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Columbia University Departments of Astronomy and Physics New York, New York 10027 ͓S0002-7537͑99͒00801-X͔ This report covers the period September 1997 through During 1997-8 the activities of the Center for Backyard August 1998 and comprises an account of astronomical Astrophysics expanded. This is a network of astronomers, research carried out in the Department of Astronomy and the primarily amateur, who do stellar photometry with small Department of Physics. telescopes in their backyards. We typically observe a star Faculty and Research Associates were James Applegate, steadily for a few months, trying to amass the densest pos- Elena Aprile, Norman Baker, William Craig, Arlin Crotts, sible coverage by stressing long observation and distribution Karl-Ludwig Giboni, Charles Hailey, Jules Halpern, David of observers in longitude. This provides a time series well Helfand, Philip Kaaret, Stephen Kahn, Marc Kamionkowski, suited to the study of periodic signals, and immunized from Laura Kay ͑Barnard͒, Karen Leighly, Lloyd Motz ͑Emeri- the ‘‘aliasing’’ problems inherent in data from a single site. tus͒, Reshmi Mukherjee ͑Barnard͒, Robert Novick ͑Emeri- The principal observers are in Belgium, Denmark, Maryland, tus͒, Frederik Paerels, Joseph Patterson, Kevin Prendergast, Arizona, Illinois, New Zealand, and Australia. Occasional Andrew Rasmussen, R. Michael Rich, Malvin Ruderman, contributions also arrive from Russia, South Africa, Israel, Daniel Savin, Edward Spiegel, Marco Tavani, Wilhelmus Japan, and Chile. A grant from the Research Corporation van der Veen, and Jacqueline van Gorkom. enabled us to make this expansion in the scale of activity. Graduate students participating in research were Elizabeth Patterson and Kemp spent most of their time on this enter- Blanton, Ari Buchalter, Alex Casti, Tzu-Ching Chang, prise. Most programs involve the study of cataclysmic bina- Xuelei Chen, Xinzhong Chen, Jean Cottam, Catherine Cress, ries, justly famous for the many periods frequently present in Deepa Majmudar, Karl Forster, Akimi Fujita, Mario their light curves. Jimenez-Garate, Himel Ghosh, Ming Feng Gu, JaeSub Hong, Our study of the long-dormant dwarf nova EG Cancri Justin Howell, John Keck, Tomotake Kozu, Kaya Mori, Don revealed orbital and ‘‘superhump’’ periods of 86.36 and Neill, Masao Sako, Adrianne Slyz, Edgar Smith, Joshua 86.90 minutes, respectively. The offset by 0.6% is the small- Spodek, Ben Sugerman, John Tomsick, Robert Uglesich, est ever seen in a cataclysmic variable, and lends support to Matt Umurhan, Leven Wadley, Frank Wang, and Fang Xu. the idea that CVs of very short period lose thermal equilib- Undergraduates participating in research were Ian Adler, rium as their thermal timescales become longer than the ti- Scott Brown, Arindam Chatterjee, Elliott Eggleston, Justin mescale for gravitational radiation. In this theory, the sec- Detray, Irina Feygina Barnard , Susan Kassin, Ali Kinkha- ͑ ͒ ondary star has a mass of 0.02 Ϫ 0.03M ᭪ , but a radius of bwala, Miriam Krauss, Andreea Petric ͑Barnard͒, Yong 0.10R᭪ . WZ Sagittae appears to be another star of this type. Moon, Scott Schnee, Charles Silver, Nigel Singh ͑Cornell͒, We studied the period excesses of all superhumping CVs, Lucianne Walkowicz ͑Hopkins͒, Beth Willman and Vanessa and found that the period excess appears to be a good signa- Yuille ͑Barnard͒. ture of the mass ratio. These CVs with a very low secondary- High school teachers participating in research were star mass exist and are even fairly numerous, possibly repre- Rachel Berger, Emily James and Britt Reichborn-Kjen- senting as much as 30% of all CVs. nerud. We also found two stars, AL Com and CP Eri, which Alex Casti, Catherine Cress, Eric Ford, Karl Forster, show large-amplitude superhumps at quiescence, contrary to Deepa Majmudar, David Schiminovich, Adrianne Slyz, Matt all known rules. These are likely to come from excitation of Umurhan, Frank Wang and Fang Xu received Ph.D. degrees. an instability at the 2:1 orbital resonance in the accretion Appointments during 1997–98 were held by Adjunct Pro- disk. fessor Michael Allison, Postdoctoral Research Scientists Large data sets, comprising typically ϳ 200 hr over ϳ 50 Valeri Egorov, Charles Liu, Uwe Oberlack, Francesco Pa- nights, have been collected on many other short-period CVs, parella, Frank Summers, Louis Tao, Ion Yadigaroglu, with the purpose of studying these precessional effects in Melinda Weil and Ming Zhao. disks. The most useful results come from the study of per- Marc Kamionkowski received the Helen B. Warner Prize manent superhumps in novalike variables. Essentially all for Astrophysics from the American Astronomical Society. novalike variables with Porb Ͻ 3.5 hr show superhumps. Their study and understanding will keep the group busy for years to come. 1. STARS & STELLAR EVOLUTION Rich, in collaboration with Jay Frogel ͑OSU͒ and A. Ren- Crotts, along with S. Heathcote ͑CTIO͒, have measured zini ͑ESO͒ has undertaken 3 large programs with the NIC- the expansion velocity of the outer circumstellar rings of SN MOS camera on board HST. The first is to image the Galac- 1987A, and the circumstellar gas just beyond. All three rings tic bulge at Ϫ 6deg latitude as deeply as possible, with the in the SN 1987A nebula appear to have the same kinematic aim of reaching the hydrogen burning limit at M H ϭ age. Crotts and Heathcote, along with J. Maza and M. Phil- ϩ 10. Preliminary reduction of the data shows that a very lips ͑CTIO͒, have also recorded changes in the spectrum of tight infrared lower main sequence has been measured, but SN 1987A in response to the impact of the ejecta upon this not down to the brown dwarfs as had been hoped. The other circumstellar nebula. two programs involve study of the stellar populations and 44 ANNUAL REPORT globular clusters in the central kpc of the M31 bulge. Prior the ISOPHOT instrument aboard the ISO satellite. After the ground-based studies find some evidence for extremely completion of the ISO mission in April 1998, twelve out of bright AGB stars, perhaps proving the presence of an inter- fourteen scheduled observations were actually done. For five mediate age population there. It has been argued that these sources we have data at 90 and 160 micron, for one source apparently bright stars are blends of stars, not intrinsically only at 90 micron and for another one only at 160 micron. luminous objects. The NICMOS imaging is intended to settle The images are roughly 15 x 35 arcmin in size and show the this debate, which has implications for the ages of spheroid distribution of dust as illuminated by the central mass losing populations in general. stars. This inner parts of the shells are clearly detected but it Rich, in collaboration with Beatriz Barbuy ͑Universidade is not clear how far out we are able to detect the dust emis- de Sao Paulo͒ has studied the Galactic Center globular clus- sion. We are currently trying to better understand the instru- ter NGC 6558. This cluster appears to be similar to NGC ment and observing mode in order to determine how far out 6522 in having a heavily populated blue horizontal branch we are still detecting the extended dust shell. and a remarkably sparsely populated red giant branch. The Van der Veen in collaboration with Martin Groenewegen cluster is a core-collapse globular cluster that has experi- ͑MPE, Munich͒, Alain Omont ͑IAP, Paris͒ and B. Lefloch enced much disk and bulge shocking in its lifetime. How- ͑IRAM͒ observed an additional six evolved mass losing ever, it is virtually impossible to understand how any process stars. Combined with observations made in 1996 and 1997 could modify the population of the cluster by transforming we now have a total sample of 16 stars which are mapped at red giants into blue HB stars, since the latter must ignite 1300 micron using the bolometer array at the IRAM 30m helium in their cores. This would be very hard to do if the telescope at ͑Pico Veleta, Spain͒. The maps are about 1 arc- envelope of an RGB star were stripped in a collision with min square in size and are obtained with a 11Љ FWHM beam. another star. The similarity of NGC 6558 with NGC 6522 These data enable us to study the most recent mass loss and other Galactic center clusters is interesting, and better history ͑last several thousand years͒ with a time resolution of color-magnitude diagrams are needed for this possible class a few hundred years. The sample includes a variety of late of globular clusters. type stars such as Carbon Stars, S-Stars, Miras and their Rich, in collaboration with M. Shara, M. Fall, and D. more massive equivalents M-supergiants. The data are now Zurek ͑STScI͒ has used WFPC2 HST images to show that being reduced and analyzed. One of the results that was im- two sets of star clusters in the Small Magellanic Cloud were mediately obvious is that while many of the circumstellar formed in very short bursts. NGC 152, 411, and 419 have envelopes appear fairly round some show strong deviations identical CMDs with an age of 1.5 Gyr, while Kron 3, NGC from spherical symmetry. 339, 361, and 416 also have identical CMDs but with an age Van der Veen in collaboration with Martin Groenewegen of 8 Gyr. These clusters are widely separated in space but ͑MPE, Munich͒ and Henry Matthews ͑JAC, Hawaii͒ ob- appear to have formed nearly simultaneously. It has been tained maps at 450 and 850 micron of four evolved stars widely believed that the SMC has had a continuous period of using the SCUBA bolometer array at James Clark Maxwell cluster formation; these observations contradict that idea. Telescope ͑JCMT, Mauna Kea, Hawaii͒. This was a pilot Reduction of 30 LMC clusters is also underway, with the sample of a variety of evolved mass losing stars to see if finding that ages of most of the younger star clusters are their circumstellar dust shells are detectable at 450 and 850 younger than those derived from ground-based data. micron. In all cases the inner dust shells are clearly detected Smith, Rich and Neill, analyzed ground-based color- and seem fairly symmetric on first sight. The data will be used in conjunction with the IRAM data at 1300 micron to magnitude diagrams of globular clusters in the Fornax dwarf study the the most recent mass loss history. Due to the dif- spheroidal galaxy and the newly discovered Sagittarius ference in dish size between the IRAM 30m and JCMT dwarf spheroidal galaxy. Both of these dwarf spheroidals ͑ ͒ ͑15m͒, the 850 micron maps have a spatial resolution similar have globular cluster systems, although the connection be- to the IRAM 1300 micron data, but the 450 micron data have tween the Sgr dwarf and its globular clusters is based on a significantly better resolution. A further advantage of the coincidence of radial velocities ͑except for M54, which is 450 and 850 micron data compared to longer wavelength is located at the maximum of stellar density of the dwarf͒. Nei- the smaller contribution of molecular emission lines. This ther globular cluster system populates the portion of the hori- makes the interpretation of the data in terms of dust mass zontal branch type vs. metallicity diagram where the possibly loss variations over time much easier and more direct. young halo globular clusters ͑e.g. Pal 12͒ are considered to lie. Rather, the blue horizontal branches and generally low metallicities place the Fornax globular clusters and Arp 2 as 2. THE FIRST SURVEY being similar to the old LMC clusters. While spanning a Helfand, along with his collaborators R.H. Becker ͑UC large range in central density, Arp 2 and Fornax 3 have very Davis͒ and R.L. White ͑STScI͒ successfully applied to the similar HB types. A search for any correlation between HB National Radio Astronomy Observatory for a continuation of type and central density revealed no evidence for the previ- their project to collect Faint Images of the Radio Sky at ously claimed tendancy for clusters with more dense cores to Twenty-cm – the FIRST VLA sky survey. As of September have bluer HB types. 