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GCN Circular 21585

LIGO/Virgo G298048: Possible features in the spectrum of GW counterpart SSS17a
2017-08-19T03:10:19Z (7 years ago)
Matt Nicholl at Harvard-Smithsonian/CfA <>
M. Nicholl (Harvard), C. Briceno (CTIO), P. Cowperthwaite (Harvard), 
E. Berger (Harvard), J. Elias (NOAO), S. Heathcote (CTIO),
J. Annis (Fermilab), D. Tucker (Fermilab), M. Soares-Santos (Brandeis),
R. Kessler (U Chicago), M Sako (U Penn)

We report optical spectroscopy of SSS17a, the counterpart to G298048, using 
the Goodman Spectrograph on the 4m SOAR telescope on 2017-08-18 starting 
at UT 23:23:38. This is ~1.5 days after the LIGO and Fermi signals. 

Previous spectra were obtained by Drout et al (GCN 21547) and more recently
Lyman et al (GCN 21582). We confirm some of the results of Lyman et al: a peak 
between 5000-6000 Angs, rapid fall-off in the blue, and no match to known 
supernova types. The colour temperature is best matched by a blackbody with 
T~5000K, after dereddening by a Milky Way E(B-V)=0.1053. The UV flux appears
suppressed relative to a simple blackbody.

This shows significant cooling compared to T~8000K one day previous, as
reported by Malesani et al (GCN 21577) and Cowperthwaite et al (GCN 21578).
The blackbody radius (~8e14 cm) indicates an average velocity ~0.1c, suggesting
the expansion has now slowed compared to those earlier estimates (0.2-0.3c).

We note the possibility of weak, broad bumps at ~5000 and ~6000 Ang in our
quick extractions.

Matt Nicholl
Postdoctoral Research Fellow
Harvard-Smithsonian Center
for Astrophysics
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