Nova Sagittarii 1998 (V4633 Sgr): a permanent superhump system or an asynchronous polar? Page: 1,170
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1170 YLipkin et al.
Photometric measurements on the bias-subtracted and flat-field-
corrected images were performed using the NOAO IRAF1 DAOPHOT
package (Stetson 1987). Instrumental magnitudes of V4633 Sgr, as
well as of a few dozen reference stars, depending on image quality,
were obtained for each frame. A set of internally consistent nova
magnitudes was obtained using the WO reduction program
DAOSTAT (Netzer et al. 1996). Good seeing conditions on 1998
September 19 were used to calibrate the magnitudes of V4633 Sgr,
as well as of about a dozen nearby comparison stars. We used the
calibrated comparison stars to convert all the measurements of
V4633 Sgr into calibrated magnitudes.
In our programme we obtained 84 nights of continuous time
series, accumulating a total of 8250 data points in I, 2392 in Vand
756 in B.
On 2000 August 4, 20 and 21, we observed V4633 Sgr in the
'fast photometry' mode (Leibowitz, Ibbetson & Ofek 1999). On the
first night we observed for 2.5 h, with time resolution of 10 s, using
no filter ('clear'). On the other two nights, we observed through an
I filter, with time resolution of 20 s. The data were reduced in the
manner described above.
2.2 Spectroscopy
V4633 Sgr was observed spectroscopically at WO on four nights:
1998 July 5 and August 30, and 1999 May 2 and July 6. The spectra
were taken with the WO Faint Object Spectrograph and Camera
(FOSC) described in Brosch & Goldberg (1994), and operated at
the f/7 Ritchey-Chretien focus of the WO 1-m telescope. The
Tektronix 1K CCD was used as the detector. We applied the
method of long-slit spectroscopy whereby both V4633 Sgr and a
bright comparison star were included in the slit (see for example
Kaspi et al. 2000). The comparison star used was non-variable to
within ~ 2 per cent. We used a 10-arcsec wide slit along with a
600 line mm-1 grism, yielding a dispersion of 4 A pixel-1 (- 8 A
resolution). On the first two nights the spectrograph was set to
cover the spectral range ~ 3600-7200A, while in the last two
nights we covered the range ~ 4000-7800A. Two exposures of the
spectrum of the nova were taken on each night.
Reduction of the bias-subtracted and flat-field-corrected spectra
was carried out in the usual manner using IRAF with its SPECRED
and ONEDSPEC packages. The spectra were dispersion-corrected
using a He-Ar arc spectrum, which was taken on each night in
between the pair of nova spectra. Each spectrum of the nova was
divided by the spectrum of the comparison star observed
simultaneously through the same slit. The two sets of nova/star
spectrum ratios obtained on each night were compared to each
other and were found to differ by no more than-~ 10 per cent. The
average of the two ratios was then taken as the representative ratio
for that night. The spectra were calibrated to an absolute flux scale
by multiplying each mean nova/star ratio by a flux-calibrated
spectrum of the comparison star. This spectrum, in turn, was flux-
calibrated using the WO standard sensitivity function and
extinction curve. These do not change appreciably from night to
night, and they are updated from time to time at WO using
spectrophotometric standard stars. The absolute flux calibration
has an uncertainty of ~ 10 per cent, but the relative flux
uncertainties within each spectrum are of order 2-3 per cent.
1IRAF (Image Reduction and Analysis Facility) is distributed by the
National Optical Astronomy Observatories, which are operated by AURA,Inc., under cooperative agreement with the National Science Foundation.
3 DATA ANALYSIS
Light curves of V4633 Sgr from discovery to 2000 July are
presented in Fig. 1. The visual LC was compiled using data taken
from VSNET.2 The I, V and B LCs were compiled using data
obtained in our programme. Note that the apparent small vertical
lines in the I LC are not error bars but dense individual successive
points observed in a single night. Bars representing the
observational errors in our measurements are below the resolution
limit of this figure.
The visual and V LCs show an apparent change in slope,
becoming more moderate about three months after maximum light
(Fig. 1). Most of the 1998 photometry was conducted around the
time the slope changed. Shortly after, in 1998 July-August, the
brightness of the star deviated systematically from the long-term
trend given by the fitted curve, forming an apparent bump in the LC
(Fig. 1, inset).
A panel of sample I-band LCs from different epochs is shown in
Fig. 2. Nightly LCs show almost no visible variation until 1998
May. Fragmented LCs in May show some variation, while in June
modulations on a time-scale of ~ 3 h are clearly visible. In July
and August, the variations took other forms. In a few nights the
variations are quasi-periodic but on a somewhat different time-
scale than in June. On a few other nights, the brightness of the star
varied monotonically during the entire nightly run. In all our
subsequent observations in 1999 and 2000, the variations returned
to the oscillation mode of 1998 June, albeit with an ever-increasing
amplitude.
3.1 The 1999 light curve
We first discuss the data of 1999 since this season is better sampled
than the other two. Fig. 3(C) shows the normalized power spectrum
(PS) (Scargle 1982) of our 1999 I-band data, after eliminating the
long-term decline by subtracting a fourth-degree polynomial from
the 1998-1999 LC.
The PS is dominated by two similar alias patterns around two
central frequencies, 7.7660 d- and 7.9628 d-1, corresponding to
the periodicities 0.128760 0.000013 d (hereafter P1) and
0.125589 0.000016 d (P2).
To derive the quoted periods, we performed a grid search in the
X2 space, fitting to the data a polynomial term representing the
secular decline of the nova and a pair of periods near the values of
Pi and P2 obtained from the PS. The grid was then examined to
find the pair of periods yielding the lowest value in the x2 space.
The errors of the two periods correspond to a 1 o confidence
level, and were derived by a sample of 2000 bootstrap simulations
(Efron & Tibshirani 1993).
We used the tests described in Retter, Leibowitz & Kovo-Kariti
(1998) to confirm the independence of the two periodicities. Similar
results were also obtained from the PSs of our 1999 Vand B data sets.
At the right-hand side of Fig. 3(C), the first overtone of P2 is
detected at 15.928 d-1, well above the noise level in its vicinity.
Such a feature is expected, as a result of the asymmetric shape of
the signal (see Section 3.6).
The lower end of the 1999 PS (Fig. 3C) is dominated by a
structure of interdependent peaks, the highest of which, designated
P3, is at 0.1976 d- (5.06 d) with a full amplitude of 0.096 mag.
This periodicity corresponds to the beat period between P1 and P2.
2VSNET =Variable Stars Network, Kyoto, Japan (http://www.kusastro.
kyoto-u.ac.jp/vsnet/).O 2001 RAS, MNRAS 328, 1169-1180
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Lipkin, Yaacov; Leibowitz, E. M.; Retter, A. & Shemmer, Ohad. Nova Sagittarii 1998 (V4633 Sgr): a permanent superhump system or an asynchronous polar?, article, August 23, 2001; Oxford, UK. (https://digital.library.unt.edu/ark:/67531/metadc861698/m1/2/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.