What is the ruling on one who had intercourse after the Fajr adhan in Ramadan, unintentionally, and unaware that this invalidates the fast?

If a fasting person engages in sexual intercourse during the daytime in Ramadan after Fajr (dawn) has broken, being ignorant that this invalidates the fast, then according to some scholars, his fast is valid, and there is no need for him to make up the day (qadaa') or offer expiation (kaffarah). Other scholars, however, are of the view that making up the day and offering expiation are obligatory, and this is the more cautious approach.