TITLE:   GCN CIRCULAR
NUMBER:  24229
SUBJECT: LIGO/Virgo S190425z: GROND observations of AT2019ebq/PS19qp  
DATE:    19/04/26 15:36:25 GMT
FROM:    Ting-Wan Chen at MPE  <jchen@mpe.mpg.de>

P. Schady (U.Bath), T.-W. Chen (MPE, Humboldt Fellow), T. Schweyer (MPE), D. B. Malesani (DAWN/NBI and DARK/NBI) and J. Bolmer (MPE) report:

We observed the field of the AT2019ebq/PS19qp (Smith et al., GCN #24210; Nicholl et al., GCN #24217; Jonker et al., GCN #24221) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). 

Observations started at 09:08 UT on 26 of April 2019, about 1.04 days after the GW S190425z detection (LIGO and Virgo Collaboration, GCN #24168), and were performed under seeing conditions of 1”.4, and at an average airmass of 1.3. We detect the candidate and based on 25 min of total exposure time in optical and 26 min in NIR, we derive the following preliminary magnitudes (all in the AB system):

r = 20.78 +/- 0.20 mag, 
i = 20.00 +/- 0.15 mag, and
z = 19.60 +/- 0.15 mag.

The magnitudes are measured using aperture photometry with a small aperture size of 0".75. We caution that the object is blended with the galaxy nucleus, and that the removal of the variable and bright host background may introduce further systematic errors.

Given magnitudes are calibrated against Pan-STARRS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.51 mag in the direction of the counterpart (Schlafly & Finkbeiner 2011). 

We acknowledge excellent help in obtaining these data from the observer Jose Ignacio Vines and supporting astronomer Angela Hempel on La Silla.