Andromeda's Parachute: Time Delays and Hubble Constant

TL;DR

This study measures precise time delays in the gravitational lens system PS J0147+4630 to estimate the Hubble constant, revealing discrepancies with accepted values and discussing potential explanations.

Contribution

It provides accurate time delay measurements for Andromeda's Parachute lens system and analyzes their implications for cosmological parameters, especially the Hubble constant.

Findings

Measured three independent time delays with about 4% uncertainty.

Found a significant discrepancy between derived Hubble constant and accepted values.

Discussed potential factors affecting the Hubble constant estimation.

Abstract

The gravitational lens system PS J0147+4630 (Andromeda's Parachute) consists of four quasar images ABCD and a lensing galaxy. We obtained $r$-band light curves of ABCD in the 2017$-$2022 period from monitoring with two 2-m class telescopes. Applying state-of-the-art curve shifting algorithms to these light curves led to measurements of time delays between images, and the three independent delays relative to image D are accurate enough to be used in cosmological studies (uncertainty of about 4%): $\Delta t_{\rm{AD}}$ = $-$170.5 $\pm$ 7.0, $\Delta t_{\rm{BD}}$ = $-$170.4 $\pm$ 6.0, and $\Delta t_{\rm{CD}}$ = $-$177.0 $\pm$ 6.5 d, where image D is trailing all the other images. Our finely sampled light curves and some additional fluxes in the years 2010$-$2013 also demonstrated the presence of significant microlensing variations. From the measured delays relative to image D and typical values of the external convergence, recent lens mass models yielded a Hubble constant that is in clear disagreement with currently accepted values around 70 km s$^{-1}$ Mpc$^{-1}$. We discuss how to account for a standard value of the Hubble constant without invoking the presence of an extraordinary high external convergence.