WHAT'S HOT IN... PHYSICS , May/June 2008

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