Gamma-Ray Bursts at Low Redshift

Long duration gamma-ray bursts (GRB) are at cosmological distance, they appear to be located near star forming regions, and are likely associated with some type of supernovae. They are also likely to be strongly beamed, which lowers their energetics by several orders of magnitude, and increases their rate by the same factor. Therefore, it is likely that one out of 100 - 1000 core collapse supernovae generates ultra-relativistic jets, which beam gamma-rays and all other early emission into two narrow cones. After a year, or so, the jets are decelerated and become non-relativistic, and their emission becomes more or less isotropic. At least two GRB: 970508 (z = 0.835) and 980703 (z = 0.966) show strong radio emission from late, and therefore non-relativistic afterglows. Such events should be readily detectable at low redshift, say z = 0.03. A search for strong radio emitters among recent nearby supernovae should be done $\sim 1$ year after the explosion. If some of these explosions generated GRB and their gamma-ray beam missed us, the bipolar structure of the radio remnant should be resolvable with VLBA.Comment: 4 pages, latex, minor changes, published in Acta Astronomica, 51,

Gamma-Ray Burst - Supernova Relation

There is growing evidence that long and hard gamma-ray bursts (GRBs), discovered at redshifts between 0.4 and 3.4, are related to some type of supernova (SN) explosions. The GRB ejecta are ultra-relativistic, and possibly beamed. There is a possibility that some SN ejecta are also beamed and/or relativistic. Prospects for farther advances guided by expected and unexpected observational developments are very good. The prospects for developing a sound and quantitative GRB theory any time soon are rather modest, if histories of quasars, radio pulsars and supernovae are used for reference. However, the current progress in the understanding of GRB afterglows (which are relativistic) and remnants (which are non-relativistic) is likely to continue, as these appear to be simpler than the GRBs. According to the current analysis of GRB 970508 the energy of gamma rays released by this event was about the same as the total energy of explosion. If correct, this result is difficult to reconcile with the internal shock models. It also implies that the global energy generation rate by GRBs is four orders of magnitude lower than the rate due to ordinary supernovae, which makes it very unlikely that the highly energetic supernova remnants were created by GRBs.Comment: 12 pages, latex Published in the Proceedings of the Space Telescope Science Institute 1999 May Symposium (13): ``Supernovae and Gamma Ray Bursts; The Largest Explosions Since the Big Bang'', p. 1; Eds.: M. Livio, N. Panagia and K. Sahu; Cambridge University Press, 200

Can HST Measure the Mass of the Isolated Neutron Star RX J185635-3754 ?

In June 2003 the isolated neutron star RX J185635-3754 will pass within 0.3'' of a 26.5 mag star, changing its position by about 0.6 mas. The displacement, caused by gravitational lensing, will be proportional to the neutron star mass. The total event duration will be approximately 1 year.Comment: 2 pages, no figures, plain te

On the Detectability of Very Massive Compact Objects with Gravitational Microlensing

If the dark halo of our galaxy is made of compact objects as massive as M = 10^6 solar masses, their detection by means of ordinary microlensing searches would take a very long time as the characteristic time scale of such a lensing event, t_0, is approximately 200 years. Fortunately, the very high magnification events of the numerous faint stars, which are normally well below the detection threshold, have short duration peaks with a characteristic time scale of only t_0 / A_{max}, where A_{max} >> 1 is the peak magnification factor. The two almost equally bright images are separated by approximately 2'' (M/10^6 solar masses )^{1/2}, and they rotate very rapidly around the lens with the relative proper motion enhanced by a factor 2 A_{max}. The same events will offer an opportunity to study spectroscopically stars which are normally far too faint to be reached.Comment: 6 pages, plain TEX, no figure

Advection Dominated Accretion Flows. A Toy Disk Model

A toy model of a disk undergoing steady state accretion onto a black hole is presented. The disk is in a hydrostatic equilibrium for all radii r > r_{in}, with the inner disk radius located between the marginally stable and marginally bound orbits: r_{ms} > r_{in} > r_{mb}. Matter flows from the disk through a narrow cusp at r_{ms} and falls freely into the black hole, carrying with it no thermal energy. At radii larger than r_{out} the disk is assumed to radiate away all locally generated heat, and therefore the disk is geometrically thin for r > r_{out}. We assume that no heat generated in the inner disk, with r_{out} > r > r_{in} can be radiated away, i.e. the disk is 100% advective, and it becomes geometrically thick in this range of radii. All enthalpy of the thick disk is used up to press the inner disk radius towards the marginally bound orbit, and to lower the efficiency of conversion of accreted mass into radiation generated only for r > r_{out}, by assumption. Conservation laws of mass, angular momentum and energy make it possible to calculate the inner thick disk radius r_{in} for any specified value of its outer radius r_{out}. As the nature of disk viscosity is not known there is some freedom in choosing the shape of the thick disk, subject to several general conditions, which include the hydrostatic equilibrium everywhere for r > r_{in}. The main purpose of this toy model is to emphasize the effect the disk thickness has on lowering the energetic efficiency of a black hole accretion.Comment: 12 pages, self-contained latex, 4 postscript figures, published 1998, Acta Astronomica, 48, 667, several errors correcte

