2 The SEDs

We have collected the X-ray to radio data and upper limits of the four GRB afterglows of 970228, 970508, 980329 and 980703, and we have then constructed the multiwavelength spectra at various epochs for each case (Fig. 1). When known, the host galaxy contribution was subtracted from the optical and infrared (IR) measurements, if significant with respect to the OT magnitude. Dereddening for foreground Galactic absorption was made with the law by Cardelli et al. (1989). The photometric flux calibration was taken by Fukugita et al. (1995) and by Bersanelli et al. (1991) for the optical and the IR data, respectively. In the following we will discuss the four GRB afterglows separately.

GRB 970228. In order to determine the optical and IR magnitudes of the OT of this GRB we subtracted from the available measurements the following host galaxy magnitudes: $B\sim26.5$ (extrapolated), V=25.75, R=25.3, I=24.45 (Fruchter et al. 1998), J=24.0 (interpolated), H=23.2 and K=22.8 (Fruchter et al. 1998); next, we dereddened the data using A_V=0.75 (Fruchter et al. 1998). Three SEDs were constructed. The first contains the X-ray data by Frontera et al. (1998a), the optical data by van Paradijs et al. (1997) and Guarnieri et al. (1997) - the latter ones were corrected for the contribution of the nearby K star - and the radio upper limits by Frail et al. (1998). Note that the optical points by Guarnieri et al. (1997) were collected about 4 hours earlier than those of van Paradijs et al. (1997), that is when, according to those authors, the OT luminosity was still rising. This could explain the difference in the flux level between the two data sets. The second SED shows the ASCA (Yoshida et al. 1997) and ROSAT (Frontera et al. 1998b) X-ray data, the optical points and upper limits by van Paradijs et al. (1997), and the radio upper limits by Frail et al. (1998a). The last SED includes V, I (interpolated between the HST V and I data sets by Fruchter et al. 1998) and R (Metzger et al. 1997) optical points, J and K (Soifer et al. 1997) IR data, and radio upper limits by Frail et al. (1998a).

  

Figure 1: SEDs of GRB afterglows at various epochs after the $\gamma$ burst. For each GRB we define $\Delta t = t_{\rm obs} - t_0$, where $t_{\rm obs}$is the epoch of the observation, in days, referring to a specific SED. A larger version of this figure can be found at the URL http://tonno.tesre.bo.cnr.it/ $\tilde{}$ masetti/seds.html

GRB 970508. Among GRB afterglows, this is the best monitored at all wavelengths, therefore five SEDs could be constructed. The first four SEDs were obtained with the data by Piro et al. (1998; X-rays), Galama et al. (1998a; U), Sokolov et al. (1998; BVRI), Castro-Tirado et al. (1998; R), Pedersen et al. (1998; R), Morris et al. (1997; K), Bremer et al. (1998; mm) and Frail et al. (1997; radio). The fifth SED contains the data gathered by Galama et al. (1998b) and the 12-$\mu$m ISO measurement by Hanlon et al. (1998). The host galaxy contribution was not removed since it was negligible at the considered epochs due to its faintness (Zharikov et al. 1998). Optical and IR data were dereddened with A_V=0.08 (Galama et al. 1998a).

GRB 980329. To obtain the first SED for this GRB afterglow we used the data by in't Zand et al. (1998; X-rays), Palazzi et al. (1998; V, R) and Rhoads et al. (1998a; I). The second SED was constructed with observations by Djorgovski et al. (1998; R), Metzger (1998; K), Smith et al. (1998; mm) and Taylor et al. (1998, radio), made about 8 days after the burst. No host galaxy contribution has been subtracted, since its brightness is uncertain. Correction for foreground Galactic absorption A_V=0.4 (Palazzi et al. 1998) was applied to the optical and IR data. We note that the revision of the I band photometric measurement on March 29.9 (Rhoads et al. 1998a) suggests a very sharp and narrow local spectral peak. If true, this would be inconsistent with a single optical-to-X-ray emission component. The change of the continuum spectral slope occurring in the optical has been tentatively explained as Lyman-$\alpha$ absorption up to $z \sim 5$ (Fruchter 1999).

GRB 980703. Two SEDs for this GRB afterglow were constructed using the X-ray data by Vreeswijk et al. (1999), the optical data by Bloom et al. (1998), Vreeswijk et al. (1999) and Rhoads et al. (1998b) and, for the first epoch, the radio data by Frail et al. (1998b). Host galaxy magnitudes V=23.04, R=22.58, I=21.95, J=20.87, H=20.27 and K=19.62 (Vreeswijk et al. 1999) were subtracted from the measurements. A foreground absorption A_V=0.019 (Vreeswijk et al. 1999) was taken into account. The optical/IR bump could be accounted for by intrinsic extinction in the OT environment at the source redshift (z = 0.966; Djorgovski et al. 1998b).