1998, 2200 hours of observations have been accumulated, Van der Veen in collaboration with several European in- covering a total of 6000 deg2 and yielding a catalog of over vestigators are currently reducing and analyzing 90 and 160 half a million radio sources with subarcsecond positions. The ␮m images of seven evolved mass losing stars obtained with images and catalogs are all publicly available at the FIRST COLUMBIA UNIVERSITY 45

Website: http://sundog.stsci.edu/. A wide variety of followup to better constrain the overall redshift distribution of the programs and science analysis projects using the survey are FIRST radio sample. Multi-fiber spectroscopy at the Her- in progress. Briefly, these include: schel Telescope has been complemented by deep R-band im- Helfand and the FIRST team completed observations for ages obtained at MDM; over 80% of the FIRST sources in the FIRST Bright Quasar Survey’s initial phase which covers four one-degree fields have identified optical counterparts, nearly 3000 square degrees of the northern sky. Over 600 and redshifts have been determined for the brighter quarter new quasars and BL Lac objects with R Ͼ 17.8 have been of this sample. The goal is to develop a sample of ϳ 1000 identified. Yadigaroglu and Helfand, in collaboration with radio sources with redshifts in order to constrain the poorly Schechter ͑MIT͒ and other FIRST team members have im- determined redshift distribution of faint radio sources. aged over 300 of these quasars in a search for gravitational Buchalter, Yadigaroglu, and Helfand have also been ob- lenses using the MDM Observatory. Two confirmed lenses taining optical imaging data in fields with large overdensities have been detected and several more candidates await fol- of radio sources in an attempt to find very large-scale struc- lowup. This frequency of lenses is an order of magnitude tures at high redshift. Candidate features on scales of 1–10 higher than in any previous lens search, suggesting that the Mpc have been identified. FBQS selection criteria are biased toward lensed objects. In Helfand, in collaboration with Brown, Kamionkowski, A. addition, Buchalter, in collaboration with J. Lehar ͑CfA͒, has Refregier ͑Princeton͒ and other collaborators are continuing obtained both MERLIN and VLA data on candidate lensed their study of weak gravitational lensing in the FIRST sur- radio lobes selected from FIRST. vey. The goal is to use distortions in the shapes of the distant Helfand and Willman, analysing photometric data on over radio sources ͑mean z ϭ 1) to infer information on the 200 FBQS objects, find that variability on timescales of power spectrum of total mass fluctuations in the intervening 10–40 years in the quasar rest frame is stronger in the blue Universe. Two major systematic effects – intrinsic correla- than in the red photometric band, and is greater for lower tion amongst the multiple components of complex radio luminosity sources than for higher luminosity ones. They sources and the effect of correlated noise in VLA images – continue to explore correlations of variability with radio lu- have been investigated and understood; both are shown to be minosity and redshift. negligible on the scales of interest (20Ј to 200Ј). The effects Helfand, Schnee, and their FIRST collaborators have of beam distortion have also been quantified, and an algo- completed a search for new radio stars in the first 5000 deg2 rithm to correct for it is being constructed. The expected of the survey. Comparing the Hipparcos and Tycho catalogs, signal in several model universes has been calculated and as well as other proper motion catalogs with the radio source shown to be within reach of the current FIRST survey list, they find a total of 26 stellar radio emitters, doubling the dataset. Preliminary calculations of the source ellipticity cor- number of previously known radio stars in this region. They relation function suggest a signal has been detected. compute the fraction of stellar radio detections brighter than 0.7 mJy as a function of optical magnitude, finding that 3. X-RAY & ⌫-RAY SOURCES ϳ 1% are detectable amongst the naked eye stars, while the Halpern is obtaining optical identifications of flat- Ϫ4 fraction falls to 10 at V ϭ 11. In all, less than one ten spectrum radio sources that have been proposed as blazar thousandth of FIRST radio sources are Galactic objects, counterparts to unidentified EGRET sources. Optical coun- marking it as the purest extragalactic sample yet collected. terparts of approximately nine radio sources have been found Blanton, Helfand, and their FIRST collaborators contin- so far, and optical spectra have been obtained yielding either ued a program to use bent-double radio galaxies as tracers redshifts or featureless continuum. These will be used to im- for galaxy clusters. In particular, they have used optical and prove the identification statistics of ␥-ray blazars, to study IR photometry to detect a cluster at z ϭ 1, and have ex- the lower limits of radio power among them, and to evaluate panded their sample of lower-redshift clusters to 75 mem- the need to postulate an additional class of high-energy ␥-ray bers. Work is now in progress to quantify the frequency of source. Halpern is also examining the brightest unidentified bent vs unbent double radio sources as a function of their high-latitude source 2EG 1835ϩ5919 through optical imag- environments; in addition to further optical imaging and ing of its entire EGRET error circle at the MDM Observa- spectroscopy, a Cycle 1 AXAF observation of one such clus- tory. Complementary X-ray observations of this region have ter will be performed in the coming year in order to measure been obtained. the cluster mass and search for signs of recent merger events At low Galactic latitude, the identity of majority of the which, in some popular models, are responsible for the bent- EGRET sources is a continuing mystery. Halpern and Hel- double morphology. fand are attempting to identify several EGRET sources at Helfand and Schnee have begun a study of radio emission low to intermediate Galactic latitude by covering their error in this complete, magnitude-limited galaxy redshift catalog. circles with ROSAT HRI and VLA pointings. If these sources Over 300 of the galaxies ( ϳ 15% of the total͒ are detected are pulsars, then they might have faint X-ray counterparts in the FIRST survey. Work is underway to distinguish be- and/or steep-spectrum radio counterparts in blank optical tween emission generated by star-formation activity and fields. However, they may also represent a new class of Ga- AGN. The goal is to help constrain the low-luminosity por- lactic object, or perhaps blazars that are relatively radio tion of the millijansky radio population. quiet, in which case they could also be indentified using this Yadigaroglu and Helfand, in collaboration with col- multiwavelength approach. X-ray observations of five leagues at the University of Cambridge have begun a project EGRET field have been obtained, and the optical identifica- 46 ANNUAL REPORT tions of the detected sources in three of these fields are al- nal jet source GRO J1655-40. Their results show that the most complete. At least one possibly interesting identifica- X-ray spectrum extends without a spectral break or cutoff to tion is being pursued. 800 keV and confirm that the black hole is nearly maximally The MDM Observatory is involved in the observations of rotating. They are currently analyzing an extensive RXTE optical afterglows of ␥-ray bursts. Approxmately six events data set on 4U 1630-47 obtained during it 1998 outburst. were pursued, with varying success due to weather and ac- Tomsick, Halpern, Kemp, and Kaaret observed the newly curacy of localization. Three afterglows were detected, in- discovered neutron-star X-ray transient XTE J2123 – 058 cluding the identification and measurement of reddening in with the MDM observatory and RXTE. They discovered the the highest redshift event ͑GRB971214 at zϭ3.42͒. orbital period and constrained the magnetic field of the neu- Tavani continued his studies of gamma-ray burst sources tron star. Comparison of the optical and X-ray distance de- ͑GRBs͒, and X-ray and gamma-ray sources. terminations suggests that the neutron star may weigh less Tavani showed that the 4-th BATSE catalog of GRBs than 1.4 solar masses. indicates the presence of complex spectral/intensity behavior Kaaret and Halpern with Piraino ͑Palermo͒ and Eracleous in GRB emission. The brightness distribution ͑logN-logP͒ of ͑Berkeley͒ discovered a possible X-ray/optical counterpart to GRBs show a clear deviation from the Euclidean expectation the EGRET ␥-ray source in Monoceros. The source contains only for very intense and hard bursts. This fact is in contrast a Be-star and thus may be similar to the radio source and with simple models of cosmological sources producing Be/X-ray binary LSI 61° sometimes associated with the GRBs. Severe selection effects may operate in the BATSE gamma-ray source 2CG 135ϩ01. sample and the inhomogeneity of the GRB population is in Sako, Kahn, Liedahl ͑LLNL͒, and Paerels ͑SRON͒ are question. If the majority of GRBs are at cosmological dis- involved in X-ray spectroscopic analyses of accretion- tances, strong intensity and spectral evolution of GRBs is powered sources. They extended the spectral analysis of the implied. ASCA observation of Cygnus X-3 to the