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TITLE:   GCN CIRCULAR
NUMBER:  31314
SUBJECT: IceCube-20211216B: observations of SN2021gpw in the errorbox of the neutrino trigger
DATE:    21/12/27 15:36:30 GMT
FROM:    Enrico Cappellaro at INAF-OAPd  <enrico.cappellaro@inaf.it>

I. Salmaso (Padova University, INAF-OAPd), S. Benetti, E. Cappellaro, A. Pastorello (INAF-OAPd), M. Fraser (University College Dublin),  P. Mazzali (JMU)
 
We obtained follow-up observations of the Type IIn SN2021gpw  (https://www.wis-tns.org/object/2021gpw) that was found to be located in the errorbox of Ice-Cube 20211216B (ATel 15123, https://www.astronomerstelegram.org/?read=15123).  
The transient was measured  at magnitude r=20.30 +/- 0.07, i=19.83+/ 0.06 on  Dec 25.21 UT (Liverpool Telescope observation).  This means that 280 days from discovery (observer frame), the absolute magnitude is still r = -17.2, i=-17.7.
In the framework of the  Nordic-optical-telescope Un-biased Transient Survey 2 (NUTS2), we also obtained a spectrum with NOT+ALFOSC on Dec. 25.22UT (range 360-960 nm, resolution 2nm). The spectrum shows a narrow H-alpha emission with broad wings (FWHM 5 nm) and a broad feature centred at 852 nm (rest frame, FWHM~30nm) that we identified with the CaII NIR triplet. Overall the spectrum is very similar to that of SN 1997cy for which the slow luminosity evolution was attributed to strong ejecta-CSM interaction  (Turatto et al. 2000, ApJ, 534, 57).
While analysis is in progress, no clear association with neutrino emission can be claimed at present.