GRB 221009A: continued REM optical/NIR observations and evidence for an achromatic steepening in the afterglow light curve

We continued to observe the field of GRB 221009A (Dichiara et al., GCN 32632; Kennea & Williams, GCN 32635; Veres et al., GCN 32636; Bissaldi et al., GCN 32637; Svinkin et al., GCN 32641; Ursi et al., GCN Circ. 32650) with the REM 60cm robotic telescope located at the ESO premises of La Silla (Chile). The observations were carried in the optical and NIR bands, from about 0.4 to 3.5 days after the GBM trigger, with a nightly cadence. A refined analysis of the first REM epoch reveals that the H-band photometry reported in Brivio et al. (GCN Circ. 32652) is affected from contamination from a field star located at an angular separation of about 5" from the NIR afterglow. We report here an updated value of H = 12.62 +/- 0.02 mag (Vega, calibrated against the 2MASS catalogue) at 0.4 days after the GBM trigger. Inspection of the H- and z-band afterglow light curves obtained with REM reveals evidence for a steepening in the decay after about 1 day. We carried out a fit of the 0.3 - 10 keV Swift/XRT afterglow light curve (https://www.swift.ac.uk/xrt_curves/01126853/ <https://www.swift.ac.uk/xrt_curves/01126853/>), rescaling the t-t0 with respect to the time of the GBM trigger, and find statistical evidence that a broken power-law decay provides a better fit with respect to a single power-law decay, with a break time at about 1 day. By performing a joint fit to the H, z and X-ray bands, with a broken power-law model we find the the overall dataset is best fit with a broken power-law with decay indices alpha1_X ~ 1.5, alpha2_X ~ 1.9, alpha1_NIR = alpha1_opt ~ 0.8, alpha2_NIR = alpha2_opt ~ 1.6 with a break time (forced to be equal in all light curves) t_b ~ 0.98 days. Under the assumption that such an achromatic break is the GRB jet-break, starting from the computed GRB Eiso (Frederiks et al., GCN Circ. 32668) we obtain a jet half-opening angle theta_jet ~ 3.5 deg and a beaming-corrected energy for the GRB of ~ 6e51 erg (using efficiency 0.2 and density n = 1 cm^(-3); Sari et al., 1999, ApJ, 519, L17; Rhoads, 1999, ApJ, 525, 737 ).