archetypal high Tavani continued the analysis of BSAX and BATSE GRB mass X-ray binary, Vela X-1. The ASCA spectrum of Vela data. Several bursts have been analyzed in detail. Both the X-1 during eclipse is dominated by emission lines that are prompt and afterglow emission has been studied and impor- produced through photoionization of the stellar wind from tant constraints on theoretical models for GRB emission the companion star. The spectrum exhibits bright recombina- have been derived. The synchrotron shock model ͑SSM͒ de- tion lines from H- and He-like ions as well as mid-Z and iron veloped by Tavani is in excellent agreement with a wealth of fluorescent lines that originate from relatively cold material. prompt and afterglow data. They analyze the spectrum with a self-consistent atomic- Tavani continued the study of unidentified gamma-ray physics-based model that accounts for population kinetics transients ͑e.g., GRO J1838-04͒ in the Galactic plane. These dominated by recombination and successfully demonstrate mysterious objects appear to have faint X-ray sources in their that the spectrum is consistent with photoionization- error boxes, but no prominent source. The study of gamma- dominated line emission. The emission measure distribution ray and radio data for the system 2CG 135ϩ1 was continued, derived from the highly ionized ions can be explained in the with encouraging results regarding the possible identification context of an explicit stellar wind model in which matter of this gamma-ray source with the radio star LSI 61 303. accelerates radially from the photosphere of the companion Tavani continued the study of the very important soft X- according to the Castor, Abbott, & Klein profile. It is found ray transient Aquila X-1. BSAX observations show a very that the differential emission measure ͑DEM͒ distribution in distinctive decay behavior after a major X-ray outburst. This wind-fed X-ray binaries can be calculated theoretically from behavior is consistent with that expected for a ‘hidden mil- a relatively small set of stellar wind parameters. Conversely, lisecond pulsar’ in a binary system, as originally proposed by the X-ray spectrum can be used to determine those param- Tavani in 1991. More analysis and theoretical work is being eters. When applied to the Vela X-1 spectrum, the mass loss carried out on this important topic. rate inferred from a DEM analysis of highly ionized material Kaaret continues to study high frequency quasi-periodic is at least a factor of four lower than previously published oscillation ͑QPOs͒ from X-ray binaries as a means to probe results. This, along with the presence of bright fluorescent the behavior of the accretion disk in the strong gravitational lines, indicates that a significant amount of cool material fields near the compact object. Together with Yu and Zhang coexists with the highly ionized ions, possibly in the form of ͑MSFC͒ and Ford ͑U. Amsterdam͒, they demonstrated a ro- clumps. They have also analyzed the ASCA spectrum of Cen- bust correlation between the X-ray spectral shape and the taurus X-3 in the context of photoionization equlibrium line QPO frequency in two distinct X-ray binaries. In collabora- emission. The detection of radiation recombination con- tion with Ford and van der Klis ͑U. Amsterdam͒, they dis- tinuum from Si XIV and the measured temperature of ϳ 120 covered QPOs from the atoll source 4U 1705-44, detected a eV suggest that the dominant excitation mechanism is pho- second QPO peak in the source 4U 1735-44, and have shown toionization followed by recombination and cascades. Appli- that the hard lags observed in neutron star X-ray binaries are cations to other high mass binaries ͑4U 1700-377 and GX very similar to those known to occur in black hole candidate 301-2͒ are currently underway. They are also performing a X-ray binaries. quantitative analysis on the ASCA spectrum of the Circinus Kaaret and Tomsick are analyzing X-ray observations of Galaxy, which is classified as a Type II Seyfert. The ob- black hole candidate X-ray binaries. They have analyzed si- served spectrum is remarkably similar to the spectrum of the multaneous RXTE and OSSE observations of the superlumi- high mass binary, Vela X-1. Bright recombination and fluo- COLUMBIA UNIVERSITY 47 rescent lines were detected which, as in the case of Vela X-1, 3͒ Gamma-ray Pulsars. A new model is being constructed indicates the presence of material from a wide range of ion- with K.S. Cheng ͑U. of Hong Kong͒ for the geometry, beam- ization. The similarity in the spectra is probably due to mass ing structure, and spectral intensity from putative outer- outflow near the active nucleus that leads to a similar ioniza- magnetosphere accelerators in gamma-ray pulsars. tion structure in the circumnuclear regions. Spectral models and data analysis techniques are being developed in prepara- 5. GALAXIES tion for the high resolving power, high statistical quality data Crotts and J. Xu compiled a map of the velocity structure that will be available from future satellite missions ͑AXAF, of the same interstellar material they previously mapped in XMM, and ASTRO-E͒. three spatial dimensions using light echoes. This combina- Kay, Halpern, Leighly, and Magalha˜es ͑IAGUSP͒ ob- tion of information allows the age and energetics of several tained spectropolarimetry measurments of the Peculiar Type different superbubbles and shells near SN 1987A to be esti- Ic Supernova 1998bw, possibly associated with GRB mated, and places the SN in the context of the structure 980425, at CTIO. They measured intrinsic linear polarization around 30 Doradus in the Large Magellanic Cloud. over the wavelength range of 3900-7500 angstroms of 0.53 Crotts, Uglesich and A. Tomaney U. of Washington col- Ϯ 0.08 percent at position angle 49 Ϯ 3 deg, after correct- ͑ ͒ lected another extensive season of imaging data describing ing for Galactic interstellar polarization using the star HD microlensing in M31. Some of their original candidate mi- 184100, which has polarization of 0.75 Ϯ 0.01 percent at crolensing events from 1995-1996 have been ruled out as position angle 176.5 Ϯ 2.5 deg. Polarization appears highest variable stars, either through more extensive ground-based in between emission features in the total flux spectrum, time coverage or through Hubble Space Telescope WFPC2 which strengthens the interpretation of the polarization as snapshots, but most have survived as likely microlensing intrinsic to the supernova. This modest polarization is less events. More such events are being found in the 1996-1997 than that of some type-II supernovae, but greater than that of and 1997-1998 ground-based data, along with about 5000 type-Ia supernovae, which are generally unpolarized. This variable stars in M31. Along with G. Gyuk, they have shown supports the interpretation of SN 1998bw, a peculiar type-Ic how a larger survey could establish the shape flattening and supernova, as a core-collapse event in which the observed ͑ core radius of the microlensing halo in M31, and establish polarization is due to moderate asymmetry in either the ͒ accurate masses for the lenses. photosphere of the ejecta or an overlying scattering enve- The nature of elliptical galaxies is further investigated by lope. However, this result does not strongly constrain argu- van Gorkom and collaborators. The focus has been on shell ments about whether some supernovae emit gamma-ray ellipticals but is now shifting to the presence of cold disks in bursts, since such emission may come from a mildly relativ- ellipticals and the possible origin of these disks: fallback istic shock associated with the radio emission and above the after major merger versus secondary infall. This year a optical photosphere, without any requirements on beaming sample of ellipticals that were not known to have shells, but or orientation. live in a gas rich environment, was observed with the VLA by Fujita, van Gorkom, Schiminovich ͑Caltech͒ and van der Hulst ͑Kapteyn Institute͒. Ten of the 16 ellipticals were de- 4. PULSARS & NEUTRON STARS tected in HI, with HI morphologies ranging from extremely Halpern is studying rotation-powered pulsars with ASCA regularly rotating disks to large tidal tails and irregular dis- and AXAF in collaboration with G. Pavlov ͑Penn State U.͒. tributions of clouds. Thus in HI, shell and non shell ellipti- Long observations of several key targets are being obtained cals are not that different, with a typical HI detection rate of to study phenomena such as the spectrum of a neutron star 50%. VLA B, C and D data were combined for the shell atmosphere, and the spectrum and pulse profile of a millisec- elliptical NGC 3656 by Balcells ͑Tenerife͒, van Gorkom and ond pulsar. These new observations are necessary to disen- Sancisi ͑Bologna͒. The HI morphology appears to be very tangle thermal and nonthermal processes that may be present similar to that of NGC 5128 and can perhaps be explained as in the same object, and to correctly derive quantities such as a highly warped HI disk, seen close to edge-on in the center. the effective temperature of the neutron star surface, the lu- HI data of a spectroscopically selected sample of E minosity of a heated polar cap, and the M/R relation of the ϩ A or post-starburst galaxies were analyzed by Chang and neutron star. van Gorkom with Zabludoff and Zaritsky ͑both UCSC͒.Of Ruderman has continued work on three topics. the five galaxies only one was detected in HI, making them 1͒ Cosmic Gamma-Ray Burst Sources. A Model is being less gas rich than ordinary spirals. The one detection has developed with W. Kluzniak ͑U. Wisconsin͒ for the GRB huge tidal tails in HI; this may be a merger remnant. energy source which could be sustained by the huge differ- The structure of nearby clusters of galaxies and evolution ential rotation energy initially present in certain newly of their galaxy population is being investigated by van formed magnetized neutron stars and also during accretion Gorkom and collaborators. Deep imaging in HI is combined induced collapse of some such neutron stars into black holes. with optical spectroscopy and photometry and, where avail- 2͒ Evolution of pulsar magnetic fields. Studies of surface able, with X-ray data. The goal is to determine the dynamical fields and spin-period glitches in pulsars resulting from su- state of the clusters by combining all the available data. Pog- perfluid core vortex and related flux-tube movement were gianti ͑Cambridge͒ and van Gorkom compared optical spec- continued. One new feature is an understanding of why ex- tra and HI morphology and detection rate in Abell 2670. cessive heat generation in Vela-like pulsars is avoided. Among the more surprising results was the