We present Very Large Telescope spectroscopic observations of the GRB 990705 host galaxy and highlight the benefits provided by the prompt phase features of gamma-ray bursts (GRBs) to derive the redshifts of the latter. In the host spectrum, we indeed detect an emission feature that we attribute to the [O II] λλ3726, 3729 doublet and derive an unambiguous redshift z = 0.8424 ± 0.0002 for this galaxy. This is in full agreement with the value z ∼ 0.86 ± 0.17 previously derived using a transient absorption edge discovered in the X-ray spectrum of GRB 990705. This burst is therefore the first GRB for which a reliable redshift was derived from the prompt phase emission itself, as opposed to redshift determinations performed using putative host galaxy emission lines or interstellar absorption lines in the GRB afterglows. Deep and high-resolution images of the host of GRB 990705 with the Space Telescope Imaging Spectrograph camera on board the Hubble Space Telescope reveal that the burst occurred in a nearly face-on Sc spiral galaxy typical of disk-dominated systems at 0.75 ≤ z ≤ 1. Assuming a cosmology with H0 = 65 km s-1 Mpc-1, Ωm = 0.3, and Ωλ = 0.7, we derive an absolute B magnitude MB = -21.75 for this galaxy and a star formation rate SFR ≈ 5-8 M⊙ yr-1. Finally, we discuss the implications of using X-ray transient features to derive GRB redshifts with larger burst samples and especially examine the case of short and dark long GRBs.Facultad de Ciencias Astronómicas y Geofísica
