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Institut für Astronomie und Astrophysik

Abteilung Astronomie

Sand 1, D-72076 Tübingen, Germany
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Dissertation Yudong Cui


Cosmic rays from the supernova remnants HESS J1731-347 and W28

Summary.
The supernova remnant (SNR) HESS J1731-347 displays strong TeV and nonthermal X-ray emission, thus the object is presently accelerating particles to very high energies. A distinctive feature of this young SNR is the nearby ( 30 pc in projection) extended source HESS J1729-345, which is currently unidentified but is in sky projection coinciding with known molecular clouds (MC). Using a new analysis tool and the new data sets from 2013, we have delivered TeV morphological and spectral results at the SNR region similar to those of the previous work in 2011. However, a different TeV morphological result at the HESS J1729-345 region, in particular, a bridge structure connecting HESS J1729-345 and the SNR is found in our TeV analysis. This morphological difference is considered within the statistical fluctuations. The main goal in my thesis is to explore whether the TeV emission from HESS J1729-345 can be explained as emission from runaway hadronic cosmic rays (CRs) that are illuminating these MCs. The observational data of HESS J1729-345 and HESS J1731-347 can be reproduced using core-collapse SN models for HESS J1731-347. Starting with different progenitor stars and their pre-supernova environment, we model potential SNR evolution histories along with the CR acceleration in the SNR and the diffusion of the CRs. A simplified three-dimensional structure of the MCs is introduced based on 12CO data of that region, adopting a distance of 3.2 kpc to the source. A Monte Carlo based diffusion model for the escaping CRs is developed to deal with the inhomogeneous environment. The fast SNR forward shock velocity, as implied from the X-ray data, can easily be explained when employing scenarios with progenitor star masses between 20M and 25M , where the SNR shock is still expanding inside the main-sequence (MS) bubble at present time. The TeV spectrum of HESS J1729-345 is satisfactorily fitted by the emission from the highest energy CRs that have escaped the SNR, using a standard Galactic CR diffusion coefficient in the interclump medium (the relative low density space where those dense MC clumps are embedded within). The TeV image of HESS J1729-345 can be explained with a reasonable three-dimensional structure of MCs. The TeV emission from the SNR itself is dominated by leptonic emission in this model. We also explore scenarios where the shock is starting to encounter the dense MS progenitor wind bubble shell. The escaping hadronic CR hypothesis for the TeV emission of HESS J1729-345 can still hold, but even in this case our model cannot easily account for the TeV emission from HESS J1731-347 in a hadronic scenario. Under realistic conditions, the CRs often exhibit strong ballistically propagation behaviors rather than a purely diffusive one in relatively small environments, e.g. the MCs near the SNR HESS J1731-347 or SNR W28. This is due to that the size of the diffusion environments is often smaller than the wavelength of some Galactic magnetic eddies and/or the Gyro-radius of the very high energy CRs. Therefore, we build a purely numerical code to simulate the CR propagation inside a given magnetic turbulence. The results of our code has shown consistency with previous works. Through comparing the numerical simulation results of our code with the GeV-TeV observational data, we would like to constrain the magnetic structures near SNRs in the future studies. Our searching for stars near/behind SNR HESS J1731-347 within the SIMBAD catalog and WISE catalog are driven by two motivations. First, SNR HESS J1731-347 may be associated with an OB star cluster. Second, ionized MCs near the SNR (an evidence for MCs being swept by the shock) could be observed through the H+3 absorption features with the help of background stars. Using the extinctions derived from the 12CO and Hi data, we find 41 (125) potential star candidates near HESS J1731-347 on the sky map with distances to Earth as about 3-5 kpc (> 5 kpc). The SNR W28 is an old SNR which exhibits thermal X-ray. The most intriguing finding near this SNR is the good match between the TeV features - HESS J1801-233 (HESS J1800-240 A,B,C) and the cold gas features - MCs located at the Northeast (to the South) of the SNR. The GeV-TeV emissions of HESS J1801-233, HESS J1800-240 A, and HESS J1800-240B are successfully explained in our hadronic W28 model. Same CR acceleration/diffusion model used for HESS J1731-347 is adopted in our W28 model as well. The TeV emissions inside/around W28 are well reproduced by the early released runaway super-TeV CRs. Assuming the GeV CRs are only released when the shock has encountered the MC clumps (high-density clumps in the molecular clouds nISM & 102 cm-3) and dissipated into the local medium, we have reproduced the GeV emission inside/around W28 through introducing two instantaneous GeV CR sources (two MC clumps being swept by the shock at 40 kyr).
 
Key words: cosmic rays; supernova remnants; SNR HESS J1731-347; SNR W28; cosmic radiation

Online-Publikation: http://hdl.handle.net/10900/76129


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