The Orbital Period of the Be/Neutron Star Binary RX J0812.4-3114

We present the results of Rossi X-ray Timing Explorer observations of the Be star X-ray binary system RX J0812.4-3114. A light curve obtained with the RXTE All-Sky Monitor shows that the source is currently in an active state with outbursts occurring at approximately 80 day intervals. The source underwent a transition from an inactive state to this regular outburst state early in 1998. An observation of RX J0812.4-3114 was obtained with the RXTE Proportional Counter Array close to the time of a predicted maximum in March 1999 and strong pulsations were detected at a period of 31.88 seconds. This confirms the result of an earlier PCA observation by Reig & Roche which was serendipitously also obtained near the predicted maximum flux of the 80 day period and also near the start of the current active state. We interpret the periodicity in the ASM light curve as indicating the orbital period of RX J0812.4-3114 with outbursts occurring around periastron passage

Automated extraction of knowledge for model-based diagnostics

The concept of accessing computer aided design (CAD) design databases and extracting a process model automatically is investigated as a possible source for the generation of knowledge bases for model-based reasoning systems. The resulting system, referred to as automated knowledge generation (AKG), uses an object-oriented programming structure and constraint techniques as well as internal database of component descriptions to generate a frame-based structure that describes the model. The procedure has been designed to be general enough to be easily coupled to CAD systems that feature a database capable of providing label and connectivity data from the drawn system. The AKG system is capable of defining knowledge bases in formats required by various model-based reasoning tools

Search for the Tunguska event in the Antarctic snow

The Tunguska explosion in 1908 is supposed to have been produced by the impact of a small celestial body. The absence of any identifiable crater together with the huge energy released by the event suggest that the impactor exploded in midair and that its material was widely spread over the Earth. The short term contribution of such exceptional events to the total accretion rate of extraterrestrial material by the Earth could be significant. Samples were chosen in a core electromechanically drilled in 1984 near South Pole Station. There, the low temperatures, preventing melting all year long, and the nearly regular snow fall rate provide good conditions for a reliable continuous record of any infalling material. In many samples Ir was below the detection limit of the instrumentation. The iridium infall averaged over 45 samples is given. In a few samples the iridium content is significantly higher than the average: the frequency and amplitude of such fluctuations can be explained by the presence on some filters of finite size cosmic particles. No significant systematic increase above the average level is observed in the part of the core corresponding to the Tunguska event. The two major results of this study are: (1) The presence of Tunguska explosion debris in the Antarctic snow is not confirmed; and (2) The estimate of the average iridium infall, is an order of magnitude lower than the Ganapathy's background but is close to the values measured in Antarctic snow and atmospheric samples by Takahashi et al. The results are also consistent with the flux of micrometeoroids deduced from optical and radar observations or derived from the study of Greenland cosmic dust collection but are lower than the flux at mid-latitude measured in paleocene-oligocene sediments from the central part of the Pacific Ocean

The Power of Words [brochure and video]

Sticks and stones may break my bones, but words will never hurt. This statement is not true for many reasons and for many people. Students across campuses throughout the nation feel the sting of hate words that hurt daily. Counselors assist students suffering from depression and suicidal thoughts everyday that stems from the usage of these harmful words. Both the user and receiver of these hurtful words are affected. Hurt Words exposes these words for what they are, gives them meaning on a personal level, show the effects of harmful language and how students, faculty and staff can help stop those words from hurting more students

The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System

A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026 using observations made with the Proportional Counter Array on board the Rossi X-ray Timing Explorer. The mass function obtained of ~16Mo together with the detection of an extended near-total eclipse confirm that the primary star is a supergiant as predicted. The orbital eccentricity is found to be very low with a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be 6.0724 +/- 0.0009 days using an improved and extended light curve obtained with RXTE's All Sky Monitor. Observations with the ASCA satellite provide an improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0" (2000) with an estimated systematic uncertainty of less than 12". A serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA observations.Comment: Accepted for publication in the Astrophysical Journa

Evaluation of several lightweight stochastic context-free grammars for RNA secondary structure prediction

BACKGROUND: RNA secondary structure prediction methods based on probabilistic modeling can be developed using stochastic context-free grammars (SCFGs). Such methods can readily combine different sources of information that can be expressed probabilistically, such as an evolutionary model of comparative RNA sequence analysis and a biophysical model of structure plausibility. However, the number of free parameters in an integrated model for consensus RNA structure prediction can become untenable if the underlying SCFG design is too complex. Thus a key question is, what small, simple SCFG designs perform best for RNA secondary structure prediction? RESULTS: Nine different small SCFGs were implemented to explore the tradeoffs between model complexity and prediction accuracy. Each model was tested for single sequence structure prediction accuracy on a benchmark set of RNA secondary structures. CONCLUSIONS: Four SCFG designs had prediction accuracies near the performance of current energy minimization programs. One of these designs, introduced by Knudsen and Hein in their PFOLD algorithm, has only 21 free parameters and is significantly simpler than the others

Rossi X-ray Timing Explorer Observations of the X-ray Pulsar EXO 1722-363 - a Candidate Eclipsing Supergiant System

Observations made of the X-ray pulsar EXO 1722-363 using the Proportional Counter Array and All Sky Monitor on board the Rossi X-ray Timing Explorer reveal the orbital period of this system to be 9.741 +/- 0.004 d from periodic changes in the source flux. The detection of eclipses, together with the values of the pulse and orbital periods, suggest that this source consists of a neutron star accreting from the stellar wind of an early spectral type supergiant companion. Pulse timing measurements were also obtained but do not strongly constrain the system parameters. The X-ray spectra can be well fitted with a model consisting of a power law with a high energy cutoff and, for some spectra, a blackbody component with a temperature of approximately 0.85 keV.Comment: Accepted for publication in The Astrophysical Journal. 27 pages including 10 figure

An XMM-Newton Observation of the Local Bubble Using a Shadowing Filament in the Southern Galactic Hemisphere

We present an analysis of the X-ray spectrum of the Local Bubble, obtained by simultaneously analyzing spectra from two XMM-Newton pointings on and off an absorbing filament in the Southern galactic hemisphere (b ~ -45 deg). We use the difference in the Galactic column density in these two directions to deduce the contributions of the unabsorbed foreground emission due to the Local Bubble, and the absorbed emission from the Galactic halo and the extragalactic background. We find the Local Bubble emission is consistent with emission from a plasma in collisional ionization equilibrium with a temperature $\log T_{LB} = 6.06^{+0.02}_{-0.04}$ and an emission measure of 0.018 cm^{-6} pc. Our measured temperature is in good agreement with values obtained from ROSAT All-Sky Survey data, but is lower than that measured by other recent XMM-Newton observations of the Local Bubble, which find $\log T_{LB} \approx 6.2$ (although for some of these observations it is possible that the foreground emission is contaminated by non-Local Bubble emission from Loop I). The higher temperature observed towards other directions is inconsistent with our data, when combined with a FUSE measurement of the Galactic halo O VI intensity. This therefore suggests that the Local Bubble is thermally anisotropic. Our data are unable to rule out a non-equilibrium model in which the plasma is underionized. However, an overionized recombining plasma model, while observationally acceptable for certain densities and temperatures, generally gives an implausibly young age for the Local Bubble (\la 6 \times 10^5 yr).Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 9 figure