IGR J17091-3624 is a low mass X-ray binary (LMXB) which received wide attention from the community thanks to its similarities with the bright black hole system GRS 1915+105. Both systems exhibit a wide range of highly structured X-ray variability during outburst, with time scales from few seconds to tens of minutes, which make them unique in the study of mass accretion in LMXBs. In this work we present a general overview into the long-term evolution of IGR J17091-3624, using Swift/XRT observations from the onset of the 2011-2013 outburst in February 2011 to the end of the last bright outburst in November 2016. We found 4 re-flares during the decay of the 2011 outburst, but no similar re-flares appear to be present in the latter one. We studied in detail the period with the lowest flux observed in the last 10 years, just at the tail end of the 2011-2013 outburst, using Chandra and XMM-Newton observations. We observed changes in flux as high as a factor of 10 during this period of relative quiescence, without strong evidence of softening in the spectra. This result suggests that the source has not been observed at its true quiescence so far. By comparing the spectral properties at low luminosities of IGR J17091-3624 and those observed for a well studied population of LMXBs, we concluded that IGR J17091-3624 is most likely to host a black hole as a compact companion rather than a neutron star.

中文翻译：

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IGR J17091-3624 返回静止状态时的 Swift/XRT、Chandra 和 XMM-Newton 观测

IGR J17091-3624 是一种低质量 X 射线双星 (LMXB)，因其与明亮的黑洞系统 GRS 1915+105 的相似性而受到社区的广泛关注。这两个系统在爆发期间都表现出广泛的高度结构化 X 射线可变性，时间尺度从几秒到几十分钟不等，这使得它们在 LMXB 质量吸积的研究中独一无二。在这项工作中，我们使用从 2011 年 2 月 2011-2013 年爆发开始到 2016 年 11 月最后一次明亮爆发结束的 Swift/XRT 观测，对 IGR J17091-3624 的长期演变进行了总体概述。我们在 2011 年爆发的衰减过程中发现了 4 次重新耀斑，但在后一次爆发中似乎没有类似的重新耀斑。我们详细研究了过去 10 年观测到的通量最低的时期，就在 2011-2013 年爆发的尾声，使用 Chandra 和 XMM-Newton 观测。我们观察到在这段相对静止期间通量的变化高达 10 倍，但没有明显的光谱软化迹象。这一结果表明，到目前为止，还没有在真正的静止状态下观察到该源。通过比较 IGR J17091-3624 在低光度下的光谱特性和在经过充分研究的 LMXB 群体中观察到的光谱特性，我们得出结论，IGR J17091-3624 最有可能将黑洞作为致密伴星而不是中子星。这一结果表明，到目前为止，还没有在真正的静止状态下观察到该源。通过比较 IGR J17091-3624 在低光度下的光谱特性和在经过充分研究的 LMXB 群体中观察到的光谱特性，我们得出结论，IGR J17091-3624 最有可能将黑洞作为致密伴星而不是中子星。这一结果表明，到目前为止，还没有在真正的静止状态下观察到该源。通过比较 IGR J17091-3624 在低光度下的光谱特性和在经过充分研究的 LMXB 群体中观察到的光谱特性，我们得出结论，IGR J17091-3624 最有可能将黑洞作为致密伴星而不是中子星。