-
C. A. Nelson,
A. J. Drake,
K. H. Cook,
D. P. Bennett,
P. Popowski,
N. Dalal,
S. Nikolaev,
C. Alcock,
T. S. Axelrod,
A. C. Becker, [......],
M. J. Lehner,
S. L. Marshall,
D. Minniti,
M. R. Pratt,
P J Quinn,
C. W. Stubbs,
W. Sutherland,
A. B. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We present Hubble Space Telescope (HST) WFPC2 photometry of 13 microlensed source stars from the 5.7 year Large Magellanic Cloud (LMC) survey conducted by the MACHO Project. 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. None of these sources is coincident with a background galaxy, which rules out the possibility that the MACHO LMC microlensing sample is contaminated with misidentified supernovae or AGN in galaxies behind the LMC. This supports the conclusion that the MACHO LMC microlensing sample has only a small amount of contamination due to non-microlensing forms of variability. We compare the WFPC2 source star magnitudes with the lensed flux predictions derived from microlensing fits to the light curve data. In most cases the source star brightness is accurately predicted. Finally, we develop a statistic which constrains the location of the Large Magellanic Cloud (LMC) microlensing source stars with respect to the distributions of stars and dust in the LMC and compare this to the predictions of various models of LMC microlensing. This test excludes at > 90% confidence level models where more than 80% of the source stars lie behind the LMC. Exotic models that attempt to explain the excess LMC microlensing optical depth seen by MACHO with a population of background sources are disfavored or excluded by this test. Models in which most of the lenses reside in a halo or spheroid distribution associated with either the Milky Way or the LMC are consistent which these data, but LMC halo or spheroid models are favored by the combined MACHO and EROS microlensing results.
03/2009;
-
C. Alcock,
R. A. Allsman,
D. R. Alves,
T. S. Axelrod,
A. C. Becker,
D. P. Bennett,
K. H. Cook,
A. J. Drake,
K. C. Freeman,
M. Geha, [......],
C. A. Nelson,
B. A. Peterson,
P. Popowski,
M. R. Pratt,
P. J. Quinn,
C. W. Stubbs,
W. Sutherland,
A. B. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We present photometry and analysis of the microlensing alert MACHO 96-LMC-2 (event LMC-14 in an earlier paper). This event was initially detected by the MACHO Alert System and subsequently monitored by the Global Microlensing Alert Network (GMAN). The ~3% photometry provided by the GMAN follow-up effort reveals a periodic modulation in the light curve. We attribute this to binarity of the lensed source. Microlensing fits to a rotating binary source magnified by a single lens converge on two minima, separated by Δχ2 ~ 1. The most significant fit X1 predicts a primary which contributes ~100% of the light, a dark secondary, and an orbital period (T) of ~9.2 days. The second fit X2 yields a binary source with two stars of roughly equal mass and luminosity and T = 21.2 days. Observations made with the Hubble Space Telescope (HST) resolve stellar neighbors which contribute to the MACHO object's baseline brightness. The actual lensed object appears to lie on the upper LMC main sequence. We estimate the mass of the primary component of the binary system, M ~ 2 M☉. This helps to determine the physical size of the orbiting system and allows a measurement of the lens proper motion. For the preferred model X1, we explore the range of dark companions by assuming 0.1 M☉ and 1.4 M☉ objects in models X1a and X1b, respectively. We find lens velocities projected to the LMC in these models of X1a = 18.3 ± 3.1 km s-1 and X1b = 188 ± 32 km s-1. In both these cases, a likelihood analysis suggests an LMC lens is preferred over a Galactic halo lens, although only marginally so in model X1b. We also find X2 = 39.6 ± 6.1 km s-1, where the likelihood for the lens location is strongly dominated by the LMC disk. In all cases, the lens mass is consistent with that of an M dwarf. Additional spectra of the lensed source system are necessary to further constrain and/or refine the derived properties of the lensing object. The LMC self-lensing rate contributed by 96-LMC-2 is consistent with model self-lensing rates. Thus, even if the lens is in the LMC disk, it does not rule out the possibility of Galactic halo microlenses altogether. Finally, we emphasize the unique capability of follow-up spectroscopic observations of known microlensed LMC stars, combined with the nondetection of binary source effects, to locate lenses in the Galactic halo.
