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INTEGRAL OBSERVATIONS OF HER X-1
D. Klochkov1, R. Staubert1 , S. Tsygankov2, A. Lutovinov2 , K. P. Postnov3 , N. I. Shakura3 , S. A. Potanin3 , C. Ferrigno4 , I. Kreykenbohm1 , and J. Wilms5
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Institut fur Astronomie und Astrophysik, University of Tubingen, Sand 1, 72076 Ё Ё 2 Space Research Institute, Moscow, Russia 3 Sternberg Astronomical Institute, 119992, Moscow, Russia 4 INAF IASF-Pa, via U. La Malfa 153, 90146 Palermo, Italy 5 Department of Physics, University of Warwick, Coventry, CV8 1GA, UK

cts/s (JEM-X, 2-20 keV) cts/s (ISGRI, 20-100 keV)

ABSTRACT First results of observations of the low mass X-ray binary Her X-1/HZ Her performed by the INTEGRAL satellite in July-August 2005 are presented. A significant part of one 35 day main-on state was covered. The cyclotron line in the X-ray spectrum is well observed and its position and shape, as well as its variability with time and phase of the 1.24 s pulsation are explored. X-ray pulse profiles for different energy bands are studied throughout the observation. The pulse period is found to vary on short time scales revealing a dynamical spin-up/spin-down behavior. Results of simultaneous optical observations of HZ Her are also discussed. Key words: X-ray binaries; accretion disks; neutron stars.

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1.

INTRODUCTION

Figure 1.

Discovered in 1972 by the UHURU satellite [6, 18], Her X-1 is one of the most extensively studied accreting pulsars. Being part of a low mass X-ray binary it shows strong variability on very different time scales: the 1.24s spin period of the neutron star, the 1.7 d binary period, the 35 d period of precession of the warpted tilted accretion disk [5, 13, 8], the 1.65 d period of the preeclipse dips [6, 18]. Due to the high orbital inclination of the system (i > 80 ) the counter-orbitally precessing warped accretion disk around the neutron star blocks the observers view the X-ray source during a substantial part of the 35 d period. This gives rise to the alternation of so-called On (high X-ray flux) and Off (low X-ray flux) states. The 35 d cycle contains two on states ­ the main-on and the short-on ­ separated by 7 В 8 d off states. The X-ray flux in the middle of the main-on is 4 В 5 times higher than that in the middle of the short-on. The sharp transition from the off-state to the main-on is called the turn-on of the source. Turn-ons are usually used for cycles counting. The 35 d period manifests itself also by the changing shape of the X-ray pulse profiles [14, 19, 3, 12] and

Low resolution light curves of Her X-1. Vertical dashed lines mark X-ray eclipses. On top of the upper panel revolution numbers are indicated. The solid curve in the bottom panel shows the ASM RXTE light curve averaged over many 35 d cycles.

through the modulation of the optical light curve [5, 7]. In spite of a large amount of observational data the system is still poorly understood. Physical mechanisms responsible for the disk precession, X-ray dips and evolution of X-ray pulse profiles are highly debated. In this work we present X-ray observations of Her X-1 performed by the INTEGRAL observatory. High sensitivity complemented by very good spectral and timing resolution makes INTEGRAL exceptionally useful for performing observations which can lead to new insight into the physics which is at work in this system.

2.

OBSERVATIONS AND DATA PROCESSING

The accreting X-ray pulsar Her X-1 was observed by INTEGRAL on July 22 ­ August 3, 2005 (MJD: 53573-


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X-ray pulse profiles of Her X-1 for different energy bands and time intervals.

