* Astronomy

Members Login
Post Info TOPIC: Birth of a black hole


L

Posts: 131433
Date:
RE: Birth of a black hole
Permalink  
 


GRB 090423 is a gamma-ray burst (GRB) detected by the Swift Gamma-Ray Burst Mission on April 23, 2009 at 07:55:19 UTC.
Read more



__________________


L

Posts: 131433
Date:
GRB 090423
Permalink  
 


A group of scientists - including one Harvard professor - have discovered the furthest known object in the universe, Gamma-Ray Burst 090423, after it was detection by NASAs Swift telescope last Thursday morning.
The object is 13.1 billion light years away and is thought to have been caused by the death of a massive star and the birth of a black hole, according to Edo Berger, an assistant professor of astronomy.


Read more

__________________


L

Posts: 131433
Date:
Permalink  
 

A stellar explosion has smashed the record for most distant object in the known universe.
The gamma-ray burst came from about 13 billion light-years away, and represents a relic from when the universe was just 630 million years old.

Read more

__________________


L

Posts: 131433
Date:
RE: Birth of a black hole
Permalink  
 




__________________


L

Posts: 131433
Date:
VLT Uncovers New Way to Form Black Hole
Permalink  
 


Nature has again thrown astronomers for a loop. Just when they thought they understood how gamma-ray bursts formed, they have uncovered what appears to be evidence for a new kind of cosmic explosion. These seem to arise when a newly born black hole swallows most of the matter from its doomed parent star.
Gamma-ray bursts (GRBs), the most powerful explosions in the Universe, signal the formation of a new black hole and come in two flavours, long and short ones. In recent years, international efforts have shown that long gamma-ray bursts are linked with the explosive deaths of massive stars.
Last year, observations by different teams - including the GRACE and MISTICI collaborations that use ESO's telescopes - of the afterglows of two short gamma-ray bursts provided the first conclusive evidence that this class of objects most likely originates from the collision of compact objects: neutron stars or black holes

Read more

__________________


L

Posts: 131433
Date:
GRB060614
Permalink  
 


Scientists using NASA data are studying a newly recognized type of cosmic explosion called a hybrid gamma-ray burst. As with other gamma-ray bursts, this hybrid blast 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 new black hole. The hybrid burst exhibits properties of the two known classes of gamma-ray bursts yet possesses features that remain unexplained.
NASA's Swift first discovered the burst on June 14. Since the Swift finding, more than a dozen telescopes, including the Hubble Space Telescope and large ground-based observatories, have studied the burst.

Read more

__________________


L

Posts: 131433
Date:
GRB 051221a
Permalink  
 


2005 December 21.0773 UT, the space-based Swift Gamma Ray Burst Explorer (Burst Alert Telescope) detected a classical short gamma ray burst. The total duration of this event (named GRB 051221a) was about 1.4 seconds. Alicia M. Soderberg (CALTECH) and her collaborators have studied the burst and its afterglow in great detail using several datasets provided by space observations, radio observations with the VLA and optical observations with Gemini. They derived a detailed set of physical parameters for this burst and their analysis helps to better understand the nature of the progenitor of short bursts and their host environments.

firstmroimage-20060929
Credit Gemini Observatory/AURA

The Gemini North Telescope staff was alerted and began observations about 2.8 hours after the burst using the Gemini Multi-Object Spectrograph (GMOS). It detected a bright optical source, and the evolution of this afterglow was followed over several days. The optical source detected by GMOS corresponds to the afterglow that faded relatively slowly. This afterglow is produced by the dynamical interaction of the ejecta with the surrounding medium.
An optical spectrum of the host galaxy was also obtained, which allowed for the redshift (distance) to be derived and it was found to be z = 0.5464. This makes GRB 051221a the most distant short gamma ray burst observed so far with a robust redshift determination. The emission lines indicate that the host galaxy is actively forming stars at a rate of about 1.6 Msun per year. This is higher than what has been found in all previous short gamma ray burst hosts. Well-known absorption features like the Ca II H and K lines are also apparent and these are indicative of an appreciably old stellar population. The mean metallicity of the host galaxy is nearly the same as our sun.
Gamma ray bursts are among the most energetic explosions in the whole universe. Gamma ray burst have been known since 1967 when they were first detected serendipitously by the Vela spacecrafts built to detect man-made nuclear explosions in the upper atmosphere or in near-Earth space. It was only 30 years later, in 1997, that their origins have been associated with very energetic events happening in distant galaxies. There are two classes of gamma ray burst: the long soft burst (surmised to be associated with the explosion of massive stars into supernovae) and the short hard bursts, (like GRB 051221a), thought to be the catastrophic energy output of merging massive stellar remnants, like neutron stars or black holes.