The Astrophysical Journal 12/2008; 552(1):259. · 6.02 Impact Factor
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K Cook,
M Huber,
S Nikolaev,
K Olsen,
A Rest,
R Smith,
A Garg,
C Stubbs,
A Becker, D Welch,
A Clocchiatti,
D Minniti
[show abstract]
[hide abstract]
ABSTRACT: The MACHO Project sought evidence for Massive Compact Halo Objects (MACHO) by surveying for the gravitational effort of the objects on light, microlensing.
07/2006
-
M. E. Huber,
S. Nikolaev,
K. H. Cook,
A. Rest,
C. Smith,
K. Olsen,
N. Suntzeff,
C. Aguilera,
C. Stubbs,
A. Garg,
P. Challis,
A. Becker,
R. Covarrubias,
A. Miceli, D. Welch,
G. Miknaitis,
J. L. Prieto,
A. Clocchiatti,
D. Minniti,
L. Morelli
[show abstract]
[hide abstract]
ABSTRACT: The SuperMACHO project, an NOAO survey, is in its 5th year of searching
for microlensing toward the Large Magellanic Cloud (LMC) using the CTIO
4m+MOSAIC (Rest et al. 2005, AJ, in press). The project samples 68
fields covering 24 square degrees over the LMC in a broad VR filter and
reaches VR ˜23 with an uncertainty of 0.1 mag. Each field was
imaged 20-30 times with a typical separation of 2 days over 3 months for
the past 4 years. Image subtraction analysis is used to identify
microlensing candidates and other variable sources in these crowded
fields where more than 16 million difference image detections are found
in the 2 TB of image data. With the difference image detections, we have
built a prototype database in MySQL and created objects using simple
clustering methods. A current total of 84,836 variable objects with
light curves characterized by various statistics (e.g., period estimates
by SuperSmoother, Fourier coefficients) are being investigated. In
cross-matching to MACHO, 21,425 sources are re-confirmed as known
variables, with the remaining being new and/or fainter detections. A
large sample of Delta Scuti variables has been identified, extending
beyond the MACHO set, providing a new catalog of good signal-to-noise
light curves for analysis. Similarly, MACHO and SuperMACHO light curves
for RR Lyrae have been compared to explore period and phase evolution
behavior. The eclipsing binary catalog from MACHO is extended nearly 3
mag deeper. We will present a summary of these and other variable stars
found in more detail.
11/2005; 37:1366.
-
P. Popowski,
K. Griest,
C. L. Thomas,
K. H. Cook,
D. P. Bennett,
A. C. Becker,
D. R. Alves,
D. Minniti,
A. J. Drake,
C. Alcock, [......],
M. Geha,
M. J. Lehner,
S. L. Marshall,
C. A. Nelson,
B. A. Peterson,
P J Quinn,
C. W. Stubbs,
W. Sutherland,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: Using 7 years of MACHO survey data, we present a new determination of the optical depth to microlensing towards the Galactic bulge. We select the sample of 62 microlensing events (60 unique) on clump giant sources and perform a detailed efficiency analysis. We use only the clump giant sources because these are bright bulge stars and are not as strongly affected by blending as other events. Using a subsample of 42 clump events concentrated in just 4.5 square degrees, we find tau = 2.17^{+0.47}_{-0.38} x 10^{-6} at (l,b) = (1.50, -2.68), somewhat smaller than found in most previous MACHO studies, but in excellent agreement with recent theoretical predictions. We also present the optical depth in each of the 19 fields in which we detected events, and find limits on optical depth for fields with no events. The errors in optical depth in individual fields are dominated by Poisson noise. We measure optical depth gradients (1.06 +/- 0.71) x 10^{-6} deg^{-1} and (0.29 +/- 0.43) x 10^{-6} deg^{-1} in the galactic latitude b and longitude l directions, respectively. Finally, we discuss the possibility of anomalous duration distribution of events in the field 104 centered on (l,b) = (3.11, -3.01) as well as investigate spatial clustering of events in all fields. Comment: 42 pages, 15 figures (6 of them in color), 12 tables; results unchanged, discussion of blending strengthened including addition of new appendix; to be published in ApJ, Vol. 630, Sept. 10, 2005 issue
The Astrophysical Journal 10/2004; 631(2):879. · 6.02 Impact Factor
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S. Nikolaev,
K. H. Cook,
C. W. Stubbs,
R. C. Smith,
A. Rest,
R. Hiriart,
K. Olsen,
A. Becker, D. Welch,
R. Covarrubias,
A. Miceli,
G. Miknaitis,
G. Prochter
[show abstract]
[hide abstract]
ABSTRACT: We present the image reduction pipeline for object-based photometry
(OBP) for the SuperMACHO project, to complement the SuperMACHO pipeline
developed for difference image analysis (DIA). As opposed to the DIA
pipeline which tracks variable sources only, the OBP pipeline monitors
every detectable object over the entire course of the survey. As a
consequence, OBP pipeline has to meet more demanding performance
specifications, both in terms of CPU cycles and storage requirements.
