# Gamma bursts

1. May 1, 2004

### cangus

can someone explain what a gamma burst is in simple terms.... thanks.

2. May 1, 2004

### meteor

Gamma bursts were first detected in 1967, but were hidden to the public, only officially announced its existence in 1973 (because they feared that were of alien civilization origin?). They are characterized by a great output of energy in gamma rays, and the most powerful GRB to date, the GRB that released more energy is GRB990123, occurred in 1999. The range of duration of a GRB varies from 30 ms. the shortest to 1000 s. the longest.

More curious facts: GRB0303029 is the closest GRB ever observed, it was 2000 ly away when exploded

GRB are thought to be caused by hypernovae, the big brothers of the more current supernovae

This recent paper associates GRBs to tidal disruption of stars by supermassive black holes (someone could clarify if tidal disruption can be a cause of hypernova, I'm not sure)
http://arxiv.org/abs/gr-qc/0404049

Last edited: May 1, 2004
3. May 2, 2004

### Nereid

Staff Emeritus
Another answer: gamma ray bursts (GRB) are short duration bursts of gammas from 'the sky'. 'Short duration' = ~a few ms to ~<1ks (as meteor said). 'Gammas' = hard X-rays to at least 100 MeV (and may extend to 1 TeV). Their intensity (photons per second) is enormous; IIRC, there is ~ 1 per day which is as 'bright' as the brightest 'non-burst' sources of gammas.

GRBs are isotropic on the sky - they come from all directions, and there is no apparent concentration towards any celestial object.

The GRBs clearly belong to two classes, called 'short duration' and 'long duration'. Almost nothing in known about the former because they have never been seen in any other part of the EM spectrum. In the absence of data, theoreticians are having a field day! IMHO, perhaps the most interesting models for short duration GRBs (or at least some of them) are magnetars (neutron stars with intense magnetic fields).

Long duration GRBs are now thought to be supernovae - there are several types of observation to support this conclusion. In some supernova models, the GRB is an intense, bi-polar jet: two beams which emerge from the poles of the star that's just gone SN. The progenitor star is massive (>10 sol?), rather than a white dwarf (so GRB would not be associated with Type 1a SN). However, it's by no means clear yet that *all* long-duration GRBs are massive SNs, nor that all massive SNs give rise to GRBs.

What is clear is that if the Earth was in the path of a nearby GRB, all life on Earth would be ended (except perhaps extremophile bacteria >20km underground); indeed, if the GRB were close enough, the Earth would be vapourised (only the gas giants would remain as planets around Sol, and much reduced at that).

4. May 2, 2004

### meteor

Nereid, you are right classifying the GRBs in two populations. This article confirms this:
http://www.space.com/scienceastronomy/dark_grb_021223.html

"They can be separated into two categories: short bursts that last a second or less, and long bursts that last longer than four seconds....

...Astronomers believe that short bursts occur when two objects merge, either a black hole with a neutron star, or two neutron stars"

Though I still believe that long GRBs are due to hypernovae.

The article also talks about some special GRBs called dark bursts. These are GRBs with no known optical or visible light component

5. May 2, 2004

### Nereid

Staff Emeritus
Thanks meteor.

Well, some 'astronomers believe that short burst occur when two objects merge, either a black hole ...' However, given the dearth of data (other than the gamma spectrum and a vague localisation on the sky), it's a 'belief' that's hard to support!

Certainly the 'hypernova' model for long duration GRBs has a lot going for it, not least some nice observations across the EM spectrum. However, IMHO, it's quite a stretch to go from good observations of a few long-duration GRBs to a blanket claim about all!

The 'dark bursts' are one reason why it's healthy to not hold your breath. Perhaps the SN occurs behind a particularly thick layer of dust, either locally or close to home? Perhaps they are so far away the optical component has been red-shifted into the IR? Perhaps there's a complicated relationship between the gamma jet and the optical after-glow??

6. May 2, 2004

### meteor

Last edited: May 2, 2004
7. May 3, 2004

### Phobos

Staff Emeritus
Well, they were discovered by military satellites monitoring nuclear explosions...so that's not exactly public information.

8. May 3, 2004

### meteor

This paper proposes GRBs as sources of UHECRs (Ultra high energy cosmic rays)
http://arxiv.org/abs/astro-ph/0402586
"In this paper, we have made an accurate investigation of proton acceleration in GRBs and we have predicted a possible signature of cosmic rays, in a sufficiently baryon-loaded fireball, via GeV $\gamma$-ray emission produced by $\pi^{0}$-meson decay. If two ungrounded assumptions are removed, namely, Bohm's scaling and a slow magnetic field decrease, the usual Fermi processes are unable to generate ultra high energy cosmic rays (UHECRs) in GRBs. We propose to develop another scenario of relativistic Fermi acceleration in the internal shock stage. We present the results of a realistic Monte-Carlo simulation of a multi-front acceleration which clearly shows the possible generation of UHECR. The amount of energy converted into UHECRs turns out to be a sizeable fraction of the magnetic energy."

Last edited: May 3, 2004
9. May 10, 2004

### meteor

http://arxiv.org/abs/astro-ph/0405123
"Compton Echoes from Gamma-Ray Bursts: Unveiling Misaligned Jets in Nearby Type Ib/c Supernovae"
This paper assures that there's a link between long GRBs and type Ib/c supernovae

10. May 18, 2004

### Chronos

Another interesting hypothesis is that short GRB's are evaporating black holes [the tiny primordial black holes Hawking theorized to have formed in the very early universe]

11. May 22, 2004

### Nereid

Staff Emeritus
It's true that a small BH should end its days in a burst of glory, a nice flash of Hawking gammas. However, I wasn't aware that the short GRBs had characteristics - spectrum, duration, etc - which matched that of evaporating BHs; do you have a reference?

12. May 22, 2004

### turbo

http://www.gsfc.nasa.gov/topstory/2003/0319hete.html

Nice NASA story regarding how a gamma ray burst was detected Oct 2002 and was followed up by observations in less-energetic wavelengths. They characterize the cause as the collapse of a Wolf-Rayet star and the ensuing formation of a black hole.

13. May 26, 2004

### Chronos

excellent questions nereid. i had not thought that far ahead. i believe the spectrum and duration of evaporating black holes would be mass dependent, but, spin, axial orientation and magnetic field strength would also appear to be important. by my recollection, penrose predicted at least some of these characteristics. i cant think of any specific papers off the top of my head, but will look.

14. May 31, 2004

### meteor

Here's reviewed that possibility
http://arxiv.org/abs/astro-ph/0110276
Evidence for a Galactic Origin of Very Short Gamma Ray Bursts and Primordial Black Hole Sources
Authors: D.B. Cline, C. Matthey, S. Otwinowski