T.-W. Chen (MPE, Humboldt Fellow), T. Schweyer (MPE), A. Rossi (INAF-OAS), K. E. Heintz (Univ. of Iceland), M. Gromadzki (Univ. of Warsaw), J. Bolmer (MPE) and P. Schady (Bath)
We observed again the field of the MASTER OT J154209.55-431742.2 (GCN#24084, Lipunov et al.) 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 07:23 UT on 10 of April 2019, about 1.25 days after the MASTER OT discovery, and were performed under seeing conditions 1".0, and at an average airmass of 1.0. We detect the candidate and based on 8.3 min of total exposure time.
We derive the following preliminary magnitudes (all in the AB system):
g = 16.70 +/- 0.01 mag,
r = 16.82 +/- 0.01 mag,
i = 17.05 +/- 0.01 mag,
z = 17.14 +/- 0.01 mag,
J = 17.11 +/- 0.01 mag,
H = 17.71 +/- 0.03 mag, and
K = 17.95 +/- 0.07 mag.
Given magnitudes are calibrated against GROND zeropoint as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.24 mag in the direction of the counterpart (Schlafly & Finkbeiner 2011).
We searched for previous observations. No source is detected in archival DSS and DSS2 red images and IR images. However, we have found that the source was detected in March 2015 by the SkyMapper Southern Sky Survey with vgriz: 17.798 +/- 0.034, 17.588 +/- 0.011, 17.593 +/- 0.030, 17.541 +/- 0.032, 17.703 +/- 0.040 (PSF magnitudes, AB) not corrected for galactic extinction (http://skymapper.anu.edu.au/object-viewer/dr1/179882566/).
We have also searched for previous observations in the VISTA Hemisphere Survey. In images obtained on April 2010 we found a very faint source with J = 20.9 +/- 0.5 mags (Vega), while in K-band we can only provide an upperlimit K>19.0 mag, calibrated against 2MASS field stars.
The SED obtained from archival images points to a very blue star. The late photometry from GROND, shows a ~5 magnitude brightening in the J band. These findings support the idea that this object is likely a bright infrared nova based on the SED and not related with the GW.
We also noticed that the source is not detected in GAIA dr1 and 2 down to g=19.5, implying that skymapper and GROND observations were obtained during an outburst.