Engineering topics

Verbunt, F.

Publications and source records attributed to Verbunt, F..

10 records

NASA NTRS1993

Further ROSAT measurements of the period of 4U 1820-30

We have made two new observations of 4U 1820-30 with the ROSAT position sensitive proportional counter (PSPC). The measurements do not provide further evidence for a secular period decrease of this source. Instead, after inclusion of our new arrival time measurements the significance of the orbital period derivative drops (from 5.5 to 4.6 sigma), its best-fit value decreases (to P-dot/P =(-5.3 +/- 1.1) x 10(exp -8)/yr) and the formal probability that statistical fluctuations have caused the observed P-dot increases to 0.5% (from 0.003%). We detect light curve changes that are large enough to be the cause of some (or all) of the period changes seen in 4U 1820-30, but not large enough to explain the discrepancy of the observed period derivative with that theoretically predicted. We explore the possibility that the secular period change observed during 1976-1991 could have been dominated by random or systematic changes in the position and shape of an occulting bulge on the disk rim and conclude that phase shifts caused by this mechanism could in principle explain the discrepancy entirely.

Klis, M. Van Der↗

NASA NTRS1993

Simultaneous ROSAT/Ginga observations of 4U 1820-30

We have made simultaneous Ginga LAC and ROSAT PSPC observations of 4U 1820-30. The 685-s orbital light curves obtained with the two instruments are very similar, indicating that the energy dependence of the orbital modulation is small. Our measurements extend the baseline over which the period variations can be measured to 15 yr. The previous possibility, that the changes in the period are themselves periodic, with a period of about 8 yr, is no longer preferred over a constant P(dot). Over the interval 1976-91 the period has decreased, rather than increasing as predicted by the standard model for the orbital evolution of the binary. The average period derivative P(dot)/P was (-0.88 +/- 0.16) x 10 exp -7/yr, different by 11 sigma from the predicted value. Under the assumption that there are no intrinsic changes in the light curve that mimic a period change, we discuss three possible explanations.

Van Der Klis, M.↗

NASA NTRS1991

Observations of Scorpius X-1 with IUE - Ultraviolet results from a multiwavelength campaign

IUE UV results are presented for the low-mass X-ray binary Sco X-1. Models that predict UV continuum emission from the X-ray-heated surface from the companion star and from an X-ray illuminated accretion disk are adjusted for parameters intrinsic to Sco X-1, and fitted to the data. X-ray heating is found to be the dominant source of UV emission; the mass-accretion rate increases monotonically along the 'Z-shaped' curve in an X-ray color-color diagram. UV emission lines from He, C, N, O, and Si were detected; they all increase in intensity from the HB to the FB state. A model in which emission lines are due to outer-disk photoionization by the X-ray source is noted to give good agreement with line fluxes observed in each state.

Vrtilek, S. D.↗

NASA NTRS1991

The orbital ephemeris and eclipse transitions of the low-mass X-ray binary EXO 0748 - 676

Using the eclipses as fiducial markers, an updated ephemeris for EXO 0748 - 676 is derived and evidence is found that between February 1985 and March 1989 the 3.82-h orbital period of EXO 0748 - 676 decreased with a time scale of -5 x 10 to the 6th yr. The sense of this change is the same as that predicted by simple models for the evolution of low-mass X-ray binaries containing main-sequence companions, but is a factor about 100 faster than expected. This rapid change in orbital period could result from the expansion of the companion due to the effects of X-ray heating. The eclipse transition durations are variable, with the shortest observed taking 1.5 s and the longest 40 s. This latter figure is about an order of magnitude too large to be due to absorption effects in the atmosphere of the secondary assuming a Roche geometry and likely stellar temperature. Either flaring activity or the presence of an X-ray heated evaporative wind or a corona may enhance the scale height of the companion's atmosphere producing the extended eclipse transitions.

Parmar, A. N.↗

NASA NTRS1990

Observations of Cygnus X-2 with IUE - Ultraviolet results from a multiwavelength campaign

IUE observations of Cyg X-2 obtained during June and October 1988 are reported and analyzed, with reference to data from simultaneous X-ray and radio observations and the predictions of theoretical models. The results are presented in extensive tables and graphs and characterized in detail. The UV flux is shown to increase monotonically as the X-ray emission moves along the Z-shaped trajectory from the horizontal to the flaring branch and the estimated mass accretion rate increases from about 6 x 10 to the -9th to 1.6 x 10 to the -8th solar mass/yr. Model calculations indicate that X-ray illumination of the accretion disk accounts for at least 80 percent of the observed UV emission, with 20 percent or less from the X-ray-heated surface of the companion star and less than 1 percent from the cool surface.

