* Astronomy

Members Login
Username 
 
Password 
    Remember Me  
Post Info
TOPIC: Gamma-Ray Bursts


L

Posts: 131433
Date:
Gamma-Ray Bursts
Permalink  
 


Scientists have used the world's largest robotic telescope to make the earliest-ever measurement of the optical polarisation of a Gamma Ray Burst (GRB) just 203 seconds after the start of the cosmic explosion. This finding, which provides new insight into GRB physics, is published in Science today (15th March 2007).
The scientists from Liverpool John Moores University and colleagues in the UK, Italy, France and Slovenia used the Liverpool Telescope on the island of La Palma and its novel new polarimeter, RINGO, to perform the measurement following detection of the burst by NASA's Swift satellite.
Gamma Ray Bursts are the most instantaneously powerful explosions in the Universe and are identified as brief, intense and completely unpredictable flashes of high energy gamma rays on the sky. They are thought to be produced by the death throes of a massive star and signal the birth of a new black hole or neutron star (magnetar) and ejection of an ultra-high speed jet of plasma. Until now, the composition of the ejected material has remained a mystery and, in particular the importance of magnetic fields has been hotly debated by GRB scientists.
The Liverpool measurement was obtained nearly 100 times faster than any previously published optical polarisation measurement for a GRB afterglow and answers some fundamental questions about the presence of magnetic fields.

Read more


__________________


L

Posts: 131433
Date:
Permalink  
 

GRB 070125
One of the first images from a new telescope in Arizona — in which Ohio State University has a stake — “resembles spots on a dog.” But it is a gamma ray burst, one of the largest explosions in the universe.

Read more

"People have thought for a long time that GRBs are black holes being born, but scientists are now thinking of other possibilities" - Neil Gehrels, Swift principal investigator at NASA Goddard.

According to several studies, a relatively weak GRB observed by Swift on Feb. 18, 2006, also was from a stellar death that produced a magnetar. That GRB was linked firmly to an exploding star: Supernova 2006aj.

Read more


__________________


L

Posts: 131433
Date:
Permalink  
 

The explosive death of a massive star has broken the record for longest-lived light show.
Observations from NASA’s Swift satellite have revealed a so-called gamma-ray burst for which the afterglow remained visible for more than 125 days.
When a star that’s 10 to 25 times as massive as the Sun checks out, it can release in a matter of seconds the same amount of energy that the sun will radiate over its 10-billion-year lifetime.

Read more

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: The nature of the outflow in gamma-ray bursts
Authors: P. Kumar, E. McMahon (University of Texas, Austin), A. Panaitescu (LANL), R. Willingale, P. O'Brien (Leicester), D. Burrows (Penn State), J. Cummings, N. Gehrels, S. Holland (NASA GSFC), S. B. Pandey (Mullard), D. Vanden Berk (Penn State), S. Zane (Mullard)

The Swift satellite has enabled us to follow the evolution of gamma-ray burst (GRB) fireballs from the prompt gamma-ray emission to the afterglow phase. The early x-ray and optical data obtained by telescopes aboard the Swift satellite show that the source for prompt gamma-ray emission, the emission that heralds these bursts, is short lived and that its source is distinct from that of the ensuing, long-lived afterglow. Using these data, we determine the distance of the gamma-ray source from the centre of the explosion. We find this distance to be 1e15-1e16 cm for most bursts and we show that this is within a factor of ten of the radius of the shock-heated circumstellar medium (CSM) producing the x-ray photons. Furthermore, using the early gamma-ray, x-ray and optical data, we show that the prompt gamma-ray emission cannot be produced in internal shocks, nor can it be produced in the external shock; in a more general sense gamma-ray generation mechanisms based on shock physics have problems explaining the GRB data for the ten Swift bursts analysed in this work. A magnetic field dominated outflow model for GRBs has some attractive features, although the evidence in its favour is inconclusive. Finally, the x-ray and optical data allow us to provide an upper limit on the density of the CSM of about 10 protons per cubic cm at a distance of about 5e16 cm from the centre of explosion.

Read more (52kb, PDF)

__________________


L

Posts: 131433
Date:
Long duration gamma-ray bursts
Permalink  
 


Long duration gamma-ray bursts (GRBs), first discovered in the 1970s, are the most explosive events in the Universe. Finding out what happens during these cataclysmic events is a major challenge, partly because they usually occur at the edge of the visible Universe and partly because the bursts last only a matter of seconds.
Observations accumulated over the last decade have led to a consensus that at least some GRBs mark the death throes of a giant star as its core collapses to form a black hole. Until now, it has generally been thought that the black hole ejects a jet of plasma (extremely hot gas) which is blasted outwards at close to the speed of light.
This theory is called into question by a new study led by Pawan Kumar from the University of Texas. The work has been accepted for publication in the journal, ‘Monthly Notices of the Royal Astronomical Society’.

