PowerPoint Presentation Isolated Neutron Stars, solid crust

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XMM-Newton Observations of HER-X1
Interacting Binaries Probing Accretion,
Evolution and Outcome 3-10 July 2004, Cefalu,
Italy
S. Zane, G. Ramsay MSSL, University College of
London, UK M. Jimenez MIT, Cambridge, USA J.W.
den Herder Space Research Organization of the
Netherlands C.J. Hailey Columbia Astrophysics
Laboratory Martin Still Patricia Boyd
NASA/Goddard Space Flight Center
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• Her X-1 is an eclipsing binary system NS A/F
  star.
• Pspin 1.24 s , Porbital 1.7 d
• X-ray intensity varies on a period of 35 d.
  Main-on 10 d short-on 5 d, separation
  period 10 d of lower intensity.
• Main-on broadband spectrum bb at 0.1 keV PL
  exponential cut-off at higher energies. Low
  NH 1019 cm-2.
• Also 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 outside the main-on are fitted by
  adding one or more partial covering absorbtion
  components (NH 1022-1023 cm-2) to the model
  above.

? 35 d cycle Occultation phases of the NS by a
(tilted, warped) precessing accretion disk
(Gerend Boynton 1976, Coburn et al. 2000).
Exceptions to the 35 d cycle in 4 occasions
1983, 1993, 1999.... and 2004 !
Anomalous Low States (ALS)
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XMM-Newton Observations of HER-X1

From Jan 2001 to Mar 2003 Her X-1 was observed
at 15 different epochs during the 35 d cycle
Since Feb. 2004 6 new observations during the
ALS.
The RXTE/ASM (2-10) keV lc of Her X-1 the thick
lines indicate the XMM-Newton observations
Ramsay et al, 2002, MNRAS, 337,
185 Jimenez-Garate et al, 2002, ApJ, 578,
391 Zane et al, 2004, MNRAS, 350, 506
Preliminary results from the ALS.
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Spin Resolved Light Curves 2-10 keV and 0.3-0.7
keV
(Best fit P computed at each ?35)

• Confirm the slow-down trend monitored by BeppoSax
  and Chandra (Oosterbroek et al, 2000, Burwitz et
  al, 2002)
• Spin modulation is weak or absent in the low
  states
• During the states of higher intensity a
  substructure in the soft X-ray. Separate peaks
  which reflect the structure seen at higher
  energies.

Beyond 2 keV, all energy bands are consistent
with being in phase each other. Turn over in the
relative phasing of soft/ hard photons evident
after 2 keV.
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The Soft/Hard light curve shift
Hard/soft phase shift expected 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
the NS beam, directed and reflected components
will be shifted by ?? 180. ? ?? 250
observed in the past during the main-on was
associated with evidence of a tilt angle.
Crosscorrelation of the bands (0.3-0.7) and (2-4)
keV ?? 80 20 at ?35 0.60 ?? 130
20 at ?35 0.02 ?? 160 20 at ?35
0.17 ?? 340 20 at ?35 0.26
?? derived from EPIC data is very different from
past main-on observations. Also, it changes
dramatically in the other two states. ? First
observation of a substantial, continuous change
in the tilt of the disk !
HST image of the bow-shock nebula around RX
J1856.5-3754 (van Kerkwick Kulkarni 2001)
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Pulse-averaged X-ray Spectra
Spectra 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. PL index 0.8-0.9
in the (2-12) keV. Substructure of the soft
emission at ?35 0.17, 0.60 we require two
blackbodies, with T0.1-0.2 keV and T0.06 keV.
Around 1 keV we include a set of lines/edges
modeling the most prominent ones resolved, at the
same ?35, by RGS.
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? 6.4 keV Fe line changes over the 35 d cycle

• Change in line width the FWHM is a factor 5-10
  larger when the source is on
• (?20-50 eV at ?350.26,0.6 to ?300 at
  ?350.17).
• Change in line center
• Probably near neutral Fe (Fe XIV or colder) in
  the low state and short-on.
• Main-on line energies correspond to Fe XX- Fe
  XXI
• (MOS 6.520.03 keV PN6.500.02 keV)

• Possible reasons
• an array of Fe K fluorescence lines for all
  charge states from Fe I-Fe XIII to Fe XXIII
• Comptonization from a hot corona (? ?1) for a
  narrower range of charge states centered ?FeXX
• Keplerian broadening
• At ?35 0.17 V 13000 km/s ? R4x108 cm for
  M1.4 M magnetospheric radius for B 1012 G.

