S. L. Marshall

The MACHO Project: Microlensing Results from 5.7 Years of Large Magellanic Cloud Observations

We report on our search for microlensing toward the Large Magellanic Cloud (LMC). Analysis of 5.7 yr of photometry on 11.9 million stars in the LMC reveals 13-17 microlensing events. A detailed treatment of our detection efficiency shows that this is significantly more than the ~2-4 events expected from lensing by known stellar populations. The timescales () of the events range from 34 to 230 days. We estimate the microlensing optical depth toward the LMC from events with 2 < < 400 days to be τ = 1.2 × 10-7, 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%-50%. A 100% MACHO halo is ruled out at the 95% confidence level for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between 0.15 and 0.9 M☉, depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9 × 1010 M☉, independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of 2. This is mostly due to Poisson noise, because with 3.4 times more exposure and increased sensitivity to long-timescale events, we did not find the expected factor of ~4 more events. In addition to 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. We note that an important source of background are supernovae (SNe) in galaxies behind the LMC.

Possible Gravitational Microlensing Of A Star In The Large Magellanic Cloud

THERE is now abundant evidence for the presence of large quantities of unseen matter surrounding normal galaxies, including our own1,2. The nature of this ’dark matter‘ is unknown, except that it cannot be made of normal stars, dust or gas, as they would be easily detected. Exotic particles such as axions, massive neutrinos or other weakly interacting massive particles (collectively known as WIMPs) have been proposed3,4, but have yet to be detected. A less exotic alternative is normal matter in the form of bodies with masses ranging from that of a large planet to a few solar masses. Such objects, known collectively as massive compact halo objects5 (MACHOs), might be brown dwarfs or ‘jupiters’ (bodies too small to produce their own energy by fusion), neutron stars, old white dwarfs or black holes. Paczynski6 suggested that MACHOs might act as gravitational microlenses, temporarily amplifying the apparent brightness of background stars in nearby galaxies. We are conducting a microlensing experiment to determine whether the dark matter halo of our Galaxy is made up of MACHOs. Here we report a candidate for such a microlensing event, detected by monitoring the light curves of 1.8 million stars in the Large Magellanic Cloud for one year. The light curve shows no variation for most of the year of data taking, and an upward excursion lasting over 1 month, with a maximum increase of ∼2 mag. The most probable lens mass, inferred from the duration of the candidate lensing event, is ∼0.1 solar mass.

The MACHO Project Large Magellanic Cloud Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo

The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (~1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with 2 < < 200 days to be τ2002=2.9+ 1.4−0.9×10−7 based upon our eight event sample. This exceeds the optical depth, τbackgnd = 0.5 × 10-7, expected from known stars, and the difference is to be compared with the optical depth predicted for a standard halo composed entirely of MACHOs: τhalo = 4.7 × 10-7. To compare with Galactic halo models, we perform likelihood analyses on the full eight-event sample and a six-event subsample (which allows for two events to be caused by a nonhalo background). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of 2.0+ 1.2−0.7×1011 M☉, which is about half of the standard halo value. We also find a most probable MACHO mass of 0.5+ 0.3−0.2 M☉, although this value is strongly model dependent. In addition, the absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs: objects from 10-4 M☉ to 0.03 M☉ contribute 20% of the standard dark halo.

The macho project: 45 candidate microlensing events from the first-year Galactic bulge data

We report the detection of 45 candidate microlensing events in fields toward the Galactic bulge. These come from the analysis of 24 fields containing 12.6 million stars observed for 190 days in 1993. Many of these events are of extremely high signal-to-noise ratio and are remarkable examples of gravitational microlensing. The distribution of peak magnifications is shown to be consistent with the microlensing interpretation of these events. Using a subsample of 1.3 million {open_quotes}clump giant{close_quotes} stars whose distance and detection efficiency are well known, we find 13 events and estimate the microlensing optical depth toward the Galactic bulge as {tau}{sub bulge}=3.9{sub {minus}1.2}{sup +1.8}{times}10{sup {minus}6} averaged over an area of {approximately}12deg{sup 2} centered at Galactic coordinates l=2.55{degree} and b=3.64{degree}. This is similar to the value reported by the OGLE collaboration and is marginally higher than current theoretical models for {tau}{sub bulge}. The optical depth is also seen to increase significantly for decreasing {vert_bar}b{vert_bar}. These results demonstrate that obtaining large numbers of microlensing events toward the Galactic bulge is feasible, and that the study of such events will have important consequences for the structure of the Galaxy and its dark halo. {copyright} {ital 1997} {ital The American Astronomical Society}

The MACHO Project: Microlensing Optical Depth toward the Galactic Bulge from Difference Image Analysis

We present the microlensing optical depth toward the Galactic bulge based on the detection of 99 events found in our Difference Image Analysis (DIA) survey. This analysis encompasses 3 yr of data, covering ~17 million stars in ~4 deg2, to a source-star baseline magnitude limit of V = 23. The DIA technique improves the quality of photometry in crowded fields, and allows us to detect more microlensing events with faint source stars. We find that this method increases the number of detection events by 85% compared with the standard analysis technique. DIA light curves of the events are presented, and the microlensing fit parameters are given. The total microlensing optical depth is estimated to be τtotal = 2.43 × 10-6, averaged over eight fields centered at l = 268 and b = -335. For the bulge component, we find τbulge = 3.23 × 10-6, assuming a 25% stellar contribution from disk sources. These optical depths are in good agreement with the past determinations of the MACHO and OGLE groups, and are higher than predicted by contemporary Galactic models. We show that our observed event timescale distribution is consistent with the distribution expected from normal mass stars, if we adopt the Scalo stellar mass function as our lens mass function. However, we note that since there is still disagreement about the exact form of the stellar mass function, there is uncertainty in this conclusion. Based on our event timescale distribution, we find no evidence for the existence of a large population of brown dwarfs in the direction of the Galactic bulge.

