一、Mephisto引力波事件光学对应体巡天

Mephisto survey for optical counterpart of gravitational wave event
 

图 1：GW 170817 引力波信号（左图；Abbott et al. 2017），光学对应体信号（右上；Coulter et al. 2017）和光学波段光变曲线（右下；Cowperthwaite et al. 2017）， The gravitational wave signal of GW 170817 (left, Abbott et al. 2017), optical counterpart of GW 170817 (top-right, Coulter et al. 2017) and light curves at optical bands of GW 170817 (bottom-right, Cowperthwaite et al. 2017)

双中子星并合和黑洞中子星并合是当前及未来地基引力波探测器最为主要的引力波源，与 GW 170817 成协的千新星的发现开辟了多信使天体物理学的时代。从物理上来看，并合抛射物中放射性物质衰变过程将产生一个快速演化的、各向同性的热辐射千新星。同时，双中子星并合过程会产生短伽马射线暴，而在短伽马射线暴喷流方向喷流与暴周介质的相互作用会产生多波段余辉辐射。在 Mephisto 巡天的过程中，我们将有希望探测到以上这些引力波电磁对应体。利用 Mephisto 多通道观测的优势，我们将优化巡天的 cadence 和曝光时间，以寻找和证认千新星和短伽马射线暴光学余辉。另外，考虑到今后大量的光学余辉会被发现，从光学余辉的数据中证认千新星信号也将是Mephisto的重要科学目标之一。

The coalescence of double neutron star (NS–NS) binaries and black hole (BH)–NS binaries are prime sources of gravitational waves (GW) for current and future ground-based detectors. The discovery of a kilonova associated with the Advanced LIGO /Virgo event GW170817 opens up new avenues of multi-messenger astrophysics. In physics, radioactive decay of the unstable nuclei in the ejecta after coalescence powers a rapidly evolving, approximately isotropic thermal kilonova. Meanwhile, along the jet beaming direction of a short gamma-ray burst (sGRB) associated with the coalescence of NS-NS or BH-NS binary, a multi-wavelength afterglow would be generated by the interaction between GRB jet and the ambient medium. With surveys of Mephisto, these electromagnetic counterparts are likely to be observed. We will the optimize the survey cadence and exposure to find and identify kilonova and optical afterglow using Mephisto's multi-color observation advantage. Considering that a large number of optical afterglow is expected to be discovered, identifying the signal of kilonova in the data of an optical afterglow is also an important scientific goal of Mephisto.

 

千新星和短伽马射线暴余辉是双中子星并合和黑洞中子星并合过程中最有可能探测到的引力波光学对应体。到目前为止，人类仅探测到一次双中子星并合事件，即 GW 170817。该事件中，千新星出现在引力波爆发之后的11小时，其最高亮度可以达到17星等。在150天后，相应的短伽马射线暴余辉被探测到，亮度达到26星等（由于该事件对应的是偏轴的短伽马射线暴，余辉亮度比典型余辉要低不少）。在Mephisto巡天过程中，我们期待能够寻找到千新星以及短伽马射线暴余辉，主要包括三大科学目标：

• 优化巡天的 cadence 和曝光时间，以获得 Mephisto 探测千新星和光学余辉的最高事件率；
• 利用颜色演化从各种天体物理暂现源中证认千新星和光学余辉；
•  从观测到的光学余辉样本数据中寻找包含千新星信号的光学余辉。

Kilonova and sGRB afterglow are the most likely optical counterparts of gravitational wave from the coalescence NS-NS or BH-NS binary. For the current only observed coalescence of NS-NS binary, GW 170817, its kilonova was found 11 hours after gravitational wave detected, the maximum apparent magnitude reached 17 mag. After about 150 days, the optical afterglow of the associated sGRB was discovered, and the apparent magnitude was 26 mag (this sGRB is off-axis, and has fainter afterglow). With surveys of Mephisto, we expect to find and identify kilonovae and optical afterglow. There are Three main scientific goals:

• Optimizing the survey cadence and exposure to find and identify kilonova and optical afterglow, and obtaining the maximum event rate;
• Identifying kilonova and optical afterglow of sGRB from all kinds of astrophysical transient via color evolution;
• Searching the optical afterglow containing kilonova signal from observed optical afterglow sample.

二、Mephisto 超新星巡天

Mephisto supernovae survey

图 2： Ia 型超新星爆炸艺术想象图（图片取自 NASA/CXC/M.Weiss），Artist's rendition of Type Ia SNe explosion (Credit: NASA/CXC/M.Weiss)

图 3： Ia 型超新星颜色变化曲线， B-V color variations of Type Ia SNe

超新星是剧烈爆发的宇宙事件（图 2）。其中的 Ia 型超新星被认为有标准峰值亮度，可以用来测量宇宙学距离。利用Ia型超新星测量宇宙学距离导致了暗能量的发现，改变了人们对宇宙的认识。但是近年来，随着越来越多的超新星样本积累，人们研究的重点集中到了 Ia 型超新星的本征亮度弥散。从理论和观测上，我们都注意到了早期的颜色曲线对 Ia 型超新星的性质和机制都有至关重要的作用（图 3）。这将对超新星的绝对亮度定标起到关键作用。因此我们计划针对这一问题优化巡天策略提出Mephisto 超新星巡天计划，以期待能积累一定量的超新星样本，特别是早期超新星。此外，近年来发现越来越多的快速光学变源，由于样本数量和观测的限制，其物理机制前身星都未知。我们希望可以积累一定量的此类事件样本，以便开展相关研究。该巡天的核心科学目标为：

• 搜寻早期超新星和快速光学变源
• 使用机器学习的方法，利用早期颜色曲线对暂现源实施分类
• 模拟Mephisto巡天中暂现源的图像，估算对不同类型发生的事件探测率

Supernovae (SNe) is an energetic explosion in the universe. The Type Ia supernova has been considered as a standardlizable candle, which can be used as a tracer of distance. With the increasing of sample size, the variations of the intrinsic luminosity of SNIa becomes the limitation. On the other hand, it has been shown that the early spectra/colour carry critical information of the progenitor and mechanism of the events. We therefore propose a Mephisto SNe survey which focus on the early phase of SNe. Moreover, with more and more unknown rapid optical transients identified, increasing the sample of such kind of events will be another kind of important work. The main goals of Mephisto SNe survey are:

• We will give a prediction of excepted number of SNe, as well as the suggested survey area
• We will use machine learning approach to rapidly classify the transients that will be identified by Mephisto
• We are making an image simulation to evaluate the detection rate of SNe or general transients in Mephisto survey