REM Optical/NIR observations of Swift J055137.6-381713

ATel #8883; M. C. Baglio (Universita' dell'Insubria, Como), P. D'Avanzo (INAF-Osservatorio Astronomico di Brera), S. Campana (INAF-Osservatorio Astronomico di Brera), S. Covino (INAF-Osservatorio Astronomico di Brera), P. Goldoni (APC/Univ. of Paris 7, CEA/Irfu), N. Masetti (INAF-IASFBo), E. Palazzi (INAF-IASFBo), T. Munoz-Darias (IAC)
on 30 Mar 2016; 14:14 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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We report on optical and near-infrared (NIR) observations of the probable new X-ray transient named Swift J055137.6-381713, first detected by Swift on 2016 March 22 (Evans et al. 2016, ATel #8863).
Observations were carried out with the REM telescope (La Silla, Chile) on Mar 26, 2016 between 01:20 UT and 02:05 UT, obtaining a totality of 12 x 150s integration images in the optical g, r, i bands, and 12 x 30s images in the NIR H band.
We clearly detect an optical/NIR source within the Swift-XRT error circle with the following coordinates (J200):

RA=05h 51m 37.825s=87.9076 deg
dec=-38d 17' 12.26''=-38.28674 deg
with an uncertainty of 0.5''.

The source clearly appears in both optical and NIR images as an extended object. Its position is consistent with the USNO/2MASS catalogued source reported in ATel #8863. We note that this source is present in the "Atlas of Radio/X-ray associations" (ARXA; Flesch, 2010, PASA, 27, 283), catalogued as a possible galaxy/QSO. The same object is also associated with the extended radio source NVSSJ055137-381708 (Condon et al. 1998, AJ, 115, 1693) and to the WISE source J055137.13-381711.1, whose mid-infrared colors are similar to the colors of gamma-ray blazars (D'Abrusco et al. 2014, ApJS, 215, 14).
We obtained for the target the following optical and NIR magnitudes:

g=18.0 +/- 0.1
r=17.4 +/- 0.1
i=17.0 +/- 0.1

H=14.9 +/- 0.1

The magnitudes are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.04 in the direction of the object (Schlafly et al. 2011).
The H-band magnitude was calibrated against the 2MASS catalog; optical calibration was performed against the APASS catalog (AB magnitudes).
According to our photometry, the target shows a brightening of ~ 1 mag in H-band with respect to the catalogued 2MASS magnitude (16.022+/-0.223).
Due to the extended nature of the source, its variability in X-rays and in the NIR, and its tentative classification as a galaxy, we tend to conclude that the X-ray transient might be a low-redshift blazar.
Further observations are encouraged.
The optical finding chart (r-band) can be found at the following URL: http://www.brera.inaf.it/utenti/davanzo/transients/Swift_J055137.6-381713/fc_J055137_rband.jpg
We acknowledge the use of the VizieR database.

Optical finding chart