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OGLE-2015-NOVA-01 is getting brighter

ATel #7394; P. Mroz, A. Udalski, L. Wyrzykowski (Warsaw University Observatory), P. Whitelock (SAAO), M. Dominik, A. Scholz (St. Andrews), S. T. Hodgkin (IoA Cambridge)
on 14 Apr 2015; 19:22 UT
Credential Certification: Przemek Mroz (pmroz@astrouw.edu.pl)

Subjects: Optical, X-ray, Cataclysmic Variable, Nova, Transient

Referred to by ATel #: 7472

OGLE-2015-NOVA-01 was alerted as a classical nova candidate on 2015 March 5 (ATel #7179). However, since that time the star is still brightening slowly, suggesting that it is not the classical nova.

The transient started between 2015 February 26 and March 3, when it brightened by 6 mag, reaching I=14 mag. This was followed by a nearly linear rise by 1.5 mag in 14 days. Since March 20, the star varies in a brightness range I=12-13 mag on a timescale of a week. Currently (April 13.25368 UT), it has I=12.8 mag. The light curve is available on the webpage of the OGLE-IV Transient Detection System.

The star was observed by the Swift satellite between 2015 April 11.75546 and 11.97359 UT with a total exposure time of 0.75 ks. No X-ray source was detected with the XRT (X-Ray Telescope) at the position of the nova, which gives 3-sigma upper limit of 0.016 cts/s in the 0.3-10 keV energy range. The star was invisible on the short (20 s) exposure taken with the UVOT (Ultraviolet/Optical Telescope) onboard Swift. Thus, it must have been fainter than 16.9 mag in the UVW1 band.

We also obtained a multicolor photometry using LCOGT 1-m robotic telescope at SAAO, South Africa, on 2015 March 19. Observations are summarized in the table below.
HJD Filter Exp. time [s] Brightness [mag]
2457100.60483 g 450 18.026 +/- 0.011
2457100.61375 r 180 15.366 +/- 0.006
2457100.61103 i 180 13.982 +/- 0.004

The light curve shape (a prolonged, flat eruption) resemble that of symbiotic novae, although a certain classification requires spectroscopy. The lack of detection in X-rays and g-r, r-i colors might be caused by a high interstellar reddening (ATel #7179). However, a symbiotic nova should be brighter in quiescence, before the eruption. Other interpretations (a red nova?) are not ruled out.

We thank the Swift PI, Neil Gehrels, for an allocation of ToO time.

The LCOGT observations were carried under proposal SAO2014B-001 which is part of the University of St Andrews and South African Astronomical Observatory allocation.

Finding charts and light curve are available here.