FAST银道面脉冲星巡天项目介绍
Tools of Radio Astronomy,6th edition（射电天文工具，第6版）
Tools of Radio Astronomy,5th edition（射电天文工具，第5版）
FAST19波束脉冲星漂移扫描巡天模拟（基于Parkes观测数据）
FAST科学数据简介
FAST第八批科学数据开放通告
FAST望远镜银道面脉冲星巡天项目
The FAST Galactic Plane Pulsar Snapshot survey:I. Project design and pulsar discoveries
资料来源于FAST官网，项目作者中国国家天文台韩金林；
1 项目简介
脉冲星是大质量恒星死亡后的残骸，是宇宙中密度最高、磁场最强、相对论效应显著的天体，是验证引力理论的天然实验室。目前国际同行已经发现约 3200 多颗脉冲星（含未发表，但网页公布的），包括约 400 多颗毫秒脉冲星（MSP）。
本项目计划利用 FAST 19 波束对可见银道面天区系统地进行4*5分钟的快照模式脉冲星遍历搜寻，即银道面脉冲星巡天Galactic PlanePulsar Snapshot survey（GPPS）,5 分钟的灵敏度为0.01mJy。FAST拥有极高的灵敏度，结合 19 波束接收机系统和快照模式，GPPS可以在5 年内完成国际上最高灵敏的银道面脉冲星搜寻巡天。项目完成时，预计发现约 1000（±200）颗脉冲星。在实现“多出成果”目标的同时，希望发现特殊的珍品脉冲星，如 RRATs、射电磁星等，争取出好成果、大成果。
本项目获得的银道面脉冲星深度巡天数据还可以用来做搜寻快速射电暴（FRB）等相关科学研究。
2 观测仪器和技术路线
The observational system of FAST for the GPPS survey
FAST observing system for GPPS surveys As shown in Figure 1, it is represented by FAST Facilities with active surfaces and mobile receivers Cabin controlled by global control system, L-band 19 beam receiver, digital backend, data storage Systems and computer clusters for data processing.
用于银道面脉冲星巡天项目（GPPS ）的FAST观测系统如上图所示，它的组成设施主要包含主动反射面控制系统和馈源支撑整体控制系统，以及L波段的19波束接收器和数字后端、存储系统以及用于数据处理的计算机集群。
An example of the pointing trajectory in the sky of
central beam of the L-band 19-beam receiver during GPPS
observations of a cover.
We designed the snapshot observation mode in four steps:
(1) first a normal pointing is made to a desired position in the sky, working as pointing No.1, and then tracking observation is carried out for some time, e.g. 5 minutes for the GPPS survey. All data from the 19-beam receiver can be recorded;
(2) second, the central beam(certainly also other beams) is offset by 3’ to the right, working as pointing No.2, and then tracking observations and data-recording can be made in the same mode;
(3) then it is offset by 3‘ to the lower-right and tracking, working as pointing No.3;
(4) finally offset by 3‘ to the left and tracking, working as pointing No.4. With these four points, a cover of the sky patch of about 0.1575 square degrees can be surveyed.
An example for the trajectory of the central beam in the sky plane during observations of a cover is displayed in Figure 5. The beam-switching between these pointings can be realized within a few seconds. To ensure the accuracy of upcoming tracking, 20 seconds are given for each beam-switching.
我们分四步设计了快照观测模式：
(1) 先正常朝向天空中的所需观测点，作为1号点，进行一段时间的跟踪观察，例如：观测 5 分钟记录来自 19 光束接收器的所有数据；
(2) 然后，中心光束（也包括其他光束）向右偏移3'，作为2号点，以相同的方式进行跟踪观察和数据记录；
(3) 然后向右下偏移 3' 并跟踪，作为 3 号点；
(4) 最后向左偏移 3' ，作为 4 号点。通过这四个点，可以测量覆盖大约 0.1575 平方度的观测区域。
上图显示了在观测过程中的中心光束轨迹移动情况，这些点之间的光束切换可以在几秒钟内实现。为了确保跟踪的准确性，每次光束切换都有 20 秒的缓冲时间。
Table1:Relevant Parameters for the GPPS Survey
By referencing the parameters for the GPPS survey listed in Table 1 we can calculate the sensitivity for detection of pulsars with an assumed pulse-width of 10% of pulsar periods,. This is the most sensitive pulsar survey up to now, the first down to a level of µJy.
