ray Astrophysics Facility (AXAF) satellite and a variety of 

 other X-ray instruments, as well as the Submillimeter Wave 

 Astronomy Satellite (SWAS). 



Further, the Science Education Department conducts several 

 programs designed to improve the teaching of precollege 

 science and mathematics, partly through the use of examples 

 from astronomy. These programs include the development of 

 curriculum materials and videos, and the training of precollege 

 educators. 



Numerous facilities serving the general astronomical com- 

 munity are located at the CfA in Cambridge as well. The 

 Institute for Theoretical Atomic and Molecular Physics, 

 established in 1988 to attract and encourage talented graduate 

 students to enter this field, emphasizes study of fundamental 

 questions in atomic and molecular physics. Other services in- 

 clude the International Astronomical Union's Central Bureau 

 for Astronomical Telegrams and the Minor Planet Center, 

 both of which disseminate information on astronomical dis- 

 coveries worldwide. The gateway for SIMBAD, an internation- 

 al astronomical computer database, is also located at the 

 Cambridge site, as is Harvard's extensive collection of 

 astronomical photographic plates, the largest in the world. In 

 addition, SAO operates the Astrophysics Data System (ADS) 

 on behalf of NASA. 



The National Aeronautics and Space Administration has 

 selected SAO to operate both the flight control center for 

 AXAF and the AXAF Science Center, which will receive, 

 analyze, and archive data from the spacecraft. SAO already 

 provides Guest Observer Facilities for investigators using the 

 Roentgen X-tay satellite (ROSAT), a ]oint venture of Ger- 

 many, the United Kingdom, and the United States. 



Research Highlights 



Research during the past year (October I, 1996, to September 

 30, 1997) included these highlights, detailed by CfA division. 



Atomic and Molecular Physics 



The Atomic and Molecular Physics Division maintains re- 

 search programs in three general areas: laboratory 

 astrophysics; fundamental atomic, molecular, and optical 

 physics; and atmospheric physics. 



AMP scientists extended their measurements of molecular 

 oxygen photoabsorption cross sections of the Herzberg I, II, 

 and III band systems. A detailed understanding of these bands 

 is important for interpretation of satellite-based observations 

 of : nightglow and for overall understanding of the oxygen 

 chemistry in the upper mesosphere and lower thermosphere. 

 These measurements will complement the definitive work by 

 these scientists on the molecular oxygen Schumann-Runge 

 bands and continua, which utilizes the vacuum-ultraviolet 

 Fast Transit Spectrograph at the Physics Department of Im- 

 perial College, London. 



Researchers developed and operated a second-generation 

 dual noble gas maser, containing dense, cohabiting ensembles 



of 'He and '"'Xe atoms. The maser population inversions for 

 the 3 He and '"'Xe ensembles are created by spin exchange col- 

 lisions between the noble gas atoms and optically pumped 

 rubidium vapor. Significantly improved frequency measure- 

 ment precision was achieved, under 100-nanohertz uncertainty 

 on the '"'Xe Zeeman transition in about 2 hours of signal 

 averaging. This performance should allow for a world-class 

 search for a '" Xe permanent electric dipole moment as a test 

 of time-reversal symmetry in elementary particle interactions. 



The same researchers have been involved in clinical studies 

 on the use of polarized noble gas for magnetic resonance imag- 

 ing. (Medical applications are limited by the availability of 

 polarized gas.) Recently, the CfA group developed a system to 

 produce approximately one liter per hour of polarized 3 He or 

 '"'Xe gas, following a design for which they received a patent 

 with their University of Michigan colleagues in 1997. They 

 also developed a sensitive Adiabatic Fast Passage system to 

 measure noble gas polarization in order to investigate the factors 

 that limit '"'Xe polarization created using diode laser arrays. 



Division scientists report improvements in stratospheric 

 (ozone layer) measurements from the SAO balloon-based Far 

 Infrared Spectrometer (FIRS-z). Researchers are now able to 

 make comprehensive measurements of the important reactive 

 chemistries affecting stratospheric ozone balance and to deter- 

 mine which types of chemistry predominate at the various al- 

 titudes in the sttatosphere. Through improved underlying 

 spectroscopy, SAO is now able to measure the important, but 

 elusive, chlorine nitrate compound. Researchers have success- 

 fully flown the instrument with improved mid-infrared 

 response, permitting measurements of several additional im- 

 portant ozone-layer and greenhouse gases. 



High Energy Astrophysics 



The High Energy Astrophysics Division develops instrumen- 

 tation for making observations from balloons, rockets, and 

 satellites of high-energy (primarily X-ray) processes in the 

 universe and analyzes and interprets the resultant data. 



Observations of six X-ray novae (XN) provide rather clear- 

 cut evidence for the existence of black holes: In each case, the 

 large and secure value of the mass function, fM > 3 M®, estab- 

 lishes a hard lower limit to the mass of the compact primary, 

 which exceeds the maximum mass of a neutron star. Now, 

 new evidence gathered and interpreted by CfA scientists ap- 

 pears to confirm that the collapsed stars in these XN possess 

 an event horizon — the defining property of a black hole. This 

 conclusion is based on the advection-dominated accretion 

 flow (ADAF) model and on a comparative study of nine XN, 

 four with black holes and five with neutron stars. X-ray data 

 for these nine XN showed, without exception, that a black 

 hole primary has a larger luminosity swing from quiescence to 

 outburst than a neutron star primary. This result is as ex- 

 pected if the former objects have evenc horizons that hide 

 most of the thermal energy and if the latter objects lack an 

 event horizon and must therefore re-radiate all the thermal 

 energy that they accreted. 



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