Multiwavelength study of EP250416a / GRB 250416C: An optically dark, long GRB with a late jet break

Abstract

We present a multiwavelength study of the γ/X-ray transient EP250416a (also designated GRB 250416C), triggered by the Einstein Probe (EP) Wide-field X-ray Telescope as well as by SVOM and Konus-Wind (KW). Observations spanning the γ-ray, X-ray, and optical bands facilitated a detailed analysis of the burst's prompt emission, afterglow evolution, and physical origin. GRB 250416C exhibits a burst duration of 30 s in the X-ray and 17.7 s in γ rays, and a joint spectral fitting of 0.5-5000 keV data yields E =342_ peak -232 ^ +90 keV. Optical spectroscopy of the afterglow, acquired with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South, yields a redshift of z=0.963. Accounting for the measured redshift, the isotropic energies are E =2.7_ X,iso -0.5 ^ +0.9 50 erg and E =7.34_ γ,iso -2.1 ^ +5.1 51 erg, aligning with the Amati relation for long GRBs. The fluence ratio ̊m S(25-50 keV)/S(50-100 keV)=0.78_ -0.12 ^ +0.10 classifies GRB 250416C as an X-ray rich GRB. The X-ray afterglow exhibits initial shallow decay (α ≈ -0.5) transitioning to a canonical decay phase (α ≈ -1), with a very late jet break at t∼ 1.5 ^ 10^6 s. This corresponds to a jet half-opening angle of þeta _j=10.6_ -1.8 +1.9 degrees. GRB 250416C is optically dark, as it shows only a faint r-band detection (r=24.16± 0.13 mag) from Gemini South-GMOS and a low optical-to-X-ray spectral index β_ ̊m OX = 0.3. This may be attributed to significant host-galaxy extinction, with a required A_V^ host =2.7± 0.3 mag derived from the extinction curve model.