Gamma Ray Bursts (GRBs) are the most energetic discrete events we observe in astronomy. Far more powerful than supernovae (exploding massive stars), they are also extremely short events, lasting between a few seconds and a few hours. Recently, with the advent of the SWIFT satellite (the image to the right is an artist’s rendering of the SWIFT satellite, credit: NASA) that can locate GRBs to a reasonable precision, the so-called “afterglow” of these events have been detected across the electromagnetic spectrum. This week’s Published Research Synopsis details the results of a single detection event, GRB 050724 belonging to the short-hard GRB class (very short gamma-ray emissions in the “hard”, or highly energetic, range of the gamma ray spectrum) and finds, among other things, that the GRB probably resulted from the collision of a neturon star with a black hole, or of a star with a neutron star.
Citation (online at CiteULike.org):
Berger, E. and 24 others (2005). The afterglow and elliptical host galaxy of the short γ-ray burst GRB 050724. Nature 438 (7070), pp. 988-990.
Synopsis:
SWIFT detected an approximately 3 second long hard GRB on July 25, 2005 dubbed GRB 050724. Immediately after receiving this detection announcement, the Very Large Array, Baade 6.5-m Magellan, and Swope 40-inch telescopes were turned to the location specified by SWIFT. These three observatories looked at the radio, near-infrared, and optical portions of the spectrum, respectively. What they saw at that location was an elliptical galaxy at a redshift of z=0.257 (meaning at a distance of about 3.3 billion lightyears), with a very bright lump at one end. That lump decayed in brightness over the next several days, gradually becoming indistinguishable from the surroundings 2 days later. The bottom image in the figure on the left shows an image taken at first detection of the GRB subtracted from the same galaxy two days later. That differenced-image is a picture of the optical glow of the expanding fireball of the GRB itself.
The precise details in of the spectrum of the GRB fireball (obtained using the Gemini North telescope on Mauna Kea) revealed that instead of being a symmetrically-expanding sphere like a supernova remnant, most of the energy of the explosion went into producing one or two jets of material that just happened to be pointed our way. This observation lead the authors to suggest that short-hard GRBs may be 50x more prevalent than we can observe, because most of these jets don’t happen to be pointed our way.
Further interpretation of the Gemini-obtained spectrum of the host galaxy and the region surrounding the GRB itself within the galaxy indicated that there was very little stellar formation going on (total new star mass less than 0.03 times the mass of our sun, in comparison, the Milky Way is estimated to have a stellar formation rate of . This means that GRB 050724 was very likely not due to the explosion of a massive young star as seen in the long-soft (relatively long duration “soft”, or less energetic gamma rays) type GRBs. This narrowed down the range of theoretically-proposed mechanisms of the short-hard GRBs to collisions between stars, neutron stars, and black holes. Precisely which of these mechanisms was at work in GRB 050724 is not known yet, and the variety of different types of short-hard GRBs observed may indicate that some or all of these theoretical proposals may be correct.
Context:
The SWIFT telescope has revolutionized the study of gamma ray bursts. What were once vaguely referred to as Gamma Ray Bursters are now turning out to be exploding massive stars with cores that instantaneously collapse into black holes (long-soft GRBs), and collisions between stars/neutron stars or neutron stars/black holes. Further study of these events is important not just for the purpose of understanding some of the most spectacular features of the universe, but also for the promise they may provide of offering a new means of measuring distances across a significant fraction of the universe. Such tools, referred to as standard candles because their brightnesses are reasonably similar, can help to study the structure of the universe itself. One type of standard candle, the Type Ia supernovae, provided the crucial observation that the acceleration of the universe is expanding, and thus that the mass and energy in our universe is dominated by so-called dark energy. Perhaps GRBs may be another such standard candle that could help to study dark energy and better understand the basic physics of our universe.
General Explanations:
Gamma rays
Gamma rays are photons with more energy than x-rays, but less than cosmic rays. Gamma rays do not easily penetrate Earth’s atmosphere, so detection and locations of GRBs requires the use of satellites.
Standard candles
If the amount of energy that a particular astronomical event releases is well known, than comparing the amount of energy detected on Earth to the amount of energy released by the event gives the distance to that event. Some types of supernovae, including the Type Ia (fusion in the core of a white-dwarf caused by the infall of new material onto the white dwarf from the atmosphere of a companion star), occur near enough that we can measure their brightnesses and determine their distances through other means. So, correcting for that distance, we know how much energy the Type 1a supernovae usually release. This type of distance measuring can be referred to as a ladder, because one type of standard candle provides the observations needed to calibrate the next type.
Galaxy types
Edwin Hubble provided us with the first systematic classification of galaxy types. He observed that some galaxies were egg-shaped (ellipsoidal), which he called early-type galaxies. Others, like the Milky Way were spiral shaped, and he called them late-type. This was because he believed that the elliptical galaxies were in an early stage of galactic evolution and would later become spiral galaxies, or barred-spiral galaxies like our own. We know now that this is probably not the way things generally go. Elliptical galaxies probably form first and then, through subsequent merger (like that which our galaxy will undergo with the Andromeda Galaxy in approximately 5 billion years) and re-consolidation, ellipticals are born.
