WISE J181834.00-284919.6: a bright infrared Nova in the Galactic bulge?

ATel #4268; R.-D. Scholz, Th. Granzer, R. Schwarz, A. Schwope, J. Storm, O. Schnurr (Leibniz Institute for Astrophysics Potsdam)
on 20 Jul 2012; 09:33 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Ralf-Dieter Scholz (rdscholz@aip.de)

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We searched the mid-infrared WISE All-Sky Source Catalog (Wright et al. 2010, AJ, 140, 1868) for brown dwarf (BD) candidates and found a very bright red object (abbreviated name WISE J1818-28) at the J2000 position RA = 18 18 34.00, DEC = -28 49 19.6 with w1=4.96, w2=3.37, w3=2.61, and w4=2.50, observed on 24 March 2010. As there was no near-infrared counterpart in DENIS (August 1998) and 2MASS (August 1999), we ruled out a very nearby cool BD and assumed a Nova candidate.

At the given position we could not find an object in any optical sky survey with limiting photographic magnitudes of about 20. We visually checked all nine DSS images (epochs between 1958 and 1996) available at http://archive.stsci.edu/cgi-bin/dss_plate_finder and the SuperCOSMOS H-alpha Survey (August 2001) and confirmed that there was no optical counterpart. The object is also not present in the optical catalogues UCAC3 (mean epoch 1995) and CMC14 (September 2003), which are both based on CCD observations with limiting magnitudes of about 17.

A search via VizieR at the CDS Strasbourg allowed us to confirm that any other relatively bright (w1<7.5 and w2<7.5) WISE source in the field (search radius of 6 arcmin) except WISE J1818-28 was also detected in the Akari mid-infrared all-sky survey (May 2006 to August 2007). In the recently released WISE 3-Band Cryo Database, providing later observations of lower accuracy in only three bands, the object appears clearly fainter on 22 September 2010: w1=9.3, w2=8.3, and w3=4.7. Systematic offsets with respect to the WISE All-Sky Source Catalog are smaller than +/-0.1 mag for other bright sources in the field, whereas for WISE J1818-28 the decline by about 4-5 mag (w1 and w2) and 2 mag (w3) is remarkable and hints at a strong burst between 2007 and 2010, probably in early 2010.

Further support for a burst in 2010 comes from the first data release of the VVV survey (Saito et al. 2012, A&A, 537, A107) that includes the region around WISE J1818-28. In the corresponding JHKs FITS files available at the ESO archive we detected the object with J~14.0, H~13.1, and Ks~11.0 (on 15 August 2010) using photometric reference stars close to the target from 2MASS. According to later observations with ESO-VIRCAM, the object faded to Ks~13.0 on 24-28 October 2010 and Ks>~16 from 02 September 2011 until 11 July 2012. We conclude that WISE J1818-28 is a strong Nova candidate.

WISE J1818-28 is located in the direction of the Galactic bulge (l=3.65, b=-6.22 deg). It is brighter (and probably less affected by reddening) than the Supernova candidate VVV-WIT-01 that lies exactly in the Galactic plane (b=-0.30 deg) and in the line of sight of (i.e. probably behind) a dark cloud (Minniti et al. 2012, ATel #4041). Based on the strong decline in the mid-infrared (WISE) data of WISE J1818-28 we expect that the peak in the near-infrared was probably at Ks<8 (in early 2010).

We have carried out optical observations with the robotic STELLA telescope (Strassmeier et al. 2010, Advances in Astronomy, id. 970306) on 25 May 2012 and 11 July 2012 using Sloan filters and detected a relatively blue counterpart with constant magnitudes of g'=16.8, r'=17.1, and i'=18.2. Its position is within the error of 0.15 arcsec consistent with the WISE position so that we do not measure a significant proper motion.

We encourage optical/infrared follow-up observations of WISE J1818-28 to follow its decline in brightness and certainly identify its nature. This bright Nova candidate could have been visible in optical observations from 2010 (e.g. in OGLE), where it may still be recovered.

(partly based on data obtained from the ESO Science Archive Facility under request number 35330)