Characterization of the hot Neptune GJ 436b with Spitzer and ground-based observations

TL;DR

This study uses Spitzer infrared photometry to analyze the secondary eclipse of GJ 436b, revealing its thermal emission, orbital eccentricity, and stellar properties, providing insights into its atmospheric and orbital characteristics.

Contribution

First infrared detection of GJ 436b's secondary eclipse with precise measurements of its brightness temperature and orbital eccentricity, enhancing understanding of its atmospheric and orbital properties.

Findings

Eclipse depth of 0.58 mmag measured

Brightness temperature estimated at 717 ± 35 K

Orbital eccentricity refined to 0.14 ± 0.01

Abstract

We present Spitzer Space Telescope infrared photometry of a secondary eclipse of the hot Neptune GJ436b. The observations were obtained using the 8-micron band of the InfraRed Array Camera (IRAC). The data spanning the predicted time of secondary eclipse show a clear flux decrement with the expected shape and duration. The observed eclipse depth of 0.58 mmag allows us to estimate a blackbody brightness temperature of T_p = 717 +- 35 K at 8 microns. We compare this infrared flux measurement to a model of the planetary thermal emission, and show that this model reproduces properly the observed flux decrement. The timing of the secondary eclipse confirms the non-zero orbital eccentricity of the planet, while also increasing its precision (e = 0.14 +- 0.01). Additional new spectroscopic and photometric observations allow us to estimate the rotational period of the star and to assess the potential presence of another planet.