} Therein Lies the Rub.

&--Tuesday, July 3, 2007 ; 8:26 PM
EldwinSchrodinger @ GRB (Gamma Ray Burst) , AGN (Active Galactic Nuclei)

loving you always ♥

Gamma-ray Bursts

Gamma-ray Bursts (GRBs) were first discovered accidentally in the late 1960s by the Vela satellites, which were actually being used to ensure no nuclear devices were being tested in the atmosphere or space. However, instead of providing proof of illegal testing, the gamma-ray events detected were found to be cosmological in origin (i.e., from outside the atmosphere). Over the past 30-40 years, much progress has been made in understanding these events. It is found that there are 2 "populations", with some bursts lasting only milliseconds, while others have a duration of a few hundred seconds. A plot of number of bursts against duration shows the "dividing line" between the groups is at about 2 seconds, with peaks in the distribution occuring at ~0.3 and 30 seconds.

Because we do not know in advance when a GRB will occur, it has not been possible to observe the shortest wavelength events (it takes more than a few seconds to point a satellite in the correct direction!); however, some information is known about typical longer duration GRBs. They are found to be external to our galaxy, millions of light-years away (see here for a list of GRBs with known redshifts), and are probably formed when a very massive star reaches the end of its life. It is known that stars with M > 10 M_sun form supernova explosions when they die, leaving behind a compact remnant, either a neutron star or a black hole, depending on their mass. When even more massive stars (> 40 M_sun?) collapse, it is possible that the explosion is not the same as a supernova, with the energy output being about 100 times greater; these have been termed hypernovae and are likely to be one way to form GRBs.

An alternative explanation for the formation of a GRB is the merging of 2 neutron stars which were previously in a binary system. As they orbit each other, rotational energy is lost and converted to gravitational energy. This causes the orbit to decay and the neutron stars spiral into each other and merge, forming a black hole. This may be the route to forming one of the shorter (< href="http://www.ligo.caltech.edu/">LIGO (the Laser Interferometer Gravitational Wave Observatory) should one day be able to detect.

Although GRBs are initially detected at gamma-ray energies, "afterglows" at longer wavelengths can also be observed. BeppoSAX, an Italian X-ray satellite, first detected X-ray emission from a GRB in the late 1990s. Afterglows are also seen in the optical and radio wavebands as well. In fact, it was the discovery of these afterglows that made the redshift determination possible.

The Swift Gamma-ray mission, launched on 20th November 2004 is a multi-wavelength observatory (optical, X-ray and gamma-ray), designed to study GRBs. See the UK Swift Science Data Centre for more details.

Active Galactic Nuclei - a basic introduction

Most of my PhD research concentrated on X-ray observations of Active Galactic Nuclei (AGN), both the lower-luminosity Seyfert Galaxies and the higher-luminosity Quasi-Stellar Objects (QSOs; sometimes also referred to as quasars). It is generally thought that AGN are powered by matter from an accretion disc fuelling a central, supermassive (~10⁶-10⁹ M_sun) black hole.

All galaxies may contain central, massive black holes, but only those which are adequately fuelled are seen as active. AGN are the brightest continuously emitting objects in the Universe (Gamma Ray Bursts are much brighter, but for only a short period of time) and can reach bolometric luminosities (that is, summed over all wavelengths) of 10⁴⁰-10⁴⁸ erg s^-1. (An erg is the unit of energy most astronomers use; it is equal to 10^-7 J.) The luminosity emitted by the nucleus can be as bright (in Seyfert galaxies) or brighter (in the case of QSOs) than all the stars of the host galaxy (~10¹¹ L_sun) put together. Because of this, the active galaxy can appear as a point source (like a single star) to a distant observer; this is what led to the term Quasi-Stellar objects.

Because AGN are so luminous, they can be seen out to high redshifts/distances. Because light travels at a finite speed (3 x 10⁸ m s^-1), the further away an object is, the longer it takes for its light to reach us. When we observe radiation from an object at a redshift of 6, we are looking back almost 95% of the age of the Universe! The closer we look to the Big Bang, the more we should be able to understand about the evolution of the Universe as a whole.