presence of a 48 ANNUAL REPORT significant number of red, yet gas rich galaxies. Bravo- the second deepest field ever imaged by HST. Rich and Fro- Alfaro, Cayatte, Balkowski ͑all Meudon͒ and van Gorkom gel combined efforts to obtain a 13-night run at MDM to made further analysis of an HI survey of the Coma cluster. obtain deep infrared photometry for the field, and had hoped The HI deficient galaxies in this cluster are among the most to reach K ϭ 22, but were prevented from getting significant interesting ever found, with completely asymmetric HI disks data because of the weather. The infrared camera on the and a post starburst galaxy with only a tiny plume of HI 2.4m ͑ONIS͒ is particularly useful for wide field imaging rising up from it’s center out of the disk. James, Reichborn- because of the combination of its size and small pixels. The Kjennerud, Valluri, van Gorkom and Duc ͑ESO͒ obtained goal is to use the infrared data to obtain photometric red- high-resolution ͑B array͒ observations of an HI-discovered shifts for galaxies detected wtih HST. These data will pro- dwarf sample in Hydra. The HI in these dwarfs is very ex- vide an additional test of the Madau star formation rate vs. tended and diffuse, with some remarkable asymmetries with redshift plot in another deep field besides the Hubble Deep respect to the stellar light. Field. Hailey, Craig and Neill are using the automated multiob- Weil continued a collaboration in which models of galac- ject spectrograph ͑AMOS͒, recently commissioned as a user tic evolution and of galactic mergers are compared to obser- instrument at Lick Observatory, to study velocity dispersions vations of diffuse, asymmetrical features observed at very and optical properties in X-ray-selected clusters of galaxies, faint levels in nearby, normal galaxies in order to investigate and their relation to cluster evolution. the mass and shape of dark matter at large galactic radii. It is Shull, Penston, Stocke ͑all Univ. of Colorado͒. van expected that further observations of HI distributions and Gorkom, Lee and Carilli ͑NRAO͒ analyzed a cluster of six kinematics of these galaxies by van Gorkom will help eluci- Ly alpha absorbers seen along the sightline toward PKS date the origin and structure of material at 50–100 kpc 2155-304. One of the suprising results is that the two stron- scales. gest absorbers agree both in velocity and velocity width with Petric and Weil are engaged in a study in which simula- the HI from the two galaxies closest to, but still at a pro- tions of mergers between galaxies are used to analyze fea- jected distance of 400 to 500 kpc to the sightline of PKS tures of an irregular galaxy, NGC 1210, which evinces 2155-304. shells, a massive ring of gas, and other indications of a recent The star formation history of field galaxies is being inves- interaction. tigated by Liu and Quillen ͑U. of Arizona͒. Using multicolor Weil tested galaxy mass estimators against numerical optical and near-IR photometry, optical spectroscopy and models of elliptical galaxies and of globular cluster systems stellar population evolutionary models, they find in a sample in ellipticals in order to determine the reliability of masses of 24 strongly starbursting field galaxies at intermediate red- and mass-to-light ratios predicted by such estimators. shift (0.3 Ͻ z Ͻ 0.5) that up to half of them may be undergoing their first burst of star formation, whereas the 6. ACTIVE GALACTIC NUCLEI rest are likely to be new bursts in galaxies with an underlying Halpern and M. Eracleous ͑Penn State U.͒ are continuing old stellar population. The rapid fading of the starbursts may their long-term spectroscopic monitoring of very broad, help explain the observed excess of strongly star-forming double-peaked Balmer lines, which are found preferentially field galaxies at that epoch compared with the present. in radio-loud AGNs. The profiles of these double-peaked Liu, Hibbard ͑NRAO͒ and Armus ͑IPAC͒ continue their lines are highly variable on time scales of months to years, a analysis of a large, unbiased sample of peculiar galaxies us- behavior which can be exploited to evaluate models for their ing multicolor UBVRK photometry. In addition, Liu, Kn- origin, and to study the dynamics of the accretion process in ezek ͑Johns Hopkins Univ.͒, Worthey ͑St. Ambrose Univ.͒ AGNs. Recent work demonstrates that variability of the and Scott ͑U. of Arizona͒ continue their analysis of a com- shapes of the emission lines must be due to dynamical mo- bined V-band imaging and optical spectrophotometric tions, and cannot be explained by reverberation ͑light echo͒ dataset of a complete subsample of the UGC galaxy catalog. effects. Results include the rejection of the binary black hole Both surveys have as their primary goal the establishment of model in four objects ͑Arp 102B, 3C 332, 3C 390.3, OX more precise indicators of galaxy evolutionary parameters in 169͒, and the discovery of a 2.2 year transient period in the the present epoch, to be used as calibrators for observations line profile of Arp 102B, which can be interpreted as the of high-redshift galaxy populations. orbit of a hot spot in an accretion disk around a black hole of 8 With Impey and Petry ͑Univ. of Arizona͒, Liu is studying 2.2 ϫ 10 M ᭪ . Possibly cyclic behavior in several objects a sample of 32 QSOs in the square degree centered on the appears to favor dynamical or wave motions in the accretion Hubble Deep Field ͑HDF͒, to use them as absorption probes disk as the cause. of the line of sight in the direction of the HDF. The QSOs Halpern and Eracleous are also continuing their study of were identified using optical multicolor photometry and fol- the X-ray spectra of broad-line radio galaxies ͑BLRGs͒ with lowup spectroscopy, with data obtained from KPNO and ASCA, comparing their Fe K␣ properties with those of ordi- Steward Observatory, respectively. nary Seyfert galaxies. They have found some BLRGs in Rich, J. Frogel ͑OSU͒ and A. Connolly ͑JHU͒ are obtain- which the Fe K␣ line is nonexistent, or weaker than in Sey- ing infrared imaging of the field containing the radio galaxy ferts, which may indicate a difference in the structure of the 53w002, at z ϭ 2.39. This field has been extensively imaged inner disk. In particular, the inner disk in BLRGs may be an by Windhorst and collaborators using HST and the optical ion torus, which would not have a cold target for the produc- WFPC2 detectors and, as recently as a year ago, this was still tion of fluorescent Fe emission. COLUMBIA UNIVERSITY 49

Leighly has completed a comprehensive X-ray spectral that there is X-ray emission associated with only one of the and variability study of narrow-line Seyfert 1 galaxies three radio hotspots in the extended radio lobes of this broad- ͑NLS1s͒ observed by ASCA. She reports results from 25 line radio galaxy. X-ray hotspots are generally very rare. observations of 23 NLS1s and compares the results with This hotspot appears in projection adjacent to a dwarf galaxy those from Seyfert 1 galaxies with broad optical lines re- and a optical spectrum of this galaxy reveals that it has ported in the literature. Time series analysis revealed that the nearly the same redshift as 3C 390.3. Thus, they postulate excess variance, or fractional amplitude of variability, is that the X-ray hotspot is caused by an impact of the AGN jet about an order of magnitude larger in NLS1s compared with on the galaxy, and that this interaction is also responsible for broad-line Seyfert 1 galaxies with the same luminosity. The the deflection of the jet at this point. They plan to test this excess variance can be used as a measure of the inverse time idea by making H I measurements at the VLA of the dwarf scale of variability, assuming that the observations are the galaxy. If there is HI present in this dwarf galaxy, we may be same length, the sampling window is the same, and the slope able to observe disturbances in the HI distribution caused by of the variability power spectrum is the same for all of the the impact. objects. This result can be interpreted as evidence for an order of magnitude larger accretion rate in NLS1s in general 7. GALAXY FORMATION AND COSMOLOGY compared with broad-line Seyfert 1 galaxies. She also dem- Crotts and Y. Fang ͑recently graduated from Colum- bia͒ onstrated that the light curves of some of the more variable have established the shape at redshift z Ϸ 2.5 of Lyman objects are detectably non-Gaussian. Detection of non- alpha forest absorbers, using pairs and triplets of QSO sight- Gaussianity in such short observations means that the light lines. Their various tests of absorber shape are more consis- curves are comprised of a only a few distinct flares rather tent with flattened structures such as sheets or pancakes, than many flares. rather than filaments as might have been expected from nu- Spectral analysis also yielded interesting results. She con- merical models of the forest. firmed that the NLS1s have significantly steeper hard X-ray Sugerman, Summers, and Kamionkowski have used a photon indices than do Seyfert 1 galaxies with broad optical large N-body and hydrodynamical simulation to investigate lines. Soft X-ray excesses were found in nearly all of those the origin of galactic angular momenta. Theory can be used objects with no evidence for soft X-ray absorption; in con- to relate the tidal torquing of an initially overdense region in trast, soft excess emission within the ASCA band is rela- the Universe to the angular momentum that a collapsed ob- tively rare in Seyfert 1 galaxies with broad optical lines. It ject will have. These predictions were compared with the appears that there is a heterogeneous origin of the soft excess primeval mass distributions from which galaxies identified in in NLS1s. There are objects with very strong soft excesses the simulation came. which also display enhanced variability compared with ob- M. Weil produced self-consistent numerical models of jects with weaker soft excesses. Interestingly, this effect does evolution in a hierarchically clustering universe in order to not appear to be associated with the strong set of correlations determine the main epoch of galaxy formation and explore governing the optical and other X-ray properties, including the origins of both spiral and elliptical galaxies. In collabo- the correlation between hard X-ray photon index and H␤ ration with V.R. Eke and G. Efstathiou ͑Cambridge͒, Weil FWHM. These properties are all consistent with the idea that investigated the influence of the cooling epoch on galaxy NLS1s are characterized by a higher accretion rate than Sey- type in a cold-dark-matter-dominated universe. Weil is cur- fert galaxies with broad optical lines. Any model