V. D'Elia (INAF-OAR and ASI-ASDC), Paolo Goldoni (APC/U. Paris 7 and SAp/CEA), Daniele Malesani (DARK/NBI), Nial R. Tanvir (U. of Leicester), Johan P. U. Fynbo (DARK/NBI) and Andrea Melandri (INAF-OAB) and report on behalf of a larger collaboration:
We observed the optical/NIR afterglow of the Swift GRB 120404A (Stratta et al., GCN 13208; Guidorzi et al., GCN 13209) with the VLT/UT2 equipped with the X-shooter spectrograph. Observations started around 04:30 UT on 2012-04-05, which is roughly 16 hr after the trigger.
In the acquisition frame (taken 15.9 hr after the GRB), the afterglow is detected at low S/N. Comparison with two closeby SDSS stars yields R ~ 21.3 (Vega), with a systematic uncertainty of ~0.3 mag.
Four exposures in a nodding pattern with an integration time of 2400 s each were obtained in poor sky conditions. The spectrum covers an approximate wavelength range between 3000 and 25000 AA. The slit width was set to 1" in the ultra-violet/blue arm, and 0.9" in the visual and near-infrared arms, resulting in a resolving power range of approximately 4500-7500.
Preliminary reduction of the spectrum shows a weak continuum with a trough at ~4720 AA, which we identify as Ly-alpha. We also detect faint absorption lines of OI 1302 and SiIV 1393/1402 at a common redshift of ~2.88. This confirms the redshift determination by Gemini-N (Cucchiara et al., GCN 13217). A second dip matching Lya at z=2.55 is also marginally detected. This is consistent with the CIV intervening absorber reported in Cucchiara et al., GCN 13217.
We are grateful for the excellent support from the Paranal Observatory staff, in particular Claudio Melo. We are in debt to Cristiano Guidorzi for providing us a FTN finding chart.