通过参考表 1 中列出的 GPPS 调查的参数，我们可以计算出监测脉冲星的灵敏度，假设脉冲宽度为脉冲星周期的 10%。这是迄今为止最关于脉冲星精密的研究课题 , 首次将精度下降到微央斯基(µJy) 的水平。
3 观测和数据处理
The sky area in the Galactic latitude of ±10◦ of the Galactic disk accessible by FAST is planned for the GPPS survey as outlined by the blue lines. High priority is given to the area of the Galactic latitude of ±5◦in the inner disk. Observations within the zenith angle ZA< 26.4◦ are now carried out with the full gain of FAST.
The distribution of covers for the GPPS survey with the full gain of FAST, i.e., observable within the zenith angle ZA < 26.4◦. Each small circle is a cover for 76 beams. Observed covers are marked in red, and covers with any known pulsars are marked with a black star inside.
After initial tests for the snapshot observation mode and data file storage in March 2019, we successfully carried out a pilot project in the FAST shared-risk open session in 2019 targeting the outer Galactic disk. Observations in 2019 have data recorded in “the standard format” for 4096 channels, 19 beams and four polarization channels(XX, X∗Y , XY ∗ Y Y ) for signals in the radio band from 1000 – 1500 MHz. To save disk space, since February 2020, only two polarization channels (XX and Y Y ) have been recorded for 2048 channels for the frequency range.A cover is named by the pointing position in Galactic coordinates of the central beam and observation date,such as G184.19–3.30 20190422. See Figure 8 for the distribution of covers and observation status, which are updated often on the GPPS webpage.
在2019年3月对快照观测模式和数据文件存储进行初步测试后，我们在 2019 年FAST风险共担公开会议上成功开展了针对银道面脉冲星巡天的试点项目。2019年的观测数据以“标准格式”记录了4096个信道、19 个波束和四个极化信道，用于1000–1500MHz无线电频段的信号。为节省磁盘空间，自2020年2月起，只记录了2048个频率范围的2048个通道的两个偏振通道。数据以中心光束在银河坐标中的位置和观测日期命名，例如 G184.19-3.30 20190422 。覆盖分布和观测状态见上图，GPPS网页也会经常更新。
The flowchart for data processing.
For pulsar search, there are three major steps involved in processing the data: (1) data preparing; (2) pulsar search; (3) results evaluation. See Figure 9 for a flow-chart of data processing.
对于脉冲星搜索，处理数据涉及三个主要步骤：（1）数据预处理； (2) 脉冲星搜索； (3)结果评价。 有关数据处理的流程图，请参见上图。
1.Data preparing
We first make a directory for the cover and link all original recovered data files in the repository for the snapshot observations including the fits file and the xlsx file, or tracking observations for verification, into a subdirectory called ori. The calibration files will be linked into a sub-directory called cal. The relevant information from fits files is extracted in the order of being timestamped with Modified Julian Date (MJD), as is the position information on the feed of the central beam from the xlsx file. These metadata are matched in MJD so that the coordinates of the telescope pointings of snapshot observations can be calculated (see Fig. 5). Based on such a list of coordinates, we can split sub-integration data of different original fits files for different pointings of every beam in a cover.
