Back to Cosmology

Introduction
In The Beginning
The Model of Gamma-Ray Bursts So Far
A New Development
Gamma-Ray Burst Missions
Gamma-Ray Bursts of Special Note
GRB970228, GRB970508, GRB971223
GRB980425
GRB990123
GRB010222
GRB030329
GRB050709
A Table to Compare - Summary of Special Case GRB's
A Model for Gamma-Ray Bursts
Short GRB's
Long GRB's
An Aside - The Effects of a GRB on Earth
Conclusion
References
Image Credits

Gamma-Ray Bursts – Putting It All Together

Introduction:

Gamma-Ray Bursts – also known as GRB’s – are one of Astronomy’s current hot topics. The evolution in understanding these enigmatic events has proceeded very rapidly from the early discoveries during the cold war to the current hypernova models and stellar remnant merging models of today. This project will go through the early stages of discovery including highlighting the space missions responsible or detecting these events. Also covered will be some handpicked GRB events to compare them with the current models.

Back to Top

In The Beginning:

Gamma-rays[1] themselves are not a mystery. In 1952, gamma-rays emanating from astronomical phenomenon were thought to emanate from interactions between cosmic rays and interstellar hydrogen (Prantzos, 2005). Gamma-ray emissions were even detected from the Crab Nebula in 1961 by the United States satellite, OSO-3. However, these were relatively lower energy – in the range of 100 to 300 keV.  Another US satellite, the OSO-7, detected gamma-rays from a solar flare in 1972. It wasn’t until 1973 that gamma-ray astronomy became a serious topic. During this time, the Cold War between the United States and Soviet Union was in full swing, and to “determine” if the Soviets were complying with the nuclear test ban treaty, the VELA series spacecraft was launched to detect higher energy gamma-rays – higher than those detected coming from the Sun or the Crab Nebula. While no atmospheric gamma-rays were detected, gamma-ray bursts lasting an average of 10 ms to 10 s were detected in random directions on the sky rather than from the Earth, with no detection occurring in the same place twice (Hurley, 2005). 

In order to solve this mystery, the COS-B satellite was sent in 1975 to study these high energy sources of gamma radiation in detail.

Figure 1

The above image (from the ESA Website) demonstrates the existence of bright gamma-ray sources within our galaxy. It was determined at this time (between 1975 and 1982 when the COS-B mission was active) that the gamma-ray sources were restricted to our own galaxy.

Back to Top

The Model of Gamma-Ray Bursts So Far:

Because these gamma-ray emissions were detected, the source of these emissions must be pretty strong – between 10³⁸ to 10⁴² ergs[2]. There was no known phenomenon to conform to this energy requirement except for neutron stars. While neutron stars themselves do not emit this much energy, their magnetic fields could provide the catalyst required to accelerate electrons fast enough and hot enough to account for this energy. Additionally, material accreted onto the neutron star – like a comet, asteroid or some other piece of debris – could be heated to the point that such energy could be generated.

This model seemed to have been confirmed by the US satellite HEAO-3 when spectroscopy of a gamma-ray pulsar was shown to contain a high energy spectral line of Aluminum (Prantzos, 2005). The aluminum spectral line was thought to be from supernova. A supernova within our galaxy seems to fit with the apparent brightness detected since at least a 3 solar mass star is required to provide the nucleosynthesis to create aluminum (Charles and Seward, 1995). The stellar remnant for such a star is a neutron star. 

Back to Top

A New Development:

In 1991, the Compton Gamma-Ray Observatory was sent to explore a wide range of high energy astrophysics from 30 keV to 30 GeV. While this observatory carried four experiments, the most remarkable was the Burst And Transient Source Experiment – or BASTE.  

 
Figure 2

What was discovered was that GRB’s were not limited to our galaxy. Instead a homogenous distribution of GRB’s existed in every region of sky. The image above shows 2,704 GRB’s detected by the BASTE experiment. Basically what this means is that the majority of detections by BASTE were outside the plane of our galaxy. It was only the resolution of these new instruments that allowed rapid follow-up of GRB events to determine there locations.

