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  3. Leaving aside the question of how black holes generate the energy in gamma-ray bursts-cosmic explosions called GRBs-their

Leaving aside the question of how black holes generate the energy in gamma-ray bursts-cosmic explosions called GRBs-their

admin2011-01-17  47
问题             Leaving aside the question of how black holes generate the energy in
       gamma-ray bursts-cosmic explosions called GRBs-their sheer brilliance has
       posed a dilemma. So much radiation emanates from the GRB’s compact space
Line    that astronomers would like to conclude that the photons are too densely packed
(5)     to allow each other to escape, but in that event, how can GRBs be observed?
           The resolution of this conundrum, developed over the past several years, is
       that the gammas are not emitted immediately; instead the initial energy release
       of the explosion is stored in the kinetic energy of a shell of particles (including
       photons as well as electrons and their antimatter counterpart, positrons). This
(10)    produces a fireball moving at a speed close to that of light and expanding to a
       diameter of 10 billion to 100 billion kilometers, then converting some of its
       kinetic energy into electromagnetic radiation, yielding a GRB.
           The initial gamma-ray emission is most likely the result of internal shock
       waves within the expanding fireball, shocks which arise when faster blobs in the
(15)    expanding material overtake slower blobs. Because the fireball is expanding so
       close to the speed of light, the principles of relativity dictate that the timescale
       witnessed by an external observer is vastly compressed, such that over a few
       seconds the observer may witnesses a burst of gamma rays that required a day to
       produce. The fireball continues to expand, eventually encountering and
(20)    sweeping up surrounding gas, then subsequently generates another shock wave
       at the boundary between the fireball and the external medium that persists as
       the fireball slows down and shrinks. This external shock nicely accounts for the
       GRB afterglow emission and the gradual degradation of this emission from
       gamma rays to x-rays to visible light, then, finally, to radio waves.
(25)         Although the fireball can transform the explosive energy into the observed
       radiation, astronomers have yet to agree on what generates the energy to begin
       with. One family of models, referred to as hypernovae, involves stars born with
       masses greater than about 20 to 30 times that of our sun; simulations show that
       the central core of such a star eventually collapses to form a rapidly rotating
(30)    black hole encircled by a disk of leftover material. Nevertheless, compact-star
       coalescence could still have a place in the big picture and a mechanism may
       account for the poorly understood short-duration GRBs. Moreover, additional
       models for GRBs are still in the running, one of which credits the birth of the
       fireball to the extraction of energy from an electrically charged black hole. This
(35)    model suggests that both the immediate and the afterglow emissions are
       consequences of the fireball sweeping up the external medium. Astronomers
       have come a long way in understanding gamma-ray bursts, but they still do not
       know precisely what causes these explosions, and they know little about the
       rich variety and subclasses of bursts.
According to the passage, the fireball which emits the initial GRBs differs from the stage in which it produces aftershocks in that
选项 A、the former lacks the accelerated velocity of the latter
B、the former is easier to observe than the latter
C、the former has higher photon density than the latter
D、the former contains a greater number of positrons than the latter
E、the former is in a state of expansion while the latter is in a state of contraction
答案E
解析
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