V1047 (N Cen 2005) rebrightening

In AAVSO Alert Notice 668, it was omitted that the rebrightening of V1047 Cen (Nova Cen 2005) was discovered and reported by the Gaia team as Gaia19cfn on June 10, 2019. The target was subsequently observed by OGLE. I apologize sincerely for the omission of this discovery information.

Good observing,

Elizabeth O. Waagen

The nova V1047 Cen (Nova Cen 2005) was discovered on 1 September 2005.
Aside from five ASAS-3 observations which allow one to bracket the time of peak to within about 8 days, most of the early data are Vis observations from the AAVSO database (observers AAX DSJ JA PAW SQH SRX SWQ). These data are shown in Figure v1047cen_05.jpg.
Assuming that the first AAVSO point is the peak (Vis = 8.5), the best fit exponential decay gives t_2 = 5.5 and t_3 = 11.9 days.

Spectra indicate that this was a nondescript Fe II nova, though the decay is quite fast.

All would have ended there, except that the GAIA project noted that the star was rebrightening in 2018 (see the correction to Alert Notice 668). This is not a nova eruption: it was very long, and only reached V~14 at peak (see Figure v1047cen_dn.jpg). The eruption amplitude in the I_c band was about 7 mag. The eruption lasted over a year, but seems to be over now. We were able to get 7 color photometry with SMARTS/Andicam before it was retired in June 2019; after that all the data come from six stalwart AAVSO
observers in the south (BHQ DDJ HMB MDAD MGW MLF), plus some OGLE I_c band data.

While the I_c-band data suggest the star is still brighter than its pre-eruption minimum, this may not be the case. There are two other stars within a couple of arcsec of V1047 Cen that now outshine it.

So the observing campaign is over, and the data analysis is ongoing.

Some science:

Novae and dwarf novae are clearly related.
Nova Per 1901 is now perhaps better known as the dwarf nova GK Per (https://www.aavso.org/vsots_gkper); the U Gem type dwarf nova V392 Per erupted as Nova Per 2018. A comparison of V1047 Cen with GK Per/N Per 1901 may be informative. The DN outbursts in GK Per last a couple of months, much less than V1047 Cen, and the outburst is typically 3 mag, not the 7 seen  in V1047 Cen. But GK Per has settled down to a steady-state, while V1047 Cen is a century younger. Time will tell how comparable these system are.

The nova is a thermonuclear detonation in the accreted hydrogen on the surface of a white dwarf star. A dwarf nova (DN) is a brightening of the accretion disk due to an instability that increases the mass transfer rate (matter in orbit must lose orbital velocity to fall onto the star). This energy goes into heating the disk, which brightens. Therefore, novae are the consequence of mass accretion driven, in part, by the DN outbursts. For a dwarf nova to erupt there must be a significant disk surrounding the white dwarf. What was unexpected in the case of V1047 Cen is that it took only a dozen years for the disk to reform and to grow to the extent that it could support a DN outburst.

The nova eruption is expected to destroy the accretion disk around the white dwarf. But they reform, apparently quickly. There's still an awful lot we do not know about the reformation of the accretion disk following the nova. Systems like V1047 Cen may inform us on the mass accretion rate immediately after the nova outburst, when the secondary is puffed up by the energy it absorbed from the explosion.

Should you keep observing V1047 Cen?

This is not an easy target. It is fainter than I~17 at quiescence, and is in a crowded area. The most recent outburst only reached V~13. There is no need for frequent observations, but a snapshot every so often would be useful. It might not erupt for another 12 years, or it could go sooner. Or later. (Astromomy demands patience.)
The timing of the outbursts will inform us about the accretion disk dynamics, and the mass transfer rate onto the white dwarf. It may currently be enhanced, driven by illumination from the still-hot white dwarf.

If you look at the complete light curve (Figure v1047cen_all.jpg), there are 3 visual observations
between June 2015 and August 2018 that seem to show the star at 14>Vis>12. Was the star hiccupping before the 2019 outburst, or is this a simple case of mistaken identity? Any observation with V or Vis brighter than about 16 should be confirmed and, if confirmed, followed up.

Figure Captions:
v1047cen_05.jpg. The outburst of Nova Cen 2005. Green asterisks are from the ASAS-3 sky survey; the open aqua points are visual observations from the AAVSO database. Other data are from the literature.
Uncertainties on the visual observations are arbitrarily set to 0.2 mag.

v1047cen_dn.jpg. The light curve of the dwarf nova outburst in V1047 Cen. Points are color-coded by band. AAVSO points are the open circles; the density of observations is evident. Thanks to Gordon Myers and Josch Hambsch for most of the data. The JHK data show when Andicam was active, before retirement on 31 July 2019. The data gap about day 8760 is the Solar gap. After reaching a plateau, the data show a gradual brightening trend with superposed irregular variations until mid January 2020, after which a
precipitous decline back towards quiescence ensued.

v1047cen_all.jpg: The complete light curve, from 2005 through June 2020. Colors and symbols are as in previous figures. The 2018-19 outburst did not get as bright as, but lasted far longer than, the 2005 nova outburst.