Browsing by Author "Griest, K"
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- ItemBinary microlensing events from the MACHO Project(2000) Alcock, C; Allsman, RA; Alves, D; Axelrod, TS; Baines, D; Becker, AC; Bennett, DP; Bourke, A; Brakel, A; Cook, KH; Crook, B; Crouch, A; Dan, J; Drake, AJ; Fragile, PC; Freeman, KC; Gal-Yam, A; Geha, M; Gray, J; Griest, K; Gurtierrez, A; Heller, A; Howard, J; Johnson, BR; Kaspi, S; Keane, M; Kovo, O; Leach, C; Leach, T; Leibowitz, EM; Lehner, MJ; Lipkin, Y; Maoz, D; Marshall, L; McDowell, D; McKeown, S; Mendelson, H; Messenger, B; Minniti, D; Nelson, C; Peterson, BA; Popowski, P; Pozza, E; Purcell, P; Pratt, MR; Quinn, J; Quinn, PJ; Rhie, SH; Rodgers, AW; Salmon, A; Shemmer, O; Stetson, P; Stubbs, CW; Sutherland, W; Thomson, S; Tomaney, A; Vandehei, T; Walker, A; Ward, K; Wyper, GWe present the light curves of 21 gravitational microlensing events from the first six years of the MACHO Project gravitational microlensing survey that are likely examples of lensing by binary systems. These events were manually selected from a total sample of similar to 350 candidate microlensing events that were either detected by the MACHO Alert System or discovered through retrospective analyses of the MACHO database. At least 14 of these 21 events exhibit strong (caustic) features, and four of the events are well fit with lensing by large mass ratio (brown dwarf or planetary) systems, although these fits are not necessarily unique. The total binary event rate is roughly consistent with predictions based upon our knowledge of the properties of binary stars, but a precise comparison cannot be made without a determination of our binary lens event detection efficiency. Toward the Galactic bulge, we find a ratio of caustic crossing to noncaustic crossing binary lensing events of 12:4, excluding one event for which we present two fits. This suggests significant incompleteness in our ability to detect and characterize noncaustic crossing binary lensing. The distribution of mass ratios, N(q), for these binary lenses appears relatively flat. We are also able to reliably measure source-face crossing times in four of the bulge caustic crossing events, and recover from them a distribution of lens proper motions, masses, and distances consistent with a population of Galactic bulge lenses at a distance of 7 +/- 1 kpc. This analysis yields two systems with companions of similar to 0.05 M..
- ItemDifference image analysis of galactic microlensing. II. Microlensing events(1999) Alcock, C; Allsman, RA; Alves, D; Axelrod, TS; Becker, AC; Bennett, DP; Cook, KH; Drake, AJ; Freeman, KC; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Peterson, BA; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, A; Vandehei, T; Welch, DLThe MACHO collaboration has been carrying out difference image analysis (DIA) since 1996 with the aim of increasing the sensitivity to the detection of gravitational microlensing. This is a preliminary report on the application of DIA to galactic bulge images in one field. We show how the DIA technique significantly increases the number of detected lensing events, by removing the positional dependence of traditional photometry schemes and lowering the microlensing event detection threshold. This technique, unlike PSF photometry, gives the unblended colors and positions of the microlensing source stars. We present a set of criteria for selecting microlensing events from objects discovered with this technique. The 16 pixel and classical microlensing events discovered with the DIA technique are presented.
- ItemMacho project limits on black hole dark matter in the 1-30 M⊙ range(2001) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennett, DP; Cook, KH; Dalal, N; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, AB; Vandehei, T; Welch, DLWe report on a search for long-duration microlensing events toward the Large Magellanic Cloud. We find none and therefore put limits on the contribution of high-mass objects to the Galactic dark matter. At a 95% confidence level, we exclude objects in the mass range of 0.3-30.0 M-circle dot from contributing more than 4 x 10(11) M-circle dot to the Galactic halo. Combined with earlier results, this means that objects with masses under 30 M-circle dot cannot make up the entire dark matter halo if the halo is of typical size. For a typical dark halo, objects with masses under 10 M-circle dot contribute less than 40% of the dark matter.
- ItemSearching for periodicities in the MACHO light curve of LMC X-2(2000) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennett, DP; Charles, PA; Cook, KH; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Groot, P; Lehner, MJ; Marshall, SL; McGowan, KE; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Sutherland, W; Tomaney, AB; Vandehei, T; van Paradijs, JUsing the exceptional long-term monitoring capabilities of the MACHO project, we present here the optical history of LMC X-2 for a continuous 6-yr period. These data were used to investigate the previously claimed periodicities for this source of 8.15 h and 12.54 d: we find upper limits of 0.10 mag and 0.09 mag, respectively.
