Indications for transit-timing variations in the exo-Neptune HAT-P-26b^★

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: cessen@phys.au.dk
² Astronomical Observatory, Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio av. 3, 10257 Vilnius, Lithuania
³ Institute of Theoretical Astrophysics, University of Oslo, Postboks 1029 Blindern, 0315 Oslo, Norway
⁴ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWA, La Plata, Argentina
⁵ Instituto de Astrofísica de La Plata (CCT-La Plata, CONICET-UNLP), Paseo del Bosque, B1900FWA La Plata, Argentina
⁶ Complejo Astronómico El Leoncito (CONICET-UNLP-UNC-UNSJ), Av. España 1512 Sur, San Juan, Argentina
⁷ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁸ Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Victoria 3800, Australia
⁹ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 18 September 2017
Accepted: 15 April 2019

Abstract

Upon its discovery, the low-density transiting Neptune HAT-P-26b showed a 2.1σ detection drift in its spectroscopic data, while photometric data showed a weak curvature in the timing residuals, the confirmation of which required further follow-up observations. To investigate this suspected variability, we observed 11 primary transits of HAT-P-26b between March, 2015, and July, 2018. For this, we used the 2.15 m Jorge Sahade Telescope placed in San Juan, Argentina, and the 1.2 m STELLA and the 2.5 m Nordic Optical Telescope, both located in the Canary Islands, Spain. To add to valuable information on the transmission spectrum of HAT-P-26b, we focused our observations in the R-band only. To contrast the observed timing variability with possible stellar activity, we carried out a photometric follow-up of the host star over three years. We carried out a global fit to the data and determined the individual mid-transit times focusing specifically on the light curves that showed complete transit coverage. Using bibliographic data corresponding to both ground and space-based facilities, plus our new characterized mid-transit times derived from parts-per-thousand precise photometry, we observed indications of transit timing variations in the system, with an amplitude of ~4 min and a periodicity of ~270 epochs. The photometric and spectroscopic follow-up observations of this system will be continued in order to rule out any aliasing effects caused by poor sampling and the long-term periodicity.