The Sagittarius dwarf galaxy as a microlensing target

We estimate the optical depth, time-scale distribution and fraction of microlensing events originating from sources in the Sagittarius dwarf galaxy (Sgr) lensed by deflectors in the Milky Way. These events have a time-scale longer by a factor ~1.3 than the MW/MW events and occur mainly on sources fainter than V~21 mag below Sgr's turn off. The fraction of events involving a source in Sgr depends on the location and extinction of the field and on the limiting magnitude of the survey. The contribution of the MW/Sgr events is negligible (<1%) at very low latitudes (|b|<2 deg.) but increases continuously towards higher |b| and becomes dominant near the highest density region of the dwarf galaxy. Sgr is present within the fields of current microlensing surveys and any optical depth map inferred from observations will become biased by the presence of Sgr towards higher |b| where the contribution of MW/Sgr events is significant. Systematic spectroscopic measurements on the sources of all the microlensing events may allow detection of this kind of event for which the degeneracy on the lens mass can be significantly reduced.Comment: 9 pages, 8 figures. Accepted for publication in A&A Main Journa

Design of experiment for the optimisation of deep reactive ion etching of silicon inserts for micro-fabrication

The following paper describes a design of experiments investigation of the deep reactive of pillar structures on a silicon wafer. The etched wafers would subsequently be used as masters for the fabrication of nickel mould inserts for microinjection moulding. Undercuts occur when the pillar base has a smaller cross-section than the apex of the pillar. They therefore affect tolerances of the subsequent nickel mould, its strength and its de-mouldability from the silicon form. The response measured in these experiments was the degree of undercut of micro-scale (10 μm x 10 μm x 40 μm, 5 μm x 5 μm x 40 μm and 2 μm x 2 μm x 40 μm) The literature suggests that gas pressure, platen power, gas flow rate, phase switching times and mask size can all affect the degree of undercut. After examination of this literature, and of manufacturers guidelines, three parameters were selected for experimental testing: platen power, C 4F 8 gas flow rate during the passivation phase and switching times. Switching times was found to be the only statistically significant parameter for both 10x10 μm and 5x5 μm pillars. The 2x2 μm pillars were not successfully replicated and could therefore not undergo statistical evaluation

X-ray Pulsations in the Supersoft X-ray Binary CAL 83

X-ray data reveal that the supersoft X-ray binary CAL 83 exhibits 38.4 minute pulsations at some epochs. These X-ray variations are similar to those found in some novae and are likely to be caused by nonradial pulsations the white dwarf. This is the first detection of pulsations in a classical supersoft X-ray binary.Comment: revised text; 11 pages and 3 figures; accepted for publication in the Astronomical Journa

Evidence for mass ejection associated with long secondary periods in red giants

Approximately 30% of luminous red giants exhibit a Long Secondary Period (LSP) of variation in their light curves, in addition to a shorter primary period of oscillation. The cause of the LSP has so far defied explanation: leading possibilities are binarity and a nonradial mode of oscillation. Here, large samples of red giants in the Large Magellanic Cloud both with and without LSPs are examined for evidence of an 8 or 24 $\mu$m mid-IR excess caused by circumstellar dust. It is found that stars with LSPs show a significant mid-IR excess compared to stars without LSPs. Furthermore, the near-IR $J$-$K$ color seems unaffected by the presence of the 24 $\mu$m excess. These findings indicate that LSPs cause mass ejection from red giants and that the lost mass and circumstellar dust is most likely in either a clumpy or a disk-like configuration. The underlying cause of the LSP and the mass ejection remains unknown.Comment: 6 pages, accepted for publication in Ap

Comments on the paper ``Bare Quark Surfacees of Strange Stars and Electron-Positron Pair Emission''

In a recent paper (Ushov, PRL, 80, 230, 1998), it has been claimed that the bare surface of a strange star can emit electron-positron pairs of luminosity \~10^{51} ergs/s for about 10s. If true, obviously, this mechanism may explain the origin of cosmic Gamma Ray Bursts. However, we point out that such a mechanism is does not work because (i) if pair production really occurs the supposed pre-existing supercritical electric field will be quenched and this discharge process may at best release ~10^{24} ergs of electromagnetic energy, and (ii) there is no way by which the trapped core thermal energy of few 10^{52} ergs can be transmitted electromagnetically on a time scale of ~10s or even on a much larger time scale. The only way the hot core can cool on a time scale of ~10 s or much shorter is by the well known process of emission of nu-antinu pairs.Comment: Final version accepted in Phy. Rev. Lett. Main conclusion that the mechanism by Usov does not work remains unchanged, [email protected]

