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)
Subjects: Optical, Cataclysmic Variable, Nova
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.