NASA NGR 44-012-209NSF GP-25901, GP-41796Astronom

Moffett, T. J.

Nather, R. E.

Vandenbo.P. A.

The Astrophysical Journal

English

Texas ScholarWorks

1974ApJ...190L..63MTHE ASTROPHYSICAL JOURNAL, 190:L63-L66, 1974 June 1 © 1974. The American Astronomical Society. All rights reserved. Printed in U.S.A. NONPERIODIC OPTICAL FLICKERING IN HZ HERCULIS T. J. MOFFETT, R. E. NATHER, AND P.A. VANDEN BouT The University of Texas at Austin and McDonald Observatory Received 197 4 January ZS; revised 197 4 March Z7 ABSTRACT ~igh-speed. s~ultaneous dual-channel ph?tometry of HZ Her and a nearby comparison star reveal nonperiodic optical ~1cker~g m the HZ :s;er sys~~ on a time scale of ~5-3°';> s. The amplitude of the flickering appears to be corre-lated with orbital phase. Optical em15s1on from a hot spot ma dISk of material around the X-ray source cannot account for the flickering. Subject headings: photometry - X-ray sources I. INTRODUCTION A number of observational and theoretical considera-tions suggest the presence of "third light" in the HZ Her/Her X-1 system. Boynton et al. (1973) require a hot component near maximum light to account for their UBVr observations. Bopp et al. (1972, 1973) find rapid variations in the XM650, 4686 emission lines. According to Joss, Avni, and Bahcall (1973) and Pringle (1973), the fact that the optical light curve is sharper and narrower than the X-ray intensity curve near minimum is due to light that originates well above the photosphere of HZ Her. Strittmatter et al. (1973) have a model for this system involving a significant contribu-tion to the total light by emission from an accretion disk surrounding Her X-1. These properties are reminiscent of the cataclysmic variable stars. In an attempt to see whether the models developed for these stars by Smak (1971) and Warner and Nather (1971) could be applied to the HZ Her system, we have tried to detect non-periodic optical flickering, common in cataclysmic variables, in the light curve of HZ Her. This effort has been successful in that HZ Her does show nonperiodic variations on a time scale of 15-300 s which are appreciably larger than the statistical and scintillation noise. However, the variation in amplitude of this flickering as a function of orbital phase rules out the hot-spot-accretion-disk model that accounts for the flickering in the cataclysmic variables. The ob-servations suggest that the "third light" originates near HZ Her on the hot side toward Her X-1 and might be the result of fluctuations in X-ray heating. II. OBSERVATIONS AND ANALYSIS Our observations were obtained with the 82-inch (208-cm) Struve reflector of the McDonald Observa-tory, using the dual-channel pulse-counting photometer (Nather 1972). No filter was inserted in the optical train, so the short-wavelength cutoff was determined pri-marily by atmospheric extinction, while the response at longer wavelengths was limited by the bi-alkali photo-cathodes of the photomultiplier tubes used. The instru-L63 ment was operated as a simultaneous two-star photom-eter, with the variable normally measured by the Amperex 56 DVP phototube in channel 1 while a near-by comparison star (located ,..,.,6' W add 1' N; V = 12.58, B - V = +0.58, U - B = +0.03) was moni-tored by the RCA 4516 phototube in channel 2. The com~arison star w::is also used for guiding, so its photo-~etnc record was mterrupted for one or two readings at mtervals of roughly 5 minutes to permit guiding cor-rections. With this exception photometric measurement was continuous,. the readings being divided into con-tiguous 4-s integrations. The two separate apertures used were aligned by al-lowing the two stars to drift through them slowly while recording the intensities of the two channel;. The resulting "light curve" was an accurate measure of the relative sizes and positions of the two apertures. The iris diaphragm in the second channel was carefully adjusted so that it was slightly smaller than, and every-where inside, the diaphragm in channel 1 to ensure that any guiding errors or seeing difficulties would be re-corded in the monitor channel for later editing. This procedure has been adopted as standard for all two-channel measurements with this instrument. The technique of simultaneous two-star measure-ment permits a direct comparison of the light curves of the two objects, in addition to certifying the reality of small changes in the variable star. Flickering was ob-served earlier in HZ Her on the same time scale reported here, but without the simultaneous measurement of a compB;rison star; since such changes might be due to changmg sky conditions or other factors, their reality was not unassailable. Earlier attempts (Groth and Nelson 1972) to detect nonperiodic variations in HZ Her met with similar difficulties. All of the observations were corrected for sky back-ground, and the effects of atmospheric extinction were removed by the application of mean extinction coeffi-cients appropriate for McDonald Observatory. The corrected data showed slow changes in level common to both variable and comparison stars, apparently the effect of variable transparency. To remove these effects, © American Astronomical Society • Provided by the NASA Astrophysics Data System 1974ApJ...190L..63ML64 T. MOFFETT, R. NATHER, AND P. VANDEN BOUT Vol. 190 we first smoothed the comparison star data by the ap-plication of a four-point running (16-s) square filter, sufficiently long to minimize the high-frequency sto-chastic and scintillation noise present but short com-pared with the transparency changes, and then divided both the variable and comparison star data by this smoothed curve. The resulting light curves for both stars are shown in figure 1, identified in orbital phase from theelementsJD0 = 2,441,644.10587 + 1~700165E In all cases the variations in the light curve of HZ Her clearly exceed those of the comparison star, which had nearly the same detected photon arrival rate as the variable, showing that flickering activity was present at the three orbital phases examined. (a) +5 Cataclysmic variables exhibit a wide variety of flickering activity arising from the mass transfer process, and in many cases the amplitude of this activity is a strong function of orbital phase. To search these data for similar behavior we must first remove the effects of other sources of variation; in this case, since the two stars had comparable counting rates, the contribution from photon arrival noise was essentially the same for both, and contributions from dark noise and from the sky were negligible. Scintillation noise contributed materially to the apparent variations in all of the runs, however, and was of differing intensity on different nights. If we assume that the comparison star has no intrinsic variations of its own, we can use its light curve, -: ~\·'.\;:':.;;\,.:• \.:::::':._ -..'.·.-,..,_ •• >:,.,."'·:~··.::·'.~· ... ~·-'' -.:. ..... ·-;"~"';.,.,, .• .... :"· ....... :~::"'::..::::·'~.:, .. -.",;: ......... ':\ ..... : /:··:":·,:...}.;..,~·-'.;'!',. /-.\·""~ ........ :..-.... 26.726 (b) 27.743 (c) +5 0 :-.,. ... :·:..: ..... _./·'··~.:, ..... -.,:_ .. :.: .. ,.., ..... :·· -5 +5 -5 28.637 26.732 26.738 26.744 27.749 27.755 27.761 I ·.· .. ··· .• -·· •. ·-. .:;,_ ... :~.ii'°',,. ...... _._.-··.·\_: ... \,,."""· .• ·':·. : .. . . .::· r - ~ •~ ..... ,, .. •/ ,.iv..·._....,,"A 28.643 28.649 28.655 DAYS (JD0 +2441900) FIG. 1.-Light curves of HZ Her (upper) and a nearby comparison star (lower) for orbital phases: (a) = 0.229, (b) = 0.835, and (c) = 0.372. The curves have been normalized by the techniques described in the text. JD0 2,441,928.637 = 1973 September 03 03hl 7m12• UT. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1974ApJ...190L..63MNo. 2, 1974 NONPERIODIC OPTICAL FLICKERING IN HZ HER L65 properly scaled, as a measure of the contribution from scintillation noise to the total observed variations. On the assumption that the percentage modulation from scintillation effects is the same for all stars-i.e., that the noise from this source is proportional to the in-tensity of the measured starlight-then variance of the variable star data ( <TF2) due only to intrinsic variations is given by ui = <THz2 - NHz - (NHz/Ne)2 (ue2 - Ne), where <THz 2 is the total variance observed; NHz and Ne are the counting rates of HZ Her and the comparison star, respectively; and ue2 is the variance of the com-parison-star data. The net intensity of the flickering in HZ Her, ex-pressed as a percentage of the total light (F = <TF/NHz) is shown, t?gether with computed orbital phase, in table 1. While the data are too sparse to draw strong c~nclusio.ns concerning the variation of the flickering with orbital phase, we note that it seems appreciably higher at p~ase 0.37 than at the other phases, and may have a maximum near phase 0.5. We also obtained data at integration times of 0.25 s (also l.isted in table 1) to permit a search for optical pulsat10ns relate? to the 1.23-s X-ray period; power-spectrum analysis of these data show no significant periodicities present at or near the expected frequency. This is consistent with the suggestion that the optical pulsing seems to be confined to orbital phases near = 0.25 and = 0. 75 (Davidsen et al. 1972). Just before or after each of the runs in table 1 we also observed HZ Her through an interference filter centered at M649 which had a transmission width (FWHM) of "'30 A. The photons detected were synchronously co-added at the X-ray pulsar rate in a multichannel s~aler with channels 40 ms wide. No significant pulsa-tions were observed to a level of 2 percent (3 u). This observation provided an easy check on the suggestion (Rees and Angel 1973) that the optical pulsing might be confined to the C m-N III emission-line complex at M650. · . TABLE 1 lnte-Beginning of gra-tion Orbital Light Curve Time Length Phase F 0D0+2,441,900) (s) (s) (midpoint) (%) 26. 708635 .......... 4 1252 0.225 1.12 26. 723289 .......... 4 2276 0.229 1.09 27. 741612 .......... 4 1876 0.835 0.81 28.633683 .......... 4 5404 0.372 1.59 19.68663 ........... .!. 2287 0.098 • 20. 735656 .......... l 1247 0.712 21.655464 .......... l 3027 0.259 28. 738833 .......... l 2120 0.422 III. DISCUSSION These observations reveal flickering in the HZ Her system, following the 1973 August 24 X-ray turn-on which is present from orbital phase 0.2 ~ ~ 0. 7 with an apparent maximum in amplitude near = 0.5. This correlation with phase rules out a hot spot in a disk of material surrounding the X-ray source as the origin of the flickering. Warner and Peters (1972) have calculated the size and location of such a hot spot for systems of various mass ratios. Their results predict maximum flickering in HZ Her orbital phases 0.75 ~ ~ 0.00, where the range is virtually independent of mass ratios (0.01 ~ Mx/MHz ~ 10.0) but allows for increased th~rmal escape velocities from the inner Lagrangian pomt due to X-ray heating, and little or no flickering at phases 0.25 ~ ~ 0.50. We emphasize that these ob-servations do not rule out models involving substantial optical emission from an entire disk of material around the X-ray source. Indeed, such systems would be ex-pected to show maximum flickering at = 0.5. The apparent maximum in the amplitude of the flickering at = 0.5, together with the observations of Boynton et al. (1973), suggest that the "third light" is associated with the X-ray heating of material close to or actually in the upper atmosphere of the hot side of HZ Her. We note that the time scales of 15-300 s are consistent with thermal cooling times of 3-30 s ob-tained by Alme and Wilson (1973) for photospheric material heated by X-rays. Tananbaum et al. (1973) report that the turn-on time for the high state of the 35-day cycle on Her X-1 oc-curred UT 1973 August 24.7 ± 1.0; therefore, all of our observations were obtained during the X-ray ON cycle. Observations during the OFF part of the 35d X-ray cycle would be useful. The absence of flickering during X-ray OFF would imply that the hard (2-20 keV) pulsed X-rays are responsible for the heating associated wit~ the fl~ckering, although not for the bulk heating which reqmres a steady energy source (Avni et al. 1973). Presence of flickering during X-ray OFF would support those models using precession of the neutron-star spin axis to account for this behavior, or else some other flickering mechanism would have to be invoked. We plan to continue our investigations of the flicker-ing behavior in HZ Her in order to obtain more com-plete phase coverage, and we urge other observers capable of simultaneous dual-channel observations to monitor this system also. This work was supported in part by NASA grant NGR 44-012-209 and grants GP-25901 and GP-41796 from the National Science Foundation. The authors wo~ld like to thank Mr. Alan Kiplinger for his generous assistance at the telescope and his advice concerning the data reduction. The clarifying suggestions of the referee are very much appreciated. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1974ApJ...190L..63ML66 T. MOFFETT, R. NATHER, AND P. VANDEN BOUT REFERENCES Alme, M. L., and Wilson, J. R. 1973, preprint. Avni, Y., Bahcall, J. N., Joss, P. C., Bahcall, N. A., Lamb, F. K., Pethick, C. J., and Pines, D. 1973, Nature Phys. Sci., 246,36. Bopp, B. W., Grupsmith, G., McMillan, R. S., Vanden Bout, P., and Wootten, H. A. 1973, Ap. J. (Letters), 186, L123. BoP-p, B. W., Grupsmith, G., and Vanden Bout, P. 1972, Ap. J. (Letters), 178, LS. Boynton, P. E., Cantera, R., Crosa, L., Deeter, J., and Gerend, D.1973, Ap.J., 186, 617. Davidsen, A., Henry, J. P., Middleditch, J,, and Smith, H. E. 1972, Ap. J. (Letters), 177, L97. Groth, E. J., and Nelson, M. R. 1972, Ap. J. (Letters), 178, Llll. Joss, P. C., Avni, Y., and Bahcall, J. N. 1973, Ap. J .. 186, 767. Nather, R. E. 1972, thesis, University of Cape Town. Pringle, 0. 0. 1973, Nature Phys. Sci., 243, 90. Rees, M., and Angel, J. R. P. 1973, private communication. Robinson, E. L., and Warner, B. 1972, M.N.R.A.S., 157, 85. Smak, J. I. 1971, Acta Astr., 21, 15. Strittmatter, P.A., Scott, J., Whelan, J., Wickramasinghe, D. T., and Woolf, N. J. 1973, Astr. and Ap., 25, 275. Tananbaum, H., Schreier, E., Levinson, R., Giacconi, R., Gursky, H., Kellogg, E., and Matilsky, T. 1973, IAU Gire., No. 2578. Warner, B., and Nather, R. E. 1971, M.N.R.A.S., 152, 219. Warner, B., and Peters, W. L. 1972, M.N.R.A.S., 160, 15. © American Astronomical Society • Provided by the NASA Astrophysics Data System

Nonperiodic Optical Flickering In Hz Herculis

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Nonperiodic Optical Flickering In Hz Herculis

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