V2362 Cyg (Nova Cygni 2006)
On April 2, 2006 H. Nishimura of Japan discovered a 10.5 magnitude nova based on two photographs taken with his Pentax camera. Similar photos (limiting magnitude 12) showed no object as recently as March 28 of that year, and CCD measurements taken soon after found the object had brightened to 8.5 on April 4/5 (Green 2006). The maximum occurred shortly thereafter, reaching magnitude 8.1. Pre-maximum spectra were obtained, and initial data suggested that this object was a fast Fe-II iron-type nova. Later observations identified the progenitor star with an emission line star (Kimeswenger et al., 2008). To date over 13,000 observations have been submitted to the AAVSO database.
On the surface, this nova doesn’t seem to be anything out of the ordinary. Fortunately for the astronomical world, V2362 Cyg proved to have quite the interesting personality. The first sign astronomers had that V2362 Cyg would be something special occurred in July 2006 (about 100 days after maximum), when its rapid 4.5 magnitude decline stopped. Between August and October the star brightened from 11.9 to 11.3, creating a renewed flurry of observations (Goranskij et al. 2006). By December 1 it had reached magnitude 9.95, an impressive two magnitude recovery from its previous minimum, and then began a second (and final) decline (Kimeswenger et al., 2006). Photometric observations from 2006 and 2007 were used by Balman, Nasiroglu, and Akyuz (2009) to determine that the orbital period of the binary system was only 1.58 hours, one of the shortest periods ever seen for a classical nova system. Clearly Nova Cygni 2006 had much to teach astronomers.
As soon as it became obvious that the system was rebrightening, connections were drawn between V2362 Cyg and another unusual nova, V1493 Aql (Nova Aql 1999) in that both had a “hump” superimposed on the decline from maximum (Goranskij et al. 2006). In 2007, astronomers from the University of Innsbruck (Austria) combined their spectroscopic data with AAVSO photometric data to develop a preliminary model of the behavior of this system. Their paper concluded by noting that “collection of all the data of various observers from the early decline until now is required to build the basis for a more sophisticated theoretical modeling” (Kimeswenger et al., 2008, L54).
This invitation was accepted by an international study that included the AAVSO’s own Arne Henden. By combining spectra and photometry with infrared data taken from the literature, they were able to further model the evolution of the system , including mass loss and dust formation (Munari et al., 2008). They also highlighted the similarities between this unusual nova and V1493 Aql.
In 2010 Strope, Schaefer, and Henden observed that there are very few complete nova light curves in the professional literature. This did not allow for a classification of novae by light curves, other than to divide them into “fast” or “slow.” Therefore spectra were mainly classified through their spectra (for example, the neon and iron novae). While the composition of novae is certainly important information, this lack of a detailed examination of novae light curves was problematic, especially given that novae have long been used as “standard candles” for the estimation of distances to nearby galaxies. Novae that have a sharper decline from maximum also have higher intrinsic brightness (absolute magnitude). Using this so-called “Maximum Magnitude versus Rate of Decline” relationship (MMRD), astronomers can estimate the absolute magnitude of a novae and therefore its distance. However, without a good foundational knowledge of how light curves (and therefore nova systems themselves) can vary, novae’s place in the pantheon of standard candles is suspect. What was needed was a large sample of novae observations from which to construct reliable light curves for a statistically significant sample of objects.
The AAVSO database represents a singular collection of carefully collected and archived magnitude estimates accumulated over more than a century. Among the stars observed are a number of novae. AAVSO visual and V-band CCD observations for the 93 most completely observed galactic novae were used by Strope, Schaefer, and Henden to construct complete or nearly complete light curves. A comparison of these light curves demonstrated that a new classification system based on the shape of the curves was possible. They divided the curves into seven types: Smooth (S), Plateau (P), Dust dips (D), Cusps (C), Oscillations (O), Flat-topped (F), and Jitters (J). Interested readers are directed to their paper for a detailed explanation of the classification system and examples of light curves. Nova Cygni 2006 was the second of three examples of the C-type class to be discovered.
Once a light curve has been constructed for a variable star or any type, astronomers can model the behavior in an attempt to explain the physical properties and processes that are responsible for the changes in brightness. In the case of the C-class novae, it has been suggested that the “cusp” (the secondary brightening) is caused by an infusion of magnetic energy from the white dwarf, followed by the quick accumulation of dust (Strope, Schaefer, and Henden , 2010). Since only 85 of the 93 novae surveyed represent an unbiased statistical sample (as defined in their paper), and V1493 Aql is the only C-type nova in this restricted sample, the work of Strope et al. suggests that C-type novae are rare (only 1% of the sample).
While only a handful of C-class novae have been observed in the Milky Way, the WeCAPP project which searched for microlensing events in M31 found 10 C-class candidates among their 91 observed novae, or 9% of the total (Lee et al., 2012). This suggests that at least in M31 C-class novae are more common than Strope, Schaefer, and Henden’s analysis suggests. Perhaps, as in the case of supernovae, we are overdue for the observation of more “cuspy” novae in our own galaxy.
Therefore Nova Cygni 2006 is a classic case of the “ugly ducking syndrome,” where our seeming outlier has turned out (with the considerable help of AAVSO data) to be a very beautiful swan indeed.
Balman, ., Nasiroglu, I., and Akyuz, A. “Detection of a Periodicity from Classical Nova Cygni 2006 (V2362) in the Optical Wavelength”,Atel#2137, July 27, 2009.
Goranskij, V.P., Metlova, N.V., and Burenkov, A.N. “Nova V2362 Cygni Increases its Brightness”, Atel #928, November 1, 2006.
Green, D.W.E. “Possible Nova in Cygnus”, CBAT Circular No. 8697, April 5, 2006.
Kimeswenger, S. et al. “Nova V2362 Cygni Rapidly Changes Spectrum”, ATel#963, December 14, 2006.
Kimeswenger, S. et al. “The Unusual Nova Cygni 2006 (V2362 Cygni)”, Astron. Astrophys., 479.3, L51-54.
Lee, C.-H. et al. “The Wendelstein Calar Alto Pixellensing Project (WeCAPP): the M31 Nova Catalogue”, Astron. Astrophys. 537.1, 1-59, 2012.
Munari, U. et al. “The Nature and Evolution of Nova Cygni 2006”, Astron. Astrophys. 492.1, 145-62, 2008.
Strope, R.J., Schaefer, B.E., and Henden, A.A. “Catalog of 93 Nova Light Curves: Classification and Properties”, AJ,140, 34-62, 2010.
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