153 research outputs found

    Towards Locating the Brightest Microlensing Events on the Sky

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    It is estimated that a star brighter than visual magnitude 17 is undergoing a detectable gravitational microlensing event, somewhere on the sky, at any given time. It is assumed that both lenses and sources are normal stars drawn from a standard Bahcall-Soneira model of our Galaxy. Furthermore, over the time scale of a year, a star 15th magnitude or brighter should undergo a detectable gravitational lens amplification. Detecting and studying the microlensing event rate among the brightest 108^8 stars could yield a better understanding of Galactic stellar and dark matter distributions. Diligent tracking of bright microlensing events with even small telescopes might detect planets orbiting these stellar lenses.Comment: 19 pages, 4 figures, accepted by Ap

    Probing For Machos of Mass 1015M10^{-15}M_\odot-107M10^{-7}M_\odot with Gamma-Ray Burst Parallax Spacecraft

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    Two spacecraft separated by \sim 1\,\au and equipped with gamma-ray burst (GRB) detectors could detect or rule out a cosmological density of Massive Compact Halo Objects (MACHOs) in the mass range 10^{-15} M_{\odot}\lsim M \lsim 10^{-7} M_{\odot} provided that GRBs prove to be cosmological. Previously devised methods for detecting MACHOs have spanned the mass range 10^{-16} M_{\odot}\lsim M \lsim 10^{7} M_{\odot}, but with a gap of several orders of magnitude near 109M10^{-9} M_{\odot}. For MACHOs and sources both at a cosmological distance, the Einstein radius is \sim 1\,\au\,(M/10^{-7} M_\odot)^{1/2}. Hence, if a GRB lies within the Einstein ring of a MACHO of mass M\lsim 10^{-7}M_\odot as seen by one detector, it will not lie in the Einstein ring as seen by a second detector \sim 1\,\au away. This implies that if GRBs are measured to have significantly different fluxes by the two detectors, this would signal the presence of a MACHO \lsim 10^{-7}M_\odot. By the same token, if the two detectors measured similar fluxes for several hundred events a cosmological abundance of such low-mass MACHOs would be ruled out. The lower limit of sensitivity, M\lsim 10^{-15}M_\odot is set by the finite size of the source. If low-mass MACHOs are detected, there are tests which can discriminate among events generated by MACHOs in the three mass ranges M\lsim 10^{-12}\,M_\odot, 10^{-12}\,M_\odot\lsim M\lsim 10^{-7}\,M_\odot, and M\gsim 10^{-7}\ M_\odot. Further experiments would then be required to make more accurate mass measurements.Comment: 8 pages, uuencoded postscript, no figure

    GRB Spikes Could Resolve Stars

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    GRBs vary more rapidly than any other known cosmological phenomena. The lower limits of this variability have not yet been explored. Improvements in detectors would reveal or limit the actual rate of short GRBs. Were microsecond "spike" GRBs to exist and be detectable, they would time-resolve stellar mass objects throughout the universe by their gravitational microlensing effect. Analyzing the time structure of sufficient numbers of GRB spikes would reveal or limit Ωstar\Omega_{star}, ΩMACHO\Omega_{MACHO}, and/or Ωbaryon\Omega_{baryon}.Comment: 18 pages, 2 figures, in press: ApJ (Letters

    An exposition on Friedmann Cosmology with Negative Energy Densities

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    How would negative energy density affect a classic Friedmann cosmology? Although never measured and possibly unphysical, certain realizations of quantum field theories leaves the door open for such a possibility. In this paper we analyze the evolution of a universe comprising varying amounts of negative energy forms. Negative energy components have negative normalized energy densities, Ω<0\Omega < 0. They include negative phantom energy with an equation of state parameter w<1w<-1, negative cosmological constant: w=1w=-1, negative domain walls: w=2/3w=-2/3, negative cosmic strings: w=1/3w=-1/3, negative mass: w=0w=0, negative radiation: w=1/3w=1/3 and negative ultralight: w>1/3w > 1/3. Assuming that such energy forms generate pressure like perfect fluids, the attractive or repulsive nature of negative energy components are reviewed. The Friedmann equation is satisfied only when negative energy forms are coupled to a greater magnitude of positive energy forms or positive curvature. We show that the solutions exhibit cyclic evolution with bounces and turnovers.The future and fate of such universes in terms of curvature, temperature, acceleration, and energy density are reviewed. The end states are dubbed Big Crunch, Big Void, or Big Rip and further qualified as "Warped", "Curved", or "Flat", "Hot" versus "Cold", "Accelerating" versus "Decelerating" versus "Coasting". A universe that ends by contracting to zero energy density is termed "Big Poof." Which contracting universes "bounce" in expansion and which expanding universes "turnover" into contraction are also reviewed.Comment: Abridged version with minor correction

    Test for Time Dilation of Intervals Between Pulse Structures in GRBs

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    If γ\gamma-ray bursts are at cosmological distances, then not only their constituent pulses but also the intervals between pulses should be time-dilated. Unlike time-dilation measures of pulse emission, intervals would appear to require negligible correction for redshift of narrower temporal structure from higher energy into the band of observation. However, stretching of pulse intervals is inherently difficult to measure without incurring a timescale-dependent bias since, as time profiles are stretched, more structure can appear near the limit of resolution. This problem is compounded in dimmer bursts because identification of significant structures becomes more problematic. We attempt to minimize brightness bias by equalizing signal-to-noise (s/n) level of all bursts. We analyze wavelet-denoised burst profiles binned to several resolutions, identifying significant fluctuations between pulse structures and interjacent valleys. When bursts are ranked by peak flux, an interval time-dilation signature is evident, but its magnitude and significance are dependent upon temporal resolution and s/n level.Comment: 5 pages in LATeX, REVTEX style, 2 embedded figures. To appear in Third Huntsville GRB Workshop Proceeding

