GROND observations of Fermi J1717-5156
ATel #4032; Arne Rau, Patricia Schady, Jochen Greiner (all MPE Garching)
on 11 Apr 2012; 15:17 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)
Subjects: Infra-Red, Optical, Blazar
We observed the location of the potential counterpart to the new Gamma-ray Transient Fermi J1717-5156 (Schinzel et al., ATel #4023; Massaro et al., ATel #4029) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP, 120, 405) at the 2.2m MPI/ESO telescope in La Silla. Observations started on April 11th 09:39 UT and had an exposure of approx 16min.
At the position of the suggested VLBI (PMN J1717-5155) and WISE (WISE J171734.65-515532.0) counterpart we find a bright optical near-IR source. We measure the following magnitudes in the AB system:
g' = 17.87 +/- 0.05
r' = 17.17 +/- 0.05
i' = 16.78 +/- 0.05
z' = 16.47 +/- 0.05
J = 15.56 +/- 0.05
H = 14.95 +/- 0.05
K = 14.48 +/- 0.07
The magnitudes were calibrated against GROND zeropoints (g'r'i'z') and 2MASS field stars (JHK) and are not corrected for the substantial Galactic foreground reddening of E(B-V)=0.325mag (Schlegel et al. 1998).
We note that the source is only marginally detected in DSS and not detected in 2MASS suggesting a brightening of approx. 3mag in the optical and at least 1mag in the near-IR.
The foreground reddening corrected GROND spectral energy distribution (SED), combined with the WISE 3.4 and 4.6mum measurements reported by Massaro et al. (ATel #4029) is best fit with a power law of the form F_lambda propto lambda^-0.95. This is in agreement with the typical spectral slopes derived in the recent GROND+Swift/UVOT SED analysis of 103 Fermi/LAT blazars by Rau et al. (2012, A&A, 538, A26) and supports the suggestion of Massaro et al. (ATel, #4029) that Fermi J1717-5156 is a blazar in outburst.
From the absence of a Lyman limit suppression in the bluest GROND filter, we estimate an upper limit on the redshift of ~3.
