On-going improvements in the Virgo strain h(t) reconstruction, online noise subtraction and early-warning pipe in preparation for the O4 run

Abstract: In this contribution, we outline the improvements on the strain h(t) reconstruction in preparation for the observing run O4 of AdvancedVirgo+. These improvements have the main goal to provide a h(t) with high precision and reduce its uncertainties on this quantity to a few percent. First, we describe how the reconstruction of the strain signal h(t) is performed in Virgo and its link to the interferometer and its calibration. We highlight how we plan to monitor the optical response of the interferometer and mirrors. We will describe how we plan to correct the bias of the reconstructed h(t) strain. We present the new online linear noise subtraction method, developed to successfully tackle correlated noise witness channels that are also present in h(t). We provide the status of the low-latency h(t) strain reconstruction, which has the main goal to reduce the latency in pre-merger early warning alerts.