4 GRB 980329

GRB 980329 was detected by the BeppoSAX satellite on 1998 March 29.16 UT. The radio and optical counterparts were discovered by Taylor et al. (1998) and Djorgovski et al. (1998), respectively. The latter claimed the detection of the host galaxy at an apparent magnitude of $R \sim25.7$. Palazzi et al. (1998), Gorosabel et al. (1999), Reichart et al. (1999) presented optical and near infrared detections of the OA and found it to decay at a rate typical for most detected OAs ( $\alpha_{\rm O} \sim 1.2$). This decay slope is in good agreement with that found in X-rays, where $\alpha_{\rm X} = 1.35\pm0.03$ (in 't Zand et al. 1998). A preliminary localisation of the host in the HST/STIS data was reported in Holland et al. (2000) (GCN #778). Unfortunately, the text incorrectly states the host to be southwest of the radio localisation, when it was actually to the northeast. The maximum measured (early) brightness of the OA was I = 20.8 and R= 23.6, leading to an extremely red colour $R{-}I = 2.8 \pm 0.4$(Reichart et al. 1999). Near-infrared (NIR) observations, on the other hand, showed that the NIR colours are approximately flat. These measurements led Fruchter (1999) to argue that the red colour could be caused by the Lyman forest if the redshift was $z\sim5$. This claim was subsequently challenged by the apparent non-detection of a Lyman forest in Keck II spectra (Djorgovski et al. 2001). Recently, Yost et al. (2002) presented supplementary multi-wavelength broad-band photometry of this burst and claim to rule out $z \ge5$ based on an afterglow model fit to the data. The NIR photometry shows that the HST/NICMOS October 1998 data (GO-7863, PI: A. Fruchter) contains a significant contribution from the OA and therefore it is not suited for host photometry. The GRB has also been found to be heavily extincted by dust (see Lamb et al. 1999; Bloom et al. 2002; Yost et al. 2002).

We retrieved NTT/EMMI R-band images of GRB 980329 from the ESO archive, obtained on March 29.99 and 30.99 (Palazzi et al. 1998). As the afterglow was detected at low signal-to-noise in the late-time images, the astrometry was in this case derived from the combined image. Seven tie objects, of which six are stellar and one a compact galaxy, were used for the astrometric solution. The astrometric error, as estimated from the residual of the tie object fit, is estimated to be about 1.25 drizzled STIS pixels, or $0\hbox{$.\!\!^{\prime\prime}$ }03$, which should be compared with an expected error of $0\hbox{$.\!\!^{\prime\prime}$ }02$, as estimated from the S/N of the OA image. The error in the transformation from STIS CL to the STIS LP image is a small fraction of a drizzled STIS pixel and can therefore be ignored. The best fit localisation in the CL image is given in Table 1.

An expanded section of the STIS CL and LP images, centered on the host, are shown in Fig. 1 with the OA position indicated. In the CL image several unresolved knots are seen on top of a low surface brightness area within an aperture of $0\hbox{$.\!\!^{\prime\prime}$ }5$. The measured ABMAG in the CL-band within this aperture is $27.5\pm0.2$. In the LP image an extended object is seen, but the knots seen in the CL-band are not detected. We find $26.6\pm0.2$ within the same aperture in the LP-band. Foreground extinction corrected photometry and detection significance estimates are given in Table 2. Photometry of the three brightest knots yields a total magnitude of $28.1\pm0.1$ in the CL-band and $27.7\pm0.2$ in LP. The flux in the LP measurement stems primarily from the underlying galaxy complex. Within an arcsecond of the OA position at least two fainter extended objects or structures are seen to the North and North-East in the CL-band. Their distance relative to the OA position is approximately 1 and 0.6 arcsec and photometry measurements yield $28.5\pm0.2$ and $29.5\pm0.5$, respectively.

Figure 1: Sub-section of the STIS CL and LP images centered on GRB 980329. A dashed ellipse indicates the 3-$\sigma$ OA localisation error while the solid circle gives the aperture used for photometry and a cross in the CL image marks the host center. The dash-dotted ellipse marks the 3-$\sigma$ error circle of Bloom et al. All images were smoothed by a 1.1 pixel Gaussian using IRAF/GAUSS. Axis units are in arcsec, North is up and East is to the left.

The STIS CL and LP measurements in addition to the Keck/ESI $R =26.53\pm0.22$, $I=26.28\pm0.27$ (Bloom et al. 2002; Yost et al. 2002) and NIRC $K=23.04\pm0.42$ measurements (Yost et al. 2002) provide an excellent opportunity to estimate the photometric redshift of the host. Using the Bayesian photometric redshift (BPZ) estimation software of Benitez (2000) and restricting z >1 (due to absence of expected emission lines in spectra of the host, see Yost et al. 2002) we find $z \sim3.6$ with the best fitting SED corresponding to an Im galaxy type. Redshifts of z<1.2 and z>4.2 are excluded at the 95% confidence level (z >5 excluded at 99.99% level). These redshift estimates are consistent with the constraint z<3.9 based on the non-detection of the Lyman forest in a Keck II spectrum of the host galaxy (Djorgovski et al. 2001) and a far-ultraviolet extinction curve constraint giving 3<z<5(Lamb et al. 1999).

It is puzzling that the CL-band shows a clear multi-component nature, with at least three unresolved knots within 0.5 $^{\prime\prime}$, whereas the LP-band does not. We find an upper limit colour for the knots of CL-LP $\le$ 0.25, while for the integrated colour of the host complex, we measure CL-LP $\approx$ 0.8. The compact knots does therefore appear very blue (the wavelength cutoff is at 5500 Å in the LP-band).

Taking the measured $E_\gamma = 5.5 \times 10^{-5}$ erg cm²(in 't Zand et al. 1998) and assuming a redshift of 3.5 we find an isotropic gamma-ray energy of $E_{\rm iso} = 1.4\times10^{54}$ erg. Assuming a total average energy for GRBs of $5\times10^{50}$ erg (Frail et al. 2001) we estimate a jet opening angle of $\theta_0 = 1.5\deg$, indicating a highly collimated beam.