Several results from applying OBP pipeline to SuperMACHO data are
presented, including comparison to MACHO photometry, comparison of light
curves of transients to those from DIA pipeline, and color-magnitude
diagrams for selected fields.
11/2003; 35:1389.
-
P. Popowski,
C. A. Nelson,
D. P. Bennett,
A. J. Drake,
T. Vandehei,
K. Griest,
K. H. Cook,
C. Alcock,
R. A. Allsman,
D. R. Alves, [......],
K. C. Freeman,
M. Geha,
M. J. Lehner,
S. L. Marshall,
D. Minniti,
B. A. Peterson,
P J Quinn,
C. W. Stubbs,
W. Sutherland, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We describe a few recent microlensing results from the MACHO Collaboration. The aim of the MACHO Project was the identification and quantitative description of dark and luminous matter in the Milky Way using microlensing toward the Magellanic Clouds and Galactic bulge. We start with a discussion of the HST follow-up observations of the microlensing events toward the LMC detected in the first 5 years of the experiment. Using color-magnitude diagrams we attempt to distinguish between two possible locations of the microlensing sources: 1) in the LMC or 2) behind the LMC. We conclude that unless the extinction is extremely patchy, it is very unlikely that most of the LMC events have sources behind the LMC. During an examination of the HST images of the 13 LMC events we found a very red object next to the source star of event LMC-5. Astrometry, microlensing parallax fit, and a spectrum suggest that in this case we directly image the lens - a low-mass disk star. Then we focus on the majority of events observed by the MACHO Project, which are detected toward the Galactic bulge. We argue that the microlensing optical depth toward the bulge is best measured using events that have clump giant sources, which are almost unaffected by blending. From this sample we derive a low optical depth toward the Galactic bulge of (1.4 +/- 0.3) x 10^{-6}, in good agreement with other observational constraints and with theoretical models. The presence of many long-duration events among the bulge candidates allows us to investigate the microlensing parallax effect. Events with the strongest parallax signal are probably due to massive remnants. Events MACHO-96-BLG-5 and MACHO-98-BLG-6 might have been caused by the 6-solar-mass black holes.
05/2003;
-
P. Popowski,
T. Vandehei,
K. Griest,
C. Alcock,
R. A. Allsman,
D. R. Alves,
T. S. Axelrod,
A. C. Becker,
D. P. Bennett,
K. H. Cook, [......],
M. Geha,
M. J. Lehner,
S. L. Marshall,
D. Minniti,
C. A. Nelson,
B. A. Peterson,
P J Quinn,
C. W. Stubbs,
W. Sutherland, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We present preliminary results of the analysis of 5 years of MACHO data on the Galactic bulge microlensing events with clump giants as sources. This class of events allows one to obtain robust conclusions because relatively bright clump stars are not strongly affected by blending. We discuss: 1) the selection of `giant' events, 2) the distribution of event durations, 3) the anomalous character of event durations and optical depth in the MACHO field 104 centered on (l,b) = (3.1, -3.0). We report the preliminary average optical depth of (2.0 +/- 0.4) x 10^{-6} (internal) at (l,b) = (3.9, -3.8), and present a map of the spatial distribution of the optical depth. When field 104 is removed from the sample, the optical depth drops to (1.4 +/- 0.3) x 10^{-6}, which is in excellent agreement with infrared-based models of the central Galactic region.
