Spectra of luminous nova M31N 2013-10h (= Swift J004304.9+411630) in brightening

ATel #5554; S. Fabrika, E. A. Barsukova, A. F. Valeev, A. Vinokurov (Special Astrophysical Observatory, Nizhnij Arkhyz), V. P. Goranskij (Sternberg Institute, Moscow University), K. Hornoch (Astronomical Institute, Ondrejov, Czech Republic), M. Henze (ESA/ESAC, Spain), A. W. Shafter (SDSU), W. Pietsch (MPE, Germany)
on 6 Nov 2013; 18:32 UT
Credential Certification: Sergei Fabrika (fabrika@sao.ru)

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We report optical spectroscopy and photometry of the recent M31 nova candidate M31N 2013-10h (= Swift J004304.9+411630 = TCP J00430483+4116306) discovered as Swift UVOT transient (ATel #5528). Koichi Nishiyama, Kurume and Fujio Kabashima, Miyaki (CBAT, 2013 10 31.4963) have confirmed the transient in the optical as 17.9 mag source in unfiltered CCD frames (on Oct. 29.571 UT they have reported only an upper limit). The confirmation was 1.2 - 1.4 days after the Swift discovery (the uncertainty is determined by Swift pointings). Using the Russian BTA telescope equipped with the SCORPIO spectrograph we have obtained spectra and photometry of the nova M31N 2013-10h on three occasions: on 2013 Nov. 1.67 UT (blue_1), Nov. 1.97 UT (red_1) and Nov. 2.66 UT (blue_2). The blue spectra have a resolution of 5 Å and spectral range 4050 - 5850 Å, the red spectrum has the same resolution and spectral range 5750 - 7500 Å.

The nova appeared to be in a brightening phase during our observations. We classify the nova as of Fe II class. Its spectrum has many blue-shifted absorption lines of hydrogen, Fe II, Si II, Mg II, Na I with weak emission components having P Cyg type profiles. The two blue spectra taken with the same configuration and separated by one day have not shown a strong evolution, however the second spectrum became redder. The expansion velocity measured as the separation between the center of the absorption and the emission peak in the blue_1 spectrum is 700 km/s in Hβ, 600-700 km/s in the brightest Fe II lines, and it is 700 km/s in Hα in the red_1 spectrum. One day later in the blue_2 spectrum the velocity decreased by 100 km/s (mostly because of the blue wing of the blue-shifted absorptions). All equivalent widths decreased respectively: from 2.1 Å to 0.6 Å in Hβ emission, from 4.6 Å to 3.6 Å in Hβ absorption. In the red_1 spectrum the Hα line has an EW = 8 Å in emission and 6 Å in absorption. In our three epochs of observation the photometry of the nova gives (errors are 0.02 - 0.03 mag): B = 16.66, V = 16.45, Rc = 16.30 (blue_1), B = 16.63, V = 16.39, Rc = 16.19 (red_1), B = 16.08, V = 15.81, Rc = 15.62 (blue_2). The nova continuously brightened and reddened during our observations.

V723 Cas, a slow Galactic nova in 1995 (Goranskij et al., Astrophys. Bull., 62, 125, 2007 and references therein) has shown a F-supergiant type spectrum in its maximum light, which is very similar to the spectrum of M31N 2013-10h. The wind velocity of V723 Cas measured, in the same way as above, was 180 km/s on 20th day after detection. The brightening phase of V723 Cas lasted for 85 days and ended with an 5-day long outburst of ~1.4 mag. One year later the star appeared as radio source, over the next 4 years the star exhibited a coronal optical spectrum, and after 10.5 years it had already become a SSS in X-rays. Regarding this analogy one might predict additional brightening peaks of M31N 2013-10h and a subsequent SSS state with a late turn-on time and a long duration (see Ness et al. 2008, ApJ, 135, 1328). However M31N 2013-10h is notably brighter (M_V = -8.8 in a pre-maximum phase) than V723 Cas (M_V = -7.6 in the outburst peak).

A very luminous nova M31N 2007-11d (Shafter et al., ApJ, 690, 1148, 2007) has estimated luminosity of M_V ~ -9.5 at maximum. This nova was a slowly rising but moderately fast declining system. In the 5 days post-discovery the spectrum showed Balmer and Fe II lines with P Cyg profiles. The Hβ line had a width FWHM ~ 1600 km/s. However the Hβ width in M31N 2013-10h in our blue_2 spectrum was ~200 km/s. We conclude that M31N 2013-10h is too luminous to represent the V723 Cas case, while it's emission line width and expansion velocity appear too low to match the luminous nova M31N 2007-11d. Further spectral and photometric monitoring of M31N 2013-10h are strongly encouraged.

Optical spectra of M31N 2013-10h