Observational follow up of well localized short gamma-ray bursts (SGRBs) has left 20−30 per cent of the population without a coincident host galaxy association to deep optical and NIR limits (≳26 mag). These SGRBs have been classified as observationally hostless due to their lack of strong host associations. It has been argued that these hostless SGRBs could be an indication of the large distances traversed by the binary neutron star system (due to natal kicks) between its formation and its merger (leading to an SGRB). The distances of GRBs from their host galaxies can be indirectly probed by the surrounding circumburst densities. We show that a lower limit on those densities can be obtained from early afterglow light curves. We find that ≲16 per cent of short GRBs in our sample took place at densities ≲10−4 cm−3. These densities represent the expected range of values at distances greater than the host galaxy’s virial radii. We find that out of the five SGRBs in our sample that have been found to be observationally hostless, none are consistent with having occurred beyond the virial radius of their birth galaxies. This implies one of two scenarios. Either these observationally hostless SGRBs occurred outside of the half-light radius of their host galaxy, but well within the galactic halo, or in host galaxies at moderate to high redshifts (z ≳ 2) that were missed by follow-up observations.

中文翻译：

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对短伽马射线暴环爆环境的约束

对定位良好的短伽马射线暴 (SGRB) 的观测跟踪使 20-30% 的人口没有与深光学和 NIR 极限（≳26 mag）重合的宿主星系关联。由于缺乏强大的宿主关联，这些 SGRB 被归类为观察性无宿主。有人认为，这些无宿主的短暴可能表明双中子星系统（由于本命踢）在其形成和合并（导致短暴）之间穿越了很长的距离。GRB 与其宿主星系的距离可以通过周围的环绕暴密度间接探测。我们表明，可以从早期余辉光曲线中获得这些密度的下限。我们发现，我们样本中 ≲ 16% 的短 GRB 发生在密度≲ 10−4 cm−3 处。这些密度代表距离大于宿主星系的维里半径时的预期值范围。我们发现，在我们的样本中被发现在观测上没有宿主的五个短暴中，没有一个与发生在其诞生星系的维里半径之外的情况一致。这意味着两种情况之一。这些观测上没有宿主的短暴发生在其宿主星系的半光半径之外，但在星系晕内，或者在后续观测中遗漏的中高红移 (z ≳ 2) 的宿主星系中。这意味着两种情况之一。这些观测上没有宿主的短暴发生在其宿主星系的半光半径之外，但在星系晕内，或者在后续观测中遗漏的中高红移 (z ≳ 2) 的宿主星系中。这意味着两种情况之一。这些观测上没有宿主的短暴发生在其宿主星系的半光半径之外，但在星系晕内，或者在后续观测中遗漏的中高红移 (z ≳ 2) 的宿主星系中。