利用不同背景估计方法对 Mrk421 的观测 查敏，张建立，陈松战 (ARGO-YBJ) 高能物理分会

2 content 工作背景的介绍 方法的简介 结果和讨论

Introduction: Mrk421 single flare: 10 I Crab ARGO has seen a 4σ effect 2008/05/ /02/18 Whipple

--continued 4 Multi-wave-length Observation of Flares of Mrk421

Method (D.E. Alexandreas NIMA (1993)) Optimize bin size method Non & Nbg estimation + the formula to estimate statistical significance Background estimation Isotropic background from charged cosmic ray; Event rate is constant except when changes in the detector configuration are made A strong function of zenith angle; Event rate may change with experimental conditions Detector threshold / dead time External changes (e.g. local atmospheric pressure) Time-swapping method Direct integral method; Equi-zenith method 5

Data information 6 数据 June 2008 ； nHit on carpet > 100;  40 0

时间交换法 (time-swapping) Assumption: A(h, ,t) = A(h, ,t 0 )  (t), where  (t 0 ) = 1 Effective acceptance map in local coordinate is tabulated within certain time period (T w ) Artificial background events are created with the randomly combination of above map and arriving time within this time period; advantage Keep most properties of background parameters Time-shuffling length （ Tw ） ? Swapping times （ Nsw ） ? Possible correction? 6.4%  180 minutes; 6.4% *180./T; limitation Conservative result Not applicable to the investigation of high declination sky. 7

8 Tw Significance (  ) 3h 

直接积分法（ Direct Integral ） E(ha,  ): the acceptance distribution in Local Equatorial Coordinates (LEC) for certain period; R(t): event rate of the detector;  (ha,ra,t) is 1 if events falls within this special bin or 0 othersize; limitation Time integration period?/ conservative result / Not applicable for the source near the North/South pole; 9

10 4h  24h 

等天顶角法（ Equi-Zenith Method ） Minimize the detecting factors from detector and environmental variation See Source/background at the same time; Free from zenith dependence; Free from acceptance/ efficiency changes; Free from Pressure and tempreture; Limitation: Giving up small zenith angle data due to number of off-source windows non-uniformity distribution for the azimuth angle 11

方位角的不一致性 12 Zenith angle dependence and <  2% geomagnetic field; other unknown factrors?

地磁场使带电次级粒子发生偏转，拉宽次级粒子的横 向分布，使得芯区粒子密度降低，从而降低了原初簇 射的触发率。 地磁场因子 13 Ivanz, CPTL(1998), Zhouyuan (HEP&NP)  1-2%

equi-declination normalization The correction happen within a day; Repeat above measurement 35 times; 35 dummy on-source windows on the orbit has the same Declination band with the on-source window. The corrected background is obtained The uncertainty of signal 14

15 10 off windows 

Summary & discussion 16 Equi-ZenithTime-swappingDirect Integral sigma

--continued Differences difference from parameters:  From method itself: 0.8  ; For a fixed point source “seeing” in 2 ways  (s 1 -s 2 )=(  1 +  2 ) 1/2 =0.447 (here  1 =  2 =0.1) Here  (s 1 -s 2 )=0.8, thus not just statistical fluctuations Data quality Different method may have different dependence on data. 17

Direct integral.vs. equi-zenith year Crab from SongZhan + JianLI

19

The optimal smooth angle Non-point source >1.58σ ~ 1.58σ+R source Point source

Angular resolution of proton Φ 70 =1.58σ N hit >20: 2.90 deg N hit >60: 1.60 deg N hit >100: 1.14 deg N hit >500: 0.56 deg N hit >1000:0.48 deg

22 Equi-Zenith Angle method Without considering the effect from non-uniform azimuth angle distribution, the number of background events can be estimated as Azimuth correction: Repeat the above measurement 35 times. Assigned as This 35 dummy on-source windows on the orbit has the same Decl. with the on-source window. The azimuth correction factor η is given by Using error spread law, the statistical error of the excess events in a on-source window is The number of background are estimated using 10 off-source windows which are at the same zenith angle as the on-source direction in the azimuthal direction. Detailed information refer to Finally, the corrected estimation of the number of background events is obtained as