A spatially resolved study of the synchrotron emission and titanium in Tycho's supernova remnant using NuSTAR

Laura A. Lopez, Brian W. Grefenstette, Stephen P. Reynolds, Hongjun An, Steven E. Boggs, Finn Erland Christensen, William W. Craig, Kristoffer A. Eriksen, Chris L. Fryer, Charles J. Hailey, Fiona A. Harrison, Kristin K. Madsen, Daniel K. Stern, William W. Zhang, Andreas Zoglauer

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    Abstract

    We report results from deep observations (~750 ks) of Tycho's supernova remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over several energy bands to identify the regions producing the hardest X-rays and to search for radioactive decay line emission from 44Ti. We find that the hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where recent Chandra observations have revealed high emissivity “stripes” associated with particles accelerated to the knee of the cosmic-ray spectrum. We do not find evidence of 44Ti, and we set limits on its presence and distribution within the SNR. These limits correspond to an upper-limit 44Ti mass of M44 <2.4 × 10-4M⊙ for a distance of 2.3 kpc. We perform a spatially resolved spectroscopic analysis of 66 regions across Tycho. We map the best-fit rolloff frequency of the hard X-ray spectra, and we compare these results to measurements of the shock expansion and ambient density. We find that the highest energy electrons are accelerated at the lowest densities and in the fastest shocks, with a steep dependence of the rolloff frequency with shock velocity. Such a dependence is predicted by models where the maximum energy of accelerated electrons is limited by the age of the SNR rather than by synchrotron losses, but this scenario requires far lower magnetic field strengths than those derived from observations in Tycho. One way to reconcile these discrepant findings is through shock obliquity effects, and future observational work is necessary to explore the role of obliquity in the particle acceleration process.
    Original languageEnglish
    Article number132
    JournalAstrophysical Journal
    Volume814
    Issue number2
    Number of pages12
    ISSN0004-637X
    DOIs
    Publication statusPublished - 2015

    Keywords

    • ISM: individual objects: (Tychoʼs SNR)
    • ISM: supernova remnants
    • X-rays: ISM

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