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Post Info TOPIC: Hercules X-1
Blobrana


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RE: Hercules X-1
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Title: Hercules X-1: variations of the cyclotron line energy with flux, with phase and with time
Authors: Ruediger Staubert

Her X-1 is one of the the most intensively studied Accreting X-ray Binary Pulsars (AXBPs). This is largely because it is a bright and persistent X-ray pulsar, showing the largest variety of observable phenomena, partly due to the favourable geometry in observing the binary nearly edge-on. Her X-1 is the binary X-ray pulsar in which the first cyclotron line (or Cyclotron Resonant Scattering Feature - CRSF) was detected in 1975. Since then, we have made an effort to observe the source and the CRSF around 40 keV as often as possible, using those X-ray satellites which cover the high energy X-ray range. Extended contributions are from the Rossi X-ray Timing Explorer (RXTE), {INTEGRAL, Suzaku and Swift. The most recent contribution with a new degree of quality is from the high energy imaging telescope on NuStar. We have found that the CRSF is variable with flux, with phase (both pulsational and precessional phase) and with time. The historical evolution of the pulse phase averaged CRSF centroid energy Ecyc since its discovery is characterized by an initial value around 35 keV, an abrupt jump upwards to beyond ~40 keV between 1990 and 1994 and an apparent secular decay thereafter. Much of this decay, however, was found to be due to an artefact, namely a correlation between Ecyc and the X-ray luminosity Lx discovered in 2007, amounting to a change of ~7% in energy for a factor of two in luminosity. In observations since 2007, however, we now find a statistically significant trend of a true secular decrease of the cyclotron line energy. In addition, we discuss recent results of pulse phase spectroscopy and evidence for the suspected variation in Ecyc with phase of the 35 d precessional period (which is observed both in the modulation of the X-ray flux and in the systematic variation in shape of the 1.24 s pulse profile.

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Blobrana


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Title: The Mass of the Compact Object in the X-Ray Binary Her X-1/HZ Her
Authors: M.K. Abubekerov, E.A. Antokhina, A.M. Cherepashchuk, V.V. Shimanskii

We have obtained the first estimates of the masses of the components of the Her X-1/HZ Her X-ray binary system taking into account non-LTE effects in the formation of the H_gamma absorption line: mx=1.8 solar masses and mv=2.5 solar masses. These mass estimates were made in a Roche model based on the observed radial-velocity curve of the optical star, HZ Her. The masses for the X-ray pulsar and optical star obtained for an LTE model lie are mx=0.85±0.15 solar masses and mv=1.87±0.13 solar masses. These mass estimates for the components of Her X-1/HZ Her derived from the radial-velocity curve should be considered tentative. Further mass estimates from high-precision observations of the orbital variability of the absorption profiles in a non-LTE model for the atmosphere of the optical component should be made.

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Blobrana


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Title: Does the neutron star in Her X-1 really show free precession?
Authors: R. Staubert, D. Klochkov, D. Vasco, K. Postnov, N. Shakura, R. Rothschild, J. Wilms

The accreting X-ray pulsar Her X-1 shows two types of long-term variations, both with a period of 35 days: 1) A modulation of the flux with a ten day long Main-On and a 5 d long Short-On, separated by two Off-states, and 2) A systematic variation of the shape of the 1.24 s pulse profile. While there is general consensus that the flux modulation is due to variable shading of the X-ray emitting regions on the surface of the neutron star by the precessing accretion disk, the physical reason for the variation of the pulse profiles had remained controversial. Following the suggestion by Truemper et al. (1986) that free precession of the neutron star may be responsible for the variation of the pulse profiles, we had developed physical models of strong feedback interaction between the neutron star and the accretion disk in order to explain the seemingly identical values for the periods of the two types of variations. In a deep analysis of pulse profiles observed by several different satellites over the last three decades we find now that the clock behind the pulse profile variations shows exactly the same erratic behaviour as the turn-on clock, even on short time scales (100 d), suggesting that there may in fact be only one 35 d clock in the system. If this is true, it presents a serious challenge for the idea of free precession of the neutron star and calls for alternative physical models for the variation in pulse shape.

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Blobrana


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Title: Her X-1: the positive cyclotron line energy / luminosity correlation
Authors: D. Vasco, D. Klochkov, R. Staubert

Studies of some bright, super-Eddington transient pulsars show a negative correlation between the energy of the cyclotron resonance scattering feature (CRSF) and the bolometric luminosity. For Her X-1, using repeated RXTE observations during 1996-2005, the inverse dependence was found: the energy of the cyclotron line increases as the luminosity increases. The X-ray flux measured by the RXTE/ASM (2-10 keV) has been assumed to represent the luminosity - more precisely: the maximum X-ray flux reached during the respective 35 d Main-On. Here, we question whether the ASM flux is really an accurate measure of the bolometric luminosity of the source. We redetermined the energy of the cyclotron line and performed spectral fits using the combined data from the PCA (3.5-60 keV) and HEXTE (20-75 keV) instruments on RXTE of the same 35 d cycles as used in the original work to determine the bolometric flux from those spectra. We confirm the result of the original analysis that the cyclotron line energy changes by ~7% for a change in flux by a factor of two.

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Blobrana


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Title: Updating the orbital ephemeris of Her X-1; rate of decay and eccentricity of the orbit
Authors: R. Staubert, D. Klochkov, J. Wilms

We present an update of the orbital ephemeris of the binary X-ray pulsar Her X-1 (4U1656+35) and determine an improved value for the rate of orbital decay. In addition, we report the first measurement of the orbital eccentricity. We have analysed pulse timing data of Her X-1 from X-ray observations by RXTE (Rossi X-Ray Timing Explorer) and INTEGRAL over the period 1996-2007. Accurate pulse arrival times were determined from solar system barycentred photon arrival times by generating pulse profiles averaged over appropriately short integration times. Applying pulse phase connection techniques, it was possible to determine sufficiently accurate local ephemeris data for seven observation periods distributed over 12 years. Combining the new local T90 values with historical values from the literature we update the orbital ephemeris of Her X-1 to T90 = MJD 46359.871940(6) and Porb = 1.700167590(2) d and measure a continuous change of the orbital period of dPorb/dt = -(4.85 ±0.13) x 10^-11 s/s. For the first time, a value for the eccentricity of the orbit of Her X-1 is measured to be e = (4.2 ±0.8) x 10^-4.

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