to explain rently using the models to test the persistence of a proposed these results must produce the clear dichotomy in spectral universal density profile for dark matter halos in systems and variability properties. Too much information is missing which include gas and star formation. Further simulations are to constrain parameters from two-phase Comptonization being used to predict the ages, distributions, metal content models for the hard X-ray photon indices from NLS1s; how- and colors of stellar systems. ever, the data appear to be consistent with a model in which Cress and Kamionkowski compared predictions for the NLS1s have the same geometry as broad-line Seyfert 1 gal- amplitude of angular clustering in cold-dark-matter models axies, but they show higher intrinsic soft photon emission with measurements from the FIRST radio survey. The radio ͑by about a factor of 10͒. This is expected also from high sources are typically at redshifts of order unity. By compar- accretion rate models of radiation-dominated disks. Warm ing with clustering of sources at lower redshifts, one can absorbers appear to be less frequent in NLS1s. This could be investigate the evolution of the matter distribution in the due to a selection effect, since the NLS1s in the sample are Universe and test various models of the origin of structure. optically or soft X-ray selected objects. The measured correlation function is in good agreement with Other NLS1 investigations include analysis of the ASCA the predictions of CDM models, and there may be some hint data from the radio-loud NLS1 RGB 0044ϩ193. This object of bias evolution in the data. apparently has a typical NLS1 X-ray spectral and variability Buchalter and Kamionkowski are calculating the three- properties; that is, there is no evidence for an extra X-ray point correlation function for redshift surveys. They studied component coming from a jet ͑in collaboration with J. Sie- how various bias scenarios can be disentangled by the three- bert and S. Laurent-Muehleisen, et al.͒. point correlation function. With Jaffe ͑Berkeley͒ they are In collaboration with D. Harris and P. Leahy, an image carrying out the first complete calculation of the projected analysis of the combined ROSAT HRI observations from the angular correlation function. These predictions will be suit- 3C 390.3 monitoring campaign was performed. They found able for comparison with measurement of the three-point 50 ANNUAL REPORT correlation function from surveys such as the APM and 8. OTHER THEORETICAL INVESTIGATIONS FIRST. Buchalter and Kamionkowski also studied weakly Kamionkowski has been part of a large collaboration led nonlinear clustering for arbitrary expansion histories. They by Bahcall ͑IAS͒ to produce a comprehensive review of the verified, by numerical calculation in a broad class of models, nuclear-reaction rates for stellar-evolution calculations and that the scaled skewness of the mass distribution provides a for solar-neutrino fluxes. Kamionkowski and Bahcall also robust test of Gaussian initial conditions, independent of the isolated a flaw in a recent calculation of the cross section for expansion history. the pp deϩ␯ reaction which gave divergent results. Kamionkowski, Wadley and Caldwell ͑Pennsylvannia͒ Xuelei Chen,→ Kamionkowski, and Bahcall re-calculated the calculated the temperature/polarization pattern induced by a electron-screening correction to nuclear fusion rates in stellar single long-wavelength gravitational-wave mode on the cos- interiors. mic microwave background ͑CMB͒. They calculated the Solar System formation. Spiegel has continued to work smallest dimensionless amplitude of a gravitational wave of with the Provenzale group in Turin on the dynamics of ac- a given frequency that could be detected by a CMB cretion disks. They have continued their simulations of fluid temperature/polarization experiment with a given noise level, motions in Keplerian shear flows in order to learn how such and compared the results with other more traditional flows would affect the distribution of dust in a primitive gravitational-wave detectors such as LIGO and LISA. Kami- solar nebula. They assume that, in the early stages of nebular onkowski and Kosowsky ͑Rutgers͒ calculated the smallest formation at least, the disk would be turbulent. As had been amplitude of a stochastic gravity-wave background, such as suspected previously, the combined effects of rotation and that produced by inflation, with a full-sky polarization map. turbulence produce anomalous diffusion that renders an ini- Kamionkowski and Kinkhabwala have calculated the tially homogeneous dust distribution quite lumpy in a few large-angle cross-correlation between the cosmic-micro- rotation periods. Thus, the mechanism may be important in wave-background ͑CMB͒ temperature and the x-ray- the formation of protoplanets. background ͑XRB͒ intensity expected in an open Universe Chaos. C. Pasquero ͑Turin͒ and Spiegel have studied the with cold dark matter ͑CDM͒ and a nearly scale-invariant dynamics of parametrically driven dynamical systems. As in spectrum of adiabatic density perturbations. They showed a pulsating star, for instance, the stability of a parametrically driven system is governed by a Hill equation — Mathieu’s that open cold-dark-matter models with densities roughly 0.3 equation in the simplest cases. The question studied in the times the critical density are inconsistent with observations present work is what happens in the nonlinear regime of a unless x-ray sources are much less weakly clustered than strongly driven system. The approach taken is modeled after other high-redshift sources. the Shil’nikov theory of autonomous nonlinear systems and Kamionkowski and Jaffe ͑Berkeley͒ calculated the ampli- it has led to the derivation of stroboscopic maps that can be tude of the Ostriker-Vishniac effect, secondary small-angle used to antipate the onset of chaos in driven systems. Appli- CMB anisotropies from reionized gas, expected in cold-dark- cations to driven fluid systems are being investigated. matter models. They found that reasonable reionization sce- Transport theory. X. ͑Jon͒ Chen and Spiegel have contin- narios predict anisotropies that should be detectable with ued their studies of radiative fluid dynamics and have de- forthcoming interferometry experiments. They showed that rived a generalized form of the radiative viscous stresses that when combined with measurement of the damping of the works well for both long and short photon mean free paths. primary anisotropies with MAP and Planck, the amplitude of They are looking into the analogous theory for neutrino the secondary anisotropy will provide the epoch at which transport in dense stellar interiors. reionization occurred. Atmospheric instabilities. Umurhan, Tao and Spiegel have Xuelei Chen and Kamionkowski are calculating the CMB studied the nonlinear development of acoustic instability in a temperature/polarization power spectra expected in thermally dissipative atmosphere. The governing equation alternative-gravity theories. They have generalized standard for the envolope of the unstable waves is a complex cosmological perturbation theory ͑in both Newtonian and Ginzburg-Landau equation and the typical case is a quintic synchronous gauge͒ to arbitrary scalar-tensor gravity theo- equation since there is often subcritical instability. This ries. They are modifying existing numerical Boltzmann equation admits isolated oscillatory structures resembling codes to handle these alternative theories. spicules, but they are currently called oscillons by pattern Xuelei Chen and Kamionkowski also calculated new theorists. cross sections for annihilation of neutralinos to three-body Tao and Spiegel studied the onset of instability in an at- final states. Neutralinos, new particles that arise in supersym- mosphere with significant radiative forces and found that in- metric particle theories, are among the leading candidates for stabilities occur under a wide range of conditions especially the dark matter in our Galaxy. The new calculations are in the acoustic modes. needed for accurate predictions for rates for indirect detec- Convection. Umurhan and Spiegel have developed the tion of these particles via observation of energetic neutrinos Boussinesq equations for the low Prandtl number limit in the from particle annihilation in the Sun and Earth. Kamion- case where the heat fluxes are prescribed on the upper and kowski and Kinkhabwala investigated the uncertainties in lower boundaries. A new pattern forming equation in the predicted WIMP detection rates which arise from imprecise case of moderate amplitudes emerges and they are studying knowledge of the spatial and velocity distribution of these the patterns it produces. particles in the Galactic halo. Slyz and Prendergast extended the BGK scheme for hy- COLUMBIA UNIVERSITY 51 drodynamics to 3D axisymmetric self-gravitating configura- cated at 10 Mpc. As a result of these efforts, diffraction- tions in cylindrical polar coordinates. limited imaging and V-band imaging for NGST are now adopted as important goals to include in addition to the so- 9. LABORATORY ASTROPHYSICS & INSTRUMENT called ‘‘core’’ mission of a 1-5 micron telescope. By the end DESIGN of 1998, NGST will submit its first formal requests for pro- posals, and, with NASA approval, may enter phase A some- Hailey, Craig and Jimenez-Garate are working on the time in 1999. Neill has been using NGST point spread func- High Energy Focusing Telescope experiment in collabora- tions to simulate artificial star fields as imaged by NGST. tion with Caltech, the Danish Space Research Institute and These fields simulate a metal rich M31 halo field with the Lawrence Livermore National Lab. Thermally slumped glass color magnitude diagram and luminosity function of 47 Tuc, mirrors are being produced for gamma-ray imaging. The fig- ure and reflectivity of these mirrors is substantially superior with the correct number of stars corresponding to the surface to that of epoxy-replica optics. A prototype mirror module is brightness of the outer M31 halo. A 10-hour integration us- being developed and individual shells have figures well un- ing the hypothetical NGST would reach 3 mag below the der 1 arcminute. Jimenez-Garate has developed a real-time turnoff in the M31 halo. More simulations are planned in laser scanning system capable of rapidly characterizing fig- 1999. ure to 10 arcsec. Design of the gondola and pointing system Gu, Kahn, and Savin and their collaborators P. Beiersdor- for HEFT is also underway. We are investigating the same fer, G.V. Brown, D.A. Liedahl, and K.J. Reed ͑Lawrence thermally-formed mirror technology for use on the Livermore National