Spitzer Space Telescope Infrared Observations of the Binary Neutron Star Merger GW170817

TL;DR

This paper reports late-time infrared observations of GW170817 with Spitzer, revealing a dimmer source than models predicted, suggesting a transition to the nebular phase or reduced thermalization efficiency.

Contribution

First infrared observations of GW170817 at late times, providing new insights into kilonova evolution and informing future observations with Spitzer and JWST.

Findings

Detected infrared source at 4.5 micron 43 and 74 days post-merger

Observed brightness dimmer than early kilonova models predicted

Fading rate and color consistent with model, indicating possible nebular phase transition

Abstract

We present Spitzer Space Telescope 3.6 and 4.5 micron observations of the binary neutron star merger GW170817 at 43, 74, and 264 days post-merger. Using the final observation as a template, we uncover a source at the position of GW170817 at 4.5 micron with a brightness of 22.9+/-0.3 AB mag at 43 days and 23.8+/-0.3 AB mag at 74 days (the uncertainty is dominated by systematics from the image subtraction); no obvious source is detected at 3.6 micron to a 3-sigma limit of >23.3 AB mag in both epochs. The measured brightness is dimmer by a factor of about 2-3 times compared to our previously published kilonova model, which is based on UV, optical, and near-IR data at <30 days. However, the observed fading rate and color (m_{3.6}-m_{4.5}> 0 AB mag) are consistent with our model. We suggest that the discrepancy is likely due to a transition to the nebular phase, or a reduced thermalization efficiency at such late time. Using the Spitzer data as a guide, we briefly discuss the prospects of observing future binary neutron star mergers with Spitzer (in LIGO/Virgo Observing Run 3) and the James Webb Space Telescope (in LIGO/Virgo Observing Run 4 and beyond).