1. Introduction

Her X-1 is an eclipsing binary system, with a neutron star (NS) primary and an A/F secondary star. The spin period is ~1.24 s and the orbital period ~1.7 d (Tananbaum et al. 1972, Giacconi et al. 1973). Her X-1 varies in X-rays on a period of 35 d, with a “main-on” state lasting ~10 d and a “short-on” state lasting ~5 d, each separated by a period of ~10 d of lower intensity. The 35 d cycle is though to be due to precession of a tilted, warped accretion disk that periodically obscures the X-rays from the central NS (Gerend & Boynton 1976, Coburn et al. 2000).

During the 35d cycle, the pulse profile shows a systematic evolution. Extensive observations have been made in X-ray, UV and optical bands using Ginga, ASCA and HST (Deeter et al. 1998, Boroson et al. 1996, Leahy et al. 2000).

During the main-on state, the broadband spectrum consists of a blackbody at ~0.1 keV plus a power law (PL) with exponential cut-off at higher energies. The galactic column density is low (NH ~1019 cm-2). In addition, it shows: i) a Gaussian feature at ~1 keV; ii) a Fe line at ~6.4 keV; iii) a cyclotron absorption line at ~40 keV (B ~3 x1012 G) (see e.g. McCray et al. 1982, Oosterbroek et al.1997, Dal Fiume et al. 1997).

Spectra taken from data outside the main-on state can be fitted by adding one or more partial covering absorbtion components (NH ~1022-1023 cm-2) to the model above. This is what is expected, for instance, during occultation phases of the NS by the accretion disk.

XMM-NEWTON EPIC OBSERVATIONS OF Her X-1G. Ramsaya, S. Zanea, M.Jimenez-Garateb, J.W.den Herderc, C.J.Haileyd

2. Observations

Her X-1 was observed using XMM-Newton at 3 epochs during the 35 d cycle (Table 1). Figure 1 shows the RXTE ASM quicklook light curve.The first observation was close to the main-on state, the third close to the short-on state and the second after the end of the main-on. This poster presents observations made using the EPIC pn and MOS1 detectors, both of which were configurated in timing mode.

A broad Fe line is detected at ~6.4 keV. Its normalization and EW (~0.2 keV) do not vary over the 35 d cycle. However, the line width changes significantly: when the source intensity is larger, the FWHM increases by a factor 5-10 (from ~0.02-0.05 at 35=0.26,0.6, to ~0.3 at 35=0.17). The Fe line is probably due to near neutral Fe (Fe XIV or colder) in the low state and short-on observations, whereas in the main-on the line energies (6.52±0.03 keV for MOS , 6.50±0.02 keV for PN) correspond to Fe XX-Fe XXI.

Line broadening and centroid displacements may be explained by: 1) an array of Fe K fluorescence lines, for a variety of charge states (anything from Fe I -Fe XIII to Fe XXIII); 2) Comptonization from a hot corona with a significant optical depth for a narrower range of charge states centered around Fe XX. Similar line broadening have been observed in some Low Mass X-Ray binaries with ASCA (Asai et al. 2000).

The Fe line broadening may also be due to Keplerian motion, if the inner disk comes into view during the main-on. The inferred Keplerian velocity is ~13000 km/s at 35= 0.17 that, for a NS mass of 1.4 M*, gives a radial distance of ~ 4x108 cm. This is close to the magnetospheric radius for B ~1012 G.

3. Spin Resolved Light Curve

a) Observations and interpretation

We first corrected the arrival time of each photon to the solar system barycenter and then the center of mass of Her X-1 (using the ephemeris of Still et al 2001). We folded data over the best fit period, computed at each 35 using a Discrete Fourier Transform in the (2-10) keV band. Light curves are shown in Figure 2 for various energies.

The softest band, (0.3-0.7) keV, shows a smooth, almost sinusoidal modulation which is more prominent at 35 =0.17. At this epoch, the hard band shows a main peak at spin= 0.7 which is asymmetric and a lower peak at spin=0.40. Close to the short-on (35=0.60) the hard emission shows a main peak (feature “A”) and a secondary, lower peak (“B”). The latter becomes more important at higher energies. All these features tend to disappear at 35 =0.26, leaving only a much reduced, almost sinusoidal modulation in all the energy bands.

