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BURSTS AND How USRA scientists helped make major advancements in high- .

During the 1960s, the second Administrator Frank J. Kerr (1918 - 2000) of the University of NASA, James E. Webb, sought a university- of Maryland was appointed by USRA to based organization that could serve the manage its programs in and needs of NASA as well as the astrophysics. Kerr was a highly-regarded community. In particular, Webb sought to astronomer, originally from Australia. He had have university researchers assist NASA in been the Director of the Astronomy Program the planning and execution of large, complex at the University projects. The result of Webb’s vision was of Maryland, the Universities Space Research Association and at the time (USRA), which was incorporated as a non- of his USRA proft association of research universities on appointment 12 March 1969. in 1983, he was Provost of As described in the previous essay, USRA’s the Division of frst major collaboration with NASA was the Physical and Apollo Exploration of the . The vehicle Mathematical by which USRA assisted NASA and the space Sciences and research community was the Lunar Engineering at Institute, later renamed the Lunar and the University. Frank Kerr Planetary Institute. In support of Another major project was undertaken in MSFC and NRL, USRA brought astronomers 1983, when USRA began to support NASA to work closely with NASA researchers in in the development of the Space the development of instrumentation and Project at NASA’s Marshall Space Flight the preparation for analyses of data for Center (MSFC). USRA also began to support various projects, including the MSFC’s Burst the Naval Research Laboratory (NRL) as and Transient Source Experiment (BATSE), they worked on the Oriented Scintillation which was another of the experiments being Spectrometer Experiment, which was one of planned for the GRO. four major experiments to be launched on NASA’s (GRO).

(Frank Kerr portrait credit: University of Maryland. Background Image: NASA’s Goddard Space Flight Center/S. Wiessinger, USRA)

Essay by W. David Cummings. Design by Erin Senoz. Gamma-ray bursts had been detected in the late 1960s and early To fnd the source of gamma-ray 1970s by the satellites, which had been placed in orbit around bursts, Dr. Gerald Fishman of the Earth to watch for possible violations of the 1963 treaty that NASA proposed the BATSE as banned tests in the atmosphere, in outer space part of the GRO (later re-named and under water. While the satellites were emplaced to monitor the Compton Gamma Ray compliance with the treaty by the Soviet Union, researchers at the Observatory (CGRO), in honor of Los Alamos National Laboratory quickly realized that the gamma-ray Arthur Holly Compton). bursts did not come from Soviet nuclear explosions. In 1973, they The CGRO was one of NASA’s “Great reported that the analysis of Vela satellite data over the three-year ;” a series of large period July 1969 - July 1972 demonstrated that bursts of gamma rays space observatories designed to as short as 0.1 second and as long as 30 seconds did not come from examine the Universe from the the Earth or the .1 to gamma- ray regime.2 So where did they come from? The energy carried by a is proportional to the of the photon, or inversely proportional Fishman turned to its . Gamma rays have the highest frequency and to USRA to help shortest wavelength of all the . Their wavelength is him build a team comparable to the size of an . The energy of a gamma that would work on various Chryssa Kouveliotou ray is hundreds of thousands times the energy of a photon in the aspects of BATSE. In January BATSE Team “visible” range, i.e., a photon that humans can see. The Earth’s 1991, USRA hired Dr. Chryssa atmosphere protects human and other life from the otherwise lethal Kouveliotou to work with Fishman and others on the BATSE team effects of gamma rays and other , so that at MSFC in Huntsville, Alabama. Dr. Kouveliotou was an expert on gamma rays can only be observed from space. Because of their high gamma-ray bursts, being the frst student with a PhD thesis on the energy, gamma rays cannot be easily focused, which means that subject. She took a leave of absence from the University of Athens, localizing gamma-ray sources is diffcult. Greece, to help develop BATSE data analysis software and to analyze and interpret these data.

Gerald Fishman with one of the BATSE detectors. (NASA)

(BATSE images and results: courtesy of NASA)

