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Spectral confirmation of the M31 novae M31N 2015-02a and M31N 2015-02b (= M31N 2006-11c)

ATel #7158; S. Fabrika, E. A. Barsukova, A. F. Valeev, A. Vinokurov, O. Sholukhova (Special Astrophysical Observatory), K. Hornoch (Astronomical Institute, Ondrejov), M. Henze (ESA/ESAC, Spain), A. W. Shafter (SDSU)
on 27 Feb 2015; 21:08 UT
Credential Certification: Sergei Fabrika (fabrika@sao.ru)

Subjects: Optical, Nova, Transient, Variables

Referred to by ATel #: 7818, 9942

We report optical spectroscopy of the recent M31 nova candidates M31N 2015-02a (ATels #7065, #7076, #7104) and M31N 2015-02b (= PNV J00413317+4110124, ATels #7113, #7116). The latter object has been identified as a recurrent nova (M31N 2015-02b = M31N 2006-11c, ATel #7142). The data were obtained on 2015 Feb 21.702 UT (2015-02b) and 21.742 UT (2015-02a) using the Russian BTA telescope equipped with the SCORPIO spectrograph. The spectra were taken with a spectral resolution of 5.2 Å in the spectral range of 3730 - 5530 Å. Photometry of the objects was obtained shortly prior to the spectroscopy, yielding B = 19.92±0.14, V = 19.60±0.09, R = 18.95±0.07 for M31N 2015-02a. The photometry of the recurrent nova M31N 2015-02b (= M31N 2006-11c) has been published recently (ATel #7142).

The nova M31N 2015-02a shows very strong hydrogen emission from lines Hβ to H8 as well as a strong forest of Fe II lines, FeII blends and CaII H,K lines. All single lines are double-peaked with a stronger red peak. We have measured the Hβ and Fe II 5018 lines, their radial velocities are -290 km/s. Other parameters are EW(Hβ) = 191 Å, FWHM(Hβ) = 1360 km/s, EW(Fe II) = 79 Å, FWHM(Fe II) = 1220 km/s. The double-peaked profile of Hβ may be well-fitted with two Gaussians with FWHM = 850 km/s (blue) and 440 km/s (red). All radial velocities are reduced to the heliocentric frame, and all widths are corrected for the spectral resolution. We conclude that the nova belongs to the Fe II class.

The recurrent nova M31N 2015-02b also shows strong hydrogen emission lines from Hβ to Hε, however it also shows the He II 4686 line and numerous He I lines (3888, 4026, 4922, 5015 Å). The radial velocity of Hβ is -290 km/s, the velocity of He II is -390 km/s. Other parameters of these two lines are EW(Hβ) = 35 Å, FWHM(Hβ) = 1200 km/s, EW(He II) = 6 Å, FWHM(He II) = 660 km/s. We find that the Hβ line profile is located on top of a very broad pedestal with a width of ±5500 km/s (the same is marginally seen in Hγ). No other lines from the line list of He/N class novae (Williams, AJ, 144, 98, 2012) are detected in the pedestal region, however in the red part of the pedestal the He I 4922 Å line is observed. We conclude that the nova belongs to the narrow-lined He/N class (see Shafter et al., ApJ, 734, 12, 2011), with a spectrum similar to that of the ultra-short-recurrence-time recurrent nova M31N 2008-12a (Darnley et al., A&A, 563, L9, 2014; Henze et al., A&A, 563, L8, 2014).