Enlightening gamma-ray bursts

Abstract

A once a day, space-based observatories see a short, bright flash of gamma rays from a seemingly random location in the sky – a gamma-ray burst (GRB). Since their discovery 30 years ago, their continued invisibility at other wavelengths has enshrined them among the great mysteries of astrophysics. Finally, the veil has lifted, through observations at other wavelengths. The Italian–Dutch BeppoSAX satellite located a GRB on 28 February quickly to arcminute precision for the first time. This enabled astronomers on La Palma to obtain images of it within 21 hours, as a 21st magnitude, rapidly fading object. The second detection of an optical counterpart, at Kitt Peak, was for the burst of 8 May. This time, a spectrum taken with the Keck telescope showed absorption lines at redshifts 0.77 and 0.835. So this burst (and, by Ockam’s razor, all bursts) is at cosmological distances from us, settling the dispute over whether GRBs are in our galaxy or far outside it. The implication is that GRBs are the most luminous objects in the universe during their brief moment of glory. The currently favoured model attributes these events to relativistic versions of supernovae: 10 erg of energy is suddenly released into a very small volume by an unknown exotic event. This energetic “fireball” then expands ultrarelativistically, with a Lorentz factor of 300 or so. When it ploughs into the surrounding gas it decelerates and radiates its kinetic energy away in gamma rays. When the blast wave slows down to lower Lorentz factors, it produces the X-ray and optical afterglows. If GRBs are caused by compact stellar remnants, their rate is likely to track the star formation rate in the universe. The flux distribution of GRBs matches this, implying that a few percent of GRBs are beyond redshift 4 and their redshift distribution resembles that of quasars. We can expect more excitement in the near future as a few exciting branches of astrophysics come together, for example in the detection of the X-ray afterglow of GRB 970616 by the Rossi XTE satellite after a concerted effort by the RXTE and BATSE teams. T formation and early evolution of young massive stars remains one of the most fascinating problems in astrophysics – a fit subject for a lively one-day meeting. Massive stars form deep in the cores of molecular clouds, when some threshold to collapse is passed. Anthony Whitworth of the University of Cardiff showed the results of numerical simulations involving colliding clouds, which matched the complexity of many star-forming complexes. But Tom Hartquist, on sabbatical at UCL, revisited the problem of the heating of molecular clouds subject to a pressure increase, and found that this may well discourage star formation rather than enhance it. As a massive star turns on, the wind from the star, or perhaps the surface of a surrounding accretion disk, will interact with ambient, possibly inflowing, gas. Observations of the cavities and shocks generated by these omnipresent outflows were presented, as well as constraints on the magnetic field structure via Zeeman splitting in OH masers. When these stars become hot enough, they ionize the dense surrounding gas producing ultracompact H II regions. The clumpy medium surrounding a star may play an important role in adding mass to the stellar wind to confine these regions. Models of this process explain the wide range of observed shapes. Attention was drawn to the preponderance of cometary shaped H II regions, especially their kinematics and the astrochemistry of the dense molecular cores at their apex where the next generation of stars may arise. A particular focus of the meeting was the nearest – and best observed – star formation region, Orion. In this well-resolved region, puzzles found elsewhere are magnified. “Bullets” of ionized gas seem to be driven from the central star at high velocities, generating spectacular fingers of emission in their wake (see picture). Michael Burton of the University of New South Wales showed the latest Hubble Space Telescope images of the “bullets”, while Jonathan Tedds of the University of Leeds presented UK Infrared Telescope measurements of the dynamics and excitation of hot molecular hydrogen in the corresponding fingers. A major challenge from the studies is to better understand these crucial feedback processes in the cycle of star formation in molecular clouds. The programme of the meeting and onward links are at “http://ast. leeds.ac.uk/~rjrw/Meeting.html”.