Let's first review a little. Gamma-ray bursts (GRBs) are identified by detection of relatively brief (usually less than a few minutes) but highly energetic emissions of gamma rays. Although there's a great deal of diversity, most events fall into one of two categories: "short", emitting strongly for less than two seconds, and "long", having strong gamma-ray emissions for more than a few seconds and higher total energy.
Short GRBs are less well understood, but they are commonly thought to result from the merger of two stellar-weight black holes or neutron stars. Short GRBs are not relevant for the present discussion.
Long GRBs are thought, with fairly general consensus, to result from certain types of supernovae (the "core-collapse" kind). One reason for this consensus is that the total energy output of a long GRB appears to be in the same neighborhood as that of a core-collapse supernova: around 1051 ergs (1044 joules, if you prefer). Also, if a GRB occurs sufficiently nearby, we can optically identify it with a bright supernova event, and there have been several instances of this.
Supernovae also come in several different kinds, which differ in their internal mechanisms. The main types are Type I, which have no evidence of hydrogen in their spectra, and Type II, which do have hydrogen lines in the spectrum.
There are further subdivisions of the Type I case: Type Ia supernovae are thought to be associated with progenitors that are white dwarfs. These are the end stage reached by most stars, which have less than about ten times the mass of the Sun (10 M⊙). Such stars lose most of their mass when they pass through an earlier red giant stage. After they have burned (via fusion) all of their hydrogen they shrink down to become white dwarfs with a mass less than about 1 M⊙.
Eventually such a star can no longer produce energy even by fusing heavier elements. The star produces energy only by gravitational contraction. Contraction stops only when degeneracy pressure (due to the Pauli exclusion principle) prevents further collapse. Degeneracy pressure, however, can support a stellar mass only if it's less than 1.38 M⊙ – the Chandrasekhar limit. If a star reaches this stage with a mass of more than 1.38 M⊙, it will collapse further into a neutron star or black hole. If this doesn't happen, yet the star later accretes mass from an external source – such as a companion in a multiple star system – the star may wind up with enough additional matter to sustain an explosive fusion reaction – and then you get a Type Ia supernova. There's some controversy now whether this is more likely to happen due to the merger of two white dwarfs or gradual accretion from a companion, but that's irrelevant for the present discussion, because Type Ia supernovae are not associated with GRBs.
The problem with Type Ia supernovae is that there's no conceivable mechanism to create one particular hallmark of a GRB – namely, jets of matter ejected from the explosion in opposite directions at speeds near the speed of light. It takes a particular mechanism called a "central engine" to accelerate matter that dramatically. This mechanism is thought to be a rapidly spinning neutron star or black hole that is surrounded by a gaseous accretion disk. The matter is sucked in by the central object, but because it has such a large amount of angular momentum it is expelled outwards in jets along the axis of rotation, instead of falling into or onto the central object.
A "core collapse" supernova is required in order to produce the right conditions for a central engine of this sort to form. (And even in this case, only a small percentage of events seem to have just the right conditions.)
To have a core collapse supernova, it's necessary for the progenitor star to have a mass more than about 10 M⊙. In this case, which is pretty rare, after a certain point, even though fusion of heavy elements may still be going on, the energy available from fusion becomes insufficient to support the entire mass of the star, and the whole thing then collapses very rapidly to a black hole or neutron star. If there is still some hydrogen remaining in the outer shell of the star immediately before collapse, you get a Type II supernova. Otherwise you get a Type Ib supernova, if there's still some helium remaining, or else a Type Ic (no helium).
If the progenitor star had a mass between about 10 M⊙ and 20 M⊙, the end result is a neutron star. Above 20 M⊙ the result is a black hole. However, as noted, it's not known what conditions are required in order to have a central engine capable of producing a GRB. From the relative frequency of observed GRBs compared to Type Ib/c or Type II supernovae, the right conditions seem to occur only 1 or 2% of the time.
The new wrinkle that two recently described supernova events exhibit is that it is possible to have relativistic jets of matter from a supernova without detectable gamma ray activity. Just how fast are we talking about in terms of the ejected matter? Well, in an "ordinary" supernova matter is ejected at speeds, at most, only 2 or 3% of the speed of light (which is still plenty fast). Yet in the recent examples, the matter is accelerated to more than 50% of the speed of light.
