The Optical Gravitational Lensing Experiment. Variable Stars in Globular Clusters - IV. Fields 104A-E in 47 Tuc

Five fields located close to the center of the globular cluster NGC 104=47 Tuc were surveyed in a search for variable stars. We present V-band light curves for 42 variables. This sample includes 13 RR Lyr stars -- 12 of them belong to the Small Magellanic Cloud (SMC) and 1 is a background object from the galactic halo. Twelve eclipsing binaries were identified -- 9 contact systems and 3 detached/semi-detached systems. Seven eclipsing binaries are located in the blue straggler region on the cluster color-magnitude diagram (CMD) and four binaries can be considered main-sequence systems. One binary is probably a member of the SMC. Eight contact binaries are likely members of the cluster and one is most probably a foreground star. We show that for the surveyed region of 47 Tuc, the relative frequency of contact binaries is very low as compared with other recently surveyed globular clusters. The sample of identified variables also includes 15 red variables with periods ranging from about 2 days to several weeks. A large fraction of these 15 variables probably belong to the SMC but a few stars are likely to be red giants in 47 Tuc. VI photometry for about 50 000 stars from the cluster fields was obtained as a by product of our survey.Comment: Latex file, l-aa style, 10 pages, 7 ps figures included. Submitted to A&A. Figure 8 available from ftp://sirius.astrouw.edu.pl/pub/jka/47Tuc/fig8.ps.

The Host Galaxy of GRB 060505: Host ISM Properties

We investigate the ISM environment of GRB 060505. Using optical emission-line diagnostic ratios, we compare the ISM properties of the GRB 060505 host region with the hosts of unambiguous long- and short-duration GRBs. We show that the metallicity, ionization state, and star formation rate of the GRB 060505 environment are more consistent with short-duration GRBs than with long-duration GRBs. We compare the metallicity and star formation rates of the GRB 060505 region with four other star-forming regions within the GRB 060505 host galaxy. We find no significant change in metallicity or star formation rate between the GRB 060505 region and the other four host regions. Our results are consistent with a compact-object-merger progenitor for GRB 060505.Comment: 7 pages, two figures; accepted for publication in ApJ

BVRI Observations of the Optical Afterglow of GRB 990510

We present $BVRI$ observations of the optical counterpart to the Gamma-Ray Burst (GRB) 990510 obtained with the Las Campanas 1.0-m telescope between 15 and 48 hours after the burst. The temporal analysis of the data indicates steepening decay, independent of wavelength, approaching asymptotically $t^{-0.76\pm 0.01}$ at early times ($t\ll 1 day)$ and $t^{-2.40\pm 0.02}$ at late times, with the break time at $t_0=1.57\pm 0.03 days$. GRB 990510 is the most rapidly fading of the well-documented GRB afterglows. It is also the first observed example of broad-band break for a GRB optical counterpart. The optical spectral energy distribution, corrected for significant Galactic reddening, is well fitted by a single power-law with $\nu^{-0.61\pm 0.12}$. However, when the $B$-band point is dropped from the fit, the power-law becomes $\nu^{-0.46\pm 0.08}$, indicating a possible deviation from the power-law in the spectrum, either intrinsic or due to additional extinction near the source or from an intervening galaxy at $z=1.62$. Broad-band break behavior broadly similar to that observed in GRB 990510 has been predicted in some jet models of GRB afterglows, thus supporting the idea that the GRB energy is beamed, at least in some cases.Comment: submitted to the ApJ Letters, 13 pages, 5 tables, 3 figures; additional data available at ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB990510/ and through WWW at http://cfa-www.harvard.edu/cfa/oir/Research/GRB

Long-term variability survey of the old open cluster NGC 6791

We present the results of a long-term variability survey of the old open cluster NGC 6791. The BVI observations, collected over a time span of 6 years, were analyzed using the ISIS image subtraction package. The main target of our observations were two cataclysmic variables B7 and B8. We have identified possible cycle lenghts of about 25 and 18 days for B7 and B8, respectively. We tentatively classify B7 as a VY Scl type nova-like variable or a Z Cam type dwarf nova. B8 is most likely an SS Cygni type dwarf nova. We have also extracted the light curves of 42 other previously reported variable stars and discovered seven new ones. The new variables show long-period or non-periodic variability. The long baseline of our observations has also allowed us to derive more precise periods for the variables, especially for the short period eclipsing binaries.Comment: 13 pages LaTeX, including 8 PostScript figures and 4 tables. To appear in June 2003 issue of The Astronomical Journa

Are the Ogle Microlenses in the Galactic Bar?

The analysis of the first two years of OGLE data revealed 9 microlensing events of the galactic bulge stars, with the characteristic time scales in the range $8.6 < t_0 < 62$ days, where $t_0 = R_E / V$. The optical depth to microlensing is larger than $( 3.3 \pm 1.2 ) \times 10^{-6}$, in excess of current theoretical estimates, indicating a much higher efficiency for microlensing by either bulge or disk lenses. We argue that the lenses are likely to be ordinary stars in the galactic bar, which has its long axis elongated towards us. A relation between $t_0$ and the lens masses remains unknown until a quantitative model of bar microlensing becomes available. At this time we have no evidence that the OGLE events are related to dark matter. The geometry of lens distribution can be determined observationally when the microlensing rate is measured over a larger range of galactic longitudes, like $-10^o < l < +10^o$, and the relative proper motions of the galactic bulge (bar) stars are measured with the HST.Comment: 10 pages, 2 figures, revised version accepted for the publication in ApJL, uses AAS LaTeX aaspp.sty macro, PostScript figures and PostScript version of the paper available through anonymous ftp from astro.princeton.edu, directory stanek/tau, or on reques