EROS and MACHO Combined Limits on Planetary Mass Dark Matter in the Galactic Halo

The EROS and MACHO collaborations have each published upper limits on the amount of planetary-mass dark matter in the Galactic halo obtained from gravitational microlensing searches. In this Letter, the two limits are combined to give a much stronger constraint on the abundance of low-mass MACHOs. Specifically, objects with masses 10−7 Mm10−3 M make up less than 25% of the halo dark matter for most models considered, and less than 10% of a standard spherical halo is made of MACHOs in the 3.5×10−7 MThe EROS and MACHO collaborations have each published upper limits on the amount of planetary mass dark matter in the Galactic Halo obtained from gravitational microlensing searches. In this paper the two limits are combined to give a much stronger constraint on the abundance of low mass MACHOs.

First Observation of Parallax in a Gravitational Microlensing Event

We present the first detection of parallax effects in a gravitational microlensing event. Parallax in a gravitational microlensing event observed only from the Earth appears as a distortion of the (otherwise symmetrical) light curve arising from the motion of the Earth around the Sun. This distortion can be detected if the event duration is not much less than a year and if the projected velocity of the lens is not much larger than the orbital velocity of the Earth about the Sun. The event presented here has a duration (or Einstein diameter crossing time) of = 220 days and clearly shows the distortion resulting from the Earths motion. We find that the projected velocity of the lens is

The MACHO project first year LMC results: The Microlensing rate and the nature of the galactic dark halo

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The MACHO Project LMC Variable Star Inventory. VII. The Discovery of RV Tauri Stars and New Type II Cepheids in the Large Magellanic Cloud

-->= 75 ± 5 km s-1 at an angle of 28° ± 4° from the direction of increasing galactic longitude, as expected for a lens in the galactic disk. A likelihood analysis of this event yields estimates of the distance to and mass of the lens: Dlens = 1.7+1.1−0.7 kpc and M = 1.3+1.3−0.6 M☉. This suggests that the lens is a remnant such as a white dwarf or neutron star. It is possible, though less likely, that the lens is a main-sequence star. If so, we can add our upper limit on the observed flux from the lens to the analysis. This modifies the estimates to Dlens = 2.8+1.1−0.6 kpc and M = 0.6+0.4−0.2 M☉.

Macho project limits on black hole dark matter in the 1-30 M⊙ range

Since July 1992, the MACHO project has been carrying out long-term photometric monitoring of over 20 million stars in the Magellanic Clouds and Galactic bulge. Our aim is to search for the very rare gravitational microlensing events predicted if the dark halo of our Galaxy is comprised of massive compact halo objects (hereafter MACHOs). We have now analyzed most of the first year{close_quote}s LMC data, comprising 9.5 million light curves of stars with an average of 235 observations each. Automated selection procedures applied to this sample show three events consistent with microlensing; the first detected is very striking (Alcock and coworkers), and two are of modest amplitude. We have evaluated our experimental detection efficiency using a range of detailed Monte Carlo simulations, including the addition of artificial stars to real data frames. Using a ``standard`` halo density profile, we find that a halo comprised entirely of MACHOs in the mass range 3{times}10{sup {minus}4} to 0.06 {ital M}{sub {circle_dot}} would predict more than 15 detected events in this data set, and objects around 3{times}10{sup {minus}3} {ital M}{sub {circle_dot}} would predict 25 events; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events aremorexa0» microlensing by halo objects, and fitting a naive spherical halo model to our data yields a MACHO halo fraction {ital f}=0.19{sup +0.16}{sub -0.10}, a total mass in MACHOs (inside 50 kpc) of 7.6{sup +6}{sub -4}{times}10{sup 10} {ital M}{sub {circle_dot}}, and a microlensing optical depth 8.8{sup +7}{sub -5}{times}10{sup -8} (68% confidence level). Should only one of these events be microlensing, this could be explained in terms of previously known populations. We have explored a wide range of halo models and find that, while our constraints on the MACHO fraction are quite model dependent, constraints on the total mass in MACHOs within 50 kpc are quite secure.«xa0lessSince July 1992, the MACHO project has been carrying out long-term photometric monitoring of over 20 million stars in the Magellanic Clouds and Galactic Bulge. Our aim is to search for the very rare gravitational microlensing events predicted if the dark halo of our Galaxy is comprised of massive compact halo objects (hereafter Machos). We have now analysed most of the rst years LMC data, comprising 9.5 million light curves of stars with an average of 235 observations each. Automated selection procedures applied to this sample show 3 events consistent with microlensing, of which one is very striking (Alcock et al. 1993) and two are of modest amplitude. We have evaluated our experimental detection e ciency using a range of detailed Monte-Carlo simulations, including addition of arti cial stars to real data frames. Using a `standard halo density pro le we nd that a halo comprised entirely of Machos in the mass range 3 10 4 to 0:06M would predict > 15 detected events in this dataset, and objects around 3 10 3M would predict 25 events; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events are microlensing of halo objects and tting a naive spherical halo model to our data yields a Macho halo fraction f = 0:19+0:16 0:10, a total mass in Machos (inside 50 kpc) of 7:6+6 4 1010M , and a microlensing optical depth 8:8+7 5 10 8 (68% CL). We have explored a wide range of halo models and nd that, while our constraints on the Macho fraction are quite model-dependent, constraints on the total mass in Machos within 50 kpc are quite secure. Future observations from this and other similar projects and accurate measurements of the Galactic mass out to large radii should combine to give much improved constraints on the Macho fraction of the halo.