High-energy jets spew from at least some cosmic explosions called short gamma-ray bursts, a new study shows, rather than radiating symmetrically in all directions. The observations will pin down the bursts鈥� energy, helping to reveal what causes them.
Gamma-ray bursts (GRBs) are colossal volleys of very-high-energy photons that can originate from any direction in the sky and they come in two classes. 鈥淟ong鈥� bursts last from seconds to minutes and are thought to occur when massive stars explode and their cores collapse into black holes.
鈥淪hort鈥� bursts last less than a second and are suspected to be caused by the merger of two neutron stars. But observations to test this idea have been difficult because until recently, telescopes were not able to respond quickly enough to catch a short GRB鈥檚 fading afterglow. The afterglow reveals a burst鈥檚 position and is caused by shockwaves from the explosion heating up surrounding gas.
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But in 2005, NASA鈥檚 Swift telescope spun around fast enough to observe about a dozen short GRBs. Two were bright and long enough 鈥� with their afterglows fading over several days 鈥� to permit detailed study with the Chandra X-ray Observatory.
Exit angle
One of those bursts, called GRB 050724, appears to radiate its energy outwards in all directions rather than in jets, says a team led by Dirk Grupe of Pennsylvania State University in University Park, US. But the other burst, called GRB 051221A, appears to beam its energy in jets that have 鈥渙pening angles鈥� of between 4掳 and 8掳.
That is important because the total energy of a burst cannot be estimated unless the shape and orientation of the explosion is known, for example whether the jets are aimed at the observer or not. PSU astrophysicist David Burrows, who led the study on the latter GRB, says: 鈥淚f you don鈥檛 understand this question of beaming, then you don鈥檛 know anything about the energy,鈥� Burrows told New 杏吧原创. 鈥淎nd without knowing the energy, it鈥檚 really hard to say much about what was actually causing the burst in the first place.鈥�
In the case of GRB 051221A, the estimated energy is in the range expected for a short GRB caused by the merger of two neutron stars.
Astrophysicist Don Lamb of the University of Chicago, US, agrees the energy estimates are crucial. He notes that several other short GRBs caught by Swift had previously shown some signs of jets, although these were not observed for as long.
Fundamental similarities
The very fact that some short GRBs appear to be beamed and others do not is interesting, says Bohdan Paczynski of Princeton University in New Jersey, US. He points out that long GRBs also show this range of behaviour. 鈥淭his points out some fundamental similarities between long and short GRBs,鈥� he told New 杏吧原创.
But it is still not clear why this might be, says Stephan Rosswog at International University Bremen in Germany. 鈥淛ets are observed in a large variety of accreting astrophysical objects,鈥� he says. 鈥淏ut there is no general consensus on how they are formed.鈥�
Magnetic fields may play a major role in some cases, he says. For short bursts, the merger of neutron stars may also involve the collision and annihilation of nearly massless particles called neutrinos. The energy created in these annihilations may produce an outflow moving at near light speed, which may be joined by neutrons and protons driven out of the merger.
鈥淭his interaction can be rather complicated and will depend on the details of the particular merger,鈥� Rosswog told New 杏吧原创, suggesting a variety of jet behaviour.
The new research has been submitted to the Astrophysical Journal.