03/2002;
-
C Alcock,
R A Allsman,
D R Alves,
T S Axelrod,
A C Becker,
D P Bennett,
K H Cook,
A J Drake,
K C Freeman,
M Geha, [......],
C A Nelson,
B A Peterson,
P Popowski,
M R Pratt,
P J Quinn,
C W Stubbs,
W Sutherland,
A B Tomaney,
T Vandehei, D Welch
[show abstract]
[hide abstract]
ABSTRACT: The nature of dark matter remains mysterious, with luminous material accounting for at most approximately 25 per cent of the baryons in the Universe. We accordingly undertook a survey looking for the microlensing of stars in the Large Magellanic Cloud (LMC) to determine the fraction of Galactic dark matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sight. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance and transverse velocity of the lens. The survey results to date indicate that between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected frequency of nearby stars acting as lenses, and demonstrates a direct determination of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MACHOs.
Nature 01/2002; 414(6864):617-9. · 36.28 Impact Factor
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D. P. Bennett,
A. C. Becker,
J. L. Quinn,
A. B. Tomaney,
C. Alcock,
R. A. Allsman,
D. R. Alves,
T. S. Axelrod,
J. J. Calitz,
K. H. Cook, [......],
C. A. Nelson,
B. A. Peterson,
P. Popowski,
M. R. Pratt,
P J Quinn,
S. H. Rhie,
C. W. Stubbs,
W. Sutherland,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We present an analysis of the longest timescale microlensing events discovered by the MACHO Collaboration during a 7 year survey of the Galactic bulge. We find 6 events that exhibit very strong microlensing parallax signals due, in part, to accurate photometric data from the GMAN and MPS collaborations. The microlensing parallax fit parameters are used in a likelihood analysis, which is able to estimate the distance and masses of the lens objects based upon a standard model of the Galactic velocity distribution. This analysis indicates that the most likely masses of 5 of the 6 lenses are > 1 Msun, which suggests that a substantial fraction of the Galactic lenses may be massive stellar remnants. This could explain the observed excess of long timescale microlensing events. The lenses for events MACHO-96-BLG-5 and MACHO-98-BLG-6 are the most massive, with mass estimates of M/Msun = 6 +10/-3 and M/Msun = 6 +7/-3, respectively. The observed upper limits on the absolute brightness of main sequence stars for these lenses are < 1 Lsun, so both lenses are black hole candidates. The black hole interpretation is also favored by a likelihood analysis with a Bayesian prior using a conventional model for the lens mass function. We consider the possibility that the source stars for some of these 6 events may lie in the foreground or background of the Galactic bulge, but we find that this is unlikely. Future HST observations of these events can either confirm the black hole lens hypothesis or detect the lens stars and provide a direct measurement of their masses. Future observations of similar events by SIM or the Keck or VLTI interferometers will allow direct measurements of the lens masses for stellar remnant lenses as well. Comment: 47 pages, with 26 included postscript figures. Includes a new likelihood analysis with a mass function prior
The Astrophysical Journal 09/2001; 579(2):679. · 6.02 Impact Factor
-
C. Alard,
J. Blommaert,
C. Cesarsky,
N. Epchtein,
M. Felli,
P. Fouque,
S. Ganesh,
R. Genzel,
G. Gilmore,
I. Glass, [......],
D. Minniti,
C Nelson,
B. Peterson,
P. Popowski,
M. Pratt,
P. Quinn,
W. Sutherland,
A. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: By cross-correlating the results of two recent large-scale surveys, the general properties of a well defined sample of semi-regular variable stars have been determined. ISOGAL mid-infrared photometry and MACHO lightcurves are assembled for approximately 300 stars in the Baade's Windows of low extinction towards the Galactic bulge. These stars are mainly giants of late M spectral type, evolving along the asymptotic giant branch (AGB). They are found to possess a wide and continuous distribution of pulsation periods and to obey an approximate log~period -- bolometric magnitude relation or set of such relations. Approximate mass-loss rates in the range of 1e-8 to 5e-7 M_sun per year are derived from ISOGAL mid-infrared photometry and models of stellar spectra adjusted for the presence of optically-thin circumstellar silicate dust. Mass-loss rates depend on luminosity and pulsation period. Some stars lose mass as rapidly as short-period Miras but do not show Mira-like amplitudes. A period of 70 days or longer is a necessary but not a sufficient condition for mass loss to occur. For AGB stars in the mass-loss ranges that we observe, the functional dependence of mass-loss rate on temperature and luminosity is found to be in agreement with recent theoretical predictions. If we include our mass-loss rates with a sample of extreme mass-losing AGB stars in the Large Magellanic Cloud, we get the general result for AGB stars that mass-loss rate is proportional to luminosity^{2.7}, valid for AGB stars with 10^{-8} to 10^{-4} M_sun per year (Abridged). Comment: to appear in The Astrophysical Journal, 51 pages, 9 figures, 3 tables; table 1 will be available in machine-readable format at the electronic ApJ
The Astrophysical Journal 01/2001; 552(1):289. · 6.02 Impact Factor
-
C. A. Nelson,
K. H. Cook,
P. Popowski,
A. J. Drake,
S. L. Marshall,
K. Griest,
T. Vandehei,
C. Alcock,
R. A. Allsman,
T. S. Axelrod, [......],
A. B. Tomaney,
D. P. Bennett,
M. Geha,
M. J. Lehner,
D. Minniti,
M. R. Pratt,
P. J. Quinn,
W. Sutherland, D. Welch,
MACHO Collaboration
[show abstract]
[hide abstract]
ABSTRACT: There are two principle geometrical arrangements which may explain Large
Magellanic Cloud (LMC) microlensing: a) halo-lensing, in which the
lensed object is part of the Milky Way galactic halo and b)
self-lensing, in which the lensed object is part of the LMC.
Self-lensing in turn may be broken into two categories: LMC-LMC
self-lensing, in which both the source and the lens reside in the LMC
and background self-lensing, in which the lens is a star in the LMC and
the source star is drawn from some population behind the LMC. Models
suggest the contribution of LMC-LMC self-lensing is small, so the nature
of LMC microlensing may be estimated from the location of the
microlensing source stars. If the source stars are in the LMC then
microlensing is dominated by halo-lensing; conversely if the source
stars are located behind the LMC then microlensing is dominated by
self-lensing. Since background populations reside behind the LMC, we
expect them to be both redder and fainter then the average population of
the LMC. We attempt to determine if the MACHO source stars come from
such a background population by comparing the HST color-magnitude
diagram (CMD) of source stars to the CMD of the average population of
the LMC and looking for the effects of extra reddening and extinction.
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. Preliminary results suggest that halo-lensing accounts for ≳
40% of the observed microlensing results. Support provided by NASA, DOE,
NSF and NPSC.
11/2000; 197:419.
-
P. Popowski,
C. Alcock,
R. A. Allsman,
D. R. Alves,
T. S. Axelrod,
A. C. Becker,
D. P. Bennett,
K. H. Cook,
A. J. Drake,
K. C. Freeman, [......],
D. Minniti,
C. A. Nelson,
B. A. Peterson,
M. R. Pratt,
P J Quinn,
C. W. Stubbs,
W. Sutherland,
A. B. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We present preliminary results of the analysis of 5 years of MACHO data on the Galactic bulge microlensing events with clump giants as sources. In particular, we discuss: 1) the selection of `giant' events, 2) distribution of impact parameters, 3) distribution of event durations, 4) the concentration of long duration events in MACHO field 104 centered on (l,b) = (3.1,-3.0). We report the preliminary average optical depth of (2.0 +/- 0.4) x 10^{-6} (internal) at (l,b) = (3.9,-3.8). We discuss future work and prospects for building a coherent model of the Galaxy.
