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GCN Circular 22096

GRB 171010A: VLT spectroscopic identification of the associated SN 2017htp
2017-11-04T00:16:56Z (7 years ago)
Antonio de Ugarte Postigo at IAA-CSIC <>
A. de Ugarte Postigo (HETH/IAA-CSIC and DARK/NBI), J. Selsing (DARK/NBI), D. Malesani (DARK/NBI), D. Xu (NAOC/CAS), L. Izzo (HETH/IAA-CSIC), K. E. Heinz (Univ. Iceland and DARK/NBI), D. A. Kann (HETH/IAA-CSIC), G. Leloudas (DARK/NBI), S. Schulze (WIS), N. R. Tanvir (Univ. Leicester), S. Covino (INAF/Brera), P. D'Avanzo (INAF/Brera), J. P. U. Fynbo (DARK/NBI), D. Hartmann (Clemson Univ.), J. Hjorth (DARK/NBI), C. Kouveliotou (GWU), L. Kaper (API, Univ. Amsterdam), A. J. Levan (Univ. Warwick), A. Melandri (INAF/Brera), P. Moller (ESO), G. Pugliese (API, Univ. Amsterdam), B. Sbarufatti (INAF/Brera and PSU), and P. Schady (MPE), S. Schmidl (TLS), on behalf of the STARGATE collaboration, report:

We observed the optical counterpart of GRB 171010A (Omodei et al., GCN 21985; Poolakkil & Meegan, GCN 21992; Sharma et al., GCN 21990) at z = 0.33 (Kankare et al., GCN 22002) using the ESO Very Large Telescope. Imaging was secured on 2017 Oct 31.25 UT (20.45 days after the GRB) using the VIMOS instrument, in the R and I filters, for a total of 10 min exposure each.

Both the candidate host galaxy of the GRB (Thorstensen & Halpern, GCN 21897; Izzo & Malesani, GCN 21988) and a second compact source, at a location consistent with that of the optical afterglow, are well visible in our images. The spatial coincidence of the second source could indicate late-time emission associated with the GRB, or the presence of an underlying star-forming region.

Spectroscopy covering both the host nucleus and the afterglow location was carried out on 2017 Nov 1.25 UT (21.46 days after the GRB; 16 days in the GRB rest frame), using the X-shooter spectrograph, for a total exposure time of 4x1200 s. The traces of both objects are visible, showing a wealth of narrow nebular emission lines, some of which especially intense at the afterglow location. We measure for the galaxy nucleus a redshift z = 0.3285, consistent with the report by Kankare et al. (GCN 22002).

Faint continuum emission is detected at the GRB location. This could be due to an associated SN, as expected for long GRBs at this redshift, or to an underlying star forming region, or a combination of both. The spectral shape is curved and redder than at the nucleus location, but extinction is low as inferred from the Balmer decrement. This opens the possibility of excess light from the SN. In the attempt to isolate its contribution, we then estimated the host contamination by rescaling the nucleus spectrum assuming it contributes the entire flux at 3500 AA (rest frame), where GRB-associated SNe have negligible emission (e.g. Hjorth et al. 2003, Nature, 423, 847; Mazzali et al. 2003, ApJ, 599, 95), due to metal line blanketing.

By comparing the residual contribution with known SN/GRB templates, we find a good resemblance with other GRB-associated SNe, such as SN 1998bw (at 6 days post maximum; e.g. Patat et al. 2001, ApJ, 555, 900) and SN 2006aj (at 5 days post maximum; e.g. Pian et al., Nature, 442, 1011). Based on this resemblance, the object has been assigned the IAU name of SN 2017htp. The brightness of the SN is about 30% (1.3 mag fainter) than SN 1998bw at a comparable epoch. An accurate computation requires however a more rigorous host subtraction which will be possible with late-time templates.

A plot showing the spectra can be viewed at this URL: <>

We acknowledge unfaltering support from the ESO staff at Paranal, in particular Marcelo Lopez, Juan Carlos Munoz, Nestor Jimenez, Eleonora Sani, Fernando Selman, and Romain Thomas.
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