Constraints on the emitting region of the gamma-rays observed in GW170817

DOI DOI PDF 被引用文献11件 参考文献53件
  • Tatsuya Matsumoto
    Racah Institute of Physics, Hebrew University, Jerusalem, 91904, Israel
  • Ehud Nakar
    The Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
  • Tsvi Piran
    Racah Institute of Physics, Hebrew University, Jerusalem, 91904, Israel

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説明

The gravitational waves from the neutron star merger event GW170817 were accompanied by an unusually weak short GRB 170817A, by an optical/IR macronova/kilonova and by a long lasting radio to X-rays counterpart. While association of short GRBs with mergers was predicted a long time ago, the luminosity of this prompt ��-ray emission was weaker by a few orders of magnitude than all known previous sGRBs and it was softer than typical sGRBs. This raise the question whether the ��-rays that we have seen were a regular sGRB viewed off-axis. We revisit this question following recent refined analyses of the ��-ray signal and the VLBI observations that revealed the angular structure of the relativistic outflow: observing angle of $\sim\,20^\circ$, a narrow jet with core $\lesssim5^\circ$ and $E_{\rm iso}>10^{52}\,{\rm ergs}$. We show here that: (i) The region emitting the observed $��$-rays must have been moving with a Lorentz factor $��\gtrsim5$; (ii) The observed ��-rays were not "off-axis" emission (viewing angle $>1/��$) emerging from the core of the jet, where a regular sGRB was most likely produced; (iii) The $��$-ray emission region was either "on-axis" (at an angle $<1/��$) or if it was "off-axis" then the observing angle must have been small ($<5^\circ$) and the on-axis emission from this region was too faint and too hard to resemble a regular sGRB.

7 pages, 4 figures, submitted to MNRAS, comments welcome!!!

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