Einstein Radii from Binary Lensing Events

We show that the Einstein ring radius and transverse speed of a lens projected on the source plane, $\hat{r}_{\rm e}$ and $\hat{v}$, can be determined from the light curve of a binary-source event, followed by the spectroscopic determination of the orbital elements of the source stars. The determination makes use of the same principle that allows one to measure the Einstein ring radii from finite-source effects. For the case when the orbital period of the source stars is much longer than the Einstein time scale, $P\gg t_{\rm e}$, there exists a single two-fold degeneracy in determining $\hat{r}_{\rm e}$. However, when $P \lesssim t_{\rm e}$ the degeneracy can often be broken by making use of the binary-source system's orbital motion. %Once $\hat{r}_{\rm e}$, and thus $\hat{v}$ are determined, one can %distinguish self-lensing events in the Large Magellanic Cloud %from Galactic halo events. For an identifiable 8\% of all lensing events seen toward the Large Magellanic Cloud (LMC), one can unambiguously determine whether the lenses are Galactic, or whether they lie in the LMC itself. The required observations can be made after the event is over and could be carried out for the $\sim 8$ events seen by Alcock et al.\ and Aubourg et al.. In addition, we propose to include eclipsing binaries as sources for gravitational lensing experiments.Comment: 18 pages, revised version, submitted to Ap

Organolithium bases in flow chemistry: a review

Flow chemistry is a continually emerging and ever-growing area of synthetic organic chemistry. It provides an orthogonal approach to traditional batch chemistry, oftentimes allowing for more efficient routes to desired target molecules. It is generally accepted that flow chemistry offers a valuable change to the process landscape. From a process perspective, there are many advantages associated with flow chemistry over traditional batch chemistry, the most prominent of which is an increased safety profile with the use of highly reactive chemical species, such as organolithiums. These reagents are highly valuable species for the efficient synthesis of pharmaceutical intermediates. Disadvantageously, use of these reagents on commercial scale is severely hindered by the highly energetic nature of the reaction intermediates and their concomitant safety risk. Flow chemistry provides a viable platform for use of these reagents, offering a high degree of control over reaction parameters. In this review, we present a comprehensive account of the published literature implementing the use of organolithium reagents as strong bases for deprotonation reactions in flow systems

Finding outlier light-curves in catalogs of periodic variable stars

We present a methodology to discover outliers in catalogs of periodic light-curves. We use cross-correlation as measure of ``similarity'' between two individual light-curves and then classify light-curves with lowest average ``similarity'' as outliers. We performed the analysis on catalogs of variable stars of known type from the MACHO and OGLE projects and established that our method correctly identifies light-curves that do not belong to those catalogs as outliers. We show how our method can scale to large datasets that will be available in the near future such as those anticipated from Pan-STARRS and LSST.Comment: 16 pages, 24 figure

MACHO Photometry of Two LMC Be X-ray Transients, EXO 0531-66 and H 0544-665

Long-term variations are well-known in Be X-ray binaries, and are attributed to non-orbital changes in the structure of the Be circumstellar (equatorial) disc. However, the timescales involved are so long (tens of days, to years) that systematic studies have been very restricted. The ~8 year MACHO monitoring of the Large Magellanic Cloud (LMC) therefore presents an ideal opportunity to undertake such studies of Be X-ray systems that lie within the monitored fields. Here we present MACHO observations of two LMC Be X-ray transients, EXO 0531-66 and H 0544-665, the light curves of which show substantial (~0.5 mag) long-term variations. However, our analysis shows little evidence for any periodic phenomena in the light curves of either source. We find an upper limit for detection of a short (1-100 d) periodicity in the V- and R-band light curves of EXO 0531-66 of 0.041 mag and 0.047 mag semi-amplitude, respectively. The upper limits for the V- and R-band data of H 0544-665 are 0.054 mag and 0.075 mag semi-amplitude, respectively. Both EXO 0531-66 and H 0544-665 become redder as they brighten, possibly due to variations in the structure of the equatorial disc around the Be star. Spectra of both sources show H$\alpha$ emission; for EXO 0531-66 we find the emission varies over time, thereby confirming its optical identification.Comment: 8 pages, 7 figures, accepted for publication in MNRA