C. D. Kilpatrick, W. Fong, A. Hajela, K. D. Alexander (Northwestern), E. Berger (Harvard), P. K. Blanchard, R. Chornock, R. Margutti, K. Paterson, J. Rastinejad (Northwestern) report:
We report on new Hubble Space Telescope (HST) observations of the near-infrared counterpart to GW170817 at a mid-time of 4.98 Jan 2021 UT, 1236.45 days from merger (GO-15886, PI Fong). These observations consist of 3 orbits observed in WFC3/IR F140W for a total of 7.8 ks on-source exposure time.
Compared to previous F606W HST imaging of the optical and near-infrared afterglow to GW170817 (Lyman et al. 2018, Nature Astronomy, 2, 751; Lamb et al. 2019, ApJL, 870, L15; Fong et al. 2019, ApJL, 883, L1), we do not visually detect a counterpart at the site of GW170817 in the latest observations. We injected artificial sources at the afterglow site and recover a 3-sigma limiting magnitude of >26.3 mag (AB and without correcting for Galactic extinction).
Based on recent Chandra detections of the X-ray counterpart to GW170817 at 1209-1213 days after merger (Hajela et al., GCN 29041, GCN 29019; Troja et al., GCN 29038) as well as radio non-detections with the VLA at 1216 days post-merger (Alexander et al., GCN 29053), we consider whether this limit is constraining in the context of the detection discussed in Hajela et al., GCN 29055. Assuming the counterpart had an unabsorbed flux of 3.46e-15 erg/cm2/s (0.3-10 keV) at that time and a Gamma~1.6 photon index, we infer that the source would have f_F140W = 1.5e-8 Jy or m_F140W = 28.5 mag (AB; unabsorbed) or 28.6 mag (AB; accounting for Galactic extinction) in the latest observations. Similarly, the radio observations yielded a 3-sigma limiting flux of 13 muJy at a central frequency of 3 GHz, which imply that m_F140W > 28 mag for the same photon index, significantly fainter than our limit.
Thus the latest observations are not constraining for this model. However, additional HST observations are planned in F814W, F110W, and F160W between now and 2021 March 1 with comparable exposure times in each filter, enabling deeper constraints on the optical and near-infrared counterpart to GW170817 over a broader range of its spectral energy distribution.
We thank the STScI staff for facilitating these observations, especially our program coordinator Shelly Meyett.