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Two papers in theoretical high-energy astrophysics stand out in this
period: #5 on gamma-ray bursts, and #6 on active galactic nuclei (AGN).
Both papers are concerned with the phenomenology of outbursts, although on
very different time scales.
Gamma-ray bursts (GRBs) are the most energetic events occurring in the
universe since the Big Bang. They pop up at random times and in random
places throughout the universe. Typically a burst lasts for a second to
hundreds of seconds. Most gamma-ray bursts occur when massive stars run out
of nuclear fuel. Their cores collapse to form black holes or neutron stars,
releasing an intense burst of high-energy gamma rays and ejecting particle
jets that rip through space at nearly the speed of light, creating a
fireball in the surrounding interstellar medium. Minutes after the onset of
a burst, a longer-lived afterglow becomes visible across the
electromagnetic spectrum, from X-rays to radio waves. Observational
astronomers monitor such afterglows in their quest to discover the physical
processes at work.
On November 20, 2004, NASA launched the Swift gamma-ray burst
mission, a multi-purpose observatory. Swift is so named because
within seconds of detecting a burst, it relays the location to ground
stations, allowing telescopes around the world the opportunity to observe
the burst's afterglow. Swift’s on-board X-Ray Telescope
(XRT) is on the case in less than a minute.
Hot Paper #5, with Bing Zhang (University of Nevada, Las Vegas) as
principal author, is on the treasure trove of early X-ray afterglow data
garnered by Swift’s XRT. For the first time, observers can
scrutinize the lightcurve of the afterglow from its onset, and thus explore
many interesting questions of GRB physics. This paper is cited because it
presents a five-component analysis of the afterglow data. In essence, the
report identifies five stages in the aftermath of a GRB that the theorists
now need to explain. The forensic story in #5 is that much information can
be gleaned about the extreme physical conditions surrounding the central
engine, a collapsed star. Swift is still rapidly accumulating data
on the early X-ray afterglows, thereby advancing the quest for the final
answers to the core questions in the study of GRBs.
In Hot Paper #6, with Darren Croton (Max Planck Institute for Astrophysics,
Garching, Germany) as leading author, the lifestyles of active galactic
nuclei are unveiled thanks to a massive computer simulation known as the
Millennium Run, a very large dark-matter simulation of the concordance cosmology
with 2160= 1.00783 x 1010 particles in a box.
The origin of structure in the universe has been a conundrum for millennia.
The only force of physics that matters when galaxies are being put together
is gravity. Theorists have long used numerical simulations to track the
assembly history and subsequent evolution of galaxies.
One of the triumphs of lambda cold dark matter cosmology is the consistent
explanation that it gives for the formation of structure in the universe
across all length scales and time scales. This model can match essentially
all structural features, from the fluctuations seen in the
cosmic microwave background, to the distribution of
galaxies at low redshift. In the model, galaxies form when gas condenses
onto merging dark-matter haloes. But the problem with that mechanism has
been its failure to predict the observed distribution of luminosities of
galaxies: there are too many bright galaxies and too many faint
galaxies.
The novel element in #6 is that AGN feedback is an important but relatively
little-explored element in the co-evolution of galaxies and the
supermassive black holes at their centers. The paper sets up the
machinery to study this co-evolution in unprecedented detail using the
very large Millennium Run. The simulation tracks the initiation of
structure formation and then its subsequent evolution, paying particular
attention to the growth and activity of the central supermassive black
holes that reside in most galaxies.
An important extension to previous work is the introduction of radio
sources associated with the black holes. The radio emission phenomenon
suppresses gas condensation at the centers of massive haloes without
requiring the formation of new stars. The net result is that the
model’s distribution of galaxy luminosities, and the history of star
formation, coincides more closely with reality when energy feedback from a
central engine is included.
Dr. Simon Mitton is a Fellow of
St. Edmund’s College, Cambridge, U.K.
Keywords: gamma-ray bursts, GRBs, black holes, Swift
X-ray telescope, Bing Zhang, Darren Croton, active galactic nuclei,
AGN