K. E. Heintz (Univ. Iceland and DAWN/NBI), L. Izzo (HETH/IAA-CSIC), A. Rossi (INAF-OAS), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), D. B. Malesani (DAWN/NBI, DARK/NBI), D. A. Perley (LJMU), C. C. Thoene (HETH/IAA-CSIC), J. P. U. Fynbo (DAWN/NBI), B. Milvang-Jensen (DAWN/NBI), D. A. Kann (HETH/IAA-CSIC), N. R. Tanvir (U. Leicester), A. J. Levan (U. Warwick), S. Schulze (Weizmann), and G. Pugliese (API/U. Amsterdam) report on behalf of the Stargate collaboration;
We observed again the optical afterglow of the bright, low-redshift GRB 180728A (Starling et al., GCN 23046; Lipunov et al., GCB 23048; Rossi et al., GCB 23055) with the ESO VLT UT2 equipped with the X-shooter spectrograph. Observations were carried out around July 30.21 UT (35.5 hr after the GRB).
In our observations, the afterglow has faded significantly compared to our previous observation (Rossi et al., GCN 23055), reaching a magnitude of R ~ 20 (Vega). Continuum is detected across the whole observed range 3000-25000 AA, despite at lower S/N. Weak emission lines from a faint host are now visible, corresponding to Halpha, Hbeta, [N II], [O III] 5008 at the GRB redshift. The small Balmer decrement is consistent with negligible dust extinction in the host galaxy.
After excluding the host emission lines and correcting for Galactic extinction, the observed continuum can be fit with a single power-law with spectral index ~ 0.6 (F_nu~nu^-beta). In particular, we do not see evidence in any of our spectra for broad undulations or deviations from a power-law behavior as reported by Buckley et al. (ATel 11897).
A figure showing our spectrum can be seen at:
We acknowledge expert support from the ESO observing staff in Paranal,
in particular Pascale Hibon, Bin Yang, and Romain Thomas.