Electronic Telegram No. 1010 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://www.cfa.harvard.edu/iau/cbat.html 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. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2007 CBAT 2007 July 22 (CBET 1010) Daniel W. E. Green