There wasn’t much time for development work. CGRO and BATSE were launched on The frst paper by members of the BATSE f these sources really board the space shuttle Atlantis on 5 April team, including Kouveliotou (who had are associated with 1991. The BATSE instrument was activated emigrated to the U.S. and remained a I on 21 April 1991, and it began to record USRA scientist) was published in distant , then gamma-ray bursts at a rate of about one per on 9 January 1992.3 The team ran tests on day. the data from 153 gamma-ray bursts that the BATSE results occurred between 21 April 1991 and 31 To further explore the question about where October 1991. The results demonstrated imply that gamma-ray gamma-ray bursts were coming from, as analytically that there is no statistically well as other questions about gamma-ray signifcant deviation from isotropy.4 bursts are some of the bursts, USRA and NASA co-sponsored the frst Huntsville Gamma-Ray Burst Workshop brightest explosions in the fall of 1991. Over 130 scientists from since the Big Bang. around the world attended and discussed their research in the context of the recently acquired BATSE data. Prior to BATSE, scientifc consensus was that the sources for gamma-ray bursts were in our own , As more gamma-ray bursts were observed the . In the simplest scenario, this over the life of the CGRO, the isotropic would predict that a plot of the positions of distribution was confrmed. During the entire the gamma-ray bursts would show most of mission, BATSE observed 2,704 gamma-ray 5 them near the disk of the Milky Way. Perhaps bursts. the most important point to come out of this frst Gamma-Ray Burst Workshop was that The BATSE results suggested that the gamma-ray bursts seemed to be coming sources of the gamma-ray bursts must from sources that are randomly and evenly be “at cosmological distances,” i.e., at distributed across the sky, i.e., the distribution distances beyond our Milky Way galaxy, seemed to be isotropic. These results and the event distribution should follow the challenged the galactic nature of the events isotropic distribution of distant galaxies. If the and argued for an extragalactic origin of the sources really were associated with distant phenomenon. galaxies, then the BATSE results implied that gamma-ray bursts are some of the brightest high velocity In 1992, the same year that the BATSE team explosions since the Big Bang. Some form a distant, previously published its paper showing the isotropic researchers argued at the time, however, that unknown Galactic “corona.” distribution of gamma-ray bursts, Robert the sources might be neutron stars in a halo This distant corona is isotropic Duncan of the University of Texas and around our galaxy. when viewed from Earth, and Christopher Thompson of the University of consequently, the population Toronto published a paper titled, Formation The theory is plausible, in part, because of of neutron stars in it can of Very Strongly Magnetized Neutron Stars: the extraordinary characteristics of neutron easily explain the angular and Implications for Gamma-ray Bursts.11 In this stars. A neutron is the remnant of a brightness distribution of the paper, they introduced the term ‘’ massive star that has used up its inner BATSE bursts…6 to designate a highly magnetized neutron nuclear fuel, the burning of which kept it star, i.e., a with a magnetic feld supported against the force of its self-. Later the same year, X-ray and optical of 1014 – 1015 , which is 100 – 1000 Once the nuclear fuel has been spent, the observations showed that some gamma-ray times greater than the magnetic feld strength collapses into an incredibly bursts were indeed associated with distant of a typical neutron star. It is unimaginably dense ball in which and are galaxies. In one case, the gamma-ray burst larger that the magnetic feld strengths of our squeezed together into and other was from a star in a galaxy about 7 billion everyday experience, for example, the less sub-atomic species. As the core collapses, -years distant; 7and in another case, than 1 gauss feld strength of the Earth, and the down-rushing, outer layers of the star about 12 billion light-years.8 The implication the typically 100 gauss feld strength of a collide with the neutron core, and then of observing gamma-ray bursts at these great magnet used to post notes on the door of a rebound, producing an explosive outpouring distances is that their sources must generate, refrigerator. of energy called a “.” The Crab in a of seconds, more energy than is an example of this process; a the Sun could emit over billions of years, In 1996, Thompson and Duncan published massive star that exploded in our galaxy as assuming that the gamma-rays are emitted at another paper in which they suggested a supernova and was observed by Chinese the same rate, in all directions. that the outbursts of SGRs are the result astronomers in 1054 CE. The neutron star of large fractures of a magnetar’s crust,12 left behind by the Crab supernova is about 30 Given this huge energy fow, perhaps the a “starquake” similar to the sudden energy kilometers across, has a rate of about 30 most interesting question is “How are these released by earthquakes, but trillions of times revolutions per second, and is located near distant gamma-ray bursts produced?” But the more powerful. the center of the nebula. BATSE team realized that there are varieties of gamma-ray bursts, so they frst tried to To check the Thompson and Duncan The halo theory posits that, depending on identify some of the categories. Kouveliotou magnetar theory, Kouveliotou needed to be the energy of the explosion and whether the was the author for the team on a paper able to measure the SGR’s magnetic feld explosion was asymmetric, a supernova could published in 1993 titled Identifcation of Two strength. The standard process for measuring impart a high-velocity kick suffcient to propel Classes of Gamma-ray Bursts.9 In it, she and the magnetic feld strength at the magnetic a neutron star from the Milky Way galaxy. her BATSE colleagues demonstrated that the pole of a spinning neutron star is based These ejected neutron stars could form a short bursts of gamma-rays, defned as those upon the assumption that the star is slowing nearly isotropic halo around the galaxy, and with durations less than 2 seconds, had a down because its energy is being radiated in principle, these ejected neutron stars could larger proportion of higher energy photons away. The idea is that if the magnetic axis is produce the distribution of gamma-ray bursts than the bursts with longer durations. The not aligned with the rotation axis, a distant seen by BATSE. The principal proponent of short bursts were said to have “hard energy observer sees a time-changing magnetic this theory was Donald Lamb of the Enrico spectra,” while the bursts of longer duration feld, which corresponds to an electric feld. Fermi Institute of the University of Chicago. In were said to have “soft energy spectra.” The combination of oscillating electric and 1997, Lamb wrote: magnetic felds results in the radiation, In addition, there were a small number of called magnetic dipole radiation. Based We do not yet know the gamma-ray bursts that were found to repeat upon this assumption, the neutron star distance scale to gamma-ray at irregular intervals. They typically had a very surface magnetic feld strength at the pole bursts. Here I discuss several short (a fraction of a second) high-energy of the spinning, magnetized neutron star observational results and burst followed by a longer, fainter glow of can be determined if the period of rotation theoretical calculations which lower-energy X-rays. They became known as and the rate of change of the period can be provide evidence about the Soft Gamma Repeaters (SGRs).10 Kouveliotou determined.13 distance scale. First, I describe suggested that SGRs represent a completely the recent discovery that different phenomenon from either of the Fortunately, these rotation rates can be many neutron stars have high other two categories of gamma-ray bursts, determined for some spinning neutron stars enough velocities to escape and she set out to understand them. when the spin axis of the neutron star is not from the Milky Way. These aligned with the magnetic axis. To a distant

