SwiftJ1943.4+0228: GROND observation of the optical/near-IR counterpart
ATel #4054; Arne Rau, Marco Nardini, Jochen Greiner (all MPE Garching)
on 19 Apr 2012; 15:13 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)
Subjects: Infra-Red, Optical, Binary, Cataclysmic Variable
We observed the new transient Swift J1943.4+0228 (Krimm et al. 2012, ATel #4049) simultaneously in g'r'i'z'J'HK with GROND (Greiner et al. 2008, PASP 120, 405) at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile). Within a 4min integration starting April 14th 09:37 we clearly detect the candidate counterpart reported by Krimm et al. (2012, ATel #4049) based on Swift/UVOT observations.
We measure the following magnitudes (all in the AB system):
g' = 18.0 +/- 0.1
r' = 17.9 +/- 0.1
i' = 18.0 +/- 0.1
z' = 18.1 +/- 0.1
J = 18.1 +/- 0.1
H = 18.1 +/- 0.1
K = 18.1 +/- 0.2
These magnitudes were derived by calibrating the images against GROND zero points (g'r'i'z') and 2MASS field stars (JHK) and are not corrected for the heavy Galactic foreground reddening of E(B-V)~0.30 mag (Schlegel et al. 1998).
The extinction-corrected g'-K spectral energy distribution is very blue (approx lambda^-2.7) consistent with an accretion disk spectrum.
The measured hydrogen absorbing column as reported by Krimm et al. (2012, ATEL #4049) suggests that Swift J1943.4+0228 lies near the end or behind the total Galactic column. At a Galactic latitude of -10 deg, this corresponds to a distance of at least 1 kpc. The relatively low luminosity of about 10^34 (d/1 kpc)^2 erg/s (if a Galactic source) suggests a cataclysmic variable origin for Swift J1934.4+0.228. However, an extra-galactic origin cannot be excluded at this stage.