06/2000;
-
The MACHO Collaboration,
C. Alcock,
R. A. Allsman,
D. R. Alves,
T. S. Axelrod,
A. C. Becker,
D. P. Bennett,
K. H. Cook,
A. J. Drake,
K. C. Freeman, [......],
C. A. Nelson,
B. A. Peterson,
P. Popowski,
M. R. Pratt,
P J Quinn,
C. W. Stubbs,
W. Sutherland,
A. B. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: The MACHO project is a search for dark matter in the form of massive compact halo objects (MACHOs). The project has photometrically monitored tens of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge in search of rare gravitational microlensing events caused by these otherwise invisible objects. In 5.7 years of observations toward the LMC some 13-17 microlensing events have been observed by the MACHO survey, allowing powerful statements to be made about the nature of the dark population in the halo of our Galaxy. A critical component of these statements is an accurate determination of the survey's detection efficiency. The detection efficiency is a complicated function of temporal sampling, stellar crowding (the luminosity function), image quality, photometry, time-series analysis, and criteria used to select the microlensing candidates. Such a complex interdependence is most naturally solved using a Monte Carlo approach. Here we describe the details of the Monte Carlo used to calculate the efficiency presented in the MACHO 5.7-year LMC results. Here we correct several shortcomings of past determinations, including (1) adding fainter source stars (2.5 magnitudes below our faintest detected "stars"), (2) an up-to-date luminosity function for the LMC, (3) better sampling of real images in both stellar density and observing conditions, (4) an improved scheme for adding artificial microlensing onto a random sample of real lightcurves, and many other improvements. [Abridged] Comment: 32 pages, Latex with 16 postscript figures, submitted to ApJS
The Astrophysical Journal Supplement Series 03/2000; 136(2):439. · 13.46 Impact Factor
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The MACHO Collaboration,
C. Alcock,
R. A. Allsman,
D. R. Alves,
T. S. Axelrod,
A. C. Becker,
D. P. Bennett,
K. H. Cook,
N. Dalal,
A. J. Drake, [......],
C. A. Nelson,
B. A. Peterson,
P. Popowski,
M. R. Pratt,
P J Quinn,
C. W. Stubbs,
W. Sutherland,
A. B. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We report on our search for microlensing towards the Large Magellanic Cloud (LMC). Analysis of 5.7 years of photometry on 11.9 million stars in the LMC reveals 13 - 17 microlensing events. This is significantly more than the $\sim$ 2 to 4 events expected from lensing by known stellar populations. The timescales ($\that$) of the events range from 34 to 230 days. We estimate the microlensing optical depth towards the LMC from events with $2 < \that < 400$ days to be 1.2 ^{+0.4}_ {-0.3} \ten{-7}$, with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8% to 50%. A 100% MACHO halo is ruled out at the 95% C.L. for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between $ 0.15 \msun$ and $ 0.9 \msun$, depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9+4-3 10^{10} msun, independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of two. Besides a larger data set, this work also includes an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing, such as supernovae in galaxies behind the LMC. [Abridged] Comment: 53 pages, Latex with 12 postscript figures, submitted to ApJ
The Astrophysical Journal 02/2000; 542:281. · 6.02 Impact Factor
-
D. P. Bennett,
C. Alcock,
R. A. Allsman,
D. Alves,
T. S. Axelrod,
D. Becker,
K. H. Cook,
A. Crouch,
A. D. Drake,
P. C. Fragile, [......],
J. L. Quinn,
P. J. Quinn,
S. H. Rhie,
P. Stetson,
C. W. Stubbs,
W. Sutherland,
S. Thomson,
A. B. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: We present the analysis of microlensing parallax events discovered by
the MACHO/GMAN and Microlensing Planet Search (MPS) Collaborations. We
find two events towards the Galactic bulge which are likely to be caused
by black hole lenses: MACHO-96-BLG-5 and MACHO-98-BLG-6. The
microlensing parallax constraint on the lens masses and the observed
upper limit on the lens brightness imply that these lenses are likely to
be black holes. If so, they would be the first isolated black holes in
the stellar mass range yet discovered. Our microlensing parallax
analysis of Large Magellanic Could (LMC) microlensing events yields a
single event, MACHO-LMC-22 which appears to have a significant parallax
signal. The microlensing parallax analysis indicates that the lens does
not reside in the Milky Way's thin disk or in the vicinity of the LMC.
The parallax analysis suggests that the lens star must reside in the
Milky Way's thick disk or dark halo. This lends support to recent
suggestions that old, blue white dwarfs seen in the Hubble Deep Fields
might be responsible for the excess microlensing signal seen toward the
LMC. This work was funded by the NSF, the NASA Origins Program and the
DOE (through LLNL).
11/1999; 31:1422.