Vrtilek, S. D.↗

NASA NTRS1989

Observations of Cygnus X-2 with IUE: Ultraviolet results from a multiwavelength campaign

The observations of the low-mass x ray binary, Cyg X-2, taken with the International Ultraviolet Explorer (IUE) in a campaign conducted in June and October of 1988 are reported. A direct relationship between the strength of the UV continuum and line emission and the placement of the x ray spectrum in one of three branches of the so-called Z-shaped curve is found by comparison with simultaneous x ray observations. All three previously known x ray spectral states are detected; the UV continuum and line emission increase monotonically along the Z with the least emission in the horizontal branch, and the most in the flaring branch. Emission lines due to HeII, CIV, NIII, NIV, NV, SiIV, and MgII are observed.

Vrtilek, S. D.↗

NASA NTRS1989

Evolution of wide binary millisecond pulsars in globular clusters

The evolution of a binary radio pulsar in a wide orbit about a low-mass degenerate dwarf companion in a globular cluster environment is investigated. An extensive series of numerical scattering experiments is used to study encounters between such a pulsar and passing cluster field stars. A wide range of initial orbital periods from 3 to 1000 days is considered with a corresponding range of masses for the degenerate dwarf companions. The calculative results are summarized in the form of scattering cross sections. In particular, cross sections are presented for three final 'channels' in which the end product of the encounter is an isolated neutron star. Cross sections for inducing various orbital eccentricities into initially circular orbits are also derived. The implications of the results for the formation of the single millisecond pulsars in M28 and M15 and for the nearly circular obit of the M4 binary radio pulsar are discussed.

Rappaport, S.↗

NASA NTRS1985

A systematic study of magnetic braking in low-mass binaries

A short summary is given of Rappaport et al. (1983) which described results of extending a simplified stellar evolution code covering the evolution of low-mass compact binaries. Magnetic braking is probably an important process in the evolution of such binaries (such as cataclysmic variables and low-mass X-ray sources). The initial simplified code describes the mass-losing star as an n = 3/2 polytrope and was developed to study the evolution of binaries with a secondary of low mass (between 0.01 and 0.4 solar mass) when the angular momentum losses are due to gravitational radiation. In the extended code, a composite polytrope model is used for the secondary, wherein the structure of the radiative core is described by an n = 3 polytrope and the convective envelope by an n = 3/2 polytrope.

Verbunt, F.↗

NASA NTRS1983

A new technique for calculations of binary stellar evolution, with application to magnetic braking

The development of appropriate computer programs has made it possible to conduct studies of stellar evolution which are more detailed and accurate than the investigations previously feasible. However, the use of such programs can also entail some serious drawbacks which are related to the time and expense required for the work. One approach for overcoming these drawbacks involves the employment of simplified stellar evolution codes which incorporate the essential physics of the problem of interest without attempting either great generality or maximal accuracy. Rappaport et al. (1982) have developed a simplified code to study the evolution of close binary stellar systems composed of a collapsed object and a low-mass secondary. The present investigation is concerned with a more general, but still simplified, technique for calculating the evolution of close binary systems with collapsed binaries and mass-losing secondaries.

Rappaport, S.↗

NASA NTRS1980

Spectroscopic observations of the optical counterpart of Centaurus X-4

The optical spectrum of the transient X-ray burst source Centaurus X-4 was observed about 5 weeks after the source reached its maximum. The brightness of the optical counterpart had decreased to V = 18.2, and the star had become appreciably redder (B - V = 0.7) compared to its color at maximum. The spectrum of Centaurus X-4 is similar to that of cataclysmic variables showing strong emission lines of H-1 and weaker lines of He-1 and He-2. The N03 lambda 4640 line is not visible. The continuum energy distribution of Centaurus X-4 shows the presence of a main-sequence star in the system, with spectral type between K3 and K7. This is consistent with the orbital period of 8.2 hr proposed by Kaluzienski et al (1980), if the main-sequence star is close to filling its Roche lobe.

Van Paradijs, J.↗