TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2800 SUBJECT: GRB 040924: VLT spectroscopy DATE: 04/10/13 14:54:11 GMT FROM: Rhaana Starling at GRACE/U of Amsterdam Klaas Wiersema (University of Amsterdam), Rhaana L.C. Starling (University of Amsterdam), Evert Rol (University of Leicester), Paul Vreeswijk (ESO), Ralph A.M.J. Wijers (University of Amsterdam) report on behalf of the GRACE collaboration: "We have obtained two spectra of the host galaxy of GRB 040924 (GCN #2735, 2734) with the FORS2 long slit spectrograph on the ESO VLT (Antu) on 10 October 2004, approx. 15.75 days after burst. The first spectrum was taken at 05:10 UT using the 300V grism, with an exposure time of 1800 s; the second spectrum was taken at 05:48 UT using the 300I grism, again with an exposure time of 1800 s. We used a 1 arcsec slit. The total wavelength range is approx. 400 - 950 nm. We find weak emission lines in the spectra, which we identify with [OII] 3727, [NeIII] 3968.9, H gamma, H beta and [OIII] 4959,5007. All of these lines are consistent with a redshift z = 0.859. The presence of [NeIII] implies a galaxy with a significant population of massive stars. The isotropic energy of the burst is E_iso=1.48x10^52 erg, using the 25-100 keV fluence value from HETE and a cosmology with H_0=70 km/s/Mpc, Omega_matter=0.3 and Omega_lambda=0.7. As the lightcurve breaks fairly early on, the true energy is likely to be much lower. The presence of continuum features relating to a supernova can only be determined after further analysis: a SN1998bw-like bump would be expected to peak at approximately t_V~23.5 days, t_R~25 days and t_I~26 days with magnitudes V~25.4, R~24.6 and I~23.8. We acknowledge excellent support from the staff of Paranal Observatory and ESO's PSO and USG."