TITLE:   GCN CIRCULAR
NUMBER:  25650
SUBJECT: LIGO/Virgo S190901ap : GRAWITA photometric observation of candidate ZTFabvizsw and GCN 25618 revision
DATE:    19/09/04 08:57:51 GMT
FROM:    Lina Tomasella at INAF,Padova  <lina.tomasella@inaf.it>

I. Salmaso, L. Tomasella, S. Benetti (INAF Padova), P. D’Avanzo (INAF Brera), E. Cappellaro (INAF Padova), M.T. Botticella (INAF Napoli), R. Martone (Ferrara University), A. Rossi (INAF Bologna), E. Brocato (INAF Teramo), on behalf of GRAWITA report:

Following Burdge et al. (GCN 25639) we revised our results published in Salmaso et al. (GCN 25618). The average seeing of Salmaso et al. frames was around 2.5-3 arcsec and ZTF19abvizsw was barely visible in a single 60 sec long exposure, while a nearby brighter star (at R.A.= 18:37:53.50, decl=+61:29:45.96) had a magnitude g=19.52+/-0.04 close to that reported by Kool et al. in GCN 25616 for ZTF19abvizsw. Therefore we indeed took erroneously the spectrum of the nearby star which is clearly unrelated to the gravitational wave trigger S190901ap.
ZTF19abvizsw stands clearly out in our best seeing frames and the preliminary magnitudes we measure are the following (Time gives the UT start):
Time (UT)               filter  mag(AB) err
—————————————————
2019-09-02 20:56:48  g   20.77     0.11
2019-09-02 20:57:58  r    20.54    0.12
2019-09-02 20:59:07  i     20.25    0.15
2019-09-02 21:00:17  z    20.65    0.15
The transient is fading rapidly in agreement with the finding of Perley et al. (CGN 25643).
We regret for any trouble this may have caused.