Early emission of rising optical afterglows the case of GRB 060904B and GRB 070420

Aims We present the time-resolved optical emission of gamma-ray bursts <ASTROBJ>GRB 060904B</ASTROBJ> and <ASTROBJ>GRB 070420</ASTROBJ> during their prompt and early afterglow phases. <BR />Methods We used time resolved photometry from optical data taken by the TAROT telescope and time resolved spectroscopy at high energies from the Swift spacecraft instrument. <BR />Results The optical emissions of both GRBs are found to increase from the end of the prompt phase, passing to a maximum of brightness at t_peak=9.2 min and 3.3 min for <ASTROBJ>GRB 060904B</ASTROBJ> and <ASTROBJ>GRB 070420</ASTROBJ> respectively and then decrease. <ASTROBJ>GRB 060904B</ASTROBJ> presents a large optical plateau and a very large X-ray flare. We argue that the very large X-flare occurring near t_peak is produced by an extended internal engine activity and is only a coincidence with the optical emission. <ASTROBJ>GRB 070420</ASTROBJ> observations would support this idea because there was no X-flare during the optical peak. The nature of the optical plateau of <ASTROBJ>GRB 060904B</ASTROBJ> is less clear and might be related to the late energy injection. Based on observations performed with TAROT at the Calern/OCA and La Silla/ESO observatories, GCN data archive and Swift public data archive.