Early evolution of the He/N super-fast Nova Sgr 2003 N.3 (V6598 Sgr), resembling that of U Sco and Nova Oph 2009 (V2672 Oph)
ATel #16141; U. Munari (INAF Padova), P. Ochner, A. Siviero, A. Farina (Univ. Padova), S. Dallaporta, P. Valisa, S. Moretti, and A. Maitan (ANS Collaboration)
on 19 Jul 2023; 08:56 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
V6598 Sgr was discovered by A. Pearce as TCP J17525020-2024150 on 2023 July 15.459 UT, who reported B=12.08, V=10.29, R=9.07, I=8.12 for Jul 15.591 UT. The object was soon classified as a nova by Masi (CBET 5278) and Garde et al. (ATel #16135) based on its very broad Halpha in emission and general spectral appearance.
We are collecting photometry of V6598 Sgr with ANS Collaboration telescopes 0310, 0311, 1205, and 2202. Our data are color-equation transformed to Landolt UBVRI equatorial system, and the g'r'i' bands to the APASS DR8 implementation of the SLOAN system. It has been necessary to carry out the measurements in PSF-fitting mode because of the presence of a close companion (5.4 arcsec away), measured at coordinates 17:52:49.57 -20:24:19.5 and V=15.19 mag, while the astrometric position of the nova is identical within 0.1 arcsec of the Chandra position for IGR J17528-2022 quoted by Hare et al. (2021, ApJ, 914, 85).
V B-V V-R V-I g' r' i'
15.873 11.007 1.701 1.259 2.556 12.116 10.280 9.151
16.861 12.206 1.464 1.573 2.903 13.125 11.020 10.030
17.881 12.931 1.309 1.857 3.005 13.847 11.340 10.786
18.864 12.937 1.404 1.668 2.763
The opposing behavior of blue and red photometric colors is due to the contamination by Halpha of the V, R, and r' passband (0.12, 0.58, and 0.78 mag, respectively, on Jul 17.88 UT). The shape of the underlying continuum is actually stable, at constant colors (B-I)~4.30 and (g'-i')~3.05 mag.
The decline of V6598 Sgr halted on day +3 from maximum, the same that happened on day +6 for Nova Oph 2009 and on day +12 for U Sco. The halt is temporary, and the decline should resume when the nuclear burning will be extinguished on the WD.
Spectroscopy of V6598 Sgr has been recorded with Asiago 1.22m + B&C (Jul 15.83, 17.84, 18.88 UT) and Varese 0.84 m + Echelle (Jul. 16.85, 17.84, 18.86 UT) telescopes. The spectra are dominated by a very red continuum, and bright and broad emission lines, the strongest being Halpha, OI 7772, 8446 and NI 8682. Their integrated flux (in units of 1E-13 erg/cm2/s) is listed in the table.
NII Hbeta HeI Halpha OI OI NI
4651 5016 7772 8446 8682
Jul. 16.85 597 239 244
Jul. 17.84 8.3 32 5.4 609 113 277 116
Jul. 18.88 6.7 25 6.2 418 35
The emission lines show a trapezoidal profile, with on top a broad central peak flanked by weaker and narrower emission peaks at -2180 and +3245 km/s (as of Jul 18.86). The FWZI of Halpha is apparently declining as 8350, 7965, and 7885 km/s on Jul. 16.85, 17.84, and 18.86 UT, respectively. The overall spectrum and the lines profiles closely resemble those of U Sco and Nova Oph 2009 at their earliest stages, as it is for the very fast decline that we measure in t(2)=2.0 days. It would be even faster (t(2)=1.5) if A. Pearce photometry for Jul 15.591 UT is considered, but it being not-transformed via color-equations to the standard system could be significantly affected by the very red color of the nova.
P-Cyg absorptions are seen with multi-components. On Jul 15.83 we measure -4095, -3625, and -3100 km/s for Hdelta, for a weighted mean of -3735 that slowed to -3425 on Jul 17.84 and -3300 on Jul 18.88. The equivalent width of the P-Cyg absorptions is rapidly declining.
The interstellar lines of NaI appear badly saturated, so they cannot be used to infer the reddening. On our Echelle spectra (corrected for telluric absorptions) we have measured an equivalent width of 0.229 Ang for the interstellar KI 7669 line, corresponding to E(B-V)=0.91 following the calibration by Munari and Zwitter (1997, A&A 318, 269), and 0.222 Ang for DIB 6614 translating to E(B-V)=0.98 adopting the calibration by Munari (2014, ASPC 490, 183), for an average E(B-V)=0.94.
