GRB 190829A: GTC confirmation of an associated Type Ic-BL Supernova

We obtained spectroscopy of GRB 190829A (Fermi GBM team, GCN #25551; Dichiara et al., GCN #25552) with OSIRIS on the 10.4 m Gran Telescopio Canarias on La Palma, Spain, based on Director's Discretionary Time proposal GTC02-19BDDT. We obtained 3 x 600 s exposures each in two grisms, R1000B and R1000R, yielding a total wavelength coverage from 3700 to 10000 AA. Observations began at 03:57 UT on 06 September 2019, 7.35 days after the trigger. After correcting for the large amount of host-galaxy reddening, using A_V = 1.5 mag and an SMC-like extinction curve, we find a spectrum showing broad undulations. An excellent match is obtained vs. an X-shooter spectrum of SN 2010bh at a similar epoch (Bufano et al., ApJ, 753, 67), especially above 5000 AA (see https://www.iaa.csic.es/~deugarte/GRBs/190829A/GRB190829A_SN.png). We further find a good match to a spectrum of SN 1998bw (Patat et al. 2001, ApJ, 555, 900) taken 6.6 days before peak light (and ~ 8 days post-GRB), but a less good match for SN 2006aj at a similar time after the GRB. We find evidence for broad absorption lines of SiII 6355, OI 7775, and CaII 8490, with the expansion speed of SiII being ~ 30,000 km/s, very similar to SN 1998bw at this time after trigger. This fully confirms the results of Terreran et al. (GCN #25664) that this rising source (Perley & Cockerma, GCN 25623, Bolmer et al., GCN 25651) is indeed the SN accompanying GRB 190829A, and we unambiguously identify it as a broad-lined Type Ic SN similar to other SNe accompanying GRBs. Comparison spectra were obtained from WISeREP (https://wiserep.weizmann.ac.il, Yaron & Gal-Yam, 2012, PASP, 124, 66.) [GCN OPS NOTE(07sep19): Per author's request, "(GCN #25664)" was added in the 4th paragraph.]