M型星AT Mic的X射线耀发统计研究

A Statistical Study of X-ray Stellar Flares of the M-type Star AT Mic

恒星耀发是剧烈的恒星活动过程,对恒星周围行星的大气构成和生命演化有重大影响。恒星耀发的辐射主要分布在光学和X射线波段,其中耀发时X射线波段流量可以增大数十至数百倍。M型耀星是最有可能存在类地行星的宿主恒星,因此,研究M型耀星的X射线耀发分布对寻找宜居的类地行星具有重要意义,并且也可以为即将发射的爱因斯坦探针卫星(Einstein Probe,EP)对恒星耀发的探测数目估计提供参考。利用X射线全天成像监视器(Monitor of All-sky X-ray Image,MAXI)的巡天监测数据研究了M型耀星AT Mic的X射线耀发的统计性质。从X射线全天成像监视器11年的巡天数据中,通过分析图像、光变曲线和能谱数据的信息,发现X射线全天成像监视器在11年间共探测到10次信噪比大于3的耀发,获得了耀发光度的统计分布。在此基础上对恒星耀发的频率进行了估计,AT Mic每年产生耀发光度LX>1030 erg·s-1的次数约为83次,爱因斯坦探针卫星每年可以观测到AT Mic的耀发幅度达到两倍以上的次数约为8次。

Stellar flares are violent processes of stellar activity,which have a significant impact on the atmospheric composition and life evolution on exo-planets around the host stars.Stellar flares are observed mainly in the optical and X-ray bands,with sudden increase of the X-ray fluxes as large as several to hundreds of times.M-type stars are the most likely host stars of earth-like planets.Therefore,on one hand,it is of great interest to study the statistical incidence of X-ray flares of M-type stars as far as the search for habitable exo-planets is concerned.On the other hand,the occurrence frequency of X-ray stellar flares,particularly for M stars,is of interest for the estimation of the detection rate of X-ray transients with the CAS′s Einstein Probe(EP)mission.We studied the statistical property of X-ray flares from the well-known M-type star AT Mic using the 11-year monitoring data from MAXI.We analyzed the X-ray images,light curves and spectra of the MAXI data on AT Mic and found 10 flares with S/N>3.The statistical distribution of the flare luminosity was obtained.Based on this result,we estimated the occurrence frequency of stellar flares from AT Mic.It is estimated that AT Mic produces about 83 flares with luminosity LX>1030 erg·s-1 every year.Einstein Probe satellite can see about 8 flares with more than twice the flare amplitude from AT Mic every year.