Although we are pretty certain that AGN have central supermassive black holes, it is not yet possible to image them directly. This will (hopefully!) be resolved by the advent of X-ray interferometry. Interferometry means that there are lots of telescopes, all linked together, observing the same object. This allows much finer resolution than can be obtained by a single instrument. Interferometry is already used in different wavebands; for example, in the UK there is a radio interferometer called MERLIN (Multi-Element Radio-Linked Interferometer Network). Thus far, the design of X-ray interferometers has been hampered by the failure to build suitable diffraction-limited optics. However, such optics are now becoming feasible, so X-ray interferometry should be on the way!

X-rays are emitted by high-energy processes, making them a very useful tool for astrophysics. In the case of AGN, the X-rays are thought to come from either the inner regions of the accretion disc, or a hot corona of electrons above the disc. The effects of strong gravity reveal themselves in X-ray spectra, allowing astronomers to analyse the environments around supermassive black holes.

Extracted from here

Labels: Cosmology

MUSIC ♥

Music Playlist at MixPod.com

TAGBOARD ♥

ABOUT ELDWIN ♥

Welcome to Eldwin Schrodinger's blog. Feel free to drop a message at my shoutout :)

WISHLIST ♥

Honda Civic
Black and white flop flops
Gen 2 ATN Monoculars
Asus eee pc
Oakley shades
Macbook pro 17"
~~ipod touch 2g~~
~~iMac 24" desktop pc~~
~~Logitech 5.1 system~~
1x30 Acog Chevron Scope
~~20nM Green laser~~

BIRTHDAYS ♥

::January::
8th - Eldwin,Stephen H.
14th - Nicholas Anthoney
17th - Wei Cheng Heng
21st - Richard D. Winters, Tiffany Chen
24th - LiHuang
26th - Mette Jørgensen

::February::
5th - Jing Song
14th - Michelle Leong, Fang Qing

::March::
3rd - Cheryl Tan(DMIT),Leon Rheeder
8th - Jason Chng
15th - Rina
17th - Lim Jun Yi,Tong ShiHui
22nd - Khazrol
23rd - Peiwen Kwok
25th - Qing Ren

::April::
3rd - Cheryl Cheang

::May::
2nd - Evelyn Chua
8th - Ivy Toh
11th - Richard Feynman
18th - Kellyn Wee
21st - Gwendolyn khong
27th - Mum's Birthday
29th - Milissa Qce
30th - Param Charleston

::June::
4th - Cecilia Kong
9th - Clarence Wong
11th - Rachel Ong
18th - Jaslyn Ng

::July::
3rd - Chelsea Low
11th - Mira
13th - Nadya Wijaja
19th - WanFong

::August::
12th - Erwin Schrodinger
16th - Kieran Lim
23rd - Min Er, Wei Kiat
30th - kristin
31st - ZhengWei

::September::
8th - ManYong
9th - Alexia Ang
21st - Jon Tan,Rayson Choo, Lim Wei Jie(KP)
26th - Prescilla
28th - Nadzirah(SP)

::October::
3rd - Jessie Ho
7th - Neils Bohr, PooSiang
9th - Meryl Cho
14th - Dwight D. Eisenhower
19th - GuanTing

::November::
1st - Pearson Wu
5th - Syikin
6th - Chik Jun Qi
12th - Candy Yeo
14th - Velvet May
23rd - Dad's Birthday

::December::
1st - Estelle
2nd - Michelle Chong
18th - Juyee Ong
21st - Cheryl Lai
23rd - Janice Tan
24th - Catherine,Marshall Lok
26th - Corrine Lau
30th - Alvin Khong
31st - Cecilia knudsen

LINKS ♥

PAST ♥

January 2007
February 2007
March 2007
April 2007
May 2007
June 2007
July 2007
August 2007
September 2007
October 2007
November 2007
January 2008
March 2008
April 2008
May 2008
June 2008
July 2008
August 2008
September 2008
October 2008
November 2008
January 2009
February 2009
March 2009
April 2009
May 2009
June 2009
July 2009
August 2009
September 2009
October 2009
November 2009
January 2010
February 2010

CREDITS ♥

Designer: Byiling (:
Basecodes : YY