TITLE:   GCN CIRCULAR
NUMBER:  26577
SUBJECT: LIGO/Virgo S191213g: Pan-STARRS monitoring shows PS19hgw (AT2019wxt) to be fading quickly 
DATE:    19/12/24 17:03:21 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt@qub.ac.uk>

M. E. Huber, (IfA) S. J. Smartt, O. McBrien, (QUB) K.C. Chambers
A.S.B. Schultz (IfA), K. W. Smith, D. R. Young, J. Gillanders. S.
Srivastav, P. Clark, D. O'Neill, M. Fulton, S. McLaughlin (QUB), T. de
Boer, J. Bulger, J. Fairlamb, C.C. Lin, T. Lowe, E. Magnier , R. J.
Wainscoat, M. Willman (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs
(Harvard), T.-W. Chen (Stockholm)

In McBrien et al. (GCN 26485) we reported the Pan-STARRS discovery of
the intrinsically faint optical transient PS19hgw (AT2019wxt) in the
80% probability contour of the compact binary merger event S191213g
(The LIGO Scientific Collaboration and the Virgo Collaboration, GCN
26402).

Spectroscopic follow-up by Izzo et al. (GCN 26491), Srivastav and
Smartt (GCN 26493), and Muller Bravo et al. (GCN 26494), Vogl et al.
(GCN 26504), Dutta et al. (GCN 26490), Vallely et al. (GCN 26508), and
Becerra-Gonzalez et al (GCN 26521) reported a blue continuum with
broad shallow He I lines. A spectroscopic classification of a Ib or
IIb SN was proposed, and a number of authors proposed it is therefore
unrelated to the GW event S191213g. 

Further observations with Pan-STARRS now show that the transient is
fading quite quickly. It faded by 1 mag in g and 0.8 mag in r over 5 days.
The SED is clearly cooling, as the fading in the redder bands is
less pronounced. 

g 58836.434471 19.41 0.09
r 58836.436117 19.32 0.07 
i 58836.437684 19.30 0.07 
z 58836.439253 19.41 0.11 
y 58836.441086 19.37 0.22

g 58841.211239 20.43  0.07
r 58841.212850 20.16  0.05
i 58841.214430 19.97  0.04
z 58841.216012 19.82  0.05
y 58841.217616 19.74  0.12

These magnitudes are based on difference imaging with the Pan-STARRS
3Pi survey data (Chambers et al, 2016, arXiv:1612.05560), and hence
are reliable and not affected by the host galaxy flux.

This is nowhere near as fast a fade as AT2017gfo, which faded by ~4
mags in 5 days in the g-band. And is slower than SN2018kzr, the next
fastest fading transient (a possible NS-WD merger, McBrien et al. 2019, ApJ,
885, L23). However it is much faster than the "faint and fast"
hydrogen free supernovae like SN2008ha, SN2010ae (Valenti et al. 2009,
Nat 459, 674 ; Foley et al. 2009, AJ 138, 376 ; Stritzinger et al.
2014, A&A, 561, 146).

Further spectroscopic observations are encouraged to monitor the
evolution and determine if this is an unusual, fast fading SN-like
transient or if it is related to S191213g. In particular, a spectrum
to show if hydrogen has developed or if the He I lines have
strengthened is required.