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VOL. 3 / NO. 1 FALL 2005 A RENAISSANCE IN GAMMA-RAY BURST RESEARCH THE STUDY OF GAMMA-RAY BURSTS fraction of supernovae also produce NEW CLUES TO THE ORIGIN OF (GRBs), which are Nature’s strongest detectable bursts of gamma-rays. Just what SHORT-HARD BURSTS explosions, is enjoying a rebirth owing to determines this bifurcation is not well On May 8, 2005, the SWIFT satellite the launch (November 2004) of NASA’s understood, but may be related to the spin of discovered the short-hard GRB 050509b, SWIFT satellite. Berkeley astronomers are the progenitor system or to the type of and relayed an accurate position from its X- taking a leading role in new developments central object created during collapse phase Ray detector to the ground in real-time. We with ground-based infrared telescope (a neutron star, for most, a black hole for a quickly marshaled an imaging and observations of GRBs following alerts from few). Professor Eliot Quataert and former spectroscopy campaign on the Keck SWIFT via internet. postdoc Todd Thompson are studying the telescopes in Hawaii, the WIYN 3.5m GRBs were discovered serendipitously in details of some of the “central engine” telescope in Arizona, the PAIRITEL 1.3m in 1967 by military satellites, and their origin models for long-duration GRBs. Arizona, and the Gemini 8m in Hawaii. remained a complete mystery for 30 years. In While great strides continue to be made PAIRITEL is the 1.3 meter Peters fact, astronomers did not even know their in understanding long-soft bursts, the Automated Infrared Imaging telescope on distances — in the solar system, or at the “Afterglow Revolution” has not been kind Mt. Hopkins in Arizona which I developed edge of the Universe — until February 1997 to those trying to study short-hard bursts over the last three years and continue to when the BeppoSAX satellite discovered a (Fig. 1). Of the tens of short-hard GRBs operate as principal investigator; PAIRITEL fading X-ray “afterglow” that was associated discovered since 1997 only four were narrowly survived the recent forest fire with a GRB. Later detection of radio and localized on the sky to an uncertainty there. The Keck data were acquired through optical afterglows accurately pinpointed smaller than the fields-of-view of moderate our UC Target of Opportunity Override GRB locations in distant galaxies. Since aperture ground-based optical or radio program (with principal investigator Kevin 1997 a great deal has been learned about the telescopes. And none of these positions were Hurley, SSL; and co-I Jason X. Prochaska, origin and physics of the majority of GRBs relayed from the discovering satellite to UCSC). The ad hoc observing team — the so-called “long-soft” GRBs (see Fig. ground-based observers in real-time. The included a bevy of UC observers including 1). The consensus opinion is now that same questions, already answered for long- Berkeley’s own Joseph Hennawi, Michael massive stars are the progenitors of all long- hard bursts, still applied: from what Cooper, Brian Gerke, Jeffery Newman, Alex soft GRBs. The dense centers of stars that distances (galactic or cosmological) do short Filippenko, Ryan Chornock, Dave Pooley, are born with masses significantly more than bursts occur? What is the physics of the and Ryan Foley. our Sun (by more than about ten times) emission mechanisms? What are the What we found was something markedly collapse after they expend their nuclear fuel. energetics of the explosions? Do short bursts different than what is typically found near This implosion leads to an explosion of the even have afterglows? Are supernovae long-soft GRBs: an apparent elliptical galaxy outer layers that glow briefly and brilliantly associated with short bursts? “host” near the center of a cluster of galaxies and are known as supernovae. A small at moderately low-redshift (z = 0.2248). Why is this important? Ellipticals are Fig. 1. Illustration of the diversity in Gamma-Ray comprised almost entirely of old stars and Burst properties. GRBs last from tens of stellar remnants, and by implication of the milliseconds to several hundreds of seconds. Those physical association, this short-hard burst that are more “red” in Gamma-Ray “color” and was then a product of “activity” of old stars; last longer than about 2 seconds comprise the “long- this is in stark contrast to their long-soft soft” class; soft means red. The shorter and “bluer” subclass are the “short-hard” GRBs; blue means cousins that are associated with young stars. hard. GRB 050509b is the first well-studied short- The fact that the GRB position was hard bursts, and has already yielded great insights to offset by about 40 kpc in projection from an the nature of such bursts. We discovered the first elliptical was also an important clue in favor contemporaneous infrared emission from a GRB by of merging remnant models. A neutron star rapidly observing the position