3C 454.3伽玛射线和近红外辐射延时分析

Analysis for Time Lag Betweenγ-ray and NIR Variations in 3C 454.3

2008年秋天,3C 454.3在γ射线能段和光学波段呈现出非常大的爆发,在这次爆发过程中Fermi/LAT和SMARTS都对其进行了观测。通过对γ射线能段与SMARTS J及B波段在这次大爆发期间获得的光变数据进行细致的DCF分析发现:这段时期内3C 454.3的J波段光变落后γ射线光变大约2d。在进行相关性分析过程中,对DCF做了稍许改进,得到一种改进的DCF-时间变换的离散相关函数(TDCF)。TDCF的峰值在T=-1.66d,无论是对TDCF取重心还是用非对称的高斯函数拟合,其结果都显示3C 454.3的J波段光变落后γ射线光变大约2d。FR/RSS Monte Carlo模拟结果也显示γ射线领先近红外(光学)光变。如果这个延时是由于电子辐射冷却产生的,那么逆康普顿散射的"种子"光子能量不能大于1.1eV。这个延时也可能是由于辐射区域的大小不同引起的,2d的延时反映了两个波段辐射区域的几何性质。高能与低能波段光变有较强的相关性证明这两个波段的辐射是由同一辐射区域产生的:γ射线辐射来自于辐射区域的内部,近红外辐射来自于包括γ射线辐射区域在内的更大区域。由于近红外的辐射区域大于γ射线辐射区域,引起光变的相对论激波传播到整个近红外辐射区域比传播到整个γ射线辐射区域所用的时间长;因此,观测到了J波段光变落后γ射线光变的现象。通过结构函数分析得到的两个波段的光变时标相差约2.5d,这与大约2d的延时符合得很好。

Discrete Correlation Function （DCF） is a powerful tool to analyze correlation between two sets. A kind of the progressive DCF technique, time-trafisformed DCF （TDCF）, is used to analyze time lag between variations of γ-ray and J or B bands for 3C 454.3. The TDCFs of Fermi/LAT γ-ray vs SMARTS J and B bands light curves both peak at τ = -1.66 days. This implies that γ-ray variations lead the J band ones by about 2 days. The period of the data is from JD=2 454 710 to JD=2 454 770. Considering the effects of flux errors and sampling characteristics, FR, RSS and the combined of them are used to Monte Carlo simulations to estimate the time lags and the uncertainties. The simulations show that the γ-rays lead the J band emission. Thus it is reliable that the γ-rays lead the J band emission. The time lag might be due to the cooling time-scale of electrons＇ synchrotron and Compton radiations. In this case, the energy of ＇seed＇ photons couldn＇t be higher than 1.1 eV. The time lag also might be related to the different sizes of their emission regions. The strong correlation of the two variations indicates that the v-rays and the J band emission are emitted from the same region. As a shock spreads in the region, it first influences the whole γ-ray emission region. At this time, the y-ray flux reaches its maximum value. The inner γ-ray emission region is only a part of the NIR emission region. Thus the NIR flux has not reached its maximum value as the γ-rays reach the maximum. As the shock spreads the whole NIR emission region, a few days later, the NIR flux reaches its maximum. Time-scale of variations indicates the size of an emission region. The structure functions show that the timescale of the Y band variations is larger than that of the γ-rays, about 2 days. This timescale difference of about 2 days is consistent with the time lag of the J band emission relative to the 7-rays. This agreement supports the above explanation of the time lag. Around JD≈2 454 712, both the γ-rays and the optical B band emission show their minimum flux, which also supports the above explanation.