Laboratory͒ have used the Lawrence- Constellation-X hard X-ray telescope. Livermore electron beam ion trap to measured the relative Hailey, Craig, Hong and Keck are analyzing data from the cross sections for Fe XXIV line emission at electron energies Gamma-ray Arcminute Telescope Imaging System between 0.7 and 3.0 keV. Good agreement with distorted ͑GRATIS͒ flight from Australia during which numerous wave and R-matrix calculations are found at energies above sources were observed including 4U1700 – 37, GRS 1915 ϩ 1.5 keV. At lower energies, the contributions of resonant 10 and 1E1740.7 – 29. Preliminary results were presented at excitation are observed and agree with R-matrix calculations. the AAS and more results are forthcoming. Keck is using a Below the excitation thresholds, the intensities of dielec- sophisticated Monte-Carlo code in an attempt to characterize tronic recombination satellites for capture into n у 5 levels very accurately the background in the narrow field of view have been measured. GRATIS instrument. Kahn and his collaborators G. V. Brown, P. Beiersdorfer, Hailey, Craig and Hong have continued work on super- D.A. Liedahl and K. Widmann ͑Lawrence Livermore Na- shields: multilayered materials for suppression of neutron tional Laboratory͒ have carried out detailed measurements, background in space-based X- and gamma-ray experiments. line identifications, and modeling calculations of the Fe XVII Hong is analyzing experimental data taken on supershield L-shell line emission spectrum between 9.8 and 17.5 Å. The performance using monochromatic beams of neutrons pro- measurements were carried out using an electron beam ion duced in a special facility at Columbia. This continues to be trap under precisely controlled conditions where electron- a promising method to control neutron backgrounds in se- impact excitation followed by radiative cascades is the domi- lected energy bands where the neutrons are a problem. nant line formation process. In addition to the strong transi- Hailey, Craig, Helfand, Kahn and Kamionkowski recently tion energies emanating from the n ϭ 3 shell, they have completed work on EPEX – the Extreme Phenomena Ex- identified and accurately determined wavelengths for transi- plorer, a proposal submitted to NASA’s MIDEX program for tions from higher shells up to n ϭ 11, including two electric the observation of gamma-ray bursts. They have also com- quadrupole transitions that have not been previously identi- pleted work on BOLT – a NASA-funded phase A study of a fied. Various theoretical values, including new distorted gamma-ray burst experiment suitable for a SMEX launch. wave calculations, are compared to the measurements which Hailey and Mori have also been modeling the time- have established definitive values for testing spectral model dependent X-ray spectrum expected from GRBs taking into predictions. They find a value of 3.04 Ϯ 0.12 for the ratio of 1 account all relativistic effects. the intensity of the 2d Ϫ 3d( P1) resonance and of the 3 Rich has been selected to develop the science case for 2p Ϫ 3d( D1) intercombination line situated at 15.01 and stellar populations research with the Next Generation Space 15.26 Å, respectively. This value is higher than the values Telescope, NGST. Rich has drafted 3 major science pro- observed in solar spectra, which supports claims that the so- grams for the Design Reference Mission of NGST. They are lar value is affected by resonant scattering. However, be- ͑a͒ measurement of the faint end of the white dwarf luminos- cause our value is significantly lower than calculated values, ity function in nearby globular clusters, providing an inde- the amount of scattering has probably been overestimated in pendent constraint on the age of the oldest stars; ͑b͒ mea- past analysis. Comparison of the measured intensity ratios of surement of the luminosity function to the end of the the transitions originating in levels of higher principal quan- hydrogen burning sequence in the Galactic bulge, dwarf tum number n with current distorted-wave calculations show spheroidals, and Magellanic Clouds, thus spanning a wide good agreement up to n ϭ 6. parameter space of metallicity and age; ͑c͒ measuring the Kahn and his collaborators P. Beiersdorfer, S.B. Utter, ages and abundance spreads in the population II stellar halos G.V. Brown, D.A. Liedahl, and C.W. Mauche at Lawrence of galaxies spanning the entire Hubble sequence, including Livermore National Laboratory, N.S. Brickhouse and A. K. classical elliptical galaxies such as NGC 3379 which are lo- Dupree at the Harvard-Smithsonian Center for Astrophysics, 52 ANNUAL REPORT and R. Mewe and J.S. Kaastra at the Space Research of the Livermore Electron Beam Ion Trap. The lifetime is impor- Neatherlands Institute have begun to establish a spectral tant for electron density plasma diagnostics which use the catalogue of the intermediate ionization states of iron, Fe IX line produced by the radiative decay of the forbidden 3 21 to Fe XXIV, in the extreme ultraviolet. Measurements are 1s2s S1 Ϫ 1s S0 transition in heliumlike oxygen. The carried out using an electron beam ion trap under precisely measured lifetime is in excellent agreement with recent theo- controlled laboratory conditions. The measurements are be- retical predictions and distinguishes among different treat- ing performed in support of the development of reliable ments of negative energy states and correlation in multi- modeling codes for the analysis of data from the Extreme configuration Dirac-Fock calculations. Ultraviolet Explorer and future space astrophysics missions Savin and his collaborators M.H. Chen and K.J. Reed sensitive to extreme ultraviolet radiation. They aim to re- ͑Lawrence Livermore National Laboratory͒ and D.S. Guo solve the controversy surrounding the short-wavelength ͑Southern University͒ have calculated the total dielectronic 2 2 spectra of stellar coronae. Preliminary measurements show a recombination ͑DR͒ coefficients for the P1/2 and P3/2 states wealth of iron lines in the 50-120 Å region. for B-like Fe21ϩ ions at electron temperatures 0.1 р T Kahn, Gu, and Savin in collaboration with P. Beiersdor- р 10000 eV. The calculations are carried out using the mul- fer, G. V. Brown, V. Decaux, D.A. Liedahl, S.B. Utter, and ticonfiguration Dirac-Fock method in intermediate coupling K. Widmann ͑Lawrence Livermore National Laboratory͒ and with configuration interaction. We find that accurate Coster- with support from NASA are performing a series of labora- Kronig energies are critical for a successful determination of tory astrophysics investigations designed to address funda- low temperature DR coefficients. We also find that the DR mental uncertainties in basic atomic physics processes rel- involving fine-structure excitations can be as important as evant to the interpretation of discrete X-ray spectra of cosmic the 2s Ϫ 2p excitation channels in the low temperature plasmas. Moderate resolution spectra acquired by the ASCA regime for some ions. These low temperature DR rates are observatory already demonstrate the inadequacy of currently important for photoionized gases. available spectral modeling codes for this wavelength band. Savin, Gu, and Kahn and their collaborators P. Beiersdor- With the upcoming launches of AXAF, XMM, ASTRO E, and fer, B. Beck G.V. Brown, D.A. Liedahl, and J. Scofield Spektrum Ro¨ntgen-Gamma, the demand for significant ad- ͑Lawrence Livermore National Laboratory͒ have used the vances in this field will increase dramatically. This research Lawrence-Livermore electron beam ion trap ͑LLNL-EBIT͒ program is based on the exploitation of the Electron Beam to produce a quasi-Maxwellian plasma. They do this by Ion Trap facility at the Lawrence Livermore National Labo- sweeping the energy of the nearly monoenergetic beam so ratory, and a uniques set of spectrometers and experimental the time spent at any energy is proportional to the Maxwell- techniques specifically developed for this purpose. Recent Boltzmann probability at that energy. To verify the accuracy experiments have been devoted to definitive measurements of the quasi-Maxwellian, they have measured line emission of line emission for iron L-shell ions in optically thin, colli- due to dielectronic recombination ͑DR͒ and electron impact sional plasmas. excitation ͑EIE͒ of Mg10ϩ and Ne8ϩ, for a range of simu- Kahn, Savin, and Gu and their collaborators D. A. lated temperatures. The ratio of DR to EIE lines in helium- Liedahl, P. Beiersdorfer, G.V. Brown, and S.B. Utter like ions is a well-understood temperature diagnostic. The ͑Lawrence Livermore National Laboratory͒ are carrying out spectroscopically inferred temperatures are in excellent a synergetic approach to modeling X-ray spectra. Plasma agreement with the simulated temperatures. The LLNL- emission models used in X-ray astronomy need to simulate EBIT offers a number of advantages over standard plasma X-ray spectra from at least thirteen elements. Development sources for studying Maxwellian plasmas. EBIT is essen- of comprehensive models requires large-scale calculations; tially driven by a Maxwellian electron distribution at a single for example, Fe M-shell spectra, K␣ fluorescence from near temperature Te ; a wide range of Te can be simulated; den- neutral ions, and dielectronic recombination satellite spectra sity effects are generally unimportant; the plasma is optically from L-shell ions. Current and recent missions (EUVE, thin; and Te is essentially constant along the line of sight. ASCA, DXS, etc.) have already demonstrated the need for Another advantage is the ability to create ions of a given major, rapid improvements in spectral models. The high- charge state and then study them in a Maxwellian plasma resolution spectra to be acquired with the next generation of under non-equilibrium conditions. X-ray observatories (AXAF, XMM, Astro-E) promise to push Savin and Kahn and their collaborators A. Wolf and oth- spectral models to their limits. Essential to ensuring the qual- ers at the Max Planck Institute for Nuclear Physics ͑Heidel- ity of calculations used in spectral codes is corroboration in berg, Germany͒ along with A. Mu¨ller and others at the Uni- the laboratory, where controlled and precisely measured versity of Giessen, ͑Giessen, Germany͒ are carrying out plasma conditions can be attained. To this end, we are capi- measurements of dielectronic recombination of relevance for talizing on a three-way synergistic relationship that links as- iron ions in photoionized gases. Iron ions provide many trophysical observations, atomic modeling, and experiments valuable plasma diagnostics for cosmic plasmas. The accu- using the Lawrence-Livermore electron beam ion trap facil- racy of these diagnostics, however, often depends on an ac- ity. curate understanding of the ionization structure of the emit- Savin and his collaborators J.R. Crespo Lo´pez-Urrutia, P. ting gas. Dielectronic recombination ͑DR͒ is the dominant Beiersdorfer, and K. Widmann ͑Lawrence Livermore Na- electron-ion recombination mechanism for most iron ions in tional Laboratory͒ have measured the lifetime of the cosmic plasmas. Using the heavy-ion storage ring at the 3 6ϩ 1s2s S1 level of the He-like O ion using the Lawrence- Max-Planck-Institute for Nuclear Physics in Heidelberg, COLUMBIA UNIVERSITY 53

