GCN Circular 23142
Subject
GRB 180728A: discovery of the associated supernova
Event
Date
2018-08-15T15:28:06Z (7 years ago)
From
Luca Izzo at IAA-CSIC <Luca.Izzo@ICRA.it>
L. Izzo (HETH/IAA-CSIC), A. Rossi (INAF/OAS), D. B. Malesani (DAWN/NBI and DARK/NBI), K. E. Heintz (Univ. Iceland and DAWN/NBI), J. Selsing (DAWN/NBI), P. Schady (Univ. Bath), R. L. C. Starling (Univ. Leicester), J. Sollerman (OKC Stockholm), G. Leloudas (DTU space), Z. Cano (BCA), J. P. U. Fynbo (DAWN/NBI), M. Della Valle (INAF-Naples), E. Pian (INAF/OAS), D. A. Kann (HETH/IAA-CSIC), D. A. Perley (LJMU), E. Palazzi (INAF/OAS), S. Klose (TLS Tautenburg), J. Hjorth (DARK/NBI), S. Covino (INAF-OAB), V. D���Elia (SSDC), N. R. Tanvir (Univ. Leicester), A. J. Levan (Univ. Warwick), D. Hartmann (Clemson U.), C. Kouveliotou (GWU) report:
We report the results of continued photometric and spectroscopic follow-up of GRB 180728A at z = 0.117 (Starling et al., GCN 23046; Lipunov et al., GCN 23048; Rossi et al., GCN 23055; Heintz et al., GCN 23067) obtained with the X-shooter instrument on the ESO/VLT UT2, Chile.
Up to now, we have observed at three epochs, specifically at 6.27, 9.32 and 12.28 days after the GRB trigger. The optical counterpart is visible in all epochs using the X-shooter acquisition camera in the g, r and z filters. We report a rebrightening of 0.5 +- 0.1 mag in the r band between 6.27 and 12.28 days. This is consistent with what is observed in many other low-redshift GRBs, which in those cases is indicative of an emerging type Ic SN.
The spectra cover the wavelength range 3,000 - 21,000 AA. All the spectra have been corrected for the Galactic extinction (A_V = 0.92), while the host extinction is likely to be small, as indicated by our early-time spectra of the afterglow (Rossi et al., GCN 23055; Heintz et al., GCN 23067). The continuum shows a strong peak around 4500 AA (rest frame; 12.28 days after the GRB), but a black body is not a good fit, as the observed peak is too narrow. The peak shifts to longer wavelengths with time. Over the three epochs, significant features develop, including a broad absorption at ~7600 AA and a deep trough around 4900 AA (all rest frame), visible in our latest spectrum. However, over the covered epochs there is not much resemblance with other broad-lined SNe seen in previous low-redshift GRBs.
For the last spectrum, we attempted the identification of a few features. In particular, we identify the broad dip at 7600 AA as due to the blend O I 8446 AA and Ca II 8492 AA, at the expansion velocity of ~30,000 km s^-1. At this velocity, we also identify the Si II 6355 doublet, as well as C II 6580. The width of the lines spans several thousands km s^-1. Independent of the interpretation of the lines, the overall shape of the continuum, together with the presence of several absorption features a few thousands km s^-1 wide, strongly indicate that this is a SN. The lack of identified H and He in the spectra suggests a classification of type Ic.
A plot of the spectrum obtained at Day 12.28 compared with GRB 980425/SN 1998bw and XRF 100316D/SN 2010bh can be found at the following link :
http://www.astro.ku.dk/~malesani/GRB/180728A/Day12_vs_98bw_2010bh_flux.png
Further observations are planned for this event, which are possible thanks to ESO director's discretionary time allocated to our project (program 2101.D-5044, PI Rossi). Further observations of this GRB/SN, particularly at different wavelengths, are strongly encouraged.
We thank the ESO staff for their kind availability in executing our series of observations, in particular we want to thank Boris Haeussler, Emanuela Pompei, John Pritchard, Luca Sbordone, Marcela Espinoza, Nestor Jimenez, Rodrigo Palomino, Steffen Mieske, Stephane Brillant. We also thank Dilyar Barat (ANU) and Francesco D���Eugenio (Univ. Ghent) for kindly providing their telescope time to observe the GRB/SN during their visitor run at ESO/VLT.