Satellite Telescopes Beam Down Cool Results

Satellite Telescopes Beam Down Cool Results

by Simon Mitton

WHAT'S HOT IN PHYSICS...

Rank	Paper	Citations This Period Sep-Oct 98	Rank Last Period Jul-Aug 98
1	G. Boella, et al., "BeppoSAX, the wide band mission for X-ray astronomy," Astron. Astrophys. Suppl. Ser., 122(2):299-307, 11 April 1997. [7 Italian and Dutch institutions] *WU854	30	†
2	T. Banks, et al., "M theory as a matrix model: a conjecture," Phys. Rev. D, 55(8):5112-28, 15 April 1997. [Rutgers U., Piscataway, NJ; U. Texas, Austin; Stanford U., CA] *WV250	28	1
3	H.Y. Hwang, et al., "Spin-polarized intergrain tunneling in La₂/₃Sr₁/₃MnO₃," Phys. Rev. Lett., 77(10):2041-4, 2 September 1996. [Lucent Technol., Murray Hill, NJ; Princeton U., NJ] *VE351	25	†
4	M.F. Kessler, et al., "The infrared space observatory (ISO) mission," Astron. Astrophys., 315(2):L27-31, November 1996. [5 European institutions] *VU266	25	†
5	G. Boella, et al., "The medium-energy concentrator spectrometer on board the BeppoSAX X-ray astronomy satellite," Astron. Astrophys. Suppl. Ser., 122(2):327-40, 11 April 1997. [Inst. Cosmic Phys. Relat. Technol., Milan, Italy; Inst. Cosmic Phys. Appl. Info., Palermo, Italy; U. Milan, Italy] *WU854	22	†
6	H.L. Lai, et al., "Improved parton distributions from global analysis of recent deep inelastic scattering and inclusive jet data," Phys. Rev. D, 55(3):1280-96, 1 February 1997. [Michigan St. U., Lansing; Argonne Natl. Lab., IL; Southern Methodist U., Dallas, TX; Florida St. U., Tallahassee; U. Oregon, Eugene] *WF297	20	†
7	J.P. Perdew, K. Burke, M. Ernzerhof, "Generalized gradient approximation made simple," Phys. Rev. Lett., 77(18):3865-8, 28 October 1996. [Tulane U., New Orleans, LA] *VP225	20	†
8	W.W.M. Allison, et al., "Measurement of the atmospheric neutrino flavor composition in Soudan 2," Phys. Lett. B, 391(3,4):491-500, 16 January 1997. [6 U.S. and U.K. institutions] *WE121	20	†
9	S. Nakamura, et al., "InGaN/GaN/AIGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate," Appl. Phys. Lett., 72(2):211-3, 12 January 1998. [Nichia Chem. Ind. Ltd., Tokushima, Japan] *YQ549	20	†
10	Y. Narukawa, et al., "Role of self-formed InGaN quantum dots for exciton localization in the purple laser diode emitting at 420 nm," Appl. Phys. Lett., 70(8):981-3, 24 February 1997. [Kyoto U., Japan; Nichia Chem. Industries, Ltd., Tokushima, Japan] *WJ626	19	4

SOURCE: ISI's Hot Papers Database. Read the full legend.

The Physics Top Ten changes its character dramatically this period, as X-ray astronomy grabs #1 and #5 in a single bound, while infrared astronomy keeps up the heat in #4. Descriptions in these newcomers of new European satellite telescopes and their instruments have crowded out most of the string theorists for the present. The high rates of citation are down to the dozens of papers reporting dramatic new observations of phenomena as diverse as gamma-ray bursts, clusters of galaxies, exploding stars, and star birth.

BeppoSAX, subject of #1 and #5, is an Italian-Dutch X-ray astronomy mission with wide spectral coverage from 0.1 - 300 keV. On board are two wide field cameras, and four narrow field instruments for detailed investigations. The wide field cameras, plus a gamma-ray burst monitor, are aimed at discovering transient phenomena. Launched on April 30, 1996 into a 600 km orbit, BeppoSAX has a nominal lifetime of five years. The satellite has excellent pointing accuracy; this advantage, coupled with unprecedented energy band, and good spatial and temporal resolutions, has led to a rich harvest of discoveries.