- ItemThe macho project Hubble Space Telescope follow-up(2001) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennet, DP; Cook, KH; Dalal, N; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, AB; Vandehei, TWe attempt to determine whether the MACHO microlensing source stars are drawn from the average population of the LMC or from a population behind the LMC by examining the Hubble Space Telescope (HST) color-magnitude diagram (CMD) of microlensing source stars. We present WFPC2 HST photometry of eight MACHO microlensing source stars and the surrounding fields in the LMC. The microlensing source stars are identified by deriving accurate centroids in the ground-based MACHO images using difference image analysis (DIA) and then transforming the DIA coordinates to the HST frame. We consider in detail a model for the background population of source stars based on that presented by Zhao, Graff, & Guhathakurta. In this model, the source stars have an additional reddening of [E(B - V)] = 0.13 mag and a slightly larger distance modulus, [Delta mu] similar to 0.3 mag, than the average LMC population. We also investigate a series of source star models, varying the relative fraction of source stars drawn from the average and background populations and the displacement of the background population from the LMC. Because of the small number of analyzed events, the distribution of probabilities of different models is rather flat. A shallow maximum occurs at a fraction s(LMC) similar to 0.8 of the source stars in the LMC. This is consistent with the interpretation that a significant fraction of observed microlensing events are due to lenses in the Milky Way halo, but does not definitively exclude other models.
- ItemThe Macho project Large Magellanic Cloud variable star inventory.: XI.: Frequency analysis of the fundamental-mode RR Lyrae stars(2003) Alcock, C; Alves, DR; Becker, A; Bennett, D; Cook, KH; Drake, A; Freeman, K; Geha, M; Griest, K; Kovács, G; Lehner, M; Marshall, S; Minniti, D; Nelson, C; Peterson, B; Popowski, P; Pratt, M; Quinn, P; Rodgers, A; Stubbs, C; Sutherland, W; Vandehei, T; Welch, DLWe have frequency-analyzed 6391 variables classified earlier as fundamental-mode RR Lyrae (RR0) stars in the MACHO database on the Large Magellanic Cloud (LMC). The overwhelming majority (i.e., 96%) of these variables have been proved to be indeed RR0 stars, whereas the remaining ones have fallen into one of the following categories: single- and double-mode Cepheids, binaries, first-overtone and double-mode RR Lyrae stars, and nonclassified variables. Special attention has been paid to the properties of the amplitude- and phase-modulated RR0 stars ( the Blazhko stars). We found altogether 731 Blazhko variables showing either a doublet or an equidistant triplet pattern at the main pulsation component in their frequency spectra. This sample overwhelmingly exceeds the number of Blazhko stars known in all other systems combined. The incidence rate of the Blazhko variables among the RR0 stars in the LMC is 11.9%, which is 3 times higher than their rate among the first-overtone RR Lyrae stars. No difference is found in the average brightness between the single- mode and Blazhko variables. However, the latter ones show a somewhat lower degree of skewness in their average light curves and a concomitant lower total amplitude in their modulation-free light curves. From the frequency spectra we found that variables with larger modulation amplitudes at the higher frequency side of the main pulsation component are 3 times more common than the ones showing the opposite amplitude pattern. A search for a modulation component with the Blazhko period in the average brightness of the individual variables showed the existence of such a modulation with an overall amplitude of approximate to 0.006 mag. On the other hand, a similar search for quadruple modulation patterns around the main pulsation component has failed to clearly detect such components at the approximate to 0.004 mag level. This means that the amplitudes of the quadruple components ( if they exist) should be, on average, at least 10 times smaller than those of the triplet components. This finding and the existence of Blazhko variables with highly asymmetric modulation amplitudes not only question the validity of the magnetic oblique rotator model but also put stringent constraints on models based on mode-coupling theories.
- ItemThe MACHO project Large Magellanic Cloud variable-star inventory.: IX.: Frequency analysis of the first-overtone RR Lyrae stars and the indication for nonradial pulsations(2000) Alcock, C; Allsman, R; Alves, DR; Axelrod, T; Becker, A; Bennett, D; Clement, C; Cook, KH; Drake, A; Freeman, K; Geha, M; Griest, K; Kovács, G; Kurtz, DW; Lehner, M; Marshall, S; Minniti, D; Nelson, C; Peterson, B; Popowski, P; Pratt, M; Quinn, P; Rodgers, A; Rowe, J; Stubbs, C; Sutherland, W; Tomaney, A; Vandehei, T; Welch, DLMore than 1300 variables classified provisionally as first-overtone RR Lyrae pulsators in the MACHO variable-star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of nine types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental and first-overtone) variables, this study increased the number of known double-mode stars in the LMC to 181. During the total 6.5 yr time span of the data, 10% of the stars showed strong period changes. The size, and in general also the patterns of the period changes, exclude a simple evolutionary explanation. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type, there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type, the number of remnant components is two; they are also closely spaced, and are symmetric in their frequency spacing relative to the central component. This latter type of variables are associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (approximate to 20%) among the fundamental-mode variables versus the low occurrence rate of their first-overtone counterparts makes it more difficult to explain the Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. None of the current theoretical models are able to explain the observed close frequency components without invoking nonradial pulsation components in these stars.