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
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:
Data were grouped in quasi-simultaneous sets and were collected from the
- 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).