The current observations by the Soderberg team supports the trend that progenitors of short burst GRBs arise from old stellar populations.

Source


Title: The Afterglow, Energetics and Host Galaxy of the Short-Hard Gamma-Ray Burst 051221a
Authors: A. M. Soderberg, E. Berger, M. Kasliwal, D. A. Frail, P. A. Price, B. P. Schmidt, S. R. Kulkarni, D. B. Fox, S. B. Cenko, A. Gal-Yam, E. Nakar, K. C. Roth

We present detailed optical, X-ray and radio observations of the bright afterglow of the short gamma-ray burst 051221a obtained with Gemini, Swift/XRT, and the Very Large Array, as well as optical spectra from which we measure the redshift of the burst, z=0.5464. At this redshift the isotropic-equivalent prompt energy release was about 1.5 x 10^51 erg, and using the standard afterglow synchrotron model we find that the blastwave kinetic energy is similar, E_K,iso ~ 8.4 x 10^51 erg. An observed jet break at t ~ 5 days indicates that the opening angle is ~ 7 degrees and the total beaming-corrected energy is therefore ~ 2.5 x 10^49 erg, comparable to the values inferred for previous short GRBs. We further show that the burst experienced an episode of energy injection by a factor of 3.4 between t=1.4 and 3.4 hours, which was accompanied by reverse shock emission in the radio band. This result provides continued evidence that the central engines of short GRBs may be active significantly longer than the duration of the burst and/or produce a wide range of Lorentz factors. Finally, we show that the host galaxy of GRB051221a is actively forming stars at a rate of about 1.6 M_solar/yr, but at the same time exhibits evidence for an appreciable population of old stars (~ 1 Gyr) and near solar metallicity. The lack of bright supernova emission and the low circumburst density (n ~ 10^-3 cm^-3) continue to support the idea that short bursts are not related to the death of massive stars and are instead consistent with a compact object merger. Given that the total energy release is a factor of ~ 10 larger than the predicted yield for a neutrino annihilation mechanism, this suggests that magnetohydrodynamic processes may be required to power the burst.

Read more (343kb, PDF)

__________________


L

Posts: 131433
Date:
Gamma-Ray Bursts
Permalink  
 


Title: Gamma-Ray Bursts
Authors: P. Meszaros
(Update v5)

Gamma-ray bursts are the most luminous explosions in the Universe, and their origin and mechanism are the focus of intense research and debate. More than three decades after their discovery, and after pioneering breakthroughs from space and ground experiments, their study is entering a new phase with the recently launched Swift satellite. The interplay between these observations and theoretical models of the prompt gamma ray burst and its afterglow is reviewed.

Read more (347kb, PDF)

-- Edited by Blobrana at 11:37, 2006-06-01

__________________


L

Posts: 131433
Date:
RE: Birth of a black hole
Permalink  
 


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' 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. "Long" bursts last from seconds to minutes and are thought to occur when massive stars explode and their cores collapse into black holes.
"Short" 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's fading afterglow. The afterglow reveals a burst's position and is caused by shockwaves from the explosion heating up surrounding gas.

But in 2005, NASA's Swift telescope spun around fast enough to observe about a dozen short GRBs. Two were bright and long enough with their afterglows fading over several days to permit detailed study with the Chandra X-ray Observatory.
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 "opening angles" of between 4 and 8.
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


The merger of two neutron stars is thought to create short GRBs and leaves trails of matter in spiral arms.
Credit: Daniel Price and Stephan Rosswog.


"If you don't understand this question of beaming, then you don't know anything about the energy. And without knowing the energy, it's really hard to say much about what was actually causing the burst in the first place" - David Burrows, PSU astrophysicist, who led the study on the latter GRB.

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.
The very fact that some short GRBs appear to be beamed and others do not is interesting. Long GRBs also show this range of behaviour.

"This points out some fundamental similarities between long and short GRBs" - Bohdan Paczynski, Princeton University in New Jersey, US.

But it is still not clear why this might be.

"Jets are observed in a large variety of accreting astrophysical objects. But there is no general consensus on how they are formed" - Stephan Rosswog, International University Bremen in Germany.

Magnetic fields may play a major role in some cases. 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.

"This interaction can be rather complicated and will depend on the details of the particular merger" - Stephan Rosswog.

The new research has been submitted to the Astrophysical Journal.

__________________


L

Posts: 131433
Date:
Short gamma-ray bursts
Permalink  
 




Gamma-ray bursts are the most powerful explosions in the universe, emitting huge amounts of high-energy radiation. For decades their origin was a mystery. Scientists now believe they understand the processes that produce gamma-ray bursts. However, a new study by Jonathan Grindlay of the Harvard-Smithsonian Centre for Astrophysics (CfA) and his colleagues, Simon Portegies Zwart (Astronomical Institute, The Netherlands) and Stephen McMillan (Drexel University), suggests a previously overlooked source for some gamma-ray bursts: stellar encounters within globular clusters.

