OGLEing MACHOs

Title: The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs
Authors: L. Wyrzykowski et al.
First Author’s Institution: Institute of Astronomy, University of Cambridge, UK & Warsaw University Astronomical Observatory, Poland

In previous astrobites posts, we’ve talked about using microlensing to find planets and to detect dark matter in other galaxies.  However, one of the earliest applications of microlensing was a bit closer to home: the detection of compact objects in the dark matter halo of our own galaxy.  We know there is a lot more mass in galaxies, including our own, than what we can see.  Because it doesn’t emit light, we call this material dark matter.

Most of the matter in our Universe is dark matter, but what is it really?  How much of it is ordinary matter that we just can’t see and how much is something more exotic?  This is astronomers we’re talking about, so these two theoretical components were given names early on.  On the side of ordinary matter, we have “massive compact halo objects,” or MACHOs.  MACHOs are made of baryonic matter, as are the Earth, Moon and stars, but unlike these familiar objects, MACHOs don’t emit radiation (or at least not a lot of it).  As a result, they aren’t detectable by normal means because all telescopes can do is detect radiation.  Black holes, brown dwarfs and free-floating planets are among the candidates for MACHOs.  Conversely, dark matter can be made of particles that are outside of our everyday experience.  The primary candidates for non-baryonic dark matter are WIMPs, or “weakly interacting massive particles.”  The theoretical particles only interact gravitationally and through the weak nuclear force so we still can’t see them with our telescopes.

Since we can’t directly image MACHOs and WIMPs (at least not yet!), how can we proceed?  One approach is to use microlensing: when a MACHO passes between us and a background point source (e.g. a star), the source will be magnified and will briefly appear brighter.  The Optical Gravitational Lensing Experiment, or OGLE, is one of several microlensing surveys searching for the momentary brightening events that might signal the presence of a black hole or a neutron star.  Because of the need for many background stars to act as sources, OGLE is looking in the direction of the Large and Small Magellanic Clouds.

The light curve of OGLE-SMC-02, a microlensing event thought to be the result of a binary black hole passing between us an a background star in the SMC. The x-axis is the date in days and the y-axis the magnitude. The data and the best-fitting models are shown in black and red (for I-band) or blue and green (for V-band). The flat line and points at the bottom show the residuals (data minus model). From Fig. 4 of the paper.

This paper presents results from observations of the SMC taken between 2001 and 2009. The entire 14-degree survey area was observed every 2-3 days in I-band and every 4-10 days in V-band. They performed an automated search that identified three candidate microlensing events, two of which were detected as they occurred. It is generally quite difficult to determine the origin of a lens. However none of these three candidates show bumps other than the microlensing event in their light curves, nor are they located in the region particularly prone to contamination.

The light curve of OGLE-SMC-02 is shown above; the lens in this case can be modeled by a 10 solar mass binary black hole ().  No other signals have yet been detected from the location of this lens but further observations could confirm this. OGLE-SMC-03 is believed to be a low-mass star. This can be verified with several years’ patience: as time passes, the lens and source will move past each other and the two objects will be resolvable. OGLE-SMC-04 is most likely a “self-lensing” event, in which both the source and lens are in the SMC.  FInally, the authors place constraints on the mass fraction of MACHOs in the galactic halo finding that, in agreement with other surveys, there’s not a whole lot. This means that we still don’t know much about dark matter and that a lot of work remains!

About Elisabeth Newton

I am an NSF Astronomy & Astrophysics postdoctoral fellow at MIT. I was a Harvard graduate student and an astrobites and ComSciCon co-founder.

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