首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
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
admin
2011-01-17
35
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.
The discussion of the means by which GHBs escape black holes that is offered in the second paragraph suggests that which of the following must be assumed if the conclusions described in lines 3 - 5 are to he validly drawn?
选项
A、The light emissions from GHBs always degrade from gamma rays to x-rays to visible light and, finally, to radio waves.
B、Particles in GHBs include not only photons but electrons and their antimatter counterpart, positrons.
C、The expansion of a fireball as it moves away from the black hole lowers photon density.
D、Shock waves almost always form at the boundary between the fireball and the external medium.
E、The timescale by which we witness astronomical phenomenon is always ruled by the principles of relativity.
答案
C
解析
转载请注明原文地址:https://kaotiyun.com/show/XDjO777K
本试题收录于:
GRE VERBAL题库GRE分类
0
GRE VERBAL
GRE
相关试题推荐
Forthepastseveralyears,theSundaynewspapersupplementParadehasfeaturedacolumncalled"AskMarilyn."Peopleareinvit
Forthepastseveralyears,theSundaynewspapersupplementParadehasfeaturedacolumncalled"AskMarilyn."Peopleareinvit
Thedevelopmentofnewenergysources______toberewarding.
Whilebeingquestionedonthecourt,themandenied______theoldlady’snecklace.
Thisquestionistoohard;itis______mycomprehension.
Whilebeingquestionedonthecourt,themandenied______theoldlady’snecklace.
In1866,sailorsonaGermanshipcalledPaulathrewabottlewithamessageintowatershundredsofmilesoffthewesterncoast
In1866,sailorsonaGermanshipcalledPaulathrewabottlewithamessageintowatershundredsofmilesoffthewesterncoast
随机试题
1岁麻疹易感儿接受被动免疫后2个月又接触了麻疹患儿,处理原则中应除外
下列选项中,最能反映墨家学派的主要思想观点的是()。
中低熔合金包埋材适用于
对慢性疾病进行现状调查,最适宜计算的指标为
患儿,10岁。发热,朝轻暮重,夜间更甚,神志迷糊,面色灰暗,经常两目上视,瞳孔无明显反应,牙关紧闭,颈项强直,四肢抽动,肢端厥冷,胸腹灼热,二便失禁,舌质紫绛,舌体卷缩僵硬,状如猪肝色,脉沉伏而细。辨证为
按《建筑施工企业安全生产许可证管理规定》规定,下列情形安全生产许可证颁发管理机关或者其上级行政机关可以撤销安全生产许可证的是()。
商业银行应兼顾信贷风险控制和提高审批效率方面的要求,合理确定授权金额及行权方式,以实现集权与分权的平衡。这是信贷授权应遵循的()原则。[2014年11月真题]
完全翻译
What’stheweatherlikenow?
A、ThatDavidisnotaseriousstudent.B、ThatDaviddoesn’tneedtoworrybecausehehasascholarship.C、ThatDavidwon’tspeak
最新回复
(
0
)