-
J. L. Quinn,
C. Alcock,
R. A. Allsman,
D. Alves,
T. S. Axelrod,
D. Becker,
D. P. Bennett,
K. H. Cook,
A. D. Drake,
K. C. Freeman, [......],
C. A. Nelson,
B. A. Peterson,
P. Popowski,
P. J. Quinn,
S. H. Rhie,
C. W. Stubbs,
W. Sutherland,
A. B. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: Microlensing event MACHO-96-BLG-5 is the longest microlensing event
observed to date, and it has a strong microlensing parallax signal which
allows an estimate of the distance and mass of the lens. The mass
estimate indicates that the that the lens is probably a black hole
stellar remnants, and if so, it would be the first isolated black hole
stellar remnant yet discovered. However, the lens mass estimate has a
systematic uncertainty due to the fact that the source star of
MACHO-96-BLG-5 is significantly blended with stars unresolved from
ground based images. Here, we present the analysis of HST WFPC2 images
of MACHO-96-BLG-5 which allows a probable identification of the lensed
source star and removes this systematic uncertainty in the lens mass
estimate. Since the blending of stellar images is common in microlensing
data, the technique presented here will be useful for removing similar
ambiguities from similar microlensing events. This work was funded by
the NSF through the Center for Particle Astrophysics and by the DOE
(through LLNL).
11/1999; 31:1423.
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P. J. Quinn,
C. Alcock,
R. A. Allsman,
D. R. Alves,
T. S. Axelrod,
A. Becker,
D. P. Bennett,
Cook, K. H., Drake, A,
K. C. Freeman,
K. Griest, [......],
S. L. Marshall,
D. Minniti,
B. A. Peterson,
M. R. Pratt,
A. W. Rodgers,
C. W. Stubbs,
W. Sutherland,
A. Tomaney,
T. Vandehei, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: The MACHO Project has been monitoring large fields in the direction of the Galactic bulge, the LMC and SMC for over 6 years. The nightly observations of this microlensing survey provide very large databases for astronomers. The MACHO database contains {approximately}5 Terabytes of data as of mid-1998. For comparison, the HST Archive contains about {approximately}2 Tb, and the Sloan. Digital Sky Survey will reach {approximately}1.0 Tb. Our database can be used to attack a variety of astrophysical problems, and has produced results relevant to microlensing, galactic structure, and stellar evolution. Of particular importance are the large numbers of variable stars (tens of thousands) found as byproducts of the microlensing search. In years to come, the microlensing databases will play a dominant role in variable star science. Important science can still be done with limited resources, even in places with no access to large telescopes (all you need is computer).
09/1998
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D. Alves,
C. Alcock,
K. Cook,
S Marshall,
D. Minniti,
R. Allsman,
T. Axelrod,
K. Freeman,
B. Peterson,
A Rodgers, [......],
T. Vandehei,
A. Becker,
C. Stubbs,
A. Tomaney,
D. Bennett,
M. Lehner,
P. Quinn,
M. Pratt,
W. Sutherland, D. Welch
05/1998;
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J. Lutz,
D. Alves,
A. Becker,
T. Vandehei,
K. Griest,
M. Lehner,
C. Alcock,
K. Cook,
S. Marshall,
D. Minniti, [......],
K. Freeman,
B. Peterson,
A. Rodgers,
M. Pratt,
C. Stubbs,
A. B. Tomaney,
D. Bennett,
P. Quinn,
W. Sutherland, D. Welch
[show abstract]
[hide abstract]
ABSTRACT: The MACHO Project has been monitoring selected fields in the galactic
bulge since 1992. About 35 planetary nebulae (PN) with observable
central stars are located within these fields. These PN are well worth
investigating both for improvements in magnitude measurements and
especially to check the possibility of variability. This is a
preliminary report on data for 22 of the PN central stars. Cousins V and
R magnitudes were measured for each of the stars; the V magnitudes range
from 13.47 to 18.20. The quality of the photometry and problems
encountered with reducing the data (e.g. objects near the edge of a
field, cosmic ray hits) will be discussed as part of our paper. One
central star, that of the PN Hf 2-2 (MACHO ID 139.32591.211), appears to
be variable with a period of 0.398571 days.
04/1998; 30:894.