"As many as one-third of all short gamma-ray bursts that we observe may come from merging neutron stars in globular clusters" - Jonathan Grindlay .

Gamma-ray bursts (GRBs) come in two distinct "flavours." Some last up to a minute, or even longer. Astronomers believe those long GRBs are generated when a massive star explodes in a hypernova. Other bursts last for only a fraction of a second. Astronomers theorize that short GRBs originate from the collision of two neutron stars, or a neutron star and a black hole.
Most double neutron star systems result from the evolution of two massive stars already orbiting each other. The natural aging process will cause both to become neutron stars (if they start with a given mass), which then spiral together over millions or billions of years until they merge and release a gamma-ray burst.
Grindlay's research points to another potential source of short GRBs - globular clusters. Globular clusters contain some of the oldest stars in the universe crammed into a tight space only a few light-years across. Such tight quarters provoke many close stellar encounters, some of which lead to star swaps. If a neutron star with a stellar companion (such as a white dwarf or main-sequence star) exchanges its partner with another neutron star, the resulting pair of neutron stars will eventually spiral together and collide explosively, creating a gamma-ray burst.

"We see these precursor systems, containing one neutron star in the form of a millisecond pulsar, all over the place in globular clusters. Plus, globular clusters are so closely packed that you have a lot of interactions. It's a natural way to make double neutron-star systems" - Jonathan Grindlay.

The astronomers performed about 3 million computer simulations to calculate the frequency with which double neutron-star systems can form in globular clusters. Knowing how many have formed over the galaxy's history, and approximately how long it takes for a system to merge, they then determined the frequency of short gamma-ray bursts expected from globular cluster binaries. They estimate that between 10 and 30 percent of all short gamma-ray bursts that we observe may result from such systems.
This estimate takes into account a curious trend uncovered by recent GRB observations. Mergers and thus bursts from so-called "disk" neutron-star binaries - systems created from two massive stars that formed together and died together - are estimated to occur 100 times more frequently than bursts from globular cluster binaries. Yet the handful of short GRBs that have been precisely located tend to come from galactic halos and very old stars, as expected for globular clusters.

"There's a big bookkeeping problem here" - Jonathan Grindlay .

To explain the discrepancy, Grindlay suggests that bursts from disk binaries are likely to be harder to spot because they tend to emit radiation in narrower blasts visible from fewer directions. Narrower "beaming" might result from colliding stars whose spins are aligned with their orbit, as expected for binaries that have been together from the moment of their birth. Newly joined stars, with their random orientations, might emit wider bursts when they merge.

"More short GRBs probably come from disk systems - we just don't see them all" - Jonathan Grindlay .

Only about a half dozen short GRBs have been precisely located by gamma-ray satellites recently, making thorough studies difficult. As more examples are gathered, the sources of short GRBs should become much better understood.


The Milky Way globular cluster M15 contains a double neutron star system, M15C, that will eventually spiral together and merge violently. Such mergers are a likely source of short gamma-ray bursts. Neutron star binaries are expected to form in globular clusters due to the likelihood of stellar exchanges in that crowded environment.
Credit: NOAO/AURA/NSF





Short gamma-ray bursts from binary neutron star mergers in globular clusters
Authors: Jonathan Grindlay (1), Simon Portegies Zwart (2), Stephen McMillan (3) ((1) Harvard-Smithsonian Centre for Astrophysics, (2) Astronomical Institute Anton Pannekoek, (3) Department of Physics, Drexel University)

The first locations of short gamma-ray bursts (GRBs) in elliptical galaxies suggest they are produced by the mergers of double neutron star (DNS) binaries in old stellar populations. Globular clusters, where the extreme densities of very old stars in cluster cores create and exchange compact binaries efficiently, are a natural environment to produce merging NSs. They also allow some short GRBs to be offset from their host galaxies, as opposed to DNS systems formed from massive binary stars which appear to remain in galactic disks.
Starting with a simple scaling from the first DNS observed in a galactic globular, which will produce a short GRB in ~300My, researchers present numerical simulations which show that ~10-30% of short GRBs may be produced in globular clusters vs. the much more numerous DNS mergers and short GRBs predicted for galactic disks.
Reconciling the rates suggests the disk short GRBs are more beamed, perhaps by both the increased merger angular momentum from the DNS spin-orbit alignment (random for the DNS systems in globulars) and a larger magnetic field on the secondary NS.

Read more (PDF)

__________________
1 2 3  >  Last»  | Page of 3  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard