2A0526-328: The white dwarf rotation period revealed - Springer Link

2A0526-328:

THE WHITE DWARF ROTATION PERIOD REVEALED

J. Schrijver, A.C. Brinkman, H. van der Woerd, Laboratory for Space Research, Utrecht, The Netherlands Watson, X-ray Astronomy Group,

M.G.

A.R. King, Astronomy Department, J. van Paradijs, Astronomical Institute The Netherlands

University of Leicester,

University of Leicester,

"Anton Pannekoek",

M. van der Klis, Space Science Department, The Netherlands

Estec,

U.K.

U.K.

Amsterdam,

Noordwijk,

ABSTRACT. We report results from EXOSAT observations of the intermediate polar system 2A0526-328 (TV Col). The hard X-ray emission (2-8 keV) is modulated with a period of 1943 s, interpreted as the white-dwarf rotation period. Soft and hard X-ray emission show intensity minima, in phase with the orbital period of 0.2286 days; analysis of the hard X-ray spectra shows that these minima are caused b y an extra low-energy absorption corresponding to a H column density of % x 1022 cm -2.

i.

INTRODUCTION

2A0526-328 (TV Col) belongs to a subclass of cataclysmic variables, referred to as 'intermediate polars' (Warner 1983). As in the case of polars (AM Her type objects), these objects are thought to be close binaries consisting of a strongly magnetized white dwarf and a low-m~ss companion star, which transfers matter to the white dwarf b y Roche-lobe overflow. In the intermediate polars the magnetic field of the white dwarf, although still important, is presumed not to play the overwhelming role as it does in polars. In particular there is no synchronicity between the orbital and white-dwarf rotation motions. Still, the magnetic field is important enough to prevent a complete accretion disk to be formed, and to force the transferred matter along the field lines. Therefore, the X-rays are expected to be modulated with the white-dwarf rotation period. 2A0526-328 has aroused considerable interest because of its

Space Science Reviews 40 (1985) 121-126. 0038-6308/85.15. 9 1985 by D. Reidel Publishing Company.

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J. SCHRIJVER ET AL.

complex periodicity. By optical photometry periods of 0.2163 days and 4.02 days were found (Motch, 1981). Radial velocity measurements in strong emission lines in the optical spectrum resulted in the detection of the 'spectroscopic' period of 0.2286 days (Hutchings et uS., 1981). In the UV spectrum the spectroscopic period was also found to be present (Bonnet-Bidaud e$ ub. 1984). The spectroscopic period has been interpreted as the binary system orbital period. Some authors did interpret the optical period of 0.2163 days as the white dwarf rotation period. The long period of 4.02 days is the beat period of the short optical period and the spectroscopic period.

2.

OBSERVATIONS

We have observed 2A0526-328 twice in November 1983 using the EXOSAT ME and LE experiments. The first observation started 26 November 1983 19:00 U.T. and lasted about 22000 s, the second one started 28 November 1983 01:15 U.T. and lasted 26000 s. The ME observations were done in the standard offset mode, with one half of the detectors pointed towards the object and the other half monitoring the background. Pulse height spectra were obtained each I0 s. The LE measurements were done using the Channel Multiplier Array with the Lexan 3000 ~ and Al/parylene filters.

3.

ANALYSIS AND RESULTS

3.1. White-dwarf

rotation period

The main result of the analysis of the ME signal is the detection of a periodic modulation of about 20% (Brinkman and Schrijver 1984). Figure 1 shows the 2-8 keV background-subtracted lightcurve folded modulo the most likely period of 1943 s, showing a quasi-sinusoidal behavioUr. The data sets from the two separate observations allow the determination of the period with a precision of about 20 s. Due to the gap between the two observations, the combined data set gives rise to a number of peaks, when the X 2 is plotted versus the folding period, as seen in Figure 2. Although 1943 s is the most likely candidate for the period, 1911 s cannot be excluded. The observed periodicity also stands clearly out in the Fourier power spectrum of the data set (Figure 3 ). The signal obtained in the LE detector is too ~eak to show modulations at a comparable level. This 32-minute period is interpreted as the rotation period of the white dwarf. Its value is of the same order of magnitude as found in other intermediate polar systems (Warner 1983, Watson e$ u~. 1984, C o o k e~ u~. 1984).

2A0526-328: THE WHITE DWARF ROTATION PERIOD REVEALED

123

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Figure I. 2-8 keV 1943 s.

150

lightcurve

of 2A0526-328

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Figure 2. X 2 as of folding period. Lgo0

function

' 20()0 Period

(s)

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2 Frequency

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4

(mHz)

Figure 3. Power spectrum of the ME light curve, corrected effects. Arrows indicate spectroscopic period ( 0. 2286 newly discovered period ( 19@3 s ).

for window days ) and

124

J. S C H R I J V E R E T A L

3.2. Orbital period Both in the ME and the LE data we observe two minima in the flux (one of them is shown in Figure 4). The time and extent of the minimum in the LE data cannot be accurately determined due to the large statistical uncertainty. The minimum in the ME light curve appears to be sharper and occurs during the LE minimum. The time difference between the two observed minima is consistent with five 'spectroscopic' periods. Also, there is a maximum in the Fourier power spectrum of the ME data set at this period. The minima occur at phase 0.9 • 0.i in the ephemeris of Hutchings e~ ub. (1981). Although the period of this slower modulation cannot be determined with high precision, it is certainly not consistent with the 'photometric' period of 0.2163 days. 41

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Figure 4. ME and LE light occurs around 23 :40 UT.

22 h 23 h UT ( 2 6 N o v 1 9 8 3 )

curves

for

first

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1h

observation.

Minimum

The X-ray spectra give more information about the nature of these minima. The X-ray flux measured by the ME detectors is sufficiently high to permit the determination of the spectral parameters. As it is the case in other intermediate polars (Warner e$ uD., 1983, Watson et uD., 1984) the spectra are rather hard, with a large low-energy absorption. We have accumulated spectra for different phases of the spectroscopic period (Figure 5). The background was subtracted by using spectra obtained during the slews before and after the measurement. The spectra can be fitted well to a power law spectrum

2A0526-328: THE WHITE DWARF ROTATION PERIOD REVEALED

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Figure 5. O b s e ~ e d count rate s ~ c t r ~ for normal flux (solid line) ~d during min~um (dash~ line), showing e~ra low-ener~ ~sorption. Inset shows 2-8 keY l i ~ t c u ~ e folded modulo 0.2286 days, with phases where s ~ c t r a were t ~ e n .

126

J. SCHRIJVER ET AL

with a photon index of 1.2 • 0.2 and a low-energy absorption. The column density corresponding to this absorption varies significantly during the 0.2286 day period. The 'normal' spectra, obtained during flux maximum, are consistent with a column density, NH, of (2 • .5) x 1022 cm -2. During the minimum, however, N H becomes (6 • .5) x 1022 cm -2. The minima can thus be explained by an extra column with density of 4 x 1022 cm -2 moving into the line of sight at a given binary phase. This would imply that the minima are connected with the binary period, and confirm that the spectroscopic period is the orbital period.

4.

CONCLUSIONS

The observation of 2AO526-328 by EXOSAT has led to the detection of two modulations in the X-ray signal. The periodic 32-minute modulation is interpreted as the white-dwarf rotation period; the time difference between the two X-ray minima is consistent with the binary system orbital period. The rotation period now detected makes this system fit well in the class of intermediate polars (periods of 5 - 30 minutes). The spectral parameters deduced also fit into the general characteristic of this group: hard, flat power spectra with large low-energy absorption. There is absorbing matter moving into and out of the line of sight with the orbital motion of the system. The observation of the true white-dwarf rotation period reopens the question of the origin of the photometric period of 0.2163 days and the beat period of 4.02 days.

ACKNOWLEDGEMENT We thank members of the Leicester X-ray Astronomy Group and of the Utrecht computer department for providing us the necessary software for the analysis of the data.

REFERENCES

Bonnet-Bidaud, J.M., Motch, C., Mouchet, M., 1984, As~r. As~roph~s., submitted Bri~, A.C., Schrijver, J., 1984, [AU CSrc. 3980 Cook, M.C., Watson, M.G., McHardy, I.M., 1984, Mon. No$. R. AsSt. Soc. 210, 7P Hutchings, J.B., Crampton, D., Cowley, A.P., Thorstensen, J.R., Charles, P.A., 1981, As$rophys. J. 249, 680 Motch, C., 1981, AsSt. As~rophys. 1OO, 277 Warner, B., 1983, Cataclysmic Variables and Related Objects, eds. M. Livio and G. Shaviv, Dordrecht, 155 Watson, M.G., King, A.R., Osborne, J., 1984, preprint