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Search Results: 1 - 10 of 17850 matches for " Andrew Gould "
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Theory of Pixel Lensing
Andrew Gould
Physics , 1995, DOI: 10.1086/177861
Abstract: Pixel lensing, gravitational microlensing of unresolved stars, is potentially much more sensitive and much more widely applicable than is generally recognized. I give explicit expressions for the pixel noise induced by a time-variable PSF, by photometric and geometric misalignment, and by discrete pixelization, and I show that these can all be reduced below the photon noise. Pixel lensing can be divided into two regimes. In the ``semi-classical'' regime, it is similar to classical lensing in that it measures the time scale of individual events. In the ``spike'' regime, it measures the total optical depth but not individual time scales. I present simple expressions for the boundary between the two regimes and for the event rate in the latter one. These expressions can be used to quickly classify all potential pixel lensing experiments. Pixel lensing can measure the luminosity function as well as the mass function of stars in target galaxies to a distance of a few Mpc. Future space-based pixel lensing could be $\sim 5$ times more effective in the infrared than in the optical, depending on developments in detector technology. Pixel techniques can also be applied to non-pixel-lensing problems including the search for unresolved variable stars and follow up observations of lensing events found in classical lensing searches. To benefit fully from pixel-lensing techniques, follow-up observations should have resolutions of at least 5 pixels per FWHM.
Extreme Microlensing
Andrew Gould
Physics , 1996,
Abstract: Extreme microlensing events, defined as events with maximum magnification $A_\max\gsim 200$ are a potentially powerful probe of the mass spectrum and spatial distribution of objects along lines of sight toward the Galactic bulge. About 75 yr${}^{-1}$ such events are expected for main-sequence sources with $I_0<19$. For many of these it is possible to measure both a ``proper motion'' and a ``parallax'' which together would yield individual mass, distance, and transverse-speed determinations of the lensing object. The proper motion is determined from finite-source effects when the lens transits, or nearly transits the source. The parallax is determined by observing the difference in the light curve as seen from two Earth observatories separated by about 1 Earth radius, $R_\oplus$. The size of the parallax effect is $\sim A_\max R_\oplus/\tilde r_e$ where $\tilde r_e$ is the projected Einstein radius. This can be of order 1\%. Detection of candidate events requires a pixel-lensing search of the entire bulge once per day, preferably by at least two observatories on different continents. Follow-up observation must be carried using optical/infrared photometry, with short (e.g.\ 1 minute) exposures on small ($\gsim 1$ m) telescopes. Extreme microlensing observations toward the Large Magellanic Cloud do not appear feasible at the present time.
Wide Field Imager in Space for Dark Energy and Planets
Andrew Gould
Physics , 2009,
Abstract: A wide-field imager in space could make remarkable progress in two very different frontiers of astronomy: dark energy and extra-solar planets. Embedding such an imager on a much larger and more complicated DE mission would be a poor science-approach under any circumstances and is a prescription for disaster in the present fiscal climate. The 2010 Decadal Committee must not lead the lemming stampede that is driving toward a DE mega-mission, but should stand clearly in its path.
LMC Microlenses: Dark or Luminous?
Andrew Gould
Physics , 1997,
Abstract: Zhao has proposed that the microlensing events observed toward the Large Magellanic Cloud (LMC) could be due to faint stars in a dwarf galaxy or tidal debris lying along the line of sight to the LMC. Zaritsky & Lin claim to have detected such a structure which, they believe, could account for most of the observed microlensing optical depth. Here I show that a large-area surface-brightness map made by de Vaucouleurs constrains any such structure to one of four possibilities. Either 1) it does not account for a significant fraction of the observed microlensing, 2) it covers the inner ~3 degrees of the LMC but does not extend beyond ~5 degrees from the LMC center, 3) it is smooth on scales of ~15 degrees in both transverse directions or 4) it has a stellar mass-to-light ratio which exceeds by a factor >10 that of known stellar populations. The second and third possibilities would not be expected to apply to tidal debris. The last merely rephrases the dark-matter problem in a new form.
Microlens Parallaxes with SIRTF
Andrew Gould
Physics , 1998, DOI: 10.1086/306981
Abstract: The Space Infrared Telescope Facility (SIRTF) will drift away from the Earth at about 0.1 AU/yr. Microlensing events will therefore have different characteristics as seen from the satellite and the Earth. From the difference, it is possible in principle to measure v-tilde, the transverse velocity of the lens projected onto the observer plane. Since v-tilde has very different values for different populations (disk, halo, Large Magellanic Cloud), such measurements could help identify the location, and hence the nature, of the lenses. I show that the method previously developed by Gould for measuring such satellite parallaxes fails completely in the case of SIRTF: it is overwhelmed by degeneracies which arise from fact that the Earth and satellite observations are in different band passes. I develop a new method which allows for observations in different band passes and yet removes all degeneracies. The method combines a purely ground-based measurement of the "parallax asymmetry" with a measurement of the delay between the time the event peaks at the Earth and satellite. In effect, the parallax asymmetry determines the component of v-tilde in the Earth-Sun direction, while the delay time measures the component of v-tilde in the direction of the Earth's orbit.
