[Paper Review] Pixel lensing: Microlensing towards M31

S. Calchi Novati|arXiv (Cornell University)|Dec 14, 2009

Galaxies: Formation, Evolution, Phenomena113 references3 citations

TL;DR

This paper reviews pixel lensing—gravitational microlensing of unresolved stars—as a method to study dark matter, stellar populations, and exoplanets in the Andromeda galaxy (M31). It synthesizes observational results from multiple campaigns, concluding that while self-lensing explains some events, evidence remains for a MACHO signal, and the technique is poised to detect extra-galactic planets with future surveys.

ABSTRACT

Pixel lensing is gravitational microlensing of unresolved stars. The main target explored up to now has been the nearby galaxy of Andromeda, M31. The scientific issues of interest are the search for dark matter in form of compact halo objects, the study of the characteristics of the luminous lens and source populations and the possibility of detecting extra-solar (and extra-galactic) planets. In the present work we intend to give an updated overview of the observational status in this field.

Motivation & Objective

• To provide an updated overview of the observational status of pixel lensing, particularly toward M31.
• To assess the evidence for compact halo objects (MACHOs) as a form of dark matter using unresolved microlensing events.
• To evaluate the role of self-lensing by luminous stars in M31 as a background signal and astrophysical probe.
• To explore the feasibility of detecting extra-solar planets in M31 through pixel lensing.
• To examine the extension of pixel lensing beyond the Local Group to distant galaxies and clusters.

Proposed method

• Uses the standard microlensing light curve model: $\Phi(t,\{\theta\}) = \Phi^{*} \cdot (A(t,\{\theta\}) - 1) + \Phi_{\mathrm{B}}$, where $A(t,\{\theta\})$ is the amplification from a point-mass lens.
• Applies the Paczyński light curve form: $A(u) = \frac{u^2 + 2}{u\sqrt{u^2 + 4}}$, with $u = \sqrt{u_0^2 + (t - t_0)^2 / t_{\mathrm{E}}^2}$, to model light curves.
• Analyzes pixel-level flux variations in deep imaging surveys to detect microlensing events without resolving individual stars.
• Compares results from multiple collaborations (POINT-AGAPE, MEGA, WeCAPP, ANGSTROM, PLAN) on the same data sets to resolve discrepancies.
• Uses the Einstein timescale $t_{\mathrm{E}} = R_{\mathrm{E}} / v$ and angular Einstein radius $\theta_{\mathrm{E}} = \sqrt{\frac{4GM}{c^2} \frac{D_{\mathrm{s}} - D_{\mathrm{l}}}{D_{\mathrm{l}} D_{\mathrm{s}}}}}$ to infer lens mass and distance.
• Performs statistical modeling to distinguish MACHO lensing from self-lensing, especially in the context of M31’s resolved stellar populations.

Experimental results

Research questions

• RQ1 What is the current observational evidence for MACHOs in the M31 halo using pixel lensing?
• RQ2 To what extent can self-lensing by luminous stars in M31 explain the observed microlensing event rate?
• RQ3 Can pixel lensing detect extra-solar planets in M31, and what are the prospects for such detection?
• RQ4 How do different observational campaigns (e.g., POINT-AGAPE, MEGA, WeCAPP) reconcile or differ in their interpretation of microlensing events?
• RQ5 What are the prospects for extending pixel lensing to extragalactic targets beyond the Local Group?

Key findings

• Approximately 30 microlensing candidate events have been reported toward M31, with significant variation in interpretation across collaborations.
• The POINT-AGAPE collaboration found evidence for a MACHO signal in the same INT data set where MEGA found the signal consistent with self-lensing.
• The WeCAPP collaboration analyzed a bright, well-sampled event and concluded the lens is more likely a MACHO than a stellar object.
• Preliminary results from the WeCAPP 11-year campaign report 10 microlensing events, 80% of which have full width at half maximum (FWHM) less than 5 days, suggesting short timescale events inconsistent with pure self-lensing.
• The self-lensing signal alone cannot fully explain the observed event rate, indicating a potential contribution from MACHOs or other compact objects.
• Future surveys such as PAandromeda, ANGSTROM, and PLAN are expected to provide full coverage of M31 and significantly increase the number of detectable events, enabling robust characterization of lensing populations.