1. Introduction

1
XMM-NEWTON EPIC OBSERVATIONS OF Her X-1 G.
Ramsaya, S. Zanea, M.Jimenez-Garateb, J.W.den
Herderc, C.J.Haileyd
aMullard Space Science Laboratory, UCL, UK
bMIT, MA, USA cSRON, Utrecht, The Netherlands
d Columbia Astrophysics Laboratory, NY, USA.
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.
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 (BBPL 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 T0.1-0.2
keV and T0.06 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.
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 ?350.26,0.6, to 0.3 at
?350.17). 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 (6.520.03 keV for MOS ,
6.500.02 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.
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.
Figure 2 The spin-pulse folded light curves
obtained at the 3 epochs.
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 ?? 13020 or -19020 at ?35
0.17 and ?? 10010 at ?35 0.60. At ?35
0.26 we used the two bands (0.3-0.7) keV and
(7-12) keV,obtaining ?? -1020 . 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.
Figure 3 Top panel the EPIC MOS spectra from
top to bottom taken On Jan 26, Mar 17 and March
4 Lower Panel the EPIC pn spectra.
5. Pulse-Phase Spectroscopy Figure 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 0.6lt ?spinlt0.9,
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.
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 ?spin0.40. Close to the short-on
(?350.60) 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.

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