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\begin{document}

\IBVShead{5172}{14 September 2001}

\IBVStitle{Nova S\lowercase{gr} 2001 No.\ 2 = V4739 S\lowercase{gr}}

\IBVSauth{Livingston, C.M.$^1$; Hearnshaw, J.B.$^1$; Gilmore, A.C.$^2$; Kilmartin, P.M.$^2$}

\IBVSinst{Department of Physics and Astronomy, University of
Canterbury, Christchurch, New Zealand}

\IBVSinst{Mt John University Observatory, University of Canterbury,
Tekapo, New Zealand}

\SIMBADobj{V4739 Sgr}

\begintext

Nova Sgr 2001 No.\ 2 was discovered by Pereira in Portugal on 2001 Aug
26.866 at magnitude $m_{V}=7.6$ (Pereira 2001). Maximum occurred a few
hours later at Aug 27.10. Apart from a handful of early visual
magnitude estimates around maximum, photoelectric $UBV(RI)_{\rm{C}}$
photometry has been obtained by Kilmartin and Gilmore at Mt John
University Observatory since Aug 27.36. The visual light curve is shown
in Fig.~1 and colour curves in Fig.~2 for the first week since
discovery.

Nova Sgr 2001 No.\ 2 appears to be the fastest classical nova ever
observed, with a $t_{2}$ value (time for 2 magnitude decline from
maximum in visual) of only $0.70 \pm 0.08$ d and $t_{3} = 1.60 \pm
0.12$ d. The light curve is a smooth steep decline from $m_{V}$(max) $=
6.5 \pm 0.1$ at $t_{0}=$ JD $2452148.60 \pm 0.05$.

Other very fast novae (see Warner 1995, Table 5.2) have all had values
of $t_{2}$ greater than 1 d. These include V838 Herculis in 1991
($t_{2}=1.2$ d) and V1500 Cygni ($t_{2}=2.9$ d). To confirm these
values, Ingram et al.\ (1992) gave $t_{2}$ as less than 3 days for V838
Her, and Young et al.\ (1976) stated that $t_{2}$ for V1500 Cyg was 2.4
d. According to Payne-Gaposchkin (1957) any nova with $t_{2}<10$ d is
in the category of being very fast.

Photometry was done with the 0.6-m $f/16$ Cassegrain O.C. reflector at Mt 
John by photon counting with a cooled EMI 9202 (S20B) 
photomultiplier. The system has been standardized to the Johnson--Cousins
$UBV(RI)_{\rm{C}}$ system by repeated measures of Cousins E-region 
standards (see Menzies et al.\ 1989 and references therein) over many 
years.

Using differential photometry from Cousins standards E745 and E746 
Kilmartin calibrated two stars near the nova on August 27. All stars 
were observed at air mass less than 1.05 with a 21\arcs\ aperture in a 
photometric sky and good seeing. The magnitudes and colours adopted 
for these stars, along with their HD numbers, are 
listed in Table~1. The standard deviation of nearly all measures was
0\fmm009 or less except for $V-I_{\rm C}$ on the last 2 nights (where it was
$\pm\, 0.2$). These HD stars are noted as constant in the Hipparcos-Tycho
database. All subsequent photometry was made differentially from the
listed stars as comparison and check respectively. 


\begin{table}[htb]
\caption{Comparison and check stars from Mt John University Observatory}
\begin{center}
\begin{tabular}{cccccccc}
\hline
Comparison \& & \lower6pt\hbox{$V$} & \lower6pt\hbox{$U-B$} & \lower6pt\hbox{$B-V$} & \lower6pt\hbox{$V-R_{\rm C}$} & \lower6pt\hbox{$V-I_{\rm C}$}\\[-4pt]
check stars & & & & & \\ 
\hline
HD169337 &7.507 & +0.465 & +0.976 & +0.669 & +1.358\\
HD169586 &6.757 & +0.092 & +0.535 & +0.305 & +0.597\\ 
\hline
\end{tabular}
\end{center}
\end{table}

We have calculated the absolute magnitude of V4739 Sgr at maximum from
the rate of decline by extrapolating the calibration of Della Valle and Livio
(1995). Fortunately $M_{V}$ is not very sensitive to $t_{2}$ for very fast
novae. The value obtained is $M_{V}=-9.07 \pm 0.17$ for V4739 Sgr, where 
the error bar arises almost entirely from the uncertainty in the
calibration rather than in the measured $t_{2}$ value.

