Archive for ‘Gamma-ray bursts’

November 9, 2011

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

According to observations of a very distant gamma-ray burst (GRB) recently reported, it appears that a couple of galaxies in the early universe, only 1.8 billion years after the big bang, contain a higher concentration of some elements heavier than hydrogen and helium than the Sun does. This is rather surprising, since the Sun is about 4.5 billion years old, and formed out of gas and dust in which heavy elements had been accumulating for about 9 billion years.

Isaac Asimov supposedly remarked, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’, but ‘That’s funny …'” That may be appropriate in this case. How did those galaxies have all those heavy elements so early?

The trigger for the observation was a GRB detected in March 2009 and designated GRB 090323. This was the type of GRB known as a “long” GRB, because the brightest phase lasts more than two seconds, and there is a diminishing afterglow that may last days. (The “short” type lasts less than two seconds and shows no afterglow.) Long GRBs are thought to result from supernova events in which a jet of relativistic particles is aimed in our direction.

Although the burst was determined to be quite distant (redshift z=3.57, corresponding to a light travel time of about 11.9 billion years), there was nothing about it at first that seemed especially unusual. However, an analysis of the spectrum of the afterglow indicated that the light from the GRB had passed through two galaxies relatively close to each other, one of which may have been the host of the GRB. What was especially odd was that the spectrum showed the presence of higher concentrations of zinc and sulfur than occur in the Sun.

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has used the brief but brilliant light of a distant gamma-ray burst as a probe to study the make-up of very distant galaxies. Surprisingly the new observations revealed two galaxies in the young Universe that are richer in the heavier chemical elements than the Sun. The two galaxies may be in the process of merging. Such events in the early Universe will drive the formation of many new stars and may be the trigger for gamma-ray bursts.

Gamma-ray bursts are the brightest explosions in the Universe. They are first spotted by orbiting observatories that detect the initial short burst of gamma rays. After their positions have been pinned down, they are then immediately studied using large ground-based telescopes that can detect the visible-light and infrared afterglows that the bursts emit over the succeeding hours and days. One such burst, called GRB 090323, was first spotted by the NASA Fermi Gamma-ray Space Telescope. Very soon afterwards it was picked up by the X-ray detector on NASA’s Swift satellite and with the GROND system at the MPG/ESO 2.2-metre telescope in Chile. From the GROND observations, the astronomers estimated the minimum rate of star formation, which has to be several times higher than the one in our Galaxy. They could, however, only determine a minimum value because the detected emission could be heavily affected (i.e. absorbed) by the presence of dust in the galaxies. The real rate of star formation, once the (unknown) dust absorption has been taken into account, could easily be 50 times higher than in the Milky Way.

The authors of the research paper point out that “These are the highest metallicities ever measured in galaxies at z > 3.” (“Metallicity” refers to the relative abundance of heavy elements.) But how unusual is this, really?

September 21, 2011

Gamma-ray bursts shed light on the nature of dark energy

Dark energy was “discovered” unexpectedly in 1997 (published in 1998), when a survey of relatively nearby Type Ia supernovae found something strange. These supernovae should all have about the same intrinsic luminosity. But instead, using assumptions current at the time about the rate of expansion of the universe, the farther away these supernovae were the less their intrinsic luminosity would have to be.

If in fact these supernovae have pretty much the same intrinsic luminosity in all cases, then they are “standard candles”. Hence a simple application of the inverse square law (apparent luminosity is proportional to the inverse square of the distance) would determine how far away the objects should be. However, by another chain of reasoning, the distance can be calculated using “Hubble’s law” from the observed redshift in the objects’ spectra, because the law states that relative velocity (which is inferred from redshift) is proportional to distance, with a constant of proportionality called “Hubble’s constant”. (The law was named after Edwin Hubble, who first conceived it.)

The problem was that this prediction of distance was too small for the observed luminosity. Otherwise stated, if the remote supernovae were actually as close as the distances calculated by Hubble’s law, they could not be as luminous intrinsically as more nearby supernovae. However, this problem would go away if there were an error in calculating the correct distance from Hubble’s law – which could happen if something now called “dark energy” existed.

In order to verify this hypothesis of dark energy, there are several reasons astrophysicists want to have another type of standard candle besides Type Ia supernovae for gauging very large distances. Ideally another type of standard candle that doesn’t depend on the behavior of Type Ia supernovae and works out to much larger distances could be identified. Astrophysicists have suspected that another type of supernova, which is responsible for a much more energetic pulse of electromagnetic radiation – a gamma-ray burst – can fill the bill.

Research just announced has studied the properties of relatively nearby gamma-ray bursts and identified certain characteristics that allow predicting the intrinsic brightness of the burst. Comparing that brightness to what’s actually observed determines the distance of the event.

Gamma-ray bursts shed light on the nature of dark energy – University of Warsaw

Dark energy is the basic constituent of the Universe today, one that is responsible for its accelerated expansion. Although astronomers observe the cosmological effects of the impact of dark energy, they still do not know exactly what it is. A new method for measuring the largest distances in the Universe developed by scientists from the Faculty of Physics, University of Warsaw and the University of Naples Federico II helps solve the mystery. A key role is played by the most powerful cosmic explosions – gamma-ray bursts.

What is the nature of dark energy, a recently discovered dominant constituent of the Universe today? Is expansion-accelerating dark energy an intrinsic property of space-time itself or rather a field unknown to science? A new distance-measuring method developed by scientists from the Faculty of Physics, University of Warsaw (FUW) and the University of Naples Federico II can provide the answer. “We are able to determine the distance of an explosion on the basis of the properties of the radiation emitted during gamma-ray bursts. Given that some of these explosions are related to the most remote objects in space that we know about, we are able, for the first time, to assess the speed of space-time expansion even in the relatively early periods after the Big Bang,” says Prof. Marek Demiański (FUW). The method was used to verify models of the structure of the Universe containing dark energy.