Context. Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Aims. Currently, only lower limits were securely set to the initial fireball Lorentz factor Γ 0 . We aim to provide a direct measure of Γ 0 . Methods. The early-time afterglow light curve carries information about Γ 0 , which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607A with the REM robotic telescope. Results. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Γ 0 ∼ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be R dec ≈ 10 17 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB.

Abstract. We present near infrared images of the bright rimmed globule IC 1396-N in the H2 2.12 µm narrow band filter as well as in broad band J, H and K filters. We detected several chains of collimated H2 knots inside the globule, having different luminosities but similar orientations in the sky. Most of the knots are associated with peaks of high velocity CO emission, indicating that they trace shocked regions along collimated stellar jets. From the H2 knots morphology and orientation, we identify at least three different jets: one of them is driven by the young protostar associated with IRAS 21391+5802, which represents the most luminous object in the region, while we were able to identify only one of the two other driving sources by means of near infrared photometry. Our photometry reveals the existence of a cluster of young embedded sources located in a south-north line which follows the distribution of the high density gas and testifies for a highly efficient star formation activity through all the globule.

Optical and near-infrared light curves of the Type IIn supernova 1999el in NGC 6951 are presented. A period of 220 days (416 days in the near-infrared) is covered from the first observation obtained a few days before maximum light. Spectroscopic observations are also discussed. Using as a distance calibrator the Type Ia SN 2000E, which occurred some months later in the same galaxy, and fitting a blackbody law to the photometric data we obtain a maximum bolometric luminosity for SN 1999el of ∼ 10 44 erg s −1 . In general, the photometric properties of SN 1999el are very similar to those of SN 1998S, a bright and well studied Type IIn SN, showing a fast decline in all observed bands similar to those of Type II-L SNe. The differences with SN 1998S are analyzed and ascribed to the differences in a pre-existing circumstellar envelope in which dust was already present at the moment of the SN outburst. We infer that light echoes may play a possibly -2significant role in affecting the observed properties of the light curves, although improved theoretical models are needed to account for the data. We conclude that mass loss in the progenitor RG stars is episodic and occurs in an asymmetric way. This implies that collapsing massive stars appear as normal Type II SN if this occurs far from major mass loss episodes, whereas they appear as Type IIn SNe if a large mass loss episode is in progress.Subject headings: supernovae: general -supernovae: individual (SN 1999el)galaxies: individual (NGC 6951) -infrared:stars 1 Based on observations collected with the AZT-24 telescope (Campo Imperatore, Italy), the 1.8-m telescope of the Osservatorio Astronomico di Padova (Asiago, Italy), the TNT telescope operated by the Osservatorio Astronomico di Collurania-Teramo (Teramo, Italy), the Italian National Telescope TNG (La Palma, Canary Islands, Spain), the 1-m telescope of the Observatorio Astronómico Nacional (Tonantzintla, Mexico), the 2.1-m telescope of the Guillermo Haro Observatory (Cananea, Mexico).

A B S T R A C TWe present the results of photometric observations of BL Lacertae at 1 m performed during the strong outburst that occurred in the summer of 1997. Measurements were carried out using the F99 filter of the Arizona system. Simultaneous observations in the B and V bands were made with other telescopes. Large and fast variations were observed on the night of August 2 when the brightness of BL Lacertae changed by ϳ0.5 mag in about 2 h: the variation amplitude is greater at higher frequency. Some implications of these results for the origin of the fast variations are discussed. We also report the first F99 calibration for three reference stars in the BL Lac field.

We present optical (UBV RI) and near-infrared (JHK) photometry, along with optical spectra, of the Type Ia supernova SN 2000E in the spiral galaxy NGC 6951. It was discovered by the staff of the Teramo Observatory during the monitoring of the SN 1999el. The observations span a time interval of 234 days in the optical and 134 days in the near-infrared (starting ∼ 16 days and ∼ 7 days before maximum B light, respectively). Optical spectra are available from 6 days before maximum B light to 122 days after it. The photometric behavior of SN 2000E is remarkably similar to that of SN 1991T and SN 1992bc: it exhibits a ∆m 15 (B) = 0.94, thus being classifiable as a slow-declining Type Ia SN and showing the distinctive features of such a class of objects both in the visible and in the near-infrared. Spectroscopically, SN 2000E appears as a "normal" Type Ia supernova, like SN 1990N. We could constrain reddening [E(B − V ) ∼ 0.5