Ready for O4 II: GRANDMA Observations of Swift GRBs during eight-weeks of Spring 2022

TL;DR

This paper describes a coordinated observational campaign by the GRANDMA network to rapidly follow up on Swift GRBs during 2022, improving response times and detection efficiency for early afterglows in preparation for LIGO-Virgo-Kagra's O4 II run.

Contribution

It introduces a dedicated infrastructure and coordinated strategy for rapid follow-up of GRBs, enhancing the GRANDMA network’s ability to detect and analyze afterglows during gravitational wave observing runs.

Findings

Detected 3 GRB afterglows out of 11 triggers

Achieved rapid response with minimized reaction times

Obtained photometric redshift and host galaxy properties for GRB 220427A

Abstract

We present a campaign designed to train the GRANDMA network and its infrastructure to follow up on transient alerts and detect their early afterglows. In preparation for O4 II campaign, we focused on GRB alerts as they are expected to be an electromagnetic counterpart of gravitational-wave events. Our goal was to improve our response to the alerts and start prompt observations as soon as possible to better prepare the GRANDMA network for the fourth observational run of LIGO-Virgo-Kagra (which started at the end of May 2023), and future missions such as SM. To receive, manage and send out observational plans to our partner telescopes we set up dedicated infrastructure and a rota of follow-up adcates were organized to guarantee round-the-clock assistance to our telescope teams. To ensure a great number of observations, we focused on Swift GRBs whose localization errors were generally smaller than the GRANDMA telescopes' field of view. This allowed us to bypass the transient identification process and focus on the reaction time and efficiency of the network. During 'Ready for O4 II', 11 Swift/INTEGRAL GRB triggers were selected, nine fields had been observed, and three afterglows were detected (GRB 220403B, GRB 220427A, GRB 220514A), with 17 GRANDMA telescopes and 17 amateur astronomers from the citizen science project Kilonova-Catcher. Here we highlight the GRB 220427A analysis where our long-term follow-up of the host galaxy allowed us to obtain a photometric redshift of $z=0.82\pm0.09$, its lightcurve elution, fit the decay slope of the afterglows, and study the properties of the host galaxy.