GCN Circular 21617

K. C. Chambers (IfA), M. E. Huber (IfA),K. W. Smith (QUB),
S. J. Smartt, (QUB), D. R. Young, M. Coughlin (Harvard), T.-W. Chen
(MPE), J. Bulger, L. Denneau, H. Flewelling, A. Heinze, E. Kankare
(QUB), T. Lowe, E. A. Magnier (IfA), A. Rest (STScI), B. Stalder
(IfA), A. S. B. Schultz, C. W. Stubbs (Harvard) J. Tonry, C. Waters,
R. J. Wainscoat, H. Weiland, M. Willman (IfA), D. E. Wright (QUB)

We report continued Pan-STARRS imaging of the transient SSS17a/DLT17ck
in NGC4339 in filters i, z and y (see Chambers et al. GCN 21553
Not found
 
 
 for
more details of filters, photometry, methods etc).

Our reference stack and calibrations provide rapid reliable photometry
with the host removed and we will continue to post updates while it is
visible from Hawaii.  A series of short exposures to avoid sky
saturation in twilight in each of i, z, and y provide average nightly
magnitudes (AB) of 

MJD             i       err 
57983.23	17.24	0.06
57984.24	17.91	0.05
57985.24	18.47	0.08

MJD             z       err 
57983.23	17.26	0.06
57984.24	17.80	0.05
57985.24	18.31	0.06

MJD             y       err 
57983.23	17.38	0.10
57984.24	17.59	0.07
57985.24	18.08	0.07

The decline rate 0.6 mag per day is still consistent with some of the
kilonova models (Kasen et al. 2015, Barnes & Kasen 2013, Tanaka et
al. 2014). 

The very rapid decline (and weak high energy emission) may explain why
transients of these types have not been recovered and recognised
independently in ground-based wide-field surveys to date, before the
LIGO-Virgo era.

For example at d=100Mpc, this transient would fade from around i=19.2
to 21.7 in four days in i-band (and would be fainter and faster in g,
r and broad composite filters).  Making it difficult to recover and
recognise over multiple nights in "~10^3 deg per night" surveys such
as PTF, Pan-STARRS, La Silla Quest, Skymapper, CRTS.  And well beyond
the wider, shallower, ASASSN and ATLAS sensitivities.