Germany, they have measured the low temperature DR rates creting gases orbiting massive black holes at the centers of for Feqϩ where q ϭ 15, 17, 18, and 19. These rates are the nuclei of active galaxies. important for photoionized gases which form in the media The Reflection Grating Array, the component developed surrounding active galactic nuclei, X-ray binaries, and cata- at Columbia, is a precision aligned array of lightweight, re- clysmic variables. Our results demonstrate that commonly flection gratings which both focus and disperse the collected used theoretical approximations for calculating low tempera- X-ray light. This work is supported by NASA as the primary ture DR rates can easily under- or over-estimate the DR rate US contribution to the XMM mission. Others involved in the by a factor of ϳ 2 or more. As essentially all DR rates used project at Columbia include researchers Paerels and Rasmus- for modeling photoionized gases are calculated using these sen, graduate students Cottam and Spodek, mechanical engi- approximations, their results indicate that new DR rates are neer Todd Decker and research associate Marcela Stern. needed for almost all charge states of cosmically abundant Both grating arrays, which are to be positioned behind elements. Measurements are underway for other charge one each of the three large XMM X-ray telescopes, were states of iron. calibrated extensively on the ground at MPE-PANTER ͑Ger- Savin and Gu and their collaborator P. Beiersdorfer at many͒ in an end-to-end configuration, and were then aligned Lawrence Livermore National Laboratory have developed a and integrated onto their respective flight telescope units at theoretical formalism for calculating the anisotropy and po- CSL ͑Belgium͒, and are currently awaiting integration into larization of photon emission due to a spiraling beam of the XMM spacecraft at Dornier ͑Germany͒. The RGS cali- electrons as may be encountered in solar and stellar flares. bration data acquired provides the valuable information re- The formalism is valid for all multipoles of atomic transi- quired to interpret the complex spectra we expect to see in tions. Measurements using an electron beam ion trap ͑EBIT͒ cosmic X-ray sources: they anchor our composite, physical can also be affected by a spiraling of the beam electrons. model for the instrument, which incorporates all available They find that spiraling effects cannot yet be discerned in assembly data, small angle X-ray scattering data from Nevis these measurements. and detailed instrument geometry ͑Spodek͒, and electromag- Savin and his collaborators D.B. Reisenfeld, P.H. Janzen, netic vector modeling of the gratings and measurable surface L.D. Gardner, and J. L. Kohl ͑Harvard-Smithson- ian Center characteristics that are tuned according to the component- for Astrophysics͒ have continued their absolute measure- level grating calibration results acquired at the Columbia ments of electron-impact excitation ͑EIE͒ of ions. EIE is the Astrophysics/Nevis X-ray facility ͑Cottam͒. This synthesis dominant mechanism for the formation of emission lines in has been incorporated into Columbia’s raytrace ͑Spodek/ many astronomical plasmas. Absolute line intensities and Paerels͒ of the RGS which has been reproduced in the XMM their ratios to one another can provide diagnostics of the Science Simulator by the XMM Science Operations Center temperature and density of the emitting plasma, and of the at ESTeC. An approximately equivalent, but less detailed abundances of the elements within the plasma. Modern space synthesis approach has been pursued in parallel ͑Rasmussen͒ observations such as the Hubble Space Telescope, the SOHO which includes qualitative spectrometer focal-plane readout solar mission, and the upcoming AXAF and XMM missions CCD response and windowed channel redistribution func- measure UV and X-ray line intensities with detectors cali- tions and thus response matrices, which are key analysis brated to a high degree of accuracy, placing more demand tools for accurate spectral interpretation. The launch of than ever on the atomic physics community to provide accu- XMM is eagerly awaited at Columbia, and Kahn’s group is rate atomic data. Theoretical calculations can provide the maintaining and developing a suite of ‘‘quick-analysis’’ soft- vast numbers of atomic rates used for the interpretation of ware which will be used in the early part of the Observato- emission line measurements, but experimental benchmark ry’s 10-year lifespan. measurements are required to test the accuracy of the calcu- Kahn, Paerels and Rasmussen head a technological study lational methods. Savin and collaborators have recently mea- phase for the Constellation-X mission, for which a grating/ sured the absolute cross section for EIE of Si2ϩ(3s21S ccd instrument ͑similar to XMM/RGS͒ was selected for Ϫ 3s3p1P) for energies below threshold to 11 eV above. study. This instrument improves severalfold the sensitivity Their results are in good agreement with 12-state close- and spectral resolution over XMM but requires new, light- coupling R-matrix calculations. weight gratings, either lithographically etched from silicon A team led by Kahn has completed fabrication and testing wafers or stretched membranes of silicon ͑Schattenberg, of two flight models of a critical component of the Reflection MIT͒, and a spectroscopic imaging readout CCD array Grating Spectrometer experiment which will fly on the Eu- ͑Ricker, MIT͒ that features fast readout time and resilience to ropean Space Agency’s X-ray Multi–Mirror Mission radiation damage encurred in space. ͑XMM͒ in January of 2000. XMM is one of four ‘‘corner- Kahn and Rasmussen are key participants in the concep- stone’’ missions of the European Space Agency’s research tual study of the Micro-Arcsecond X-ray Interferometer Mis- program in the space sciences. The Reflection Grating Spec- sion ͑MAXIM͒, along with others from Colorado, Caltech, trometer will provide the first high sensitivity, high resolu- CfA and NASA-Goddard. MAXIM is an interferometer op- tion X-ray spectra of cosmic sources. It will enable detailed, erating with microarcsecond angular resolution, which will quantitative measurements of such important information as be able to image event horizons around very massive black the temperature, density and chemical content of cosmic holes in the active galactic nuclei of external galaxies. Inner- plasmas ranging from the shock-heated interstellar media most radii of accretion disks in X-ray binaries and virtually surrounding recent supernova explosions to the irradiated ac- all classes of ͑small volume͒ X-ray sources will be clearly 54 ANNUAL REPORT resolved, ushering in a whole new era of observational as- mission. Due to the diffuse nature of this emission, INTE- tronomy. Although this mission is more than a decade away, GRAL’s limited sensitivity for extended emission may well recent advances in the development of other space interfero- prevent its detection, while XENA’s wide field-of-view, im- metric missions using the required precision technology aging capabilities, and high sensitivity would make it a ͑e.g., SIM͒ promise the feasibility of MAXIM. promising instrument to first detect and image the Fe-60 Aprile, Egorov, Giboni, Kozu, Oberlack, and Ventura lines. ͑INFN-Padova University, Italy͒, have continued the devel- Another concept study, ‘‘XENA-100,’’ was submitted to opment of a Balloon-Borne Liquid Xenon Gam- ma-Ray Im- NASA as a liquid xenon Compton telescope on a ultra-long aging Telescope ͑LXeGRIT͒ for high energy astrophysics. duration balloon flight mission of about 100 days. Such mis- The telescope images cosmic gamma-rays in the range of sion would yield significantly improved sensitivity compared about 0.3 to 30 MeV through their Compton interactions in a to INTEGRAL, and would explore the MeV regime at sen- liquid xenon time projection chamber ͑LXeTPC͒. This ho- sitivies as low as a factor 30 below the current ͑and first͒ mogenous detector is position-sensitive in all three dimen- Compton telescope in space, COMPTEL. The extended ob- sions, providing the capability to greatly reduce background, servation time would allow many science goals to be ad- the main limitation of sensitivity in this energy range. Its dressed, ranging from gamma-ray line sources ͑Fe-60, Ti-44, performance has been verified with calibration sources. The Al-26, annihilation line, nuclear de-excitation lines,...͒ in LXeTPC, developed by the Columbia team with collabora- the Milky Way and supernovae in the Virgo cluster to con- tors at Waseda University, the University of New Hamp- tinuum sources such as black-hole candidate binaries, GRBs shire, and NASA/MSFC, was integrated as a balloon payload and AGNs. and successfully flown in 1997 on high altitude balloons Aprile, Xu, and Mukherjee with Digel at NASA/GS FC, from Palestine, TX. The two initial flights clearly demon- continued the analysis of EGRET data from all viewing pe- strated the feasibility of the technology in a space environ- riods of the Orion cloud region, to determine the molecular ment. This experience led to a variety of modifications in the mass calibrating ratio X ϭ N(H2)/Wco and the gamma-ray design of the trigger logic, data-acquisition system, data re- emissivity per nucleon in the ISM, with better precision al- ceiving and storage, and the balloon gondola itself. Data lowed by the increased exposure. Possible energy depen- analysis techniques have been explored further, and will be- dence and spatial variations of these quantities, as well as come a major issue in the upcoming year, including the ap- significant excesses due to point sources were studied. The plication of imaging techniques developped for the COMP- refined analysis, which now uses all available data on the TEL instrument on CGRO. The group is heading for a turn- region, also includes sources in the 3rd EGRET catalog around flight in Spring 1999 from Fort Sumner, NM, with which are close enough to the region of interest to affect the the goal of measuring the in-flight instrumental background, residual maps. previously assessed