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GCN Circular 21577

Subject
LIGO/Virgo G298048: optical spectral energy distribution of SSS17a
Date
2017-08-18T23:00:31Z (7 years ago)
From
Daniele Malesani at DARK/NBI <malesani@dark-cosmology.dk>
Daniele Malesani, Darach Watson, and Jens Hjorth (all DARK/NBI), report:

Analysis of the optical and near-infrared data reported through the GCNs 
allows to build the spectral energy distribution of the candidate 
optical counterpart (Coulter et al., GCN 21529; Allam et al., GCN 21530; 
Yang et al., GCN 21531) of the LIGO/Virgo event G298048 (GCN 21509). A 
standard Galactic extinction curve with A_V = 0.34 has been adopted but 
no rest-frame extinction has been included.

We caution that the available data are based on a preliminary analysis 
and are not always strictly simultaneous, though the reported 
variability (at least in the optical) is modest (Arcavi et al., GCN 
21565). Overall, the SED shows a blue continuum in the NIR to optical 
region, reaching a peak around the g band and rapidly decreasing in the 
UV as constrained by the Swift/UVOT measurements (Evans et al., GCN 
21550). The spectral slope in the UV is quite steep, being well 
represented by an exponential tail or a very steep power law (spectral 
index ~3.7 on Aug 18.16 UT; and ~ 5 on Aug 18.57 UT).

A blackbody with T ~ 8500 K broadly represents the available data at a 
mean epoch Aug 18.1 UT (black line in the plot). Assuming an isotropic, 
spherically symmetric emitter, the radius of the blackbody source would 
be approximately 10^15 cm for a distance of 40 Mpc. At 55 ks after the 
explosion, this would imply a relativistic expansion velocity. As noted 
by Cenko et al. (GCN 21572), the later Swft/UVOT measurement indicate 
cooling in the source, implying further expansion and confirming the 
relativistic velocity.

The SED is clearly atypical for optical afterglows of long and short 
GRBs, as also highlighted by the lack of detection in the X-ray band 
(Evans et al., GCN 21550).

A plot showing the counterpart SED is posted online at:

http://www.astro.ku.dk/~malesani/GRB/170817A/SED_opt.png

Data were grouped in quasi-simultaneous sets and were collected from the 
following reports:
- Aug 18.0 UT (red triangles): Yang et al. (GCN 21531); Nicholl et al. 
(GCN 21541); Tanvir & Levan (GCN 21544); Simon et al. (GCN 21551); 
Singer et al. (GCN 21552);
- Aug 18.16 UT (blue circles): Evans et al. (GCN 21550);
- Aug 18.42 UT (green squares): Wolf et al. (GCN 21560);
- Aug 18.57 UT (cyan diamonds): Cenko et al. (GCN 21572).
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