Gravitational microlensing results from MACHO

The Astrophysical Journal, 1996

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 analysed most of the rst year'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 3 events consistent with microlensing, of which one is very striking ) 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 0:06 M would predict > 15 detected events in this dataset, and objects around 3 10 3 M 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

Astronomy & Astrophysics, 1998

We present final results from a search for gravitational microlensing of stars in the Magellanic Clouds by very light machos (MAssive Compact Halo Objects) in the Galactic Halo. The EROS programme used a CCD camera devoted to the detection of events with a duration ranging from 15 minutes to a few days; none of the 350,000 light curves exhibits a

The Astrophysical Journal, 1996

The MACHO project has been monitoring about ten million stars in the Large Magellanic Cloud in the search for gravitational microlensing events caused by massive compact halo objects (Machos) in the halo of the Milky Way. In our standard analysis, we have searched this data set for well sampled, long duration microlensing lightcurves, detected several microlensing events consistent with Machos in the 0.1 M ⊙ < ∼ m < ∼ 1.0 M ⊙ mass range, and set limits on the abundance of objects with masses 10 −5 M ⊙ < ∼ m < ∼ 10 −1 M ⊙ . In

The Astrophysical Journal, 2000

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. A detailed treatment of our detection efficiency shows that this is significantly more than the ∼ 2 to 4 events expected from lensing by known stellar populations. The timescales ( t ) of the events range from 34 to 230 days. We estimate the microlensing optical depth towards the LMC from events with 2 < t < 400 days to be τ 400 2 = 1.2 +0.4 −0.3 × 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% 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 M ⊙ and 0.9 M ⊙ , depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9 +4 −3 × 10 10 M ⊙ , independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of two. 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. 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. We note that an important source of background are supernovae in galaxies behind the LMC.

1995

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 analysed most of the rst year'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 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 0:06 M would predict > 15 detected events in this dataset, and objects around 3 10 3 M 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 10 10 M , 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.

1995

We have monitored 8.6 million stars in the Large Magellanic Cloud for 1.1 years and have found 3 events consistent with gravitational microlensing. We place strong constraints on the Galactic halo content in the form of compact lensing objects in the mass range 10 4 M to 10 1 M . Three events is fewer than expected for a standard spherical halo of objects in this mass range, but appears to exceed the number expected from known Galactic populations. Fitting a naive spherical halo model to our data yields a MACHO fraction f = 0:20 +0:33 0:14 , which implies a total MACHO mass (inside 50 kpc) of 8:0 +14 6 10 10 M , and a microlensing optical depth 9 +15 7 10 8 ( 68% CL).

We present the results from the Optical Gravitational Lensing Experiment II (OGLE-II) survey (1996-2000) towards the Large Magellanic Cloud (LMC), which has the aim of detecting the microlensing phenomena caused by dark matter compact objects in the Galactic halo [massive compact halo objects (MACHOs)]. We use high-resolution Hubble Space Telescope images of the OGLE fields and derive the correction for the number of monitored stars in each field. This also yields blending distributions which we use in `catalogue-level' Monte Carlo simulations of the microlensing events in order to calculate the detection efficiency of the events. We detect two candidates for microlensing events in the All Stars Sample, which translates into an optical depth of 0.43 +/- 0.33 × 10-7. If both events were due to MACHO, the fraction of mass of compact dark matter objects in the Galactic halo would be 8 +/- 6 per cent. This optical depth, however, along with the characteristics of the events seems to be consistent with the self-lensing scenario, i.e. self-lensing alone is sufficient to explain the observed microlensing signal. Our results indicate the non-detection of MACHOs lensing towards the LMC with an upper limit on their abundance in the Galactic halo of 19 per cent for M = 0.4Msolar and 10 per cent for masses between 0.01 and 0.2Msolar.

The Astrophysical Journal, 1997

We report the first discovery of a gravitational microlensing candidate towards a new population of source stars, the Small Magellanic Cloud (SMC). The candidate event's light curve shows no variation for 3 years before an upward excursion lasting ∼ 217 days that peaks around January 11, 1997 at a magnification of ∼ 2.1. Microlensing events towards the Large Magellanic Cloud and the Galactic bulge have allowed important conclusions to be reached on the stellar and dark matter content of the Milky Way. The SMC gives a new line-of-sight through the Milky Way, and is expected to prove useful in determining the flattening of the Galactic halo.

Nuclear Physics B - Proceedings Supplements, 1996

The MACHO collaboration has recently analyzed 2.1 years of photometric data for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis has revealed 8 candidate microlensing events and a total microlensing optical depth of τmeas = 2.9 +1.4 −0.9 × 10 −7 . This significantly exceeds the number of events (1.1) and the microlensing optical depth predicted from known stellar populations: τ back = 5.4 × 10 −8 , but it is consistent with models in which about half of the standard dark halo mass is composed of Machos of mass ∼ 0.5M⊙. One of these 8 events appears to be a binary lensing event with a caustic crossing that is partially resolved, and the measured caustic crossing time allows us to estimate the distance to the lenses. Under the assumption that the source star is a single star and not a short period binary, we show that the lensing objects are very likely to reside in the LMC. However, if we assume that the optical depth for LMC-LMC lensing is large enough to account for our entire lensing signal, then the binary event does not appear to be consistent with lensing of a single LMC source star by a binary residing in the LMC. Thus, while the binary lens may indeed reside in the LMC, there is no indication that most of the lenses reside in the LMC.