5. Pulse-Phase SpectroscopyFigure 4 shows the Fe line parameters as a function of the spin phase. We used a Gaussian plus PL model in the range (5-8) keV. At 35 =0.26 there is little evidence for modulation in the emission line parameters. However, at 35 =0.60 the EW and line normalisation is lowest at the peak in hard X-ray intensity. At 35

=0.17 data have the best signal/noise ratio. Here the soft flux below 0.7keV, the line normalisation, the FWHM and the EW exhibit a common minimum at 0.6< spin<0.9, which is shifted in phase respect to the hard emission.

XMM Newton data agree with Choi et al. (1994), but not with Oosterbroek et al. (2000) who found the Fe line correlated with the hard (power law) emission.

We find evidence to support to the idea that the 6.4 keV Fe line originates from the relatively cold mattial of the illuminated spot which is also the site where soft emission is reprocessed.

aMullard Space Science Laboratory, UCL, UK; bMIT, MA, USA; cSRON, Utrecht, The Netherlands; d Columbia Astrophysics Laboratory, NY, USA.

b) The soft-hard light curve shift

In the main-on and short-on states we find a turn over in the relative phasing of soft and hard emission is evident after ~2 keV. Beyond ~2 keV, all the various energy bands are in phase.

A crosscorrelation of the phase folded/binned data relative to the two bands (0.3-0.7) keV and (2-4) keV gives = 130°±20° or -190°±20° at 35 =0.17 and = 100°±10° at 35 =0.60. At 35 =0.26 we used the two bands (0.3-0.7) keV and (7-12) keV,obtaining = -10°±20 °.

The hard/soft shift in phase is explained if soft photons result from reprocessing of hard X-rays in the inner part of the accretion disk. If a non-tilted disk intercepts (and reprocesses) part of the NS beam, the directed and reflected components will be shifted by = 180°. Therefore, the value of ~250° observed in the past during the main-on was associated with the evidence of a tilt angle. For precessing disks, the tilt of the disk (and as well) should vary with 35 (Gerend & Boyndon 1976).

The value of derived from EPIC data is considerably different from past observations during the main-on state. Also, it changes in the other two states (especially in the lower-state observation). This suggests we are observing, for the

first time, a substantial, continuous variation in the tilt of the disk.

Table 1. Summary of the XMM-Newton observations of Her X-1. For the orbital phase we used the ephemeris of Deeter et al. (1991); for the 35d phasing we define 35 =0 at the main-on turn-on.

Observationdate

EffectiveExposure(ksec)

EPIC PNMean Count(counts/sec)

Orbital Phase 35 d Phase

2001 Jan 26 10 398.4 0.20-0.26 0.172001 Mar 4 11 21.4 0.47-0.56 0.262001 Mar 17 11 54.3 0.52-0.60 0.60

4. The Pulse-Averaged X-Ray Spectra

The EPIC PN and MOS spectra are shown in Figure 3. The best fits agree with past observations; spectra taken at the three 35 are consistent with a single input emission model (BB+PL+ gaussian lines) and an absorption component associated with the intervening matter.

The PL index in the (2-12) keV band is ~0.8-0.9. The values of NH and of the covering factors for the intervening matter are consistent with Coburn et al. (2000). The thermal emission has a complex substructure: in the two spectra with best signal/noise ratio (35 =0.17, 0.60) we require two blackbodies, with T~0.1-0.2 keV and T~0.06 keV. We also require more than one partial covering absorber. Around ~1 keV we have included a set of lines/edges to model the most prominent lines/edges resolved, at the same 35, by RGS. Details of the RGS observations will be reported in Hailey et al. (2001).

The EPIC pulse profiles are similar to those obtained using Ginga (Deeter et al. 1998) and, according with Scott et al. (2000), can be explained in terms of successive obscurations of a multi-component X-ray beam by a counterprecessing, twisted and tilted accretion disk. The X-ray beam consists of two main components: a direct pencil beam (that originates close to the poles, at the NS surface) plus a reversed fan beam focussed in the antipodal direction.A similar configuration was predicted by Brainerd & Meszaros (1991), studying the backscattering of magnetic polar cap radiation by the accreting material and its subsequent gravitational focussing around the NS.

Figure 2: The spin-pulse folded light curves obtained at the 3 epochs.

Figure 4: The results of Pulse phase spectroscopy through the spin cycle at the 3 epochs.

Figure 3: Top panel: the EPIC MOS spectra – from top to bottom takenOn Jan 26, Mar 17 and March 4: Lower Panel: the EPIC pn spectra.