Showing papers by "Ram Sagar published in 1999"
University of Amsterdam1, University of Alabama in Huntsville2, Stony Brook University3, Marshall Space Flight Center4, Rice University5, University of Groningen6, ASTRON7, University of Cambridge8, Spanish National Research Council9, International Trademark Association10, University of Hertfordshire11, National Science Foundation12, University of California, Berkeley13, Netherlands Institute for Space Research14, Imperial College London15, University of Kent16, Tel Aviv University17, Harvard University18, University of Arizona19, Raman Research Institute20
Abstract: Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.
150 citations
International Trademark Association1, Spanish National Research Council2, University of Copenhagen3, University of Bologna4, University of Ferrara5, University of Iceland6, University of Liège7, Kitt Peak National Observatory8, University of Bonn9, Max Planck Society10, Chinese Academy of Sciences11
TL;DR: Broad-band observations of the intense gamma ray burst GRB 990123 started approximately 8.5 hours after the event and continued until 18 February 1999, suggesting some color dependence that could be related to a cooling break passing the ultraviolet-optical band at about 1 day after the high-energy event.
Abstract: Broad-band (ultraviolet to near-infrared) observations of the intense gamma ray burst GRB 990123 started ∼8.5 hours after the event and continued until 18 February 1999. When combined with other data, in particular from the Robotic Telescope and Transient Source Experiment (ROTSE) and the Hubble Space Telescope (HST), evidence emerges for a smoothly declining light curve, suggesting some color dependence that could be related to a cooling break passing the ultraviolet-optical band at about 1 day after the high-energy event. The steeper decline rate seen after 1.5 to 2 days may be evidence for a collimated jet pointing toward the observer.
115 citations
TL;DR: In this article, the metal-rich Galactic bulge globular cluster NGC 6553 and an adjacent field region were analyzed using a VI CCD photometry of about 40,000 stars down to 23 mag.
Abstract: We have carried out VI CCD photometry of about 40 000 stars down to 23 mag in an area of about centered on the metal-rich Galactic bulge globular cluster NGC 6553 and an adjacent field region. Our photometry agrees fairly well with HST data. The cluster population dominates over the field population up to a radial distance of ~ 3′ from the cluster centre. The distance and reddening for the field population present in the direction of the cluster are derived for the first time. These values put the cluster NGC 6553 in the background of the young (age ~ 800 Myr) Galactic disk stars but in the foreground of the old Galactic bulge populations. The similar values of 0.9 for both the disk and the cluster indicate the absence of interstellar matter over a distance of ~ 3 kpc between the disk and the cluster. An analysis of the giant branch morphology confirms that the metallicity of the cluster population is similar to solar. The red giant branch (RGB) of both cluster and Galactic bulge extend beyond and currently available theoretical isochrones reproduce its shape only up to 4.5. Our analysis indicates that the presence of differential interstellar extinction across the cluster face causes some elongation and tilt in the HB and produces scatter in the giant branch. The ratio of duration of the RGB-bump phase relative to the life time of the star during the HB phase derived from present observations is in good agreement with the theoretical predictions.
27 citations