Optical and Near-Infrared Follow-up Observations of GRB980329

Abstract

We imaged the field of GRB980329 in the optical and in the near-infrared starting 20 hours after the event, at the ESO NTT, at the NOT, and at the TIRGO. In the first night we detect an object of R = 23.6 +- 0.2 within the BeppoSAX NFI error box at the same position as a transient VLA source proposed as the radio afterglow of this GRB. The source faded by 1.6 +- 0.5 magnitudes in 2.1 days, similarly to the decays of previous GRB optical afterglows. This transient is likely the optical counterpart of GRB980329. In the near-infrared we detect signal at 2-sigma significance, whose position is only marginally consistent with that of the VLA source. The spectrum of the transient bears the signatures of substantial absorption within the GRB host galaxy. The afterglow energetics are interpreted as synchrotron radiation from an expanding blast wave.