of the Crab. Advection dominates close to the pulsar and is likely to play a role in the X-ray half-light radius. The source of diffusion and mixing of particles is uncertain, but may be related to the Rayleigh-Taylor instability at the outer boundary of a young PWN or to instabilities in the toroidal magnetic field structure.Comment: 13 pages, ApJ, in press, corrected typ