γ射线暴X射线耀发的研究进展

An Investigation of Research Progress for X-ray Flare in Gamma-ray Burst

γ射线暴是宇宙中恒星尺度的最剧烈爆发现象。γ射线暴瞬时辐射结束后,进入余辉辐射阶段。X射线耀发是γ射线暴X射线辐射衰减过程中出现的短时标闪耀现象。X射线耀发的脉冲轮廓具有不对称性,其上升时标小于下降时标。在部分γ射线暴中,X射线耀发的亮度达到瞬时辐射的亮度。X射线耀发的持续时间与峰值时间具有线性关系。X射线耀发的光谱比X射线余辉的光谱硬。早期X射线耀发与晚期X射线耀发相比,其脉冲轮廓较窄,光谱较硬。X射线耀发产生的物理过程类似于γ射线暴瞬时辐射的物理过程。在火球(fireball)模型中,内部壳层之间发生碰撞,产生的内激波加速电子,电子的同步辐射产生X射线耀发。当火球扫过星际介质,外激波加速电子时,电子的同步辐射也可产生X射线耀发。在光球(photospere)模型中,能量耗散发生在光学厚的区域,热辐射的光谱峰值落在X射线能段附近,γ射线暴的喷流在光球半径处会产生X射线耀发。如果射线暴喷流由坡印亭能流主导,喷流就会与星际介质相互作用,磁场的不稳定性使磁场发生耗散,产生的能量形成X射线耀发。γ射线暴的喷流具有几何效应。一部分同步辐射可能发生在喷流辐射面的高纬度处。由于曲率效应(curvature effect),各向异性辐射与各向同性辐射相比,X射线耀发的峰值出现较晚。此外,在γ射线暴发生后,黑洞会间歇性地吸积外部介质。在吸积过程中,黑洞周围的磁场会调节吸积的速率和喷流中的能量,这是出现多个X射线耀发的原因。

Gamma-ray burst(GRB)is the most violent explosion in the universe.It has strong gamma-ray emission during short timescale.For some GRBs after prompt emission,X-ray flare is shown in the X-ray decay phase.The pulse of X-ray flare,similar to that of GRB,has an asymmetric structure.Usually the rise time of X-ray flare is less than the decay time of X-ray flare.The X-ray flare duration ranges from hundreds of seconds to thousands of seconds.In some GRBs,the X-ray flare is as bright as the prompt emission.For one X-ray flare,the duration is linearly related to the peak time.The spectrum of X-ray flare is harder than that of normal X-ray afterglow.The early X-ray flare has a narrow pulse and hard spectrum,compared with the late X-ray flare.The physical mechanisms on the X-ray flare are similar to those of GRB prompt emission.In the fireball model,fast shell collides with slow shell,and the internal shock is generated.The shock accelerates electrons,and the synchrotron radiation generates the X-ray flare.The fireball external shock sweeps up the interstellar medium,and the synchrotron radiation produces X-ray flare as well.In the photosphere model,X-ray flux emerges at the photospheric radius,because the jet has enormously dissipative energy near the optically thick region.The peak energy of thermal emission settles down in the X-ray band,and this makes X-ray flare.In the Poynting-dominated models,GRB jet has interaction with the surrounding interstellar medium,producing the instable magnetic field,which can release huge energy of the magnetic field.The X-ray flare is powered by the dissipative magnetic energy.In the case of anisotropic jet model,due to curvature effect,the anisotropic emission makes a late X-ray flare.Multiple X-ray flares in one GRB can be caused by the intermittent accretion of black hole after GRB occurs.During the accretion,the magnetic field around the black hole can modify the accretion rate and the energy released by GRB jet.