by Monte-Carlo calculations, and the ob- Giboni, Aprile and Rochwarger studied the performance servation of celestial sources such as the Crab pulsar and of newly developed Schottky Cadmium Telluride ͑CdTe͒ de- Cygnus X-1. Two undergraduate students, Yong Moon from tectors, in view of their potential applications as imaging Columbia College, and Masaki Yamashita from Waseda uni- arrays with excellent spectroscopic response in gamma-ray versity, contributed to this work during the Summer of 1998. astrophysics and nuclear medicine. The spectroscopy and the Moon developed a new software package to display the LX- efficiency of these room temperature semiconductor detec- eGRIT instrumentation and control data during the flight. tors was studied as a function of bias voltage, temperature, Yamashita carried out experiments with a 3-liter volume liq- and source-detector geometry. Results from experiments uid xenon gridded ionization chamber to study the effect of with a variety of gamma-ray sources were analyzed and pub- impurities in the Xe gas on the charge collection efficiency lished in two papers. in the liquid. Mukherjee and Yuille spent some time working on the In response to the NASA University Explorer ͑UNEX͒ development of a laser calibration system for STACEE ͑So- program announcement of opportunity, Aprile, Egorov, Gi- lar Tower Atmospheric Cherenkov Effect Experiment͒. This boni, Helfand, Kahn, Kozu, and Oberlack with collaborators system has been taken to the STACEE site at Sandia Na- at Waseda University, the University of New Hampshire, tional Labs, Albuquerque. The calibration system, when University of Padova and INFN, Clemson University, and completed, will be used to measure the timing, gain, and SRON Utrecht have studied the performance of a next- linearity of the photomultiplier tubes used by STACEE. generation Compton telescope concept based on the LX- Mukherjee and M. Bo¨ttcher ͑Rice͒, together with other eTPC imaging technology. The studies which led to the collaborators, have been working on the analysis of the spec- ‘‘XENA’’ proposal as a UNEX long-duration ballon flight tral energy distribution of the compact radio-loud quasar, mission, demonstrated the competitive capabilities of such PKS 0528ϩ134, detected in high energy gamma-rays by an instrument even within the limited time frame of a 14-day EGRET. They find that the multiwavelength spectrum of balloon flight from the Southern Hemisphere. The comple- PKS 0528ϩ134 can be modeled as follows: the radio to UV mentarity of a XENA mission to the upcoming European emission can be explained as synchrotron emission from satellite mission INTEGRAL, based on coded-aperure instru- relativistic electrons in a uniform relativistically moving ments, was particularly emphasized. The search for Fe-60 in plasma. The high energy emission is due to the inverse the Galaxy with its far-reaching implications for nucleosyn- Compton scattering of seed photons off the relativistic elec- thesis theories was identified as a main science driver of such trons. The electron cooling process is a combination of the COLUMBIA UNIVERSITY 55 synchrotron self-Compton and external radiation Compton ‘‘The Nature of OH/IR Stars in the Galactic Centre,’’ mechanisms, based on the different dependences of the two A&A, 329, 991. components on the Doppler boosting factor. Bracco, A., Chavanis, P.H., Provenzale, A., and Spiegel, Tavani is the Principal Investigation of a proposed new E.A. 1998, ‘‘Aggregation of Particles in a Turbulent gamma-ray mission, AGILE, planned to operate during the Keplerian Flow,’’ Phys. Fluids, submitted. years 2001-2005 in the 30 MeV - 50 GeV band within the Bracco, A., Provenzale, A., Spiegel, E.A., P.A., and Yecko, framework of the Italian Space Agency program for small P.A. 1998, ‘‘Spotted Disks,’’ in Theory of Black Hole missions. This mission would be a natural bridge between Accretion Disks, M. Abramowicz, G. Bjornsen and J. CGRO and GLAST. Pringle, eds., Cambridge Univ. Press, in press. Kaaret, Novick, and Tomsick continue work on the Stellar Brown, G.V., Beiersdorfer, P., Liedahl, D.A., Widmann, K., X-ray Polarimeter ͑SXRP͒ for the Spectrum-X-Gamma mis- and Kahn, S.M. 1998, ‘‘Laboratory Measurements and sion. The flight model of the SXRP has been completed and Modeling of the Fe XVII X-ray Spectrum,’’ Ap.J., 502, has undergone calibration and environmental testing. When 1015. launched, the SXRP will provide an order of magnitude in- Buchalter, A., Helfand, D.J., Becker, R.H., and White, R.L. crease in polarization sensitivity relative to any previously 1998, ‘‘Constraining ⍀0 with the Angular Size-Redshift flown X-ray polarimeter. Relation of Double-Lobed Quasars in the FIRST Survey,’’ Ap.J., 494, 479. PUBLICATIONS Caldwell, R.R., Kamionkowski, M., and Wadley, L. 1998, Aprile, E. et al., 1998, ‘‘XENA - A Liquid Xenon Compton ‘‘The First Space-Based Gravitational-Wave Detectors,’’ Telescope for Gamma-Ray Astrophysics in the MeV Re- Phys. Rev. D, submitted. gime,’’ Hard X-rays and Gamma-Ray Detector Physics Campana, S., Colpi, M., Mereghetti, S., Stella, L., and and Applications, SPIE, Vol. 3446, 88-99. Tavani, M. 1998, ‘‘The Neutron Stars of Soft X-ray Tran- Aprile, E. et al., 1998, ‘‘The Electronics Readout and Data sients,’’ A&A Reviews, in press. Qcquisition System of a Liquid Xenon Time Projection Campana, S., Stella, L., Mereghetti, S., Colpi, M., Tavani, Chamber as a Balloon-borne Gamma-ray Compton Tele- M., Ricci, D., Dal Fiume, D., and Belloni, T. 1998, ‘‘Aq- scope,’’ Nuclear Instruments and Methods in Physics Re- uila X-1 from Outburst to Quiescence: the Onset of the search A, 412, 425-436. Propeller Effect and Signs of a Turned-On Rotation- Athanassoula, E. and Prendergast, K. H., 1998, ‘‘The forma- Powered Pulsar,’’ Ap.J. (Lett.), in press. tion of Dwarf Ellipticals: Comparison of Numerical Cappi, M., Matsuoka, M., Otani, C., and Leighly, K.M. Simulations and Observation,’’ submitted. 1998, ‘‘The Complex Spectrum of 3C 273: ASCA Obser- Bahcall, J.N. and Kamionkowski, M. 1997, ‘‘The Proton- vations,’’ PASJ, 50, 213. Proton Reaction, Solar Neutrinos, and a Relativistic Field Carollo, M., Danziger, I.J., Rich, R.M., and Chen, X. 1997, Theoretic Model of the Deuteron,’’ Nucl. Phys. A, 625, ‘‘Nuclear Properties of Kinematically Distinct Cores,’’ 893. Ap.J., 491, 545. Bahcall, J.N., Chen, X., and Kamionkowski, M. 1997, ‘‘The Casti, A.R., Morrison, P., and Spiegel, E.A. 1998, ‘‘Negative Electron-Screening Correction for the Proton-Proton Re- Energy Modes and Gravitational Instability of Interpen- action,’’ Phys. Rev. C, 57, 2756. etrating Fluids,’’ Ann. N.Y. Acad. Sci., in press. Balmforth, N.J., Provenzale, A., Spiegel, E.A., Tresser, C., Chen, K., Ruderman, M., and Zhu, T. 1998, ‘‘Neutron Star Wu, C.-W., M. Martens, M. 1998, ‘‘Red Spectra From Magnetic Field Evolution, Crust Movement and White and Blue Noise,’’ Proc. Roy. Soc. London, B, sub- Glitches,’’ Ap.J., 492, 267. mitted. Chen, M.H., Reed, K.J., Guo, D.S., and Savin, D.W. 1998, Becker, R.H., M.D. Gregg, M.D., Laurent-Muehleisen, S.A., ‘‘Dielectronic Recombination for Boronlike Ions,’’ Phys. White, R.L., Helfand, D.J., McMahon, R.G., Oegerle, W. Rev. A, accepted. Friedman, S., Richards, G., York, D., Rockosi, C., and Chen, X. and Kamionkowski, M. 1998, ‘‘Three-Body Anni- Impey, C. 1997, ‘‘BAL Quasars in the VLA FIRST Sur- hilation of Neutralinos Below Two-Body Thresholds,’’ vey,’’ in Mass Ejection from AG N, eds. N. Arav, I. Journal of High Energy Physics, 07, 001. Schlosman, and R.J. Weymann, ASP Conf. Ser. 128, p31. Chiang, J. and Mukherjee, R. 1998, ‘‘The Luminosity Func- Beiersdorfer, P., Utter, S.B., Brown, G.V., Liedahl, D.A., tion of the EGRET Gamma-Ray Blazars,’’ Ap.J., 496, Mauche, C.W., Kahn, S.M., Brickhouse, N.S., Dupree, 752. A.K., Mewe, R., and Kaastra, J.S. 1998, ‘‘Spectral Cata- Craig, W.W., Decker, T.R., Hailey, C.J., and Jimenez- logue of the Intermediate Ionization States of Iron in the Garate, A.M. 1998, ‘‘Hard X-ray Optics for the HEFT Extreme Ultraviolet,’’ in Proceedings of the NASA Labo- Balloon Borne Payload: Prototype, Design & Status,’’ to ratory Space Science Workshop, Cambridge, MA, April appear in SPIE Conference Proceedings, Vol. 3445, on 1998. EUV, X-ray, and Gamma-Ray Instrumentation for As- Blanton, E.L., Gregg, M.D., Helfand, D.J., Becker, R.H. and tronomy. White, R.L. 1998, ‘‘FIRST Bent-Double Radio Sources: Crespo Lo´pez-Urrutia, J., Beiersdorfer, P., Savin, D.W. and Tracers of High Redshift Clusters,’’ Ap.J., submitted. Widmann, K. 1998, ‘‘Precision Measurement of the Life- 3 Blommaert, J.A.D.L., van der Veen, W.E.C.J., van Langev- time of the 1s2s S1 Metastable Level in Heliumlike elde, H.J., Habing, H.J., and Sjouwerman, L.O. 1998, O6ϩ,’’ Phys. Rev. A, 58, 238. 56 ANNUAL REPORT

Cress, C. and Kamionkowski, M. 1998, ‘‘Interpreting the Tirado, A., Cusumano, G., Dal Fiume, D., Heise, J., Hur- Clustering of Radio Sources,’’ MNRAS, 297, 486. ley, K., Nicastro, L., Orlandini, M., Owens, A., Palzaai, Crotts, A.P.S. 1998, ‘‘Spectroscopy of QSO Triplets: Prob- E., Parmar, A.N., Zand, J. in’t, and Zavattini, G. 1998, ing the Shape of Lyman Alpha Forest Absorbers,’’ ‘‘Spectral Properties of the Prompt X-ray Emission and B.A.A.S., 192, 51.10. Afterglow from the Gamma-Ray Burst of 28 February Crotts, A.P.S., and Fang, Y.1998, ‘‘Reobservation of Close 1997,’’ Ap.J. (Lett.), 493, L67. QSO Groups: The Size Evolution and Shape of Ly ␣ Fruchter, A., Tavani, M. et al. 1998, ‘‘HST/STIS Observa- Forest Absorbers,’’ ApJ, 502, 16. tions of the Optical Counterpart to GRB 970228,’’ in Pro- Crotts, A., Heathcote, S., Maza, J., and Phillips, M. 1998, ceedings of the 4th BATSE Symposium on Gamma-Ray ‘‘Supernova 1987A in the Large Magellanic Cloud,’’ Bursts, eds. C.A. Meegan & P. 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‘‘Large Scale Surveys in Cosmology,’’ ed. S. Colombi Kamionkowski, M. 1998, ‘‘Possible Relics from New Phys- and Y. Mellier, in press. ics in the Early Universe: Inflation, The Cosmic Micro- Helfand, D.J. 1998, ‘‘A Cradle Census: Evidence for Young wave Background, and Particle Dark Matter,’’ to appear Neutron Stars in Supernova Remnants,’’ in Neutron Stars in The Early and Future Universe, proceedings of the and Supernova Remnants, ed. F. Pacini, in press. CCAST Workshop, Beijing, China, June 22–27, 1998, Hong, J., Hailey, C.J., and Craig, W.W. 1998, ‘‘Develop- edited by Minghan Ye ͑Gordon Breach, New York, ment of Neutron Shields for Gamma-Ray Detectors,’’ to 1998͒. appear in SPIE Conference Proceedings, Vol. 3445, on Kamionkowski, M. and Jaffe, A. 1998, ‘‘New Troubles for EUV, X-ray, and Gamma-Ray Instrumentation for As- Inflation?’’ Nature ͑in press͒. tronomy. 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