Preparation & Characterization of Dimethylformamidyl Substituted Titanium( IV) Compounds

456-45

Optical and Infrared Observations of SGR 1806-20

The soft gamma-ray repeater (SGR) 1806$-$20 is associated with the center-brightened non-thermal nebula G~10.0$-$0.3, thought to be a plerion. As in other plerions, a steady \Xray\ source, AX~1805.7$-$2025, has been detected coincident with the peak of the nebular radio emission. Vasisht et al.\ have shown that the radio peak has a core-jet appearance, and argue that the core marks the true position of the SGR. At optical wavelengths, we detect three objects in the vicinity of the radio core. Only for the star closest to the core, barely visible in the optical but bright in the infrared ($K=8.4\,$mag.), the reddening is consistent with the high extinction ($A_V\simeq30\,$mag.) that has been inferred for AX~1805.7$-$2025. From the absence of CO band absorption, we infer that the spectral type of this star is earlier than late~G/early~K. The large extinction probably arises in a molecular cloud located at a distance of 6$\,$kpc, which means that the star, just like AX~1805.7$-$2025, is in or behind this cloud. This implies that the star is a supergiant. Since supergiants are rare, a chance coincidence with the compact radio core is very unlikely. To our knowledge, there are only three other examples of luminous stars embedded in non-thermal radio nebulae, SS~433, \mbox{Cir X-1} and G~70.7+1.2. Given this and the low coincidence probability, we suggest that the bright star is physically associated with SGR~1806$-$20, making it the first stellar identification of a high-energy transient.Comment: 7 pages, AASTeX (needs LaTeX style files aaspptwo.sty and epsf.sty, plus PostScript figure). In case of problems, contact [email protected]

A Giant Glitch in the Energetic 69 ms X-ray Pulsar AXS J161730-505505

We present new results on the recently discovered 69 ms X-ray pulsar AXS J161730-505505, the sixth youngest sample of all known pulsars. We have undertaken a comprehensive X-ray observing campaign of AXS J161730-505505 with the ASCA, SAX, and XTE observatories and follow its long term spin-down history between 1989 and 1999, using these, archival GINGA and ASCA data sets, and the radio ephemeris. The spin-down is not simply described by a linear function as originally thought, but instead we find evidence of a giant glitch (|Delta P/P| > 10E-6) between 1993 August and 1997 September, perhaps the largest yet observed from a young pulsar. The glitch is well described by steps in the period and its first derivative accompanied by a persistent second derivative similar to those in the Vela pulsar. The pulse profile of AXS J161730-505505 presents a single asymmetric peak which is maintained over all observation epochs. The energy spectrum is also steady over time, characterized by a highly absorbed power-law with a photon index 1.4 +/- 0.2, consistent with that found for other young rotation powered pulsars.Comment: 6 pages with 2 figures, LaTex, emulateapj.sty. To appear in the Astrophysical Journal Letter

The Discovery of an Anomalous X-ray Pulsar in the Supernova Remnant Kes 73

We report the discovery of pulsed X-ray emission from the compact source 1E 1841-045, using data obtained with the Advanced Satellite for Cosmology and Astrophysics. The X-ray source is located in the center of the small-diameter supernova remnant (SNR) Kes 73 and is very likely to be the compact stellar-remnant of the supernova which formed Kes 73. The X-rays are pulsed with a period of ~ 11.8 s, and a sinusoidal modulation of roughly 30 %. We interpret this modulation to be the rotation period of an embedded neutron star, and as such would be the longest spin period for an isolated neutron star to-date. This is especially remarkable since the surrounding SNR is very young, at ~ 2000 yr old. We suggest that the observed characteristics of this object are best understood within the framework of a neutron star with an enormous dipolar magnetic field, B ~ 8x10^14 G

A giant, periodic flare from the soft gamma repeater SGR1900+14

Soft gamma repeaters are high-energy transient sources associated with neutron stars in young supernova remnants. They emit sporadic, short (~ 0.1 s) bursts with soft energy spectra during periods of intense activity. The event of March 5, 1979 was the most intense and the only clearly periodic one to date. Here we report on an even more intense burst on August 27, 1998, from a different soft gamma repeater, which displayed a hard energy spectrum at its peak, and was followed by a ~300 s long tail with a soft energy spectrum and a dramatic 5.16 s period. Its peak and time integrated energy fluxes at Earth are the largest yet observed from any cosmic source. This event was probably initiated by a massive disruption of the neutron star crust, followed by an outflow of energetic particles rotating with the period of the star. Comparison of these two bursts supports the idea that magnetic energy plays an important role, and that such giant flares, while rare, are not unique, and may occur at any time in the neutron star's activity cycle.Comment: Accepted for publication in Natur

Nonmonotonic dependence of the absolute entropy on temperature in supercooled Stillinger-Weber silicon

Using a recently developed thermodynamic integration method, we compute the precise values of the excess Gibbs free energy (G^e) of the high density liquid (HDL) phase with respect to the crystalline phase at different temperatures (T) in the supercooled region of the Stillinger-Weber (SW) silicon [F. H. Stillinger and T. A. Weber, Phys. Rev. B. 32, 5262 (1985)]. Based on the slope of G^e with respect to T, we find that the absolute entropy of the HDL phase increases as its enthalpy changes from the equilibrium value at T \ge 1065 K to the value corresponding to a non-equilibrium state at 1060 K. We find that the volume distribution in the equilibrium HDL phases become progressively broader as the temperature is reduced to 1060 K, exhibiting van-der-Waals (VDW) loop in the pressure-volume curves. Our results provides insight into the thermodynamic cause of the transition from the HDL phase to the low density phases in SW silicon, observed in earlier studies near 1060 K at zero pressure.Comment: This version is accepted for publication in Journal of Statistical Physics (11 figures, 1 table