Tephrochronologic Constraints on Temporal Distribution of Large Landslides in Northwest Argentina

Two morphologic settings in the northwestern Argentine prone to giant mountain-front collapse-deeply incised narrow valleys and steep range fronts bordering broad piedmonts-were analyzed through detailed investigations of fossil landslides and related fluvio-lacustrine sediments. Nine different rhyodactic tephra layers were defined by geochemical fingerprinting of glass, morphology of pumice, stratigraphic relationships, and mineralogy. The age of three tephra could be determined either directly by 40Ar/39Ar dating or relatively by 14C dating of associated sediments: Paranilla Ash (723+/-89 ka), Quebrada del Tonco Ash ( approximately 30 ka), and Alemanía Ash ( approximately 3.7 ka). These units permit correlation of several spatially separate landslide deposits. Landslide deposits in narrow valleys were generated in the late Pleistocene between 40 and 25 ka and in the Holocene since ca. 5 ka and correspond to periods characterized by increased humidity in subtropical South America. In contrast, the age of large landslides in piedmont regions is significantly greater but more difficult to define by tephrochronology. However, selected deposits from this second environment have cosmogenic nuclide exposure ages of 140-400 ka. Because of the large distance of the collapsed mountain fronts from eroding streams and because of important Quaternary displacement along the mountain-bounding faults, we suggest that strong, low-frequency seismic activity is the most likely trigger mechanism for most of the landslides in this environment.     

Measurement of a Peak in the Cosmic Microwave Background Power Spectrum from the North American Test Flight of Boomerang

We describe a measurement of the angular power spectrum of anisotropies in the cosmic microwave background (CMB) at scales of 0&fdg;3 to 5 degrees from the North American test flight of the Boomerang experiment. Boomerang is a balloon-borne telescope with a bolometric receiver designed to map CMB anisotropies on a long-duration balloon flight. During a 6 hr test flight of a prototype system in 1997, we mapped more than 200 deg(2) at high Galactic latitudes in two bands centered at 90 and 150 GHz with a resolution of 26&arcmin; and 16&farcm;5 FWHM, respectively. Analysis of the maps gives a power spectrum with a peak at angular scales of 1 degrees with an amplitude 70 μK(CMB).     

Extended Far-Infrared CO Emission in the OMC-1 Core of Orion

We report the discovery of quasi-periodic oscillations (QPOs) at roughly 187 and 150 Hz in the X-ray intensity of X-ray nova XTE J1859+226. The source was observed during a recent outburst with the Rossi X-Ray Timing Explorer. Besides these high-frequency QPOs, we have also detected QPOs (and sometimes their harmonics) at 6-7 Hz and significant broadband variability at low frequencies. These properties, as well as the observed hard X-ray spectrum, make XTE J1859+226 a black hole candidate (BHC). The detection of QPOs at two distinct frequencies greater, similar100 Hz in two observations separated by about 4 hr provide additional insights into the high-frequency QPO phenomenon in BHCs. The importance lies in the proposed interpretations, which invariably involve the effects of strong gravity near a black hole. We compare our results to those of other BHCs and discuss the impact of the observational data on the models in a global context.     

Discovery of a New Pulsating X-Ray Source with a 1549.1 Second Period, AX J183220-0840

A new pulsating X-ray source, AX J183220-0840, with a 1549.1 s period was discovered at R.A.=18h32m20s and decl.=-8&j0;40'30" (J2000, with an uncertainty of 0&farcm;6) during an ASCA observation on the Galactic plane. The source was observed two times, in 1997 and in 1999. A phase-averaged X-ray flux of 1.1x10-11 ergs cm-2 s-1 and a pulsation period of 1549.1+/-0.4 s were consistently obtained from these two observations. The X-ray spectrum was represented by a flat, absorbed power law with a photon index of Gamma approximately 0.8 and an absorption column density of NH approximately 1.3x1022 cm-2. Also, a signature of iron K-shell line emission with a centroid of 6.7 keV and an equivalent width of approximately 450 eV was detected. From the pulsation period and the iron-line feature, AX J183220-0840 is likely to be a magnetic white dwarf binary with a complexly absorbed thermal spectrum with a temperature of about 10 keV.     

BeppoSAX and Chandra Observations of SAX J0103.2-7209 = 2E 0101.5-7225: A New Persistent 345 Second X-Ray Pulsar in the Small Magellanic Cloud

We present two weak lensing reconstructions of the nearby (zcl=0.055) merging cluster Abell 3667, based on observations taken approximately 1 yr apart under different seeing conditions. This is the lowest redshift cluster with a weak lensing mass reconstruction to date. The reproducibility of features in the two mass maps demonstrates that weak lensing studies of low-redshift clusters are feasible. These data constitute the first results from an X-ray luminosity-selected weak lensing survey of 19 low-redshift (z<0.1) southern clusters.     

Large Proper-Motion Infrared

When some magnetic field lines connect a Kerr black hole with a disk rotating around it, energy and angular momentum are transferred between them. If the black hole rotates faster than the disk, ca&solm0;GMH>0.36 for a thin Keplerian disk, then energy and angular momentum are extracted from the black hole and transferred to the disk (MH is the mass and aMH is the angular momentum of the black hole). This way, the energy originating in the black hole may be radiated away by the disk. The total amount of energy that can be extracted from the black hole spun down from ca&solm0;GMH=0.998 to ca&solm0;GMH=0.36 by a thin Keplerian disk is approximately 0.15MHc2. This is larger than approximately 0.09MHc2, which can be extracted by the Blandford-Znajek mechanism.     

Large-Scale Structure of the Carina Nebula

Observations obtained with the Midcourse Space Experiment (MSX) satellite reveal for the first time the complex mid-infrared morphology of the entire Carina Nebula (NGC 3372). On the largest size scale of approximately 100 pc, the thermal infrared emission from the giant H ii region delineates one coherent structure: a (somewhat distorted) bipolar nebula with the major axis perpendicular to the Galactic plane. The Carina Nebula is usually described as an evolved H ii region that is no longer actively forming stars, clearing away the last vestiges of its natal molecular cloud. However, the MSX observations presented here reveal numerous embedded infrared sources that are good candidates for sites of current star formation. Several compact infrared sources are located at the heads of dust pillars or in dark globules behind ionization fronts. Because their morphology suggests a strong interaction with the peculiar collection of massive stars in the nebula, we speculate that these new infrared sources may be sites of triggered star formation in NGC 3372.     

奥斯特/GPS掩星与地面雷达联合观测电离层电子密度的初步结果

1999年2月丹麦发射了一颗奥斯特小卫星,它装载了两台GPS接收机,其中一台用于电离层和中性大气掩星测量,另一台用于自主定位。本文给出了1999年10月奥斯特/GPS掩星与日本MU雷达和数字测高仪的联合观测结果。奥斯特掩星反演得到的电子密度剖面与地面雷达观测的进行了比较,结果表明:天基和地基雷达观测的电子密度是一致的。文章还给出利用GPS的单频信号反演电离层电子密度剖面技术和3维射线追踪技术模拟掩星结果。     

一次成功的应力预测地震

在冰岛成功地“应力预测”了１次５级地震,做法是根据Ｓ波分裂的时间延迟变化来估计地震的时间和强度,即估计在受应力作用的体积（也是地震释放应变的体积）内微裂隙在应力作用下达到破裂临界点的和强度。当地的研究提供了大致的震中位置。本文叙述了应力预测依据的准则。