OR.
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The region emitting Fe K? lines is likely to be
different for lines observed at different beat
phases
Low state line is gt5 weaker and modulated with
the orbital phase. - Correlation with the fast
rising UVW1 disc origin scenario?
Main on norm. and EW are correlated with the
soft X-ray emission
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Low state Evidence for a new feature at 7 keV
(in addition to that at 6.4 keV) Signature of the
hot corona?
Evolution of the 7 keV line along the 35d cycle
7 keV line confirmed by Chandra at ? 35 0.44,
0.46 (Jimenez et al)
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Constraint on the region emitting the 7 keV line
a widely extended photo ionized plasma?

• 1) Upper limit
• 7 keV ? log ?gt3.3
• ? Lx/(nr2) ionization parameter
• Lx 1.6 x1037 erg/s
• ? nr2 gt 8x1033 cm-1
• 2) Lower limit
• Simplest single -T plasma
• L line 7.7x1031 erg/s
• T 230 eV
• n2V gt3 x1055 cm-3 (EM)
• 3) R gt R Alfven 3.6 107 cm

Good Candidate predicted by illuminated disk
models Accretion disk corona r 108- 1010
cm, n 1015 - 1016 cm-3 Model predictions a) A
spatially extended photo ionized layer on the top
of a Shakura-Sunyaev disk, b) more external
layers (Corona) where the Compton temperature
goes up to few x106 K (Jimenez-Garate et al, 2001)
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RGS Spectra, Low and short on state
Jimenez-Garate et al, ApJ, 578, 391

• Low and short-on states
• Faint PL continuum
• Narrow emission lines from C VI, N VI, N VII, O
  VII, O VIII, Ne IX (NeX?)
• No velocity broadening (within the resolution)
• Weak RRCs of OVII and NVII ? photo ionized gas
• From helium-like emission of Ne IX, O VII, N VI
  G(fi)/r 4 ? photo ionized gas.

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The narrow line emission region photo ionized
disk atmosphere or illuminated face of the donor
HZ?

• Thermal and ionization balance ? upper limit on
  the e density
• EM from line fluxes ? lower limits on the e
  density
• He? line ratios and UV photo excitation
  calculations ? upper limits to the radius
  enclosing the region

Confidence limits (90) on density and location
of the narrow line emission region, obtained
from spectroscopic analysis and modelling. The
limits shown correspond to O VII, and similar
limits with N VI and Ne IX can be set
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Low Emission detected with high resolution
spectroscopy during the Low State may originate
in the illuminated accretion disk atmosphere
(blue) and hot corona (pink)

• Fe line at 7 keV signature of the hottest
  layers of the corona
• Narrow lines detected with RGS photo ionized
  layers just above the disk

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Feb 2004 HerX-1 entered its 4th ALS
Which is still going on!
XTE lc from April 15 to Jun 21, in 5 days
average There were two periods of increased
apparent activity, but it does not look
definitive now
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ALS Spectra similar to those of the standard
low state
Also RGS spectra qualitatively similar No
pulsations detected Work in progress
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Summary

• XMM spectra consistent with obscuration model by
  a precessing disk
• Low State identified with illuminated accretion
  disk atmosphere and hot corona from high
  resolution spectroscopy.
• Detection of a Fe line at 7 keV during the low
  state signature of the hottest layers of the
  corona
• RGS narrow line emission during the low state.
  Photo ionized layers above the disk
• 35 d variability of the 6.4 keV Fe line.
  Emitting regions probably different during low
  and high state.
• UV modulation along the orbital period in both
  OM filters.
• Before the ALS spin-down and change in the
  hard/soft phase shift ? was the inner disk
  changing?
• Spectra taken during the ALS remarkably similar
  to those taken during the standard low state