Spectroscopy and photometry of X Ser (Nova Ser 1903) during its current DN stunted outburst

ATel #9463; U. Munari (INAF Padova) and S. Dallaporta (ANS Collaboration)
on 6 Sep 2016; 16:18 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The old nova X Ser (Nova Ser 1903) is known to display stunted dwarf-nova outbursts (Honeycutt et al. 1998, AJ 115, 2527), for which multiband photometry - and spectroscopy in particular - seem acutely rare. The under-populated AAVSO lightcurve of X Ser records four such events during 2009-2015, and other three were noticed during the 1991-2000 monitoring by Honeycutt (2001, PASP 113, 473). A new outburst has just been discovered by R.J. Modic (VSNET) who recorded X Ser at V=14.54 on Aug 28. Stunted outbursts have spacings and widths similar to classical dwarf-nova outbursts but of much smaller amplitude, the latter fact tentatively ascribed to the presence of a brighter background source that causes a reduced contrast on an otherwise normal dwarf-nova outburst (Honeycutt 2001).

On Sep 5.80 UT we have obtained UBVRI photometry of X Ser (on Landolt's system of equatorial standards) with ANS Collaboration telescope N.36, resulting in U=14.111, B=14.287, V=14.187, Rc=14.012, and Ic=13.809 (average error 0.008 mag). It confirms the outburst status of X Ser, and provides the first five-color data point of apparently any such event. The current B-V=+0.10 is moderately bluer than the B-V=+0.28 obtained by Szkody (1994, AJ 108, 639) for X Ser in quiescence.

Also on Sep 5.80 UT, we have obtained a 3500-8000 Ang spectrum of X Ser with the Asiago 1.22m telescope (at 2.31 Ang/pix dispersion). On top of a smooth blue continuum with minimal absorption by the Balmer continuum, several sharp lines appears in strong emission. The stronger is HeII 4686 Ang (integrated Flux 5.1 x 10[-14] erg/cm2 s Ang, Equivalent Width 4.83 Ang), followed by Halpha (F=4.6 x 10[-14], E.W.=8.72) and Hbeta (F=1.4 x 10[-14], E.W.=1.46). The strongest HeI lines is 6678 (F=0.75 x 10[-14], E.W.=1.47). Hydrogen Hgamma, Hdelta, Hepsilon, H8 and H9 are also visible in emission as for HeI lines at 5016, 5876, and 7065 Ang (but not 4471). The spectrum is closely similar to another one of X Ser we obtained with the same telescope on June 17, 2012 during the early decline from the 2012 stunted outburst, when the star was passing through V=14.61 mag.

Comparing with the quiescence fluxed spectra of X Ser obtained in 1997 and 1999 by Thortensen and Taylor (2000, MNRAS 312, 629), the integrated flux of emission lines has increased during the present outburst by 19, 4.3, 2.8, and 2.0 times for HeII 4686, Halpha, Hbeta, and HeI 6678, respectively, while the flux in the continuum (at 5500 Ang) has increased by 12 times. There is therefore a component triggered by the outburst which is hard enough to (almost) completely ionize Helium. The fast approaching conjunction with the Sun will probably stop observations before X Ser will have time to venture through (advanced) decline and its Helium recombination.