Outburst evolution, historic light curve and a flash-ionized nebula around the WZ Sge-type object PNV J03093063+2638031^⋆

¹ INAF Osservatorio Astronomico di Padova, 36012 Asiago ( VI), Italy
e-mail: ulisse.munari@oapd.inaf.it
² Physics Department, University of Rijeka, Radmile Matejčić 2, 51000 Rijeka, Croatia
³ ANS Collaboration, c/o Osservatorio Astronomico, via dell’Osservatorio 8, 36012 Asiago ( VI), Italy

Received: 19 June 2015
Accepted: 7 September 2015

Abstract

We have monitored the 2014 superoutburst of the WZ Sge-type transient PNV J03093063+2638031 for more than four months, from V = 11.0 maximum brightness down to V = 18.4 mag, close to quiescence value, by obtaining BVR_CI_C photometry and low resolution fluxed spectroscopy. The evolution was normal and no late-time “echo” outbursts were observed. The absolute integrated flux of emission lines kept declining along the superoutburst, and their increasing contrast with the underlying continuum was simply the result of the faster decline of the continuum compared to the emission lines. Inspection of historical Harvard plates covering the 1899–1981 period did not reveal previous outbursts, neither “normal” nor “super”. We discovered an extended emission nebula (radius ~1 arcmin) around PNV J03093063+2638031, that became visible for a few months as the result of photo-ionization from the superoutburst of the central star. The nebula is not present on Palomar I and II sky survey images and it quickly disappeared when the outburst was over. From the rate at wich the ionization front swept through the nebula, we derive a distance of ~120 pc to the system. The nebula is density bounded with an outer radius of 0.03 pc, and the absolute magnitude of the central star in quiescence is M_V ~ 14.2 mag. The electron density in the nebula is estimated to be 10⁵ cm^-3 from the observed recombination time scale. Given the considerable substructures seen across the nebula, a low filling factor is inferred. Similar nebulae have not been reported for other WZ Sge objects and the challenges posed to models are considered.