(Background image of : courtesy of NASA) observer, the radiation from such a neutron the X-ray powering SGR 1806-20 was a star appears in pulses for the same reason magnetar. that pulses of light appear to come from a lighthouse. The lighthouse lamp emits a The origin of gamma-ray bursts remains a steady beam, but the lamp rotates so that subject of intense scientifc enquiry, but most the observer sees fashes of light from the astrophysicists agree that the origin of SGRs has lighthouse. Similarly, a spacecraft that been solved because of the work of Kouveliotou, can detect X-rays in a certain energy range Thompson, and Duncan. SGRs are produced by will see pulses of X-rays coming from a so- “starquakes,” the cracking and re-forming of the called “X-ray pulsar” because the neutron crust of highly-magnetized neutron stars that are star emitting the radiation is rotating. in the Milky Way galaxy or in a nearby galaxy.

Kouveliotou and her BATSE teammates Astrophysicists also agree that magnetars are studied radiation from an SGR that important because of the extreme environment was one of three discovered in 1979, they create. The U. S. National High Magnetic designated as SGR 1806-20 (the numbers Field Laboratory (MAGLAB) has produced give the astronomical coordinates that magnetic feld strengths up to 4.5 x 105 gauss. Neutron star emitting radiation along its magnetic axis while rotating along another axis. (Bill Saxton, NRAO/AUI/NSF)