Optimal Microlensing Observations
Andrew Gould
Physics , 1998, DOI: 10.1086/307214
Abstract: One of the major limitations of microlensing observations toward the Large Magellanic Cloud (LMC) is the low rate of event detection. What can be done to improve this rate? Is it better to invest telescope time in more frequent observations of the inner high surface-brightness fields, or in covering new, less populated outer fields? How would a factor 2 improvement in CCD sensitivity affect the detection efficiency? Would a series of major (factor 2--4) upgrades in telescope aperture, seeing, sky brightness, camera size, and detector efficiency increase the event rate by a huge factor, or only marginally? I develop a simplified framework to address these questions. With observational resources fixed at the level of the MACHO and EROS experiments, the biggest improvement (factor ~2) would come by reducing the time spent on the inner ~25 deg^2 and applying it to the outer ~100 deg^2. By combining this change with the characteristics of a good medium-size telescope (2.5 m mirror, 1" point spread function, thinned CCD chips, 1 deg^2 camera, and dark sky), it should be possible to increase the detection of LMC events to more than 100 per year (assuming current estimates of the optical depth apply to the entire LMC).
An Upper Limit on the Granularity of the Local Stellar Halo
Andrew Gould
Physics , 2003, DOI: 10.1086/377525
Abstract: I use the statistical properties of 4588 nearby halo stars to show that if the local stellar halo is composed of cold streams from disrupted dwarf satellites as is predicted by popular cosmologies, then at 95% confidence there are at least 1350 such streams. Moreover, no stream can contain more than 2.3% of the local stars, while average streams must contain less than 0.08%.
Microlensing Search of $10^6$ Quasars
Andrew Gould
Physics , 1995, DOI: 10.1086/176553
Abstract: By monitoring $10^6$ quasars one could search for lensing by stars and Massive Compact Halo Objects (Machos) out to redshifts $z\sim 4$. If Machos have a present cosmological density $\Omega_{L,0}=1\%$, then the expected event rate is $\Gamma\sim 200\,\yr^{-1}$. The expected event rate for known stars in galaxies is $\Gamma\sim 20\,\yr^{-1}$ assuming that their present cosmological density is $\Omega_{L,0}=0.3\%$. Typical event times are $t_e\sim 3\,\yr$ for Machos and $t_e\sim 10\,\yr$ for stars. By comparing the optical depths to quasars at different redshifts, one could measure the star-formation and Macho-formation history of the universe. By comparing the time scales of events found parallel and perpendicular to the Sun's motion relative to the microwave background, one could measure or constrain the characteristic scale of large scale motions. The lensing events themselves would help probe the structure of quasars on scales of 50--1500 AU. The monitoring program could be carried out with a single dedicated 1 m telescope with a 4 deg${}^2$ camera. Quasar lensing events can be unambiguously distinguished from quasar variability because in the former case the broad lines are unaffected while in the latter they respond to the variation in the continuum on times scales $\sim 1\,\yr$.
Transverse Velocities of Galaxies From Microlens Parallaxes
Andrew Gould
Physics , 1994,
Abstract: The transverse velocity of a spiral galaxy can be measured to an accuracy $\sim 60\,\kms$ by making parallax observations of quasars being microlensed by stars in the disk of the galaxy. To make the measurement, a quasar must be located behind the disk of the galaxy between about 1 and 2 scale lengths from the center. The quasar must then be monitored for microlensing events and the events followed simultaneously from the Earth and a satellite in solar orbit, preferably at $\sim 30\,$AU. A systematic search in a volume within $21,000\,\kms$ could locate quasars with $B<23$ behind a total of $\sim 1900$ galaxies. The rate of lensing events (and hence galaxy velocity measurements) would be $\sim 3\,{\rm yr}^{-1}$. The events would have typical characteristic times $\omega^{-1}\sim 3\,$yr. Under the assumption that the mass spectrum of lensing objects is the same in other spiral galaxies as in our own, the observations could be used to measure the Hubble parameter to an accuracy of 5\%.
A New Macho Search Strategy
Andrew Gould
Physics , 1994, DOI: 10.1086/175893
Abstract: I propose a radical revision in the search strategy for Massive Compact Objects (Machos) toward the Galactic bulge: monitor the entire $\sim 300$ square deg of the bulge and tune the search primarily to microlensing events of bright $M_V\lsim 2$ stars. By employing a small second telescope to follow up the events detected with low dimensionless impact parameter $\beta\lsim 0.2$, one could measure the proper motion $\Omega$ of $\sim 10\,\rm yr^{-1}$ events and significantly constrain $\Omega$ for a similar number. If, in addition, the events were followed from a Macho parallax satellite, it would be possible to measure individual masses, distances, and transverse velocities for the events with proper motions. If a fraction $\xi$ of the bulge is composed of low-mass objects in the range $10^{-3}\lsim M_{\rm low}/M_\odot\lsim 10^{-1}$, then mass measurements could be made for $\sim 10(\xi/0.1)(0.01 M_\odot \VEV{M_{\rm low}^{-1}})\,\rm yr^{-1}$ of these, thus allowing a direct measurement of the mass function in the sub-stellar range. In the absence of a parallax satellite, ground-based observations could significantly constrain (but not measure) Macho parallaxes. These constraints, when combined with the proper-motion measurements, would in turn constrain the mass, distance, and transverse speed of the Machos. The proposed strategy should therefore be adopted even before a parallax satellite is launched.
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