The reddening can be estimated using $(B-V)_{0}$ at time $t_{2}$, which
is $-0.02 \pm 0.04$ for novae, as found by van den Bergh and Younger
(1987). The interpolated $(B-V)_{\rm{obs}}$ colour index at $t_{2}$ is
$0.44 \pm 0.02$ giving $E_{B-V}=0.46 \pm 0.04$ and hence $A_{V}=3.2
E_{B-V} = 1.47 \pm 0.13$. A relatively high value is also suggested by
the strong IS NaD line (Vanlandingham 2001).

The distance to the nova then follows and is $d = 6600 \pm 700$ pc with a
distance modulus ($5 \log d - 5$) of $14.1 \pm 0.2$. Given that the star has
($l, b$) $=$ ($3\fdeg2, -8\fdeg0$) this distance places it near the
galactic centre. 

Capaccioli et al.\ (1989) have found that the absolute visual magnitude
of classical novae 15 days after maximum is $M_{15}=-5.69 \pm 0.14$,
independent of speed class. With an apparent magnitude of $m_{15}=13.42
\pm 0.02$ (the small uncertainty is due to the small error in time of
maximum) the distance modulus would be $17.6 \pm 0.2$, much greater than 
before, and implying a distance far beyond the galactic centre. We
conclude that the value of $M_{15}$ may be substantially less luminous
for very fast novae, as was also suggested by van den Bergh and Younger
(1987) for V1500 Cyg. Hence for extremely fast novae ($t_{2}<2$ d) it is
reasonable to suggest that the $M_{15}$ calibration may not be valid.

\IBVSfig{10cm}{5172-f1.ps}{Visual light curve of V4739 Sgr. $\times$
photoelectric photometry (MJUO); $+$ visual estimates from IAU Circ.\ 7692;
$\bigtriangleup$ CCD photometry from IAU Circ.\ 7692,7702; $\downarrow$ visually
estimated upper limit from IAU Circ.\ 7692}