53585). The observations include 5 orbital revolutions of the satellite: revs. 338, 339, 340, 341 and 342. They partially cover a main-on state of the source: 35 d phases pre 0 - 0.11 and pre 0.20 - 0.28 (pre = 0 is the phase of the turn-on of the source). In our analysis we use the data obtained with the instruments JEM-X ( 20 keV) and ISGRI ( 20 keV). The data processing is performed with the version 5.1 Offline Science Analysis (OSA) software distributed by ISDC [2]. Additional gain correction of the photon energy based on the position of the tungsten background line was done. A 1% systematic error to both JEM-X and ISGRI data sets has been added 1 . For the pulse-phase-resolved spectroscopy we also used the software developed at IASF, Palermo [11]. The spectral analysis is performed with the XSPEC package v. 11.3. The results of the analysis of averaged spectra are independently checked using the software package developed at Space Research Institute, Moscow [see e.g. 21]. Figure 1 shows low resolution light curves obtained with ISGRI (top) and JEM-X (bottom). The turn-on occurred at MJD 53574.7 corresponding to orbital phase orb 0.7. The solid line on top of the JEM-X light curve represents the ASM RXTE light curve averaged over many 35 d cycles and stretched vertically to match the one from JEM-X. Pre-eclipse dips (sharp flux decrease just before the eclipse) are clearly seen. All JEM-X data from revolution 341 are rejected because of a strong solar flare.

3.

PULSE PROFILES

We extracted X-ray pulse profiles for different energy bands and time intervals. Only the data corresponding to the on-state of the source are presented (revs. 339, 340 and 341). We also excluded dips and X-ray eclipses. During the off-state (revs. 338 and 342) the data are very noisy and show marginal sinelike pulsations. It is seen (Fig. 2) that pulse profiles change significantly in shape during the period of observations. Their shape is typical for Her X-1. Analogous pulse profiles were observed by RXTE at different times [see e.g. 9]. The value of the pulse period found with the standard epoch folding method is 1.237758(36) s. This period has been used for constructing the pulse profiles and performing the pulse-phase-resolved spectroscopy (see below). Due to limited precision of the epoch folding method it cannot be used to study the short-time-scale variations of the pulse period. Such variations can be explored by measuring the relative phase shift of corresponding details between different pulse profiles. From the analysis of RXTE observations of Her X-1 it was found that the most stable and best measured feature of the pulse profile is the steep decay after the trailing "shoulder" of the main peak near pulse phase 0.9. The position of this decay does not depend on the shape or position of peaks and varies with time much less than any other detail of the pulse profile. It therefore provides a good phase reference. Figure 3 shows the pulse phase of the decay as a function of time. If the period of pulsations were constant

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http://isdc.unige.ch/index.cgi?Support+documents


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Photon index = 0.95 (+/-0.2) Cutoff energy= 25.2 (+/-1.0) keV Fold energy = 8.8 (+/- 0.3) keV Cycl. line centroid = 38.5 (+/-0.7) keV Cycl. line width = 8.6 (+1.1/-1.2) keV Cycl. line depth = 0.66 (+0.09/-0.10) Iron line centroid = 6.31 (+/-0.2) kev Iron line width = 0.8 (+/- 0.2) keV

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Figure 3.

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Pulse phase of the sharp decay after the trailing "shoulder" of the main peak used as a phase reference of the pulse period (see text). The apparent change of the slope (dashed lines) indicates a pulse period change by 2 в 10-7 s on a time scale of few days.

Top panel: the averaged X-ray spectrum of Her X-1 (best fit model parameters with 1 uncertainties are shown); middle panel: residuals after fitting the spectrum without the cyclotron line; bottom panel: residuals after fitting the spectrum with the adopted model (see text).

Figure 4.

the phase would be linear with some constant slope (with slope zero for the correct period). The apparent change of the slope (dashed lines in Fig. 2) indicates a change of the period by 2 в 10-7 s on a time scale of few days.

6.

SIMULTANEOUS OPTICAL OBSERVATIONS

4.

AVERAGED SPECTRUM

For constructing the average spectrum, we used the data from revolutions 339 and 340. During this interval the source is in the On state and both JEM-X and ISGRI data are available. As before, eclipses and dips are excluded. The resulting spectrum is modeled by a power law with an exponential cutoff (highecut in XSPEC). A gaussian emission line is added at 6.4 keV to account for the iron line. The cyclotron line is firmly observed and modeled by a gaussian absorption line. The averaged spectrum is shown in Fig. 4 along with the best fit model parameters and 1 uncertainties. 2ed = 0.78/165 d.o.f. The r position of the line (38.5 ± 0.7keV) is consistent with that measured by RXTE at a similar time [16, 17].