Another remarkable observatory - launched in the year 1994, the BeppoSAX satellite managed to capture the transient events of GRB’s – sometimes within hours of detection. While the actual GRB event was not captured, the residual hot X-Ray afterglow was detected.

The previous model of GRB’s being the result of neutron stars and their magnetic fields may no longer be valid. GRB’s are bright and if they occur well outside our galaxy, they must fit a new energy requirement. The magic number:

10⁵² ergs or greater.

There are not many sources capable of producing this much energy, and this new requirement has placed the GRB as the brightest, most violent object in the Universe. In most cases, a GRB will be brighter than the combined light output of its host galaxy (Freedman and Kaufman, 2005). 

The ten images below models what a GRB might look like, and how long they last (http://imagers.gsfc.nasa.gov/ems/gamma.html):  

The graph next to each of the images shows the average photon counts – including the galaxy image which is in the center of the image – the GRB occurs near the top. Notice the counts of frame 6, as well as the overall duration. This burst lasted only about a second. The relative short-lived event like this makes them very hard to directly detect them as they burst. Our only hope is to piece together a series of satellites to form a network to detect GRB’s, and to quickly slew a telescope[3] to the source with the hopes of looking for either the residual X-ray emission or find an optical counterpart. The network that exists now is called the Third Interplanetary Network. It is the goal of this network to detect a GRB event within 10 seconds – or less.

Back to Top

Gamma-Ray Burst Missions:

It is probably important to mention that to detect GRB’s we need to be above the Earth’s atmosphere – or at least 100 kilometers above the surface (http://imagers.gsfc.nasa.gov/ems/gamma.html):

Our atmosphere naturally blocks many of the harmful rays – including gamma-rays - so we must detect and observe these events far above the surface.

(Figure 14)

There are, and have been, a number of observatories and satellites used in search of GRB’s. The list below shows all of the observatories that have either solely or partly contributed to the collection of data and the detection of GRB’s. The missions of significant contributions are in bold. 

Back to Top

Gamma-Ray Bursts of Special Note:

This section will cover a few of the GRB’s that have helped mold our current models – which will be covered later.

But first, what is in a name? The naming convention for Gamma-Ray Bursters are GRB followed by the date – two digit year, two digit month and two digit day. This is named after the date of initial discovery. This is also called the “BASTE term.”

Back to Top

GRB’s 970228, 970508, and 971223 (Wijers and Galama, 1999)(Galama et al., 2000):

GRB 970228 is unique in that it is the first GRB to have an optical counterpart. The majority of detections by BASTE and BeppoSAX were of X-Ray afterglows. These three GRB’s were considered long and radiated energy from the gamma-ray spectrum through to radio spectrum. According to Wijers and Galama (1999), the nature of the optical counterpart is due to synchrotron emissions resulting from the reduced speed of the ejecta (thought to be a supernova). Reexaminations of the power curves of these GRB’s resulted in a similar appearance to a Type 1c supernova power curve.

Back to Top

GRB 980425 (Waxman, 2004)(Guetta et al., 2004):

This unusual GRB is one of the closest GRB events ever recorded – only 100 million light-years away. While the brightness was that of a cosmological GRB, the close proximity of this event was probably the result of the relativistic jets being viewed off-axis. Interestingly, this event also carries the designation SN1998bw – meaning that this was also recorded as a supernova. This particular supernova has become a member of a handful of GRB’s that occur near to us – at:

.

Figure 17 is an optical image of SN1998bw/GRB 980425. At first glance, this event looks just like any other supernova. Only by comparing the light-curve distribution from BeppoSAX and BASTE to other known GRB’s was the determination that this event is a GRB.

Back to Top

GRB 990123 (Briggs at al., 1999)(Dai and Lu, 1999):

This particular GRB is special in that it was the first GRB to be simultaneously detected in the gamma-ray, x-ray, radio and optical spectrums. It is thought the result of this is due to us looking directly into the beam of the source object (star).

Figure 18 shows what appears to be residual material left over from the shock waves of non-thermal, relativistic electrons – results from the internal shock waves as a by product of collisions of material at different velocities.

This residual material is actually the surrounding medium that got in the way.

The BeppoSAX X-ray image (figure 19) of this event shows a curious, near perfect circular object which seems to support the thought of this even beaming directly towards our planet. The energy output of this event is estimated to be:

.

This particular GRB is at the “usual” cosmological distances – greater than a redshift of 1.6.

Figure 20 demonstrates the double-spectrum of an energetic GRB, thought to be both the relativistic and non-relativistic shockwaves emanating from the source. This will become the typical spectral appearance of a GRB.

Back to Top

GRB 010222 (Krzysztof et al., 2001)(Frail et al., 2002)(Galama et al., 2003)(Bjornsson et al., 2002):

This particular GRB is also a distant event at a redshift of z = 1.5. What makes this particular event special is that its host galaxy is a starburst galaxy – that is a galaxy that is actively forming high mass stars.

In addition, this particular event was also a very bright one; however, the decay rate has been one of the slowest ever measured – taking almost twice as long for the decay as compared to figure 16.

Measurements of the electron distribution of this event support the jet model of a GRB – discussed later.

Back to Top

GRB 030329 (Guetta et al., 2004(Vanderspek et al., 2004):

This unusual event occurred at z = 0.17, that is only 2 billion light years distant. In addition, the light curves and spectra of this event are very similar to SN1998bw.

Figure 23 shows an example of the light curve of this event as compared to the optical light curve of SN1998bw.

The matching of the supernova spectrum with the GRB spectrum is important in determining the source of GRB’s.

Back to Top

GRB 050709 (http://www.eso.org/outreach/press-rel/pr-2005/pr-26-05-p2.html)

The HETE-2 satellite detected a 70 millisecond GRB 2,400 million light years away in a young, star-forming dwarf galaxy.

By observing the light curve for 20 days, it was determined that this (and other) short GRB events cannot be explained by the hypernova model. Instead, this GRB was probably the result of merging neutron stars.

Back to Top

A Table to Compare – Summary of Special Case GRB’s:

These particular GRB events have provided valuable insight to determine the source of these events. In particular:

• There are a few cases where a GRB is relatively close to us although the vast majority are at “cosmological” distances
• There is an optical counterpart of a GRB which supports a relation to supernova
• Matching spectra between GRB’s and supernova also support the relation between GRB’s and supernova
• Short and Long GRB events (see below) have different mechanisms – short GRB’s equal neutron star merging while long GRB’s are probably intense supernova called “hypernova”

The next section will cover the current models of GRB’s.

Back to Top

A Model for Gamma-Ray Bursts:

GRB’s are the Universe’s most violent event. Luckily, the vast majority of these events are at cosmological distances, with the most distant GRB detected at a z = 6.3 – that means the event occurred when the Universe was “only” 900 million years old. There is no apparent reason for this other than statistical data of the collected GRB detections – although a very novel idea is inferred by the rate of GRB’s at a high redshift. The introduction of metal into the spectrum of GRB’s in the early universe is though to be the result of massive Population III stars (Murakami et al., 2005). This supports the double-ionizations as detected by the Wilkinson Microwave Anisotropy Probe (WMAP).

There are currently two types of GRB’s:

• Short Gamma-Ray Bursts
• Long Gamma-Ray Bursts

Both of these events have a different mechanism.

Back to Top

Short Gamma-Ray Bursts:

A short GRB is any event that lasts less than 2 seconds. GRB 050709 and data from the HETE-2 observatory has really provided the breakthrough of determining the source of a short gamma-ray burst. These short events are very powerful, but rare to catch – that is until HETE-2. What makes the short GRB so special is that there are not jets of relativistic energy fueling the event. That is where a massive star would have a variety of layers to provide the energy; a short GRB is missing this. Therefore, the current model of a short GRB is a merging of two neutron stars (http://www.eso.org/outreach/press-rel/pr-2005/pr-26-05.html).

What supports the short GRB model most of all is the lack of a significant optical afterglow that would have resulted from a supernova. Such afterglows are present in long GRB events.

Back to Top

Long Gamma-Ray Bursts:

Over 2/3’s of all GRB events are long GRB’s (Gou et al., 2004). These events last longer than 2 seconds. As a result, most of our data in determining the nature of GRB’s is based on long GRB events. The model for long GRB’s is a hypernova resulting from a star with a solar mass of 30. An optical counterpart to a long GRB and matching spectra to supernova (see the comparison table above) support the supernova model. Additionally, the SWIFT gamma-ray satellite has detected near-infrared afterglows as well as Iron spectral lines[4] from long GRB’s – data that also supports the hypernova model (Gou et al., 2005)(Berger et al., 2005).

In this case, the SWIFT observatory plays a key role in finding not only GRB events, but also searching for the faint afterglows that would otherwise be hidden from Earth-based telescopes.

Figure 28 (above) sums up the hypernova model quite nicely. The formation of a black hole[5] is inferred from a very high mass star that also emanates a burst of gamma-rays (the GRB’s) in the form of two jets.

Figure 29 goes into detail the jet model of a GRB. Make a note of the gamma symbol in the yellow, the letter X in the orange and the letter V outside the jet. These correspond to the frequency of the jet at the given location. The gamma-ray emission is the result of the initial supernova explosion – evident of the iron lines in the spectra. Additionally, there are presumed to be several concentric shells of material moving at a variety of speeds. These speeds indicate the energy that is emitted. The slower the expanding shell, the more energy is built up hence the long afterglow into the radio spectrum.

Data from the XMM-Newton X-Ray observatory and supercomputer simulations have “solved” the GRB mystery (http://www.pparc.ac.uk/nw/press/gamma.asp). Supporting the data from SWIFT and the HETE-2, a long GRB is the result of a collapsar, or hypernova – a supernova of a high mass star that forms a black hole.

Back to Top 

An Aside – The Effects of a GRB on Earth:

An interesting article appeared in the November 2005 issue of Astronomy Magazine. The article discusses the possibility of life in our galaxy and includes a section on the effects of a nearby GRB to life on Earth.

This particular scenario places a long GRB (lasting 10 seconds) event at only 6500 light years from Earth (Nadis, 2005). The result is a destruction of our Ozone and a smog that envelopes the planet for several years. The UV-B radiation from the Sun is no longer blocked which results in damage to DNA.  The steps involved (Nadis, 2005):

• Within a few weeks, Nitrous Oxide and Nitrous Dioxide (NO and NO2) are produced by the GRB – these compounds destroy the Ozone layer
• UV-B radiation increases by 50% destroying DNA
• After a few months, the NO and NO2 cover the entire planet in a brown smog – the Ozone begins to regenerate but only at a 20% level
• After 10 years, the NO and NO2 sink to a level where they are rained out as acid rain
• In 15 years, the atmosphere returns to normal, but the Earth has now entered a global cooling trend as a result of the smog. Most of the land and marine life are killed over the course of these 15 years after the GRB event
   

A GRB is a violent event, and is also deadly to life on Earth if a GRB were to occur near us. There is speculation that such an event may have contributed to the dinosaur extinction, but there is little data to support this.

Back to Top

Conclusion:

Much progress has been made in determining the true nature of a Gamma-Ray Burst. Continual observations by the SWIFT and HETE-2 space-based observatories with follow up observation using radio and optical telescopes have provided valuable data to mold the models that explain this phenomenon. As it stands, there are two types of Gamma-Ray Busts with two different models:

• Short Gamma-Ray Bursts – GRB events lasting 2 seconds of less, the result of which is the merging of compact objects like neutron stars
• Long Gamma-Ray Busts – events lasting longer than 2 seconds that are the result of a collapsar or hypernova, a type of supernova of a very high mass star.