Proceedings of the International Astronomical …, 2004

The gravitational microlensing experiments in the direction of Large Magellanic Cloud (LMC) predict a large amount of white dwarfs (∼ 20%) filling the galactic halo. However, the predicted white dwarfs have not been observed at the galactic halo. To interpret the microlensing results and resolving the mentioned problems, we use the hypothesis of spatially varying mass function of MACHOs, proposed by Kerins & Evans (1998) (hereafter KE). However the KE model is not compatible with the duration distribution of events (Rahvar 2004a). Here we use more realistic power-law model of MF, dn/dm ∝ m α for the MACHOs of halo. The index of MF in this model changes from −2.7 for stars with m > 1M at the central part of galactic halo to the substellar regime with an upper limit of −1 at the edge of halo. We show that in contrast to the abundant brown dwarfs of galactic halo, heavy MACHOs can be responsible for the microlensing events in the direction of LMC.

Helvetica Physica Acta - HELV PHYS ACTA, 1996

The French collaboration EROS and the American-Australian collaboration MACHO have reported the observation of altogether ≈10 microlensing events by monitoring during several years the brightness of millions of stars in the Large Magellanic Cloud. In particular the MACHO team announced the discovery of 8 microlensing candidates by analysing their first 2 years of observations. This would imply that the halo dark matter fraction in form of MACHOs (massive astrophysical compact halo objects) is of the order of 45 - 50%. The most accurate way to get information events detected so far by the MACHO collaboration in the Large Magellanic Cloud the average mass turns out to be 0.27 Msun.

1994

We review recent gravitational microlensing results from the EROS, MA-CHO, and OGLE collaborations, and present some details of the very latest MA-CHO results toward the Galactic Bulge. The MACHO collaboration has now discovered in excess of 40 microlensing events toward the Galactic Bulge during the 1993 observing season. A preliminary analysis of this data suggests a much higher microlensing optical depth than predicted by standard galactic models suggesting that these models will have to be revised. This may have important implications for the structure of the Galaxy and its dark halo. Also shown are MACHO data of the first microlensing event ever detected substantially before peak amplification, the first detection of parallax effects in a microlensing event, and the first caustic crossing to be resolved in a microlensing event.

Nature, 2001

1996

M 31 is a very tempting target for a microlensing search of compact objects in galactic haloes. It is the nearest large galaxy, it probably has its own dark halo, and its tilted position with respect to the line of sight provides an unmistakable signature of microlensing. However most stars of M 31 are not resolved and one has to use the "pixel method": monitor the pixels of the image rather than the stars. AGAPE is the implementation of this idea. Data have been collected and treated during two autumns of observation at the 2 metre telescope of Pic du Midi. The process of geometric and photometric alignment, which must be performed before constructing pixel light curves, is described. Seeing variations are minimised by working with large super-pixels (2.1 ) compared with the average seeing. A high level of stability of pixel fluxes, crucial to the approach, is reached. Fluctuations of super-pixels do not exceed 1.7 times the photon noise which is 0.1% of the intensity for the brightest ones. With such stable data, 10 microlensing events are expected for a full "standard halo". With a larger field, a regular and short time sampling and a long lever arm in time, the pixel method will be a very efficient tool to explore the halo of M 31.

Astrophysical Journal, 2003

Recently obtained kinematic data has shown that the Large Magellanic Cloud (LMC) possesses an old stellar halo. In order to further characterize the properties of this halo, parametric King models are fit to the surface density of RR Lyrae stars. Using data from both the MACHO and OGLE II microlensing surveys, the model fits yield the center of their distribution at RA = 05:21.1+-0.8, Dec = -69:45+-6 (J2000) and a core radius of 1.42+-0.12 kpc. As a check the halo model is compared with RR Lyrae star counts in fields near the LMC's periphery previously surveyed with photographic plates. These data, however, require a cautious interpretation. Several topics regarding the LMC stellar halo are discussed. First, the properties of the halo imply a global mass-to-light ratio of M/L_V = 5.3+-2.1 and a total mass of 1.6+-0.6 10^10 M_sun for the LMC in good agreement with estimates based on the rotation curve. Second, although the LMC's disk and halo are kinematically distinct, the shape of the surface density profile of the halo is remarkably similar to that of the young disk. For example, the best-fit exponential scale length for the RR Lyrae stars is 1.47+-0.08 kpc, which compares to 1.46 kpc for the LMC's blue light. In the Galaxy, the halo and disk do not resemble each other like this. Finally, a local maximum in the LMC's microlensing optical depth due to halo-on-disk stellar self-lensing is predicted. For the parameters of the stellar halo obtained, this maximum is located near MACHO events LMC-4 and LMC-23, and is large enough to possibly account for these two events, but not for all of the observed microlensing.