Read more

__________________


L

Posts: 131433
Date:
GRB060505
Permalink  
 


Two brilliant flashes of light from nearby galaxies are puzzling astronomers and could indicate that gamma-ray bursts, which signal the birth of a black hole, are more diverse than once thought.

First seen 40 years ago, gamma-ray bursts are the brightest explosions in the universe. They appear to fall into two distinct categories, short and long, depending on whether they shine for less than or more than about two seconds. Observations accumulated over the last decade led to a consensus that the long variety occurred when a massive star at the end of its life collapses to form a black hole. In addition to making a burst of gamma-rays, the explosion also produces a bright supernova. The short ones, not accompanied by a supernova, are thought to herald the merger of two neutron stars or a neutron star and a black hole, resulting in a bigger black hole.
The two new gamma-ray bursts are of the long variety but, surprisingly, did not show any evidence of supernova activity. This flies in the face of what was an emerging consensus about the origin of long bursts, according to University of California, Berkeley's Joshua Bloom, assistant professor of astronomy. To Bloom, this indicates that there are more than two ways to produce a gamma-ray flash and a black hole.

Read more

__________________


L

Posts: 131433
Date:
RE: Gamma-Ray Bursts
Permalink  
 



Scientists have discovered what appears to be a new kind of cosmic explosion, the subject of four articles in this week's issue of Nature. They call the explosion a hybrid gamma-ray burst.
As with other gamma-ray bursts, this hybrid burst is likely signalling the birth of a new black hole. It is unclear, however, what kind of object or objects exploded or merged to create the black hole or, perhaps, something even more bizarre. The hybrid burst exhibits properties of the two known classes of gamma-ray bursts yet possesses features that cannot be explained.
The burst was discovered by NASA's Swift satellite on June 14, 2006, and has since been studied with over a dozen telescopes, including the Hubble Space Telescope, ESO's Very Large Telescope, the Gemini South Telescope as well as with smaller telescopes such as the Danish 1.5m telescope at La Silla in Chile.

"We have lots of data on this, dedicated lots of observation time, and we just can't figure out what exploded. All the data seem to point to a new but perhaps not so uncommon kind of cosmic explosion" - Neil Gehrels of NASA Goddard Space Flight Centre in Greenbelt, Md., lead author on one of the Nature reports.

"Gamma-ray bursts are the most powerful known explosions in the universe. Yet they are random and fleeting, never to appear twice, and only in recent years has their nature been revealed with fantastic new data from the Swift satellite. And now Swift has presented us with a new intriguing burst that will test our understanding of such phenomena" - Professor Keith Mason, CEO of the Particle Physics and Astronomy Research Council.

Gamma-ray bursts fall into two categories, long and short. The long bursts last for more than two seconds and appear to be from the core collapse (supernova) of massive stars forming a black hole. The surrounding star can feed the newly formed black hole for many seconds leading to a long period of energy release. Most of these bursts come from near the edge of the visible universe. The short bursts, under two seconds long and often lasting just a few milliseconds, appear to be caused by the merger of two neutron stars or a neutron star with a black hole, which subsequently creates a new or bigger black hole. The small amount of remaining merger material can only feed the black hole for about a second and hence gives a short period of energy release.
The hybrid burst, called GRB 060614 after the date it was detected, was 1.6 billion light years away in the constellation Indus. The burst lasted for 102 seconds, placing it soundly in long-burst territory. But the burst lacked the hallmark of a supernova, or star explosion, commonly seen shortly after long bursts.

"Neither of our standard models fits GRB060614. We see no bright optical light after a few days that we would have expected from a supernova, and yet if the burst is due to a merger the new black hole is expected to feed, and hence release gamma-rays, for only a few seconds. This one is a real puzzle!" - Dr Paul O'Brien from the University of Leicester.

Certain properties of the burst concerning its brightness and the arrival time of photons of various energies, called the lag-luminosity relationship, suggest that burst behaved more like a short burst (from a merger) than a long burst. Yet no theoretical model of mergers can support a sustained release of gamma-ray energy for 102 seconds.

"Everyone was happy that there were two kinds of gamma-ray bursts but this throws a spanner in the works" - Patricia Schady of UCL's Mullard Space Science Laboratory.

"The lack of a supernova from this relatively nearby event was so puzzling, that some astronomers tried to explain it by proposing that the burst was actually at a great distance behind the galaxy it was found in - a cosmic coincidence. However, the images from the UK-built UltraViolet-Optical Telescope on Swift prove conclusively that this is not the case" - Dr Mat Page, also of UCL-MSSL.

The burst is perhaps not unprecedented. Archived data from the 1990s from the Compton Gamma-ray Observatory possibly reveal other hybrid "long-short" bursts, but no follow-up observations are available to confirm this. Another burst in May appears to also have had no supernova associated with it. Named GRB 060505, the burst lasted for 4 seconds but unfortunately did not have the lag-luminosity measured that makes GRB 060614 appear to be a hybrid.