While Gamma-Ray Bursts are primarily a cosmological event, there are a handful of events that have occurred closer to Earth. It is inferred that such an event probably assisted in the extinction of life on Earth millions of year ago.

The models that describe the Gamma-Ray Burst are in their infant stages, but this is a perfect example of observational data supporting computer and mathematical models. Additionally, further study using tools like SWIFT and HETE-2 will provide the continuum of data required to solidify these models.

Back to Top

References:

Berger, E. et al. “The Discovery of the Optical and Near-IR Afterglows of the First SWIFT Gamma-Ray Bursts.” The Astrophysical Journal, 629:328-333, August 10, 2005. 

Bjornsson, G.; Hjorth, J.; Pedersen, K. and J.U. Fynbo. “The Afterglow of GRB 010222: A Case of Continuous Energy Injection.” The Astrophysical Journal, 579:L59-L62, November 10, 2002.  

Briggs, M.S. et al. “Observations of GRB 990123 by the Compton Gamma Ray Observatory.” The Astrophysical Journal, 524:82 – 91, October 10, 1999. 

Chandra X-Ray Observatory. “X-Ray Astronomy Field Guide.” Internet 

Charles, Philip and Frederick Seward. Exploring the X-Ray Universe. Cambridge University Press, 1995. 

Dai, Z.G. and T. Lu. “The Afterglow of GRB 990123 and a Dense Medium.” The Astrophysical Journal, 519:L155-L158, July 10, 1999.

Frail, D.A. et al. “GRB 012222: A Burst Within a Starburst.” The Astrophysical Journal, 565:829-835, February 1, 2002. 

Freedman, Rodger and William Kaufmann II. Universe. Seventh Edition.” W.H. Freeman and Company. New York, 2005. 

Galama, T.J. et al. “Evidence for a Supernova in Reanalyzed Optical and Near-Infrared Images of GRB 970228.” The Astrophysical Journal, 536:185-194, June 10, 2000.

Galama, T.J. et al. “Hubble Space Telescope and Ground-Based Optical and Ultraviolet Observations of GRB 010222.” The Astrophysical Journal, 587:135-142, April 10, 2003. 

Guetta, Dafne; Perna, Rosalba; Stella, Luigi and Mario Vietri. “Are All Gamma-Ray Bursts Like GRB 980425, GRB 030329, and GRB 031203?” The Astrophysical Journal, 615:L73-L76, November 10, 2004. 

Hendon, Arne. “Observing Gamma-Ray Bursters by Amateurs.” Encyclopedia of Astronomy & Astrophysics, 2005. 

Hurley, Kevin. “Gamma-Ray Bursts.” Encyclopedia of Astronomy & Astrophysics, 2005.

Gehrels, N. et al. “The SWIFT Gamma-Ray Burst Mission.” The Astrophysical Journal, 611:1005-1020, August 2004. 

Gou, L.J.; Meszaros, P.; Abel, T. and B. Zhang. “Detectability of Long-Gamma-Ray Burst Afterglows from Very High Redshifts. “ The Astrophysical Journal, 604:508-520, April 1, 2004. 

Gou, L.J., Meszaros, P. and T.R. Kallman. “Detectability of Gamma-Ray Burst Iron Lines by SWIFT, CHANDRA, and XMM-Newton.” The Astrophysical Journal, 624:889-897, May 2005.

Guetta, Dafne; Rosalba, Perna; Stella, Luigi and Mario Vietri. “Are All Gamma-Ray Bursts Like GRB 980425, GRB 030329, and GRB 031203?” The Astrophysical Journal, 615:L73-L76, November 10, 2004. 

Jakobsson et al. “SWIFT Identification of Dark Gamma-Ray Bursts.” The Astrophysical Journal, 617:L21-L24, December 2004. 