1.数据预处理
我们首先为封面创建一个目录，并将存储库中用于快照观察的所有原始恢复数据文件（包括 fit 文件和 xlsx 文件，或跟踪观察以进行验证）链接到名为 ori 的子目录中。 校准文件将链接到名为 cal 的子目录中。 拟合文件中的相关信息按照使用修改后的儒略日期 (MJD) 加时间戳的顺序提取，与来自 xlsx 文件的中央光束馈送的位置信息一样。 这些元数据在 MJD 中匹配，以便可以计算快照观测的望远镜指向的坐标。 基于这样的坐标列表，我们可以将不同原始拟合文件的子积分数据拆分为封面中每个光束的不同指向。
2.Searching for pulsars and pulses
Three main approaches have been realized to search for pulsars and individual pulses: PRESTO (Ransom 2011), SIGPROC (Lorimer 2011) and our single pulse detection module developed in-house. Most of the data processing is carried out via multi-jobs (Tange 2020). Currently, the PRESTO module and the single pulse module have been applied to searching for all covers, and the SIGPROC module has been tested and will be applied for re-searching for all GPPS survey data by utilizing a new computer cluster that will be available in March 2021.
2.寻找脉冲星群和脉冲星
已经实现了三种主要的方法来搜索脉冲星群和单个脉冲：PRESTO (Ransom 2011)、SIGPROC (Lorimer 2011) 和我们内部开发的单脉冲检测模块。 大部分数据处理是通过多作业进行的（Tange 2020）。 目前，PRESTO 模块和单脉冲模块已用于搜索所有覆盖天区，SIGPROC 模块也通过了测试，于2021 年 3 月用新的计算机集群重新搜索所有 GPPS 测量数据。
3.Evaluation of searching results
We found that the AI code developed by Zhu et al. (2014) is very efficient for discriminating a pulsar signal from RFI. A new AI module has been developed and tested, and is applied in parallel. After this AI-sifting of candidates, only a small number of false candidates have to be discarded during manual checking of pulsar candidates. The relevant parameters can be extracted from .bestprof and tabulated. These candidates from each survey observation are cross-matched with known pulsars, and certainly only real candidates for new pulsars will be further manually examined and undergo folding by pdmp. If the result is good, the candidate will be observed again for verification.
3.检索结果的评价
我们发现朱等人开发的人工智能代码。 （2014）对于从 RFI 中区分脉冲星信号非常有效。 一个新的人工智能模块已经被开发和测试，并被并行应用。 在对候选者进行 AI 筛选之后，在人工检查脉冲星候选者时，只需丢弃少量错误候选者。 相关参数可以从 .bestprof 中提取并制成表格。 来自每个调查观测的这些候选者与已知脉冲星交叉匹配，当然只有真正的新脉冲星候选者才会被进一步手动检查并通过 pdmp 进行折叠。 如果结果良好，将再次观察候选人进行验证。
Verification Observations and Processing
Thanks to the excellent pointing accuracy of FAST, the position of any good candidate detected from one beam can be determined with an accuracy better than 1.5‘ in radius. For strong pulsars detected in a few nearby beams in the snapshot observations, more accurate coordinates can be determined according to their beam center positions in the sky and the S/N of a pulsar detected from these beams.
验证观察和处理
由于 FAST 具有出色的指向精度，可以以优于 1.5 英尺半径的精度确定从一束检测到的任何良好候选者的位置。 对于快照观测中在附近的几个光束中检测到的强脉冲星，可以根据它们在天空中的光束中心位置以及从这些光束中检测到的脉冲星的信噪比来确定更准确的坐标。
4 新脉冲星的发现
Follow-up observations are going on for many newly discovered pulsars, especially binaries. On the website for the FAST GPPS survey are releases of new discoveries and updates of survey status and pulsar parameters.
许多新发现的脉冲星，尤其是双星，正在进行后续观测。 FAST GPPS 调查的网站上发布了新发现以及调查状态和脉冲星参数的更新。
http://zmtt.bao.ac.cn/GPPS/GPPSnewPSR.html