How is the speed actually measured? Well, the interesting thing is that it's easy to measure at radio frequencies, using radio telescopes. Let's consider the case of the supernova known as SN 2009bb, to be specific, which was first observed on March 21, 2009. This one showed up in a galaxy called NGC 3278, which is about 130 million light-years away.
Astronomers didn't even have to act especially quickly to do the measurement – in fact it's best to do it several weeks after the event. Using radio interferometry it's straightforward to observe the size of the expanding sphere of radio emissions. Divide the radius of the sphere by the time since the initial event and you have (roughly) the rate of expansion.
Actually, it's slightly more complicated than that, because relativistic speeds are involved. It's necessary to apply equations of special relativity. Suppose R is the apparent radius of the sphere, and the time interval is Δt. Then the apparent velocity of expansion is R/Δt. It is even possible for this apparent velocity to exceed c (the speed of light).
However, this apparent velocity is not actually the rate at which the ejected matter is moving. Suppose that rate is denoted by v. Let β=v/c, the ratio of the actual speed to the speed of light. Define the quantity (Lorentz factor) γ=1/√(1-β2). Then what's actually true is that γβc=R/Δt. In the case of SN 2009bb the apparent velocity that was measured was ~0.85c. Solving for β you get β~0.65. That is, ejected matter was moving at about 65% of the speed of light. This matter was just lightweight electrons, but it still takes one honking explosion to move even electrons that fast – perhaps 30 times as fast as what one gets in an "ordinary" supernova.
Here's the research abstract:
A relativistic type Ibc supernova without a detected γ-ray burst
Long duration γ-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of γ-rays and a long-lived radio afterglow. Until now, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected because of limited satellite sensitivity or beaming of the collimated emission away from our line of sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for type Ibc supernovae with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. A comparison with our radio survey of type Ibc supernovae reveals that the fraction harbouring central engines is low, about one per cent, measured independently from, but consistent with, the inferred rate of nearby GRBs.
There's a practical application of this discovery. It means that astronomers can use radio telescopes to identify supernova events in which a central engine is involved, even if no gamma rays are detectable. Eventually this will help figure out more precisely what conditions are necessary in order to create the central engine.
There are several possible reasons why gamma rays may not be detectable in such events. It could be that sufficiently powerful gamma rays simply are not produced in some cases. Or else they are produced, but quickly absorbed in the neighborhood of the event so that we never see them. Finally, since the gamma rays are presumably directed in a narrow beam, like the ejected matter, the beam simply is not along our line of sight.
A very similar result has also just been reported for the supernova SN 2007gr. This one is in a somewhat closer galaxy, NGC 1058, about 34 million light-years away. In this case, matter was ejected with β~0.52, corresponding to an apparent rate of 60% of the speed of light. Again, no associate gamma ray emission was detected.
The research report:
A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr
The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration γ-ray bursts (GRBs). Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft γ-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations and searches. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c.
|Soderberg, A., Chakraborti, S., Pignata, G., Chevalier, R., Chandra, P., Ray, A., Wieringa, M., Copete, A., Chaplin, V., Connaughton, V., Barthelmy, S., Bietenholz, M., Chugai, N., Stritzinger, M., Hamuy, M., Fransson, C., Fox, O., Levesque, E., Grindlay, J., Challis, P., Foley, R., Kirshner, R., Milne, P., & Torres, M. (2010). A relativistic type Ibc supernova without a detected γ-ray burst Nature, 463 (7280), 513-515 DOI: 10.1038/nature08714|
|Paragi, Z., Taylor, G., Kouveliotou, C., Granot, J., Ramirez-Ruiz, E., Bietenholz, M., van der Horst, A., Pidopryhora, Y., van Langevelde, H., Garrett, M., Szomoru, A., Argo, M., Bourke, S., & Paczyński, B. (2010). A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr Nature, 463 (7280), 516-518 DOI: 10.1038/nature08713|
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