    Distribution of Spectral Characteristics and the Cosmological Evolution of GRBs

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    We investigate the cosmological evolution of GRBs, using the total gamma ray fluence as a measure of the burst strength. This involves an understanding of the distributions of the spectral parameters of GRBs as well as the total fluence distribution - both of which are subject to detector selection effects. We present new non-parametric statistical techniques to account for these effects, and use these methods to estimate the true distribution of the peak of the nu F_nu spectrum, E_p, from the raw distribution. The distributions are obtained from four channel data and therefore are rough estimates. Here, we emphasize the methods and present qualitative results. Given its spectral parameters, we then calculate the total fluence for each burst, and compute its cumulative and differential distributions. We use these distributions to estimate the cosmological rate evolution of GRBs, for three cosmological models. Our two main conclusions are the following: 1) Given our estimates of the spectral parameters, we find that there may exist a significant population of high E_p bursts that are not detected by BATSE, 2) We find a GRB co-moving rate density quite different from that of other extragalactic objects; in particular, it is different from the recently determined star formation rate.Comment: 20 pages, including 10 postscript figures. Submitted to Ap

    Calibration of Tests for Time Dilation in GRB Pulse Structures

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    Two tests for cosmological time dilation in γ\gamma-ray bursts -- the peak alignment and auto-correlation statistics -- involve averaging information near the times of peak intensity. Both tests require width corrections, assuming cosmological origin for bursts, since narrower temporal structure from higher energy would be redshifted into the band of observation, and since intervals between pulse structures are included in the averaging procedures. We analyze long (>> 2 s) BATSE bursts and estimate total width corrections for trial time-dilation factors (TDF = [1+zdimz_{\rm dim}]/[1+zbrtz_{\rm brt}]) by time-dilating and redshifting bright bursts. Both tests reveal significant trends of increasing TDF with decreasing peak flux, but neither provides sufficient discriminatory power to distinguish between actual TDFs in the range 2--3.Comment: 5 pages in LATeX, REVTEX style, 2 embedded figures. To appear in Third Huntsville GRB Workshop Proceeding

    On Source Density Evolution of Gamma-ray Bursts

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    Recent optical afterglow observations of gamma-ray bursts indicate a setting and distance scale that many relate to star-formation regions. In this paper, we use and a set of artificial trigger thresholds to probe several potential GRB source density evolutionary scenarios. In particular, we compare a uniform subset of BATSE 4B data to cosmological scenarios where GRBs evolve as the comoving density, the star formation rate, the QSO rate, and the SN Type Ic rate. Standard candle bursts with power-law spectra and a universe without vacuum energy were assumed. Our results significantly favor a comoving density model, implying that GRB source density evolution is weaker than expected in these evolutionary scenarios. GRB density might still follow star-formation rates given proper concurrent GRB luminosity evolution, significant beaming, significant error in standard candle assumptions, or were a significant modification of star formation rate estimates to occur.Comment: 12 pages, 4 figures, accepted by Ap

    Flux Enhancement of Slow-moving Particles by Sun or Jupiter: Can they be Detected on Earth?

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    Slow-moving particles capable of interacting solely with gravity might be detected on Earth as a result of the gravitational lensing induced focusing action of the Sun. The deflection experienced by these particles are inversely proportional to the square of their velocities and as a result their focal lengths will be shorter. We investigate the velocity dispersion of these slow-moving particles, originating from distant point-like sources, for imposing upper and lower bounds on the velocities of such particles in order for them to be focused onto Earth. We find that fluxes of such slow-moving and non-interacting particles must have speeds between ~0.01 and ~0.14 times the speed of light, cc. Particles with speeds less than ~0.01 c will undergo way too much deflection to be focused, although such individual particles could be detected. At the caustics, the magnification factor could be as high as ~10E+6. We impose lensing constraints on the mass of these particles in order for them to be detected with large flux enhancements to be greater than E-9 eV. An approximate mass density profile for Jupiter is used to constrain particle velocities for lensing by Jupiter. We show that Jupiter could potentially focus particles with speeds as low as ~0.001c, which the Sun cannot. As a special case, the perfect alignment of the planet Jupiter with the Sun is also considered.Comment: 20 Pages, 5 figure

    Limits on the cosmological abundance of supermassive compact objects from a millilensing search in gamma-ray burst data

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    A new search for the gravitational lens effects of a significant cosmological density of supermassive compact objects (SCOs) on gamma-ray bursts has yielded a null result. We inspected the timing data of 774 BATSE-triggered GRBs for evidence of millilensing: repeated peaks similar in light-curve shape and spectra. Our null detection leads us to conclude that, in all candidate universes simulated, ΩSCO<0.1\Omega_{SCO} < 0.1 is favored for 105<MSCO/M<10910^5 < M_{SCO}/M_{\odot} < 10^9, while in some universes and mass ranges the density limits are as much as 10 times lower. Therefore, a cosmologically significant population of SCOs near globular cluster mass neither came out of the primordial universe, nor condensed at recombination.Comment: 14 pages including 3 figures, appeared 2001 January 2
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