COSMOGRAIL XVI: Time delays for the quadruply imaged quasar DES J0408-5354 with high-cadence photometric monitoring

TL;DR

This study measures precise time delays in the quadruply imaged quasar DES J0408-5354 using high-cadence photometry, providing robust data for cosmological applications and demonstrating an effective observational strategy.

Contribution

It introduces a new high-cadence observational approach for accurate time delay measurement in quadruply imaged quasars, enhancing robustness against microlensing effects.

Findings

Measured one time delay with 1.8% accuracy in 7 months

Determined two additional delays with respective uncertainties of 8.2% and 41%

Demonstrated the effectiveness of high-cadence monitoring for gravitational lens time delays

Abstract

We present time-delay measurements for the new quadruply imaged quasar DES J0408-5354, the first quadruply imaged quasar found in the Dark Energy Survey (DES). Our result is made possible by implementing a new observational strategy using almost daily observations with the MPIA 2.2m telescope at La Silla observatory and deep exposures reaching a signal-to-noise ratio of about 1000 per quasar image. This data quality allows us to catch small photometric variations (a few mmag rms) of the quasar, acting on temporal scales much shorter than microlensing, hence making the time delay measurement very robust against microlensing. In only 7 months we measure very accurately one of the time delays in DES J0408-5354: Dt(AB) = -112.1 +- 2.1 days (1.8%) using only the MPIA 2.2m data. In combination with data taken with the 1.2m Euler Swiss telescope, we also measure two delays involving the D component of the system Dt(AD) = -155.5 +- 12.8 days (8.2%) and Dt(BD) = -42.4 +- 17.6 days (41%), where all the error bars include systematics. Turning these time delays into cosmological constraints will require deep HST imaging or ground-based Adaptive Optics (AO), and information on the velocity field of the lensing galaxy.