of the very long- binary is formed after both stars in a massive duration GRB 041219a. The nomenclature of star binary die in supernova events. Most GRBs is their yymmdd date followed by letter systems are disrupted in the explosions, but a indicating order during day. (continued on page 2) GAMMA-RAY BURST RESEARCH (continued from page 1) few survive to form gravitationally bound binary systems that are ejected from their birthplaces at speeds of several hundred km/s. Over time, the binary orbit decays as the system radiates gravitational energy. After hundreds to thousands of million years, these neutron star pairs coalesce. The combination of delayed merger time and fast motion allows, and basically requires, systems such as these to coalesce far from where they are born (see Fig. 2). No one knows exactly what happens when a double Figure 2: Simulation (left) of a neutron star binary merger event in a massive elliptical galaxy. The binary system travels at high velocity from its birth site and traverses the galaxy for about 1 billion years, trapped in the neutron star system coalesces, but the rest deep potential of the galaxy. Eventually the compact objects merge with each other. At right, our observations mass energy available and dynamical from the Keck telescope showing the X-ray afterglow position of GRB 050509b. The offset from the apparent timescale are well in-line with the properties host galaxy is consistent with this merging neutron star progenitor model. of short-hard gamma-ray bursts. In these observations we have effectively cosmology. A number of us are also coverage of early afterglows and will help us solved the distance scale puzzle (and, by interested in exploring the connection of to identify the redshifts of distant GRBs in extension the energetics question) for short- GRBs to supernovae. Kevin Hurley and near-real time. ◆ hard bursts. From all of the data we and Steve Boggs at SSL continue to explore the Josh Bloom (mail to: [email protected]) others have assembled thus far, the merging high-energy emission of GRBs and related recently joined the UC Berkeley astronomy faculty as neutron star model now looks to be the phenomena. For years, Alex Filippenko, an assistant professor. He arrived from Cambridge, “best bet” for short-hard GRBs. However, Weidong Li, Saurabh Jha and the Katzman MA where he was a Junior Fellow at the Harvard inferences from this one burst are by no Automatic Imaging Telescope (KAIT) team Society of Fellows. He did his graduate work at means the smoking gun for the merging have studied the early afterglow emission at Caltech and Cambridge University. Josh is married to neutron star model. The position is also Ana Bellomo, originally from Barcelona, Spain, and optical wavelengths. The KAIT-PAIRITEL the two are now settling down in their Berkeley consistent with faint blue galaxies at higher telescope pair will provide unprecedented Bungalow home. redshift, and so there is a small chance (<1%) that GRB 050509b arose from greater distances and from perhaps young 2005 COMMENCEMENT stars. New short burst localizations in the year ahead will strengthen our conclusions. ASTRONOMY AWARD WINNERS Maureen Elizabeth Teyssier was awarded the Department Citation for outstanding FUTURE LOOKS BRIGHT FOR GRAY scholarly effort. BURST RESEARCH The Dorothea Klumke-Roberts Award is given annually to an outstanding undergraduate The Berkeley GRB group is now student in Astronomy. The 2005 Award was shared between Mohan Ganeshaligam and reaching a point where we can hope to Charles Conroy. address, in parallel, many of the critical The Mary Elizabeth Uhl Award for distinguished PhD research in Astronomy or Physics open questions. I am very interested in using was given to Mark Reuben Krumholtz for his thesis “Computational and Theoretical GRBs as probes of the distant universe and Investigations of Star Formation” supervised by Chris McKee, and Joshua Simon for his thesis “Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales” supervised by Leo Blitz. SACKLER Julie Comerford, Joe Converse and Katie Peek are this year’s Outstanding Graduate Student Instructors. LECTURE Katie Peek was also presented an award for Teaching Effectiveness, one of just 15 awarded on the campus. ◆ THIS YEAR OUR RAYMOND AND BEVERLY SACKLER DISTINGUISHED LECTURE in Astronomy was given on NEW GRADUATE STUDENTS April 6, 2005. Steven Squyres, Katey Alatalo is a graduate of the minutes from Disneyland. He majored in Professor at Cornell University and the University of Michigan where she worked Astrophysics and Mathematics at Rice Principal Investigator of the Mars on the Robotic Optical Transient Search University. He did his senior research Exploration Rover Mission, presented Experiment (ROTSE). She arrived in the project on gamma-ray bursts and is inter- an inspiring talk on the findings of the summer to work with Josh Bloom.

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