Optical observations of the source simultaneous with INTEGRAL have been performed with the 1.25m ZTE telescope in SAI Crimea station in July 28-30 2005 (MJD:53579-53581, pre 0.12 - 0.18). The aim was to search for rapid variability with the period of X-ray pulsations [optical 1.24 s pulsations have been reported by some authors, see e.g. 10] During our observations we failed to find any pulsations with the amplitude higher than 0.3% in the V band.

7.

DISCUSSION AND CONCLUSIONS

5.

PULSE-PHASE-RESOLVED SPECTROSCOPY

In order to measure parameters of the cyclotron line as a function of the pulse phase we have split the data into four time intervals. During each interval the pulse profile does not change significantly and can be directly compared with the line parameters behavior. The result is presented in Fig. 5. The left panel shows the position of the cyclotron line with the pulse phase, the right panel shows the same for the width of the line. Dotted lines represent the corresponding ISGRI pulse profiles. Both parameters apparently show a positive correlation with the pulse-phase-resolved flux.

The shape of the X-ray pulse profiles of Her X-1 and their evolution with time is a subject of great discussions in the literature. There are two basic models explaining their behavior. Both of them assume a complicated structure of the emitting region on the surface of the neutron star. In the first model systematic variations of the pulse profile are due to a partial obscuration of the neutron star by the inner edge of the precessing accretion disk [see e.g. 12, 1]. In the second model the evolution of pulse shapes is interpreted by free precession of the neutron star [see e.g. 19, 8]. To discriminate between these models one needs a larger "library" of high quality pulse profiles in different energy bands equally distributed in time. The INTEGRAL observations contributed substantially to this task. From the historical monitoring of the cyclotron line in the X-ray spectrum of Her X-1 there is evidence for a flux-related change of the cyclotron line energy. In particular, the RXTE observations in combination with data from INTEGRAL (this work) show an apparent correla-


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Left: The position of the cyclotron line as a function of the pulse phase for four time intervals. Dotted line shows corresponding ISGRI pulse profiles in the range 20­100 keV. Right: The same for the width of the line (-parameter of the gaussian absorption line model).

tion between the cyclotron line energy and the X-ray flux in the middle of the main-on [16, 17]. The correlation is positive, contrary to what is observed in some high luminosity transient pulsars [21, 20]. The pulse-phase-resolved analysis shows a significant variability of the line position with pulse phase, confirming previous results of Voges [22] and Soong et al. [15]. The INTEGRAL observations do not confirm the presence of a harmonic cyclotron line at 75 keV as reported by Di Salvo et al. [4].

[ [ [ [

ACKNOWLEDGMENTS [ The work was supported by the DFG grants Sta 173/31-2 and 436 RUS 113/717/0-1 and the corresponding RBFR grants RFFI-NNIO-03-02-04003 and RFFI 06-02-16025, as well as DLR grant 50 0R 0302 [ [ [ REFERENCES [1] Blum, S. & Kraus, U. 2000, A&A, 529, 968 [2] Courvoisier, T. J. L., Walter, R., Beckmann, V., et al. 2 0 0 3 , A& A, 4 1 1 , L 5 3 [3] Deeter, J., Scott, D. M., Boynton, P. E., et al. 1998, ApJ, 502, 802 [4] Di Salvo, T., Santangelo, A., & Segreto, A. 2004, Nuclear Physics B Proceedings Supplements, 132, 446 [5] Gerend, D. & Boynton, P. E. 1976, ApJ, 209, 562 [6] Giacconi, R., Gursky, H., Kellogg, E., et al. 1973, ApJ, 184, 227 [7] Howarth, I. D. & Wilson, B. 1983, MNRAS, 202, 347 [ [ [ [ [

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