"Two of the six long gamma ray bursts seen at close distances appear to have no supernova. This may be a more common type of explosion than we expected, possibly a new mechanism for star death" - Dr Pall Jakobsson of the University of Hertfordshire.

Scientists remain divided on whether this was a long-short burst from a merger or a long burst from a star explosion with no supernova for whatever reason. Most conclude, however, that some new process must be at play: either the model of mergers creating second-long bursts needs a major overhaul, or that the progenitor star from an explosion is intrinsically different from the kind that makes supernovae.

Source

__________________


L

Posts: 131433
Date:
GRB 060206 and 060210
Permalink  
 


Title: "Anomalous" Optical GRB Afterglows are Common: Two z~4 Bursts, GRB 060206 and 060210
Authors: K. Z. Stanek, X. Dai, J. L. Prieto, D. An, P. M. Garnavich, M. L. Calkins, J. Serven, G. Worthey, H. Hao, A. Dobrzycki, C. Howk, T. Matheson
(revised v3)

We report on two recent z~4 gamma-ray bursts (GRBs), GRB 060206 and GRB 060210, for which we have obtained well-sampled optical light curves. Our data, combined with early optical data reported in the literature, shows unusual behaviour for both afterglows. In R-band GRB 060206 (z=4.045) experienced a slow early decay, followed by a rapid increase in brightness by factor ~2.5 about 1 hour after the burst. Its afterglow then faded in a broken power-law fashion, with a smooth break at t_b=0.6 days, but with additional, less dramatic (~10%) ``bumps and wiggles'', well detected in the densely sampled light curve. The R-band afterglow of GRB 060210 (z=3.91) is also unusual: the light curves was more or less flat between 60 and 300 sec after the burst, followed by ~70% increase at ~600 sec after the burst, after which the light curve declined as a t^{-1.3} power-law. Despite earlier reports to the contrary, we find that for GRB 060206 X-rays also more or less follow the optical decay, but with significant variations on short timescales. However, the X-ray afterglow is contaminated by a nearby, variable source, which especially at late times obscures the behaviour of the X-ray afterglow. We argue that ``anomalous'' optical afterglows are likely to be the norm, and that the rapid variations often seen in Swift-XRT data would also be seen in the optical light curves, given good enough sampling. As a result, some of the often employed procedures, such as deriving the jet opening angle using a smooth broken power-law fit to the optical light curves, in many cases might have a poor statistical significance. We argue that the early increase in brightness for both bursts might be due to the turn-on of the external shock. Existence of such features could provide valuable additional information about the burst.

Read more (50kb, PDF)

__________________


L

Posts: 131433
Date:
RE: Gamma-Ray Bursts
Permalink  
 


Title: Hubble diagram of gamma-ray bursts: Robust evidence for a Chaplygin gas expansion-driven universe with phase transition at z ~= 3
Authors: Herman J. Mosquera Cuesta, Habib Dumet M., Rodrigo Turcati, Carlos A. Bonilla Quintero, Cristina Furlanetto, Jefferson Morais

The Hubble diagram (HD) of Gamma-Ray Bursts (GRBs) having properly estimated redshifts is compared with the predicted one for the Chaplygin gas (CG), a dark energy candidate. The CG cosmology and that of Friedmann and Lambda-CDM models are studied and confronted to the GRBs observations. The model-to-sample chi^2 statistical analysis indicates the CG model as the best fit. The present GRBs HD plot exhibits a marked trend: as one goes back in time, it gets much closer to the predict HD for a Friedmann universe. This clear trend conclusively demonstrates that a transition from decelerate to accelerate expansion did take place. However, contrarily to claims based on supernovae type Ia, the transition redshift lies somewhere between ~ 2.5 < z ~= 3.5 rather than at z ~ 0.5-1. All of these striking features of the GRBs HD constitute the most robust demonstration that the Chaplygin gas can in fact be the universe's driving dark energy field.

Read more (341kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: GRBs search results with the ARGO-YBJ experiment operated in Scaler Mode
Authors: G. Di Sciascio and T. Di Girolamo for the ARGO-YBJ Collaboration

The ARGO-YBJ experiment is almost completely installed at the YangBaJing Cosmic Ray Laboratory (4300 m a.s.l., Tibet). The lower energy limit of the detector (E ofs about 1 GeV) is reached with the scaler mode, i.e., recording the single particle rate at fixed time intervals. In this technique, due to its high altitude location and large area (about 6700 m˛), this experiment is the most sensitive among all present and past ground-based detectors. In the energy range under investigation, signals due to local (e.g. solar GLEs) and cosmological (e.g. GRBs) phenomena are expected as significant enhancements of the counting rate over the background. Results on the search for GRBs in coincidence with satellite detections are presented.

Read more (PDF)

__________________
«First  <  15 6 7 8  >  Last»  | Page of 8  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard