A search for a 78.4 day modulation in the high energy X-ray flux observed with OSO-8 and in the U-band optical polarization is reported. It is suggested that if such a modulation does exist, it is more likely to be related to the rotation of the free modes of oscillation of the primary than to the existence of a third body in the system

Caraveo, P.

Coe, M. J.

Crannell, C. J.

Dennis, B. R.

Dolan, J. F.

Frost, K. J.

Orwig, L. E.

English

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Technical Memorandum TM 79716
(EAST--T11-79116) THE 78.4 DAY PERIOD OF 	 N79-
CYONUS 18-1 (NASA) 11 p HC 1102/bF 1101
CSCL 03s
Unclas
'	 U3/89 17933
I
The 78.4-Day Period of Cygnus
XR-1
J. F. Dolan, P. Caraveo, M. J. Coe,
C. J. Crannell, B. R. Dennis, K. J. Frost and
L. E. Orwig
FEBRUARY 1979
National Aeronautics and
Space Administration
Goddard Spans FILM Center
Greenbelt, Maryland 20771 o
5{
f
NASA/GSFC
ABSTRACT
We report here a search for a 78.4-day modulation in the high-energy X-ray  flux observed with
OSO-8 and in the U-band optical polarisation observed by Dolan, with negative results. We suggest
that if such modulation does exist, it is more likely to be related to the rotation of the free modes
of oscillation of the primary than to the existence of a third body in the system.
ii
4
CONTENTS
4
Page
Abstract....................................................................	 ii
The 78.4-Day Period of Cygnus XR-1 ............................................... 1
References............................. ..................................... 	 6
t,
FIGURES
Figure
1 'f.-ray count rates observed from Cyg XR-1 plotted modulo the 78.4-day period of
Kemp et al.' (a) Individual observations of the 23 to 153 keV count rates taken in
1976 (crosses) and 1977 (circles). (b) Weighted mean count rate of the observations
in (a) over intervals of 0.1 in phase. (c) The 3 to 6 keV flux observed by Kemp et at. 3 .... 4	 4
2 The polarisation of HDE 226868 observed in the U-band filter folded modulo the
78 A-day period of Kemp et al, 3 (a) The weighted means of the polarisation observed
1973 to 1975 by Dolans binned in intervals of 0.1 in phase. (b) Modulation in the
Stokes parameter Qo in the U-band observed by Kemp et al. 3 ....................... 5
iii
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i
THE 78.4-DAY PERIOD OF
CYGNUS XR-1
Cygnus XR-1 /HDE 226868 is a multiple star system in which the secondary is the best astro-
nomical candidate for a black hole. The existence of a third body in the system could affect the
mass derived for the secondary from the analysis of spectroscopic radial velocity curves.' Hence, 	
_.:.•?
the presence of a periodicity in the system other than that of the 5.6-day binary orbit bears strongly
upon the identification of the secondary with a black hole, especially if such a periodicity can be
related to the orbital period of a third body. The existence of a 78A (or 39.2) day periodicity in
the optical and low-energy x-ray wavelength region of the emission from this system has been
reported by Kemp et al."A The existence of a39.2-day modulation in the optical and low-energy
x-ray luminosities has been questioned by Walker et al.' We report here a search -for a 78.4-day
modulation in the high-energy x-ray flux observed with OSO-8 and in the U-band optical
polarisation observed by Dolan', with similar negative results. We suggest, however, that if such
modulation does exist, it is more likely to be related to the rotation of the free modes of oscillation
of the primary than to the existence of a third body in the system.
Analysis of the power spectrum of the U-band polarisation of HDE 226868 on 225 nights
during the years from 1974 througn 1977 led to the original detection by Kemp et al.' of a
39.2-day modulation in the magnitude of polarisation with peak-to-peak amplitude of 0.25% (or 	
1
0.0054 mag.). A modulation with a 78.4-day period was detected by Kemp et al. I in the power	
r
spectrum analysis of the U-band photometry of the system taken during 43 nights in 1974 and 84
nights in 1977. The peak-to-peak modulation was approximately 0.0 15 magnitudes. A reanalysis
of the ultraviolet-filter photometry at the 78.4-day period produced a double-peaked modulation;
the 0.0054 magnitude modulation in the polarisation was equivalent to a peak-to-peak amplitude of
approximately 8% of the mean polarisation. In a similar analysis of the power spectrum of the
x-ray flux in the energy range 3 to 6 keV from Cyg XR-1 observed between October 1974 and
December 1977 with the All-Sky Monitor on Ariel V, Kemp et a1. 3 detected a 78.4-day modu-
lation with peak-t*VA amplitude of approximately 15% of the mean Aux. The modulation
appears to be in phase with the U-band photometric modulation. This periodic modulation was
detected only when the x-ray source was in an intensity `low state."
Cygnus XR-1 was observed with the high energy x-ray spectrometet on OSO-8 during periods
of 10 to 20 days in each of the years from 1975 to 1977 (ref. 7). The source exhibited a transition
from the intensity `low state" to the intensity "high state" during the observation in 1975 (ref. 8).
Consequently, only the data obtained in 1976 and 1977, when the source was continuously in a
low state, were analysed for a 78.4-day modulation. The counting rates from the source between
23 and 153 keV, binned in intervals of one-tenth the 5.5995-day binary period, were folded
modulo the 78.4-day period with zero phase as given by Kemp et al.3 (Fig. 1 a). The weighted
mean counting rate over intervals of 0.1 in phase are shown in Fig. I b for direct comparison with
the 3 to 6 keV x-ray data of Kemp et al. (Fig. l c). The modulation observed by Kemp et al. at
lower energies is not immediately apparent in our data. The variations evident in Fig. 1 b have a
peak-to-peak range of 19% of the mean flux between 23 and 153 keV. If any 78.4 day modulation
exists in the high-energy x-ray flux from Cyg XR-1, it must be at an amplitude significantly lower
than this. Because of the irregular spacing of the data and the fact that all observing runs were short
compared to the 78.4-day period, no more stringent limits could be placed on the existence of a
78.4-day modulation by the use of power spectrum analysis.
Observations of the polarisation of HDE 226868 in the U-band were obtained over 23 nights in
1973, 1974 and 1975 by Dolan-' The observations were folded modulo the 78.4-day period with
the same zero phase as above. The weighted mean polarisation, in intervals of 0.1 in phase, is shown
in Fig. 2a. The modulation detected by Kemp et al.' is shown for comparison in Fig. 2b. The
peak-to-peak amplitude of the variations we see is 34% of the mean polarisation. Variations
typically of this magnitude were ascribed by Dolan' to Rayleigh scattering of light from inter-
componrnt gas streams known to exist in the system. A chi-squared test shows that the data points
making up the polarisation light curve are consistent with a random distribution about either a non-
varying polarisation or the polarisation curve fitted to their data by Kemp et al.' normalised to our
I
2
mean value. Once again, the irregular spacing and greatly varying statistical weights of the
individual observations preclude the use of a power spectrum analysis to set more stringent limits
on any periodic modulation.
Kemp et at. 3 propose a physical interpretation of a 78.4-day period in terms of either a
ternary system, the precession of an accretion disk, or some kind of pulsation. These authors
discuss at some length the three body possibility, although they point out that their analysis of
spect oscopic radial velocity observations of the visible primary limits any third body in the system
:p
with a 78.4-day period to a mass of less than (0.8 f 0.6) Mo. We believe that a more likely inter-
pretation of a 78.4-day periodicity in the system, if confirmed, is related to the funcamental rotatior
properties of the free modes of oscillation of the primary. The non-degenerate star is believed to
be pulsating in a broad array of gravitational modes (g-modes), global in scale and lacking spherical
symmetry (cf. ref. 9). Under slow rotation and sufficient non-linear coupling, the g-modes can
combine to form a much smaller set of non-linear modes departing from azimuthal symmetry. As
the main anti-nodes of these non-linear modes rotate past each other, the oscillatory power density
at these locations is temporarily enhanced. The occurrence of transitions between intensity states
of the x-ray source is ascribed by Dolan et al.' to sudden changes in the amount of mass transferred
whenever the main anti-nodes are aligned with the sub-secondary point on the surface of the
primary. In this picture, the 78.4-day period would be the time between effective alignments of
certain g-modes.
I
As noted by Kemp et al. 3 , excess mass transfer between the binary components modulated
with a 78.4-day period could explain all the modulation they see in the x-ray and UV emission. The
continued existence of this period over a duration of three years implies that some forcing function
may be maintaining ;.c. The 78.4-day period is an exact integral multiple of the 5.6-day orbital
period of the secondary"; tidal forces occurring every periastron passage may provide such a
forcing function. Continued observations are necessary to determine whether the 78.4-day
modulation can be confirmed, and whether it can be related to the rotation properties of the
primary's free modes of oscillation.
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REFERENCES
I
1. Bahcall, J. N., Dyson, F. J., Katz, J. I., and Paczynski, B. Astrophys J., 189, L17 (1974).
2. Kemp, J. C., Herman, L. C., Rudy, R. J., and Barbour, M. S. Nature, 270, 227 (1977).
3. Kemp, J. C., Herman, L. C., and Barbour, M. S. Astron. J.; 83, 962 (1978).
4. Walker, E. N., Watson, M. G., and Holt, S. S. Nature, 270, 229 (1977).
5. Dolan, J. F., Astroph.ev. Space Sci., 52, 201 (1977).
6. Dennis, B. R., Frost, K i., Lencho, R. J., and Orwig, L. E. Space Sci. Instr. 3,325 (1977).
7. Dolan, J. F., Crannell, C. J., Dennis, B. R., Frost, K. J., and Orwig, L. E. Astrophys. J., in press
(1979).
8. Dolan, J. F., Crannell, C. J., Dennis, B. R., Frost, K. J., and Orwig, L. E. Nature, 267, 813
(1977).
9. Wolff, C. Astrophys. J., 193,721 (1974); ibid, 205, 612 (1976); Wolff, C. L., and Kondo,
Y. ibid, 219, 605 (1978).
6


The 78.4 day period of Cygnus XR-1

The 78.4 day period of Cygnus XR-1

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