Theory of Exploring the Dark Halo with Microlensing 1: Power--Law Models

Abstract

The detection of microlensing has opened the way for the development of new methods in galactic astronomy. This series of papers investigates what microlensing can teach us about the structure and shape of the dark halo. In this paper we present formulas for the microlensing rate, optical depth and event duration distributions for a simple set of axisymmetric disk-halo models. The halos are based on the "power--law models" which have simple velocity distributions. Using these models, we show that there is a large uncertainty in the predicted microlensing rate because of uncertainty in the halo parameters. For example, models which reproduce the measured galactic observables to within their errors still differ in microlensing rate towards the Magellanic Clouds by more than a factor of ten. We find that while the more easily computed optical depth correlates well with microlensing rate, the ratio of optical depth to rate can vary by a factor of two (or greater if the disk is maximal). Comparison of microlensing rates towards the Large and Small Magellanic Clouds (LMC and SMC) and M31 can be used to aid determinations of the halo flattening and rotation curve slope. For example, the ratio of microlensing rates towards the LMC and SMC is $\sim 0.7-0.8$ for E0 halos and $\sim 1.0-1.2$ for E7 halos (c.f. Sackett \& Gould 1993). Once the flattening has been established, the ratio of microlensing rates towards M31 and the LMC may help to distinguish between models with rising, flat or falling rotation curves. Comparison of rates along LMC and galactic bulge lines-of-sight gives useful information on the halo core radius, although this may not be so easy to extract in practice. Maximal disk models provide substantially smaller halo optical depths, shorter event durations and even larger model uncertainties.

arXiv comment19 pages, tex, 12 figures(uuencoded + text.ps)

Cited by in corpus (29)

• Supersymmetric Dark Matter
• The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations
• The MACHO Project: 45 Candidate Microlensing Events from the First Year Galactic Bulge Data
• Updated Constraints on Asteroid-Mass Primordial Black Holes as Dark Matter
• Updating the MACHO fraction of the Milky Way dark halo with improved mass models
• First detection of a gravitational microlensing candidate towards the Small Magellanic Cloud
• The WIMP annual modulation signal and non-standard halo models
• Astrophysical uncertainties on stellar microlensing constraints on multi-Solar mass primordial black hole dark matter
• Microlensing
• Binary events and extragalactic planets in pixel microlensing
• Satellite Parallaxes of Lensing Events Towards the Galactic Bulge
• The case for primordial black holes as dark matter
• Microlensing toward crowded fields: Theory and applications to M31
• Axionic Boson Stars in Magnetized Conducting Media
• Event Rate and Einstein Time Evaluation in Pixel Microlensing
• Perturbation theory with dispersion and higher cumulants: framework and linear theory
• Searching for Machos (and other Dark Matter Candidates) in a Simulated Galaxy
• The Minimum Total Mass of MACHOs and Halo Models of the Galaxy
• Statistical Interpretation of LMC Microlensing Candidates
• Microlensing by the Halo MACHOs with Spatially Varying Mass Function
• Ultra High Energy Cosmic Rays from Axion Stars
• SpheCow: flexible dynamical models for galaxies and dark matter haloes
• The Galactic Halo from Microlensing
• Microlensing of Globular Clusters as a Probe of Galactic Structure
• Expected EAGLE event rate towards the Magellanic Clouds
• Probing the mass function of halo dark matter via microlensing
• Reply to "Comment on 'Scalar-tensor gravity coupled to a global monopole and flat rotation curves' "
• EROS/MACHO Gravitational Microlensing Events Toward LMC in Evans Halo Model
• The inverse problem in microlensing: from the optical depth to the galaxy models parameters