locate the object in the sky). Kouveliotou The magnetic feld strength of SGR-1806-20 and her colleagues started the investigation he origin of is larger than that by a factor of 1.8 trillion. In by using data from the Rossi X-ray Timing an article for Scientifc American written in Explorer (RXTE), a U.S. satellite launched Tgamma-ray bursts 2003, Kouveliotou, Thompson, and Duncan on 30 December 1995 that was designed briefy discussed the extreme environment of specifcally to study time-varying, X-ray remains a subject magnetars: emission from cosmic sources. In November 1996, they used RXTE to measure the of intense scientific In such strong felds, bizarre things happen. change in the X-ray emission from SGR enquiry, but most X-ray photons readily split in two or merge 1806-20. They found that the X-ray together. The vacuum itself is polarized, emission came in pulses, and they carefully astrophysicists agree becoming strongly birefringent, like a measured the time between the pulses. calcite crystal. are deformed into They found that the period of rotation for that the origin of Soft long cylinders thinner than the quantum- the neutron star was 7.476551 seconds. relativistic wavelength of an .15 They conducted the same analysis on Gamma Repeaters archived data from the Japanese Advanced has been solved For his contributions to gamma-ray astronomy Satellite for Cosmology and Astrophysics through BATSE, Gerald Fishman was awarded (ASCA) spacecraft. In this analysis, they because of the work the Rossi Prize of the High Energy Astrophysics found the period of SGR 1806-20 observed Division of the American Astronomical Society in in 1993 to be 7.4685125 seconds. The of Kouveliotou, 1994. He also won the Shaw Prize, which some period determined from the 1993 data was consider to be the Asian equivalent of the Nobel slightly shorter than the period from the Thompson, and Prize, in 2011. 1996 data, thus the spinning neutron star was observed to be slowing down at the Duncan. For her work in proving the existence of rate of 0.0026 seconds per year.14 magnetars, Chryssa Kouveliotou shared the 2003 Rossi Prize with Christopher Thompson Kouveliotou determined the magnetic feld and Robert Duncan. She was elected to the U. S. strength of SGR 1806-20 by using her spin National Academy of Sciences in 2013. rate values with a formula that was based on the assumption that the neutron star The pioneering research of these scientists was slowing down because it was radiating exemplify USRA’s role in realizing James Webb’s electromagnetic energy in the manner of vision: to bring together the best minds in a spinning dipole. She found the magnetic academe, government agencies and other feld strength of SGR 1806-20 to be organizations in the development of knowledge approximately 8 x 1014 gauss, much larger Artistic rendering of a magnetar, with its surface in space science and technology. than the magnetic feld associated with any cracks and strong magnetic feld. (NASA/GSFC Conceptual Image Lab) other known X-ray pulsar, confrming that 1 Klebesadel, R. W., Strong, I. B., and Olson, R. A. (1973). Observatons of Gamma-Ray Bursts of Cosmic Origin. The Astrophysical Journal, 182, L85-L88. 2 The Great Observatories also include the Hubble , the , and the Chandra X-ray Observatory. 3 Meegan, C. A., Fishman, G. J., Wilson, R. B., Paciesas, W. S., Pendleton, G. N., Horack, J. M., Brock, M. N., & Kouveliotou, C. (1992). Spatal distributon of γ-ray bursts observed by BATSE. Nature, 355(6356), 143-145. 4 In the accompanying fgure, which was presented in the BATSE team’s paper, the format of the plot allows for a two-dimensional presentaton of the whole sky, from the north to the south galactc pole in lattude and a full 360 degrees in galactc longitude. The fgure shows the positons of the 153 gamma-ray bursts. 5 The accompanying fgure shows the distributon for the 2,704 gamma-ray bursts observed, color-coded for the energy of the bursts. 6 Lamb, D. O. (1997). The distance scale to gamma-ray bursts. Reviews in Modern Astronomy, 10, 101-126; p. 101. 7 Metzger, M. R., Djorgovski, S. G., Kulkarni, S. R., Steidel, C. C., Adelberger, K. L., Frail, D. A., Costa, E., & Frontera, F. (1997). Spectral constraints on the redshif of the optcal counterpart to the γ-ray burst of 8 May 1997. Nature, 387(6636), 878-880. 8 Kulkarni, S. R., Djorgovski, S. G., Ramaprakash, A. N., Goodrich, R., Bloom, J. S., Adelberger, K. L., Kundic, T., Lubin, L., Frail, D. A., Frontera, F., Feroci, M., Nicastro, L., Barth, A. J., Davis, M., Filippenko, A. V., & Newman, J. (1998). Identfcaton of a host galaxy at redshif z = 3.42 for the γ-ray burst of 14 December 1997. Nature, 393(6680), 35-39. 9 Kouveliotou, C., Meegan, C. A., Fishman, G. J., Bhat, N. P., Briggs, M. S., Koshut, T. M., Paciesas, W. S., and Pendleton, G. N. (1993). Identfcaton of two classes of gamma-ray bursts. The Astrophysical Journal, 413, L101-L104. 10 Schilling, G. (2002) Flash!: The hunt for the biggest explosions in the universe. (Translated by Naomi Greenberg-Slovin.) Cambridge University Press, Cambridge, ; p. 199. 11 Duncan, R. C. and Thompson, C. (1992). Formaton of very strong magnetzed neutron stars: Implicatons for gamma-ray bursts. The Astrophysical Journal, 392, L9-L13. 12 Thompson, C. and Duncan, R. C. (1996). The sof gamma repeaters as very strongly magnetzed neutron stars. II Quiescent , X-ray, and Alfvén emissions. The Astrophysical Journal, 473, 322-342. 13 Michel, F. C. (1991) Theory of Neutron Star Magnetospheres, University of Chicago Press. 14 Kouveliotou, C., Dieters, S., Strohmayer, T., van Paradijs, J., Fishman, G. J., Meegan, C. A., Hurley, K., Kommers, J., Smith, I., Frail, D., & Murakami, T. (1998) An X-ray pulsar with a superstrong magnetc feld in the sof γ-ray repeater SGR1806 – 20, Nature, 393(6682), 235-237; p. 235. 15 Kouveliotou, C., Duncan, R. C., & Thompson, C. (2003) Magnetars. Scientfc American, February, pp. 35-41; p. 40.