The greatest prize has been solving the puzzle of gamma ray bursts (GRBs): are they local or cosmological? Science Watch turned to Cettini Maccarone of the Italian National Research Council, Palermo. She highlighted "the historic breakthrough in the quest for the origin of GRBs, giving confirmation of their extragalactic nature. This came about thanks to the extremely effective combination of four instruments (the GRB monitor, a wide field camera, and two spectrometers) and the flexibility of operation. Fast determination of the position of a GRB led to the discovery of an X-ray afterglow from the burst site, and the source position indicated that GRBs come from remote galaxies. For this achievement the BeppoSAX team received the 1998 Bruno Rossi Prize from the American Astronomical Society." The medium energy spectrometer (#5) provided the vital link with the afterglow discovery in December 1997.

Many serendipitous X-ray sources have turned up in the wide field and deep surveys. BeppoSAX has notched up 400 new sources in 50 square degrees of sky. Main astrophysical environments currently under scrutiny include the hot coronae around stars, spatially resolved observations of supernova remnants to get their temperature profiles, and non-thermal X-ray sources at the centers of compact galaxies and active galactic nuclei.

While BeppoSAX detects photons from hot spots in the universe, the European Space Agency’s International Space Observatory (ISO) was designed for cool sites. ISO (described in #4) was the first true orbiting infrared observatory, which ceased operations on April 8, 1998 when its helium cryostat ran dry. With up to 1,000 times the sensitivity and 100 times the angular resolution of its predecessor, ISO gathered a treasure trove of data: in 29 months it logged 26,000 observations by astronomers in 19 countries. A fantastic waterworld in space came under ISO's gaze.

As Martin Kessler (European Space Agency, Astrophysics Division, Madrid), the lead author of #4 tells Science Watch, "ISO, which lasted almost a year longer than specified, was a great scientific and technical success. Its 60cm-diameter telescope was equipped with four versatile instruments, cooled by superfluid liquid He to 2-4 K. ISO's scientific harvest includes the following. It showed us that water is ubiquitous in the cosmos by measuring water vapor transitions in objects such as Mars, Titan (Saturn’s largest moon), the giant planets, comets, the cold interstellar medium, and circumstellar envelopes." Water vapor detected in the spectrum of the old star W Hydrae confirmed a prediction that highly-evolved stars puff out large quantities of water.

Kessler adds, "ISO made detailed investigations of star-forming regions in our own and external galaxies. These included detection of pre-stellar cores and measurement of the distribution function for the initial masses of stars. There are substantial numbers in the brown dwarf range." Brown dwarfs are very faint starlike objects, some 10 or more times the mass of Jupiter.

Star birth in the dark cloud Lynds 1689, was seen through the initial warming of a cloud of gas and dust twice the mass of the Sun, which reached 13 K as a prelude to true collapse and eventual star formation. ISO has also given a clear picture of dusty rings around the nearby young stars. The temperature and mass show the rings are made of very cool dust. With a mass 0.1 - 20 times that of the Moon, these rings are comparable in size to the dust ring at the edge of the solar system, but with much less mass.

The theme of star birth continues with ISO data on remote galaxies, which is able to distinguish between black holes or starbursts as the energy source in ultra-laminas galaxies. Furthermore it can uncover star formation that is hidden by dust from the view of optical telescopes. The most distant galaxies probed show what star formation was like when the universe had only 1/8th its present age. In the Hubble Deep Field, ISO found star formation rates far higher than in nearer normal galaxies. This suggests that galaxies do evolve over time, with their star formation rates slowing with increasing age.

Dr. Simon Mitton is Director of Science and Professional Publishing
at Cambridge University Press, Cambridge, UK.