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

F. Courbin , V. Bonvin , E. Buckley-Geer , C.D. Fassnacht , J. Frieman , H. Lin , P.J. Marshall , S.H. Suyu

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T. Treu T. Anguita V. Motta G. Meylan E. Paic M. Tewes A. Agnello D.C.-Y. Chao M. Chijani D. Gilman K. Rojas P. Williams A. Hempel S. Kim R. Lachaume M. Rabus T. M. C. Abbott S. Allam J. Annis M. Banerji K. Bechtol A. Benoit-L\'evy D. Brooks D. L. Burke A. Carnero Rosell M. Carrasco Kind J. Carretero C. B. D'Andrea L. N. da Costa C. Davis D. L. DePoy S. Desai B. Flaugher P. Fosalba J. Garcia-Bellido E. Gaztanaga D. A. Goldstein D. Gruen R. A. Gruendl J. Gschwend G. Gutierrez K. Honscheid D. J. James K. Kuehn S. Kuhlmann N. Kuropatkin O. Lahav M. Lima M. A. G. Maia M. March J. L. Marshall R. G. McMahon F. Menanteau R. Miquel B. Nord A. A. Plazas E. Sanchez V. Scarpine R. Schindler M. Schubnell I. Sevilla-Noarbe M. Smith M. Soares-Santos F. Sobreira E. Suchyta G. Tarle D. L. Tucker A. R. Walker W. Wester

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keywords quasardelaystimedatadaysimagedj0408-5354quadruply

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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.

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Global structure of the time delay likelihood
stat.ME 2026-02 unverdicted novelty 6.0

Time delay likelihoods modeled with Gaussian processes develop a boundary-driven W-shape with a global maximum at the true delay and rises at observation window edges, misleading nested sampling and biasing H0 high.