\clearpage

\begin{table}[htb]
\caption{Photoelectric photometry of V4739 Sgr from Mt John University Observatory}
\vspace{-2pt}
\begin{center}
\begin{tabular}{crcrcr} 
\hline
HJD (2450000\,+) & $V$ mag & $U-B$ & $B-V$ & $V-R_{\rm C}$ & $V-I_{\rm C}$ \\
\hline
2148.839 &7.46 & $-0.71$ & 0.55 & 0.53 & 0.99\\
2148.853 &7.51 & $-0.74$ & 0.56 & 0.55 & 1.01\\
2148.869 &7.58 & $-0.75$ & 0.55 & 0.56 & 1.04\\
2148.875 &7.59 & $-0.76$ & 0.56 & 0.56 & 1.04\\ 
2148.908 &7.66 & $-0.78$ & 0.55 & 0.60 & 1.10\\
2148.918 &7.69 & $-0.80$ & 0.54 & 0.61 & 1.12\\
2148.927 &7.71 & $-0.81$ & 0.55 & 0.61 & 1.11\\
2148.935 &7.75 & $-0.81$ & 0.54 & 0.62 & 1.13\\
2149.006 &7.94 & $-0.86$ & 0.53 & 0.68 & 1.22\\
2149.014 &7.95 & $-0.87$ & 0.53 & 0.70 & 1.24\\
2149.043 &8.02 & $-0.87$ & 0.52 & 0.72 & 1.26\\
2149.052 &8.04 & $-0.87$ & 0.53 & 0.72 & 1.27\\
2149.061 &8.05 & $-0.88$ & 0.53 & 0.73 & 1.28\\
2149.070 &8.06 & $-0.89$ & 0.54 & 0.73 & 1.29\\
2149.079 &8.07 & $-0.89$ & 0.54 & 0.74 & 1.29\\
2149.088 &8.09 & $-0.87$ & 0.52 & 0.75 & 1.30\\
2149.097 &8.10 & $-0.87$ & 0.52 & 0.75 & 1.31\\
2149.106 &8.12 & $-0.89$ & 0.55 & 0.77 & 1.33\\
2149.115 &8.16 & $-0.90$ & 0.53 & 0.77 & 1.33\\
2149.818 &9.12 & $-0.93$ & 0.28 & 1.09 & 1.53\\
2149.827 &9.13 & $-0.89$ & 0.27 & 1.09 & 1.56\\
2149.866 &9.27 & $-0.86$ & 0.26 & 1.13 & 1.56\\
2150.843 &9.94 & $-0.84$ & $-0.02$ & 1.37 & 1.44\\
2150.847 &9.95 & $-0.85$ & $-0.04$ & 1.39 & 1.46\\
2150.851 &9.95 & $-0.87$ & $-0.02$ & 1.38 & 1.45\\
2150.856 &9.94 & $-0.85$ & $-0.02$ & 1.36 & 1.45\\
2151.997 &10.58 & $-0.68$ & $-0.20$ & 1.47 & 1.25\\
2152.002 &10.49 & $-0.67$ & $-0.11$ & 1.43 & 1.23\\
2152.873 &11.07 & $-0.64$ & $-0.36$ & 1.53 & 1.14\\
2152.878 &11.06 & $-0.65$ & $-0.36$ & 1.54 & 1.16\\
2152.999 &11.12 & $-0.65$ & $-0.35$ & 1.55 & 1.16\\
2153.004 &11.14 & $-0.65$ & $-0.38$ & 1.56 & 1.15\\
2153.910 &11.39 & $-0.66$ & $-0.40$ & 1.64 & 1.21\\
2153.914 &11.37 & $-0.66$ & $-0.38$ & 1.60 & 1.19\\
2154.855 &11.55 & $-0.71$ & $-0.32$ & 1.68 & 1.14\\
2154.861 &11.54 & $-0.71$ & $-0.32$ & 1.66 & 1.15\\
2160.830 &12.75 & $-0.79$ & $-0.32$ & 1.41 & 1.02\\
2160.836 &12.76 & $-0.69$ & $-0.30$ & 1.42 & 1.02\\
2161.853 &12.94 & $-0.75$ & $-0.32$ & 1.37 & 0.76\\
2161.858 &12.98 & $-0.68$ & $-0.37$ & 1.42 & 0.64\\
2162.921 &13.25 & $-0.81$ & $-0.43$ & 1.39 & 1.12\\
2162.924 &13.22 & $-0.75$ & $-0.39$ & 1.35 & 1.01\\
2163.862 &13.49 & $-0.82$ & $-0.47$ & 1.45 & 0.8\\
2163.869 &13.51 & $-0.82$ & $-0.48$ & 1.45 & 1.0\\
2164.846 &13.56 & $-0.83$ & $-0.33$ & 1.30 & 0.7\\
2164.850 &13.62 & $-0.78$ & $-0.43$ & 1.35 & 0.5\\
\hline
\end{tabular}
\end{center}
\end{table}

\clearpage

\IBVSfig{10cm}{5172-f2.ps}{Photoelectric colour curves of V4739 Sgr
from Mt John}

\references

Capaccioli, M., Della Valle, M., D'Onofrio, M., Rosino, L., 1989, {\it
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Della Valle, M., Livio, M., 1995, {\it ApJ}, {\bf 452}, 704

Ingram, D., Garnavich, P., Green, P., Szkody, P., 1992, {\it PASP}, {\bf
104}, 402

Menzies, J.W., Cousins, A.W.J., Banfield, R.M., Laing, J.D., 1989, 
{\it South African Astr.\ Obs.\ Circ.}, {\bf 13}, 1

Payne-Gaposchkin, C.H., 1957, The Galactic Novae, (North Holland Pub.\ Co., Amsterdam)

Pereira, A.J.S., 2001, {\it IAU Circ.}, No.\ 7692

van den Bergh, S., Younger, P.F., 1987, {\it Astron.\ Astrophys.\
Suppl.\ Ser.}, {\bf 70}, 125

Vanlandingham, K., 2001, {\it IAU Circ.}, No.\ 7696

Warner, B., 1995, Cataclysmic Variable Stars, (Cambridge Univ.\ Press.,
Cambridge)

Young, P.J., Corwin, H.G. Jr, Bryan, J., de Vaucouleurs, G., 1976, {\it ApJ}, {\bf 209}, 882

\end{document}
