VVV-WIT-02: A Large-Amplitude Transient of Unknown Nature in the VVV Survey

ATel #5954; I. Dekany (Pontificia Universidad Catolica de Chile, The Milky Way Millennium Nucleus), D. Minniti (Pontificia Universidad Catolica de Chile, Vatican Observatory, The Milky Way Millennium Nucleus), R. K. Saito (Universidade Federal de Sergipe)
on 6 Mar 2014; 19:04 UT
Credential Certification: Istvan Dekany (idekany@astro.puc.cl)

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We report the discovery of WIT-02, a large-amplitude transient event observed by the VVV Survey. The initially faint and red progenitor object (< K_s >=15.52 and <J-K_s>=2.06) that was observed on March 29, 2010, produced a large-amplitude outburst sometime before it was re-observed by the VVV Survey as a K_s~9.3 mag saturated source on July 28, 2011. After a mostly continuous decline since then, it reached an apparently constant quiescent state by mid-2013, slightly brighter than the progenitor.
From its VVV K_s-band time-series observations, we derived an accurate position for the object of RA=17:53:02.08, DEC=-24:51:59.19. According to the VVV reddening maps (Gonzalez et al., 2012, A&A, 543, 13) and assuming the Nishiyama et al. (2009, ApJ, 696, 1407) extinction law, the approximate color excess for a 2 arcmin region around the target position and at the mean distance of the bulge red clump stars is E(J-K_s)~0.2 mag, which corresponds to an estimated absolute reddening of A(K_s)~1.0 mag.
Within a few weeks from the observed maximum brightness, our data show a short-lasting (few days) bump in the light-curve. However, these magnitudes are beyond the saturation limit of VIRCAM and therefore subject to systematics. During this possible bump, the observed color indices of the object were (Z-K_s)~4.3 and (Y-K_s)~3.0 (the K_s magnitude was measured ~1 day after the Z and Y ones). During the first phase of decline, the light-curve can be approximated with a power law of K_s ~ t^4.6, significantly different from the universal pre-bump (~ t^1.75) and post-bump (~ t^3.5) decay rates of classical novae (Hachisu & Kato 2006, ApJSS, 167, 80). WIT-02 was re-observed ~150 days later and had already fainted to K_s = 14.91 mag, and continued decaying with a smaller slope, until the light-curve apparently flattened out during the subsequent ~400 days, by March 2013. The new quiescent magnitude of the object was < K_s>=15.27 during that month, which is about 0.25 mag brighter than its average magnitude recorded before eruption.
Our search for earlier data of this object within a radius of 10" shows that it had no observable counterpart in previous images from 2MASS, GLIMPSE, Spitzer, nor MSX. However, we found a bright source at an angular distance of only 3.4 arc seconds from the VVV position of WIT-02 in the DENIS catalog with the identifier J175302.3-245200. It was observed on April 14, 1996, with magnitudes K_s=12.05 and (J-K_s)=1.39. VVV images do not show any other detectable object around WIT-02 within the above radius, and all objects in its 10" vicinity are several magnitudes fainter than the DENIS magnitudes. This suggests that WIT-02 probably underwent a previous outburst ~14 years before the one reported here, and raises the possibility that it is a peculiarly fast recurrent nova, but follow-up spectroscopic observations are required to confirm this.

Acknowledgments: We gratefully acknowledge the use of data from the ESO Public Survey program ID 179.B-2002 taken with the VISTA telescope, and data products from the Cambridge Astronomical Survey Unit.

VVV-WIT-02 Table of Observations