[IAUC] CBET 1010: 20070722 : V5558 SAGITTARII

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                                                  Electronic Telegram No. 1010
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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V5558 SAGITTARII
     U. Munari, M. Orio, and M. Valentini, Istituto Nazionale di Astrofisica,
Padova Astronomical Observatory; K. L. Page and J. P. Osborne, Department of
Physics and Astronomy, University of Leicester; and S. Dallaporta, P. Valisa,
G. DallaVia, and A. Stanzione, ANS (Asiago Novae and Symbiotic stars)
collaboration write that V5558 Sgr = N Sgr 2007 spent the first seventy days
following its discovery (on Apr. 14.78 UT; cf. IAUC 8832) on an unusual, very
protracted pre-maximum halt, during which it rose in brightness by just 1
magnitude, and they report on their CCD photometry of the nova, acquired
almost daily with ANS telescopes since the object's discovery.  On June 27
(when the nova was at V = 7.93, B-V = +0.90, V-Ic = +1.43), it suddenly
entered a new phase characterized by a much steeper rise in brightness that
took the nova to a maximum attained around July 10.0 at V = 6.53, B-V = +0.96,
and V-Ic = +1.22.  Starting at July 12.0 UT (when it was at V = 6.59, B-V =
+1.09, and V-Ic = +1.32), V5558 Sgr entered a rapid decline that brought the
nova down to V = 8.37, B-V = +1.12, V-Ic = +1.94 by July 19.96.  The decline
rate seems to have considerably slowed around July 19-20, in correspondence
with the nova regaining the brightness of the extended pre-maximum halt.
Only continued monitoring will tell if the occurring of the last month have
been an isolated flare on top of an unusual, very slow, and flat photometric
evolution or instead the start of a normal decline toward quiescence.
     So far, the lightcurve has been highly reminiscent of that displayed by
Nova Cas 1995 = V723 Cas (cf. Munari et al. 1996, A.Ap. 315, 166).  V5558 Sgr
has shown large spectral changes in data obtained with the 1.82-m telescope
at Asiago and the 0.6-m telescope of the Schiapparelli Observatory in Varese.
At maximum on July 10, the spectra were still characterized by a smooth
underlying continuum with numerous weak absorptions, while Balmer series and
FeI I lines were in emission, the strongest of which belong to multiplets 42,
48, 49, and 74.  On July 12, He I started emerging in emission.  By July 14,
the forest of weak absorptions had gone, the emission lines reinforced, and
this trend was followed on July 16.  Comparing July 12 and 17 spectra, the
integrated flux ratio He I 587.6-nm/Fe II 501.8-nm (multiplet 42) remained
constant at 2.2, while the He I 587.6-nm/H_beta ratio increased from 0.32 to
0.52.  The most relevant spectral changes, however, were those displayed by
the line profiles.  Since discovery and until a few days ago, the emission
lines were very sharp, with P-Cyg absorption displaced by just -120/-130 km/s.
At maximum brightness on July 10, the P-Cyg absorption component of the H_alpha
profile still had a mere velocity of -130 km/s.  Then, by July 12 a fast, hot
wind emerged, with suddenly bloated-up emission-line profiles that peaked to
a FWHM of 1150 km/s on July 14, still with the sharp, low-velocity absorption
superimposed.  By July 17 the FWHM of the fast, hot wind had decreased to 850
km/s, and by July 20 to 780 km/s.  This internal high-velocity wind is expected
to slam onto the exterior, slower, and mostly neutral wind.  This should
produce a shock front.
     Tight optical and photometric monitoring is in progress with ANS
telescopes to catch signatures of this event.  Early signatures of this shock
have already been searched by observing the nova with the SWIFT satellite.  On
July 18.736, the satellite exposed for 2364 sec on the nova and detected it
with a 99.92-percent confidence level.  Assuming an optically thin plasma
spectrum (kT = 5 keV) and absorption column of N(H) = 2 x 10**21 cm**(-2)
[note E(B-V) = 0.36 from CBET 965], the observed count rate of (2.1 +/- 0.9)
x 10**(-3) counts/s translates into an x-ray luminosity of L = 2.0 x 10**31
x D**2 erg/s in the range 0.2-10 keV, where D is the distance in kpc.


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                         (C) Copyright 2007 CBAT
2007 July 22                     (CBET 1010)              Daniel W. E. Green