SMILE: Search for MIlli-LEnses

TL;DR

This paper presents a search for milli-arcsecond gravitational lenses in VLBI data to detect compact dark matter objects or primordial black holes in the mass range of 10^6 to 10^9 solar masses, using citizen science and follow-up observations.

Contribution

It introduces a novel citizen science approach combined with VLBI data analysis to identify milli-arcsecond gravitational lens candidates, expanding methods for dark matter and primordial black hole detection.

Findings

Identified 40 milli-arcsecond lens candidates from VLBI data.

Two candidates show potential as gravitational lenses based on spectral analysis.

Follow-up observations are ongoing to confirm lensing nature.

Abstract

Dark Matter (DM) halos with masses below $\sim10^{8}$ $M_{\odot}$, which would help to discriminate between DM models, may be detected through their gravitational effect on distant sources. The same applies to primordial black holes, considered as an alternative scenario to DM particle models. However, there is still no evidence for the existence of such objects. With the aim of finding compact objects in the mass range $\sim$ 10$^{6}$ -- 10$^{9}$$M_{\odot}$, we search for strong gravitational lenses on milli (mas)-arcseconds scales (< 150 mas). For our search, we used the Astrogeo VLBI FITS image database -- the largest publicly available database, containing multi-frequency VLBI data of 13828 individual sources. We used the citizen science approach to visually inspect all sources in all available frequencies in search for images with multiple compact components on mas-scales. At the final stage, sources were excluded based on the surface brightness preservation criterion. We obtained a sample of 40 sources that passed all steps and therefore are judged to be milli-arcsecond lens candidates. These sources are currently followed-up with on-going European VLBI Network (EVN) observations at 5 and 22 GHz. Based on spectral index measurements, we suggest that two of our candidates have a higher probability to be associated with gravitational lenses.