Discovery of the Optical Afterglow of Xrf 040812 : Vlt

— We present Chandra and VLT observations of the X−Ray Flash XRF 040812. The X−ray analysis reveals with high precision the position of a hard, fading source. A careful analysis of our I−band VLT images taken starting 17 hours after the burst led to the discovery of the optical afterglow superimposed to a bright (I = 21.5) host galaxy. The optical afterglow is seen decaying with an index of 1.1. We do not detect any jet break and supernova rebright-ening in the optical light curve. The bright apparent luminosity of the host galaxy allows us to get a rough estimate of the redshift, comparing with a set of GRB/XRF host galaxies with known luminosity and redshift. Such comparison suggests a redshift of XRF 040812 in the range 0.3 ≤ z ≤ 0.7. This is also consistent with the lack of emission features in our spectrum. The low inferred redshift is in agreement with the idea that XRFs are low-luminosity, closer events. 1. – Observations and discovery of the afterglow The X-Ray Flash XRF 040812 was discovered by the INTEGRAL satellite on 2004 Aug 12.251 UT, with the IBIS/ISGRI instrument in the 15-200 keV band [1]. Our optical observations at the VLT could start only 0.7 days after the XRF. In order to overcame any saturation problem due to the presence of a very bright star (V∼11) we placed an occulting bar on the detector even if this meant to hide part of the INTEGRAL error box. Two epochs of Chandra observations (5 and 10 d after the burst) revealed the presence of a fading, uncatalogued source within the INTEGRAL error box [2]. Following on the Chandra results, we re-analized our VLT data discovering that the position of the Chandra candidate fell just behind the occulted area of the detector in both epochs. Luckily, c Societ` a Italiana di Fisica