The ESA-Gaia satellite is regularly producing Alerts on objects where photometric variability has been detected after several passages over the same region of the sky. The physical nature of these objects has often to be determined with the help of complementary observations from ground-based facilities. We have compared the list of Gaia Alerts (from the beginning in 2014 to Nov. 1st, 2018) with archival LAMOST and SDSS spectroscopic data. A search radius of 3″ has been adopted. In using survey data, the date of the ground-based observation rarely corresponds to the date of the Alert, but this allows at least the identification of the source if it is persistent, or the host galaxy if the object was only transient like a supernova (SN). Some of the objects have several LAMOST observations, and we complemented this search by adding also SDSS DR15 data in order to look for long-term variability. A list of Gaia Nuclear Transients (GNT) from Kostrzewa-Rutkowska et al. (Mon. Not. R. Astron. Soc. 481(1):307, 2018 ) has been included in this search also. We found 26 Gaia Alerts with spectra in LAMOST+SDSS labelled as stars, among which 12 have multi-epoch spectra. A majority of them are Cataclysmic Variables (CVs). Similarly, 206 Gaia Alerts have associated spectra labelled as galaxies, among which 49 have multi-epoch spectra. Those spectra were generally obtained on a date widely different from the Alert date, and are mostly emission-line galaxies with no particularity (except a few Seyferts), leading to the suspicion that most of the Alerts were due to a SN. As for the GNT list, we found 55 associated spectra labelled as galaxies, among them 13 with multi-epoch spectra. In these two galaxy samples, in only two cases, Gaia17aal and GNTJ170213+2543, was the date of the spectroscopic observation close enough to the Alert date: we find a trace of the SN itself in their LAMOST spectrum, both being now classified here as a type Ia SN. Compared to the galaxy sample from the Gaia alerts, the GNT sample has a higher proportion of AGNs, suggesting that some of the detected variations are also due to the AGN itself. Similarly for Quasars, we found only 30 Gaia Alerts but 68 GNT cases associated with single epoch quasar spectra in the databases. In addition to those, 12 plus 23 are quasars where multi-epoch spectra are available. For ten out of these 35, their multi-epoch spectra show appearance or disappearance of the broad Balmer lines and also variations in the continuum, qualifying them as “Changing Look Quasars” and therefore significantly increasing the available sample of such objects.

中文翻译：

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使用 LAMOST 和 SDSS 表征一些 Gaia 警报

ESA-Gaia 卫星定期对在同一天空区域经过几次后检测到光度变化的物体发出警报。这些物体的物理性质通常需要借助地面设施的补充观测来确定。我们将盖亚警报列表（从 2014 年开始到 2018 年 11 月 1 日）与档案 LAMOST 和 SDSS 光谱数据进行了比较。已采用 3 英寸的搜索半径。在使用调查数据时，地面观测的日期很少与警报的日期相对应，但是如果它是持久的，这至少可以识别源，或者如果物体只是像超新星一样短暂，则可以识别宿主星系(SN)。一些物体有几个 LAMOST 观测值，我们还通过添加 SDSS DR15 数据来补充这一搜索，以寻找长期可变性。来自 Kostrzewa-Rutkowska 等人的 Gaia 核瞬变 (GNT) 列表。(Mon. Not. R. Astron. Soc. 481(1):307, 2018 ) 也包含在此搜索中。我们在LAMOST+SDSS中发现了26个光谱标记为恒星的盖亚警报，其中12个具有多历元光谱。其中大多数是灾难性变量 (CV)。类似地，206 个 Gaia Alerts 关联的光谱标记为星系，其中 49 个具有多历元光谱。这些光谱通常是在与警报日期大不相同的日期获得的，并且大多数是没有特殊性的发射线星系（除了少数赛弗特星系），导致怀疑大多数警报是由于超新星引起的。至于 GNT 列表，我们发现了 55 个标记为星系的相关光谱，其中13个具有多历元光谱。在这两个星系样本中，只有 Gaia17aal 和 GNTJ170213+2543 两种情况的光谱观测日期与警报日期足够接近：我们在其 LAMOST 光谱中发现了 SN 本身的踪迹，两者现在都在此处归类为Ia 型 SN。与来自盖亚警报的星系样本相比，GNT 样本的 AGN 比例更高，这表明检测到的一些变化也是由 AGN 本身引起的。类似地，对于类星体，我们只发现了 30 个 Gaia Alerts，但在数据库中发现了 68 个与单个时期类星体光谱相关的 GNT 案例。除此之外，12 加 23 是可以获得多历元光谱的类星体。对于这 35 个中的 10 个，它们的多历元光谱显示了宽巴尔默线的出现或消失以及连续谱的变化，