Krzysztof, Z. et al. “Rapid UBVRI Follow-Up of the Highly Collimated Optical Afterglow of GRB 010222.” The Astrophysical Journal, 563:592-596, December 20, 2001. 

Meszaros, R. “Theories of Gamma-Ray Bursts.” Annual Review of Astronomy and Astrophysics – Pre-Print, 2001.

Mukherjee, Soma et al. “Three Types of Gamma-Ray Bursts.” The Astrophysical Journal, 508:314-327, November 1998. 

Murakami, T.; Yonetoku, D.; Umemura, M.; Matsubayashi, T. and R. Yamakazi. “The Reionization History and Earlt Metal Enrichment Inferred from the Gamma-Ray Burst Rate.” The Astrophysical Journal, 625:L13-L16, May 30, 2005. 

Nadis, Steve. “Do We Live in a Cosmic Shooting Gallery?” Astronomy Magazine, November 2005, pages 34 to 41.

Prantzos, Nikos. “Gamma-Ray Astronomy.” Encyclopedia of Astronomy & Astrophysics, 2005.

Vanderspek, R. et al. “HETE Observations of the Gamma-Ray Burst GRB 030329: Evidence for an Underlying Soft X-Ray Component.” The Astrophysical Journal, 617:1251-1257, December 20, 2004.

Waxman, Eli. “The Nature of GRB 980425 and the Search for Off-Axis Gamma-Ray Burst Signatures in Nearby Type 1b/c Supernova Emission.” The Astrophysical Journal, 602:886-891, February 20, 2004.

Weeks, Trevor. “Gamma-Ray Telescopes.” Encyclopedia of Astronomy & Astrophysics, 2005.

Wijers, R.A.M.J. and T.J. Galama. “Physical Parameters of GRB 970508 and GRB 971214 from their Afterglow Synchrotron Radiation.” The Astrophysical Journal, 523:177-186, September 20, 1999.

Back to Top

Image Credits:

Figure 1: http://www.rssd.esa.int/index.php?project=COSB&page=index

Figure 2: http://cossc.gsfc.nasa.gov/docs/cgro/batse/

Figure 3: http://bepposax.gsfc.nasa.gov/bepposax/first/grb970228.html

Figure 4: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 5: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 6: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 7: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 8: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 9: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 10: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 11: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 12: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 13: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 14: http://imagers.gsfc.nasa.gov/ems/gamma.html

Figure 15: http://www.mpe.mpg.de/~jcg/grb970228.html

Figure 16: http://www.mpe.mpg.de/~jcg/grb970228.html

Figure 17: http://www.mpe.mpg.de/~jcg/grb980425.html

Figure 18: http://www.mpe.mpg.de/~jcg/grb990123.html

Figure 19: http://www.mpe.mpg.de/~jcg/grb990123.html

Figure 20: http://www.mpe.mpg.de/~jcg/grb990123.html

Figure 21: http://www.mpe.mpg.de/~jcg/grb010222.html

Figure 22: http://www.mpe.mpg.de/~jcg/grb010222.html

Figure 23: http://www.mpe.mpg.de/~jcg/grb030329.html

Figure 24: http://www.mpe.mpg.de/~jcg/grb030329.html

Figure 25: http://www.eso.org/outreach/press-rel/pr-2005/pr-26-05-p2.html

Figure 26: http://www.eso.org/outreach/press-rel/pr-2005/pr-26-05-p2.html

Figure 27: http://www.eso.org/outreach/press-rel/pr-2005/pr-26-05-p2.html

Figure 28: http://www.nasa.gov/images/content/133208main_gamma_h_star_history_final.jpg

Figure 29: http://www.oamp.fr/ECLAIRS/01home/home.htm

Figure 30: http://www.pparc.ac.uk/nw/press/gamma.asp

Figure 31: http://www.nasa.gov/vision/universe/starsgalaxies/gammaray_extinction.html

Back to Top