Issue |
A&A
Volume 489, Number 1, October I 2008
|
|
---|---|---|
Page(s) | 263 - 279 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:200809772 | |
Published online | 16 June 2008 |
Detailed high-energy characteristics of AXP 1RXS J170849-400910
Probing the magnetosphere using INTEGRAL, RXTE, and XMM-Newton
1
SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: Hartog@sron.nl
2
Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
Received:
12
March
2008
Accepted:
10
June
2008
1RXS J170849-400910 is one of four anomalous X-ray pulsars
which emit persistent luminous radiation in soft X-rays (<10 keV)
as well as in hard X-rays (>10 keV). In this paper we present
detailed spectral and temporal characteristics over the whole X-ray
band. For this purpose data have been used from INTEGRAL, RXTE and XMM-Newton.
The hard X-ray (>10 keV) time-averaged total spectrum, accumulated
over four years with the imager IBIS-ISGRI onboard INTEGRAL adding up
to 5.2 Ms net exposure, can be described by a power law with a
photon index Γ = 1.13± 0.06 and extends to
~175 keV. The 20-175 keV flux is (7.76 ± 0.34)
10-11 erg cm-2 s-1 which exceeds the 2-10 keV (unabsorbed) flux by a
factor of ~2.3. No evidence for a spectral break is found below
300 keV. Also, no significant long-term time variability has been
detected above 20 keV on time scales of 1 and 0.5 year. Pulsed
emission is measured with INTEGRAL up to 270 keV, i.e. to much higher
energies than the total emission, with a detection significance of
12.3σ (20-270 keV). The pulse profiles from 0.5 keV up to
270 keV show drastic morphology changes below ~20 keV. Three
different pulse components can be recognized in these pulse
profiles: 1) a hard pulse peaking around phase 0.8 which contributes
to the pulse profiles above ~4 keV; 2) a softer pulse which
peaks around phase 0.4 not contributing in the hard X-ray domain and 3) a very soft pulse component below 2 keV. A combined
time-averaged pulsed spectrum (2.8-270 keV) from INTEGRAL, RXTE-PCA and
HEXTE (collected over nine years) can be described with a soft and a
hard power-law component:
= 2.79 ± 0.07 and
= 0.86 ± 0.16. In the pulsed spectrum extracted
from a 25.5 ks net exposure XMM-Newton observation we find a
discontinuity between 2 keV and 3 keV. Above these energies the
spectrum is consistent with the spectrum taken with RXTE-PCA. The pulse
profiles and the total-pulsed spectrum prove to be stable over the
whole nine-years time span over which the data have been taken. Also
detailed phase-resolved spectroscopy of the pulsed emission confirms
the long-term stability as the spectra taken at different epochs
connect smoothly. The phase-resolved spectra reveal complex spectral
shapes which do not follow the shape of the total-pulsed
spectrum. The spectral shape gradually changes with phase from a
soft single power law to a complex multi-component shape and then to
a hard single power law. The spectrum switches from a very hard
(Γ = 0.99 ±0.05) to a very soft (Γ = 3.58 ± 0.34)
single power-law shape within a 0.1-wide phase interval. The
discontinuity measured between 2 keV and 3 keV with XMM-Newton is a
result of a curved component. This component which is most apparent
within phase interval 0.7-0.9 significantly contributes in the
energy range between 4 keV and 20 keV. It has a very steep spectrum
below 5 keV with a photon index Γ ~ -1.5. From the
phase-resolved spectra we identify three independent components with
different spectral shapes which together can accurately describe all
phase-resolved spectra (2.8-270 keV). The three shapes are a soft
power law (Γ = 3.54), a hard power law (Γ = 0.99) and
a curved shape (described with two logparabolic functions). The
phase distributions of the normalizations of these spectral
components form three decoupled pulse profiles. The soft component
peaks around phase 0.4 while the other two components peak around
phase 0.8. The width of the curved component (~0.25 in phase)
is about half the width of the hard component. After 4U 0142+61, RXS J1708-40
is the second anomalous X-ray pulsar for which such detailed
phase-resolved spectroscopy has been performed. These results give
important constraints showing that three dimensional modeling
covering both the geometry and different production processes is
required to explain our findings.
Key words: stars: neutron / X-rays: individuals: 1RXS J170849-400910 / gamma rays: observations / X-rays: individuals: 4U 0142+61
© ESO, 2008
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