Current Status of the Microlensing Surveys

The ongoing microlensing searches have generated more photometric measurements of pulsating stars than all previous observing projects combined. In particular, OGLE has made ~ 340,000 B, V, and I-band measurements of ~ 1,300 Cepheids in the Large Magellanic Clouds accessible over Internet. Microlensing searches contributed to the development of very efficient image subtraction software which works best in crowded fields. This suggests the use of a period - flux amplitude rather than period - luminosity relation for the Cepheids for distance determination, as the flux amplitude is directly measurable with the image subtraction, and it is not biased by crowding. Future projects will dramatically increase the data rate, will provide all-sky coverage and a complete census of variables, including pulsating stars, to the ever fainter limits. Time will show which approach, a small number of large teams or a large number of small teams, will be more productive.Comment: 9 pages with 3 figures, latex, revised and extended on November 29, 1999, to be published in the ASP Conference Series Vol. XXX, 1999: ``The Impact of Large Scale Surveys on Pulsating Star Research'', L. Szabados and D. Kurtz, ed

Gravitational Microlensing by the Globular Cluster Stars

The microlensing of background stars by compact objects in globular clusters is analyzed. The main strength of the proposed search is the direct relationship between the lens mass and the time scale of the microlensing event. The main problem is the low overall rate of expected events which implies that a ground based search should last for about a decade to generate a non trivial number of events, like a few dozen. A space borne experiment could increase the rate by a large factor by increasing the number of stars which could be monitored thanks to the much better seeing. The clusters M22 with the galactic bulge background, and 47 Tuc with the SMC background, are the two examples of possible targets.Comment: 6 pages; plain TeX. Accepted for publication in Acta Astronomica 44/

Massive Variability Searches: The Past, Present and Future Massive Variability Searches

Many decades ago a search for variable stars was one of the main areas of astrophysical research. Such searches, conducted with CCD detectors rather than with photographic plates, became a by-product of several projects seeking gravitational microlensing events towards the Magellanic Clouds and/or the Galactic Bulge: EROS, MACHO, and OGLE. These searches demonstrated that is is possible and practical to process in near real time photometry of tens of millions of stars every night, and to discover hundreds of thousands of variable stars. A limited subset of new variable star catalogs was published, but no comprehensive database of all photometric results became public domain so far. In the last few years a much broader, but shallower searches have been undertaken, and many other are at various stages of implementation or planning. There is a need to develop a system that would allow all these data to be processed and to be posted on the Internet in real time. Full information related to variability of point sources is made of a relatively few data types, hence it may be relatively easy to handle. Yet, it may be diverse enough to be interesting to a large number of users, professional as well as amateur, making it possible to do real time virtual observing, as well as data mining.Comment: Published in the Proceedings of the MPA/ESO/MPE Workshop: `Mining the Sky' held at Garching, Germany July 31 - August 4, 2000; Springer, Eds: A.J. Banday, S. Zaroubi, and M. Bartelmann, p. 481; latex, 6 page

Gravitational Microlensing: Black Holes, Planets; OGLE, VLTI, HST and Space Probes

OGLE and other projects are likely to discover first stellar mass black holes and the first planets through gravitational lensing in the next year or two. It is important to have follow-up projects ready, using diverse observing methods. The best for black hole detection would be a measurement of image splitting with VLTI, or any other optical interferometer. Alternative approach is to measure non-linear motion of the light centroid with the HST, or even with a ground based telescope. Every year OGLE detects several very long duration microlensing events brighter than I = 16 mag and K = 14 mag. The two images may be separated by up to 10 mas. Ground based detection of strong caustic crossing planetary events will provide mass ratios and proper motions for the detected systems. For most events photometric parallax needed for mass determination will require a space instrument at least as far as the L2 point, to provide long enough baseline.Comment: 10 pages, latex 2 figure

The Distance to Pleiades

The distance to Pleiades remains controversial. There is a simple way to resolve the dispute definitely by measuring the distance to one of its brightest members, Atlas, which is astrometric and spectroscopic binary.Comment: 3 pages, latex, 1 figure, submitted to Acta Astronomic