VVV-WIT-01: An Extreme Transient of Unknown Astrophysical Origin in the VVV Survey

ATel #4041; D. Minniti (Pontificia Universidad Catolica de Chile), P. W. Lucas (University of Hertfordshire), N. Cross (Royal Observatory Edinburgh), V. D. Ivanov (European Southern Observatory), I. Dekany (Pontificia Universidad Catolica de Chile), R. Kurtev (Universidad de Valparaiso)
on 14 Apr 2012; 12:11 UT
Credential Certification: Dante Minniti (dante@astro.puc.cl)

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The VVV survey is mapping the Galactic Bulge and Southern plane in the near-infrared with VIRCAM at the 4.1m VISTA telescope at ESO Paranal Observatory since 2010 (vvvsurvey.org; Minniti et al. 2010, New Astronomy, 15, 433). We report the discovery of WIT-01, an extreme transient that was first observed by the VVV survey in March 2010 at Ks=12.0 mag, and that showed a large amplitude decline in the near infrared since then. This is a very red object (H-Ks=5.0 mag), that was invisible at shorter wavelengths down to the VVV survey limits (ZY ~20.5, J ~ 19.5). We have measured an accurate position based on the VVV images with scale of 0.34 arcsec/pixel, and typical seeing of 1 arcsec. The J2000 coordinates are: RA = 16 10 53.45, DEC = -51 55 32.4. The only SIMBAD match found around this position is the Infrared Dark Cloud SDC G331.062-0.294. Multiple Ks-band images were acquired spanning from March 2010 until March 2012. Additional single-epoch images were acquired in JH on March 2010 and in ZY in July 2010. The exposure times were 80 sec for the initial ZYJHKs-band images, and 16 sec for the following epochs in the Ks-band. The mean magnitude in the March 2010 runs was Ks~12.3, and in the July 2010 runs it was Ks~16.7. The object has faded beyond detection (with Ks>18.5 mag) in the following 2011 and 2012 runs, including in the last VVV image of 03 April 2012. The source was not present in previous images from 2MASS, DENIS, GLIMPSE, nor MSX, but the WISE photometry of March 2010 shows a very bright source at that position, with mean magnitudes W1(3.3 um)=7.93, W2(4.6 um)=5.61, W3(12 um)=4.76, W4(23 um)=4.06. This object could be a very reddened (Av>70 mag) core collapse Supernova in the Milky Way disk, or an eruptive Pre Main Sequence variable star, although at this point other alternatives (such as a reddened LBV, a Nova, or even a new kind of variable star), cannot be discarded. The purpose of this ATEL is to alert the community about this event that should have peaked in early 2010, and to encourage further observations.