TITLE:   GCN CIRCULAR
NUMBER:  21557
SUBJECT: LIGO/Virgo G298048: Likelihood of SSS17a Being an Unrelated Extragalactic Transient in NGC 4993
DATE:    17/08/18 12:15:23 GMT
FROM:    Ryan Foley at UCSC  <foley@ucsc.edu>

R. J. Foley (UC Santa Cruz) reports

In the interest of focusing follow-up observing resources, we perform a quick assessment of how “unique” the potential EM counterpart to the LIGO/Virgo G298048 trigger (LVC GCNs 21509, 21513), SSS17a (Coulter et al., LVC GCN 21529), is in the context of other extragalactic astrophysical transients.

The optical spectrum (Drout et al., LVC GCN 21547) and colors (Nicholl et al., LVC GCN 21541; Chambers et al., LVC GCN 21553) are inconsistent with a Type Ia supernova at any epoch.  Its nominal host galaxy, NGC 4993, is an S0 galaxy, which typically have little star formation (although there are dust lanes near the SN position; Foley et al., LVC GCN 21536).  If at the distance of its nominal host galaxy, NGC 4993, correcting for only Milky Way extinction, SSS17a had a peak luminosity of M_g ~ -16.0 mag (e.g., Simon et al., LVC GCN 21551), which is much more luminous than novae, much less luminous than SNe Ia near peak, and similar to some core-collapse SNe near peak.  The lack of a strong source in recent imaging (Cowperthwaite et al., LVC GCN 21533) indicates that SSS17a is <~ 2 days old.  At the very least, there was no source to deep limits from Hubble images ~4 months ago (Foley et al., LVC GCN 21536).  These data provide strongly constraining information on the nature of SSS17a.

Approximately 7.3% of all supernovae in S0 galaxies are core-collapse galaxies (Foley & Mandell, 2013, ApJ, 778, 167).  Using the luminosity of NGC 4993 and the formulae of Li et al. (2011, MNRAS, 412, 1473), we find that ~17% of all supernovae in S0 galaxies with a similar luminosity to that of NGC 4993 are core-collapse SNe.

Using the Li et al. local volumetric core-collapse rate, we expect ~0.1 core-collapse SN per year in the 90% LIGO localization volume.  A core-collapse SN exploding within 2 days of the LIGO trigger and within this volume has a probability of roughly 5 x 10^-5.  Including all SNe, this number only increases to 8 x 10^-5.  The probability of a core-collapse SN exploding since the Hubble images were obtained inside the LIGO volume is ~0.003.  We note that these calculations do not account for possible overdensities, and NGC 4993 is a member of a relatively massive group of galaxies.

Finally, examining the core-collapse luminosity functions of Li et al. (2011, MNRAS, 412, 1441), ~50% of Type Ib/c supernovae and ~65% of Type II supernovae in S0-Sbc galaxies have peak absolute magnitudes greater than -16.0 mag.  That is, roughly half of all core-collapse SNe in S0-Sbc galaxies never reach the reported magnitude of SSS17a.

Combined, we estimate that the likelihood of SSS17a being an unrelated core-collapse SN in NGC 4993 to be ~5 x 10^-6.  Excluding the strongest constraints related to possible recent non-detections, the likelihood of SSS17a being an unrelated core-collapse SN in NGC 4993 is ~3 x 10^-4.

We conclude that SSS17a is a rare event, and warrants significant additional follow-up observations.