Spectral analysis of the binary
x-ray pulsar 4U 1538-52 observed by
RXTE
José J. Rodes, José M. Torrejón and Guillermo Bernabéu
Departament of Physics, Systems Engineering and Signal Theory
University of Alicante
Abstract
We present pulse-phase resolved X-ray spectra of the high mass
binary 4U 1538-52 using all the available data from the Rossi X-ray
Timing Explorer (RXTE) archives. This binary X-ray pulsar was
observed from the RXTE in 1997 January and 2001 January for a
complete orbital period. Also, 4U 1538-52 was observed for a year
taking a spectrum by month from 1996 November to 1997 December.
We have not found evidence for the first harmonic at ≈ 40 keV in
the spectra, both phase resolved and averaged (as Mihara 1995 and
Coburn 2001). However, we found evidence for a feature between 50
and 60 keV in the RXTE data (also reported by Robba et al. 2001 from
BeppoSAX). So we conclude that this feature may be a real feature
and although weak we show that it is visible in phase averaged spectra.
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Introduction
4U 1538-52 is a high mass X-ray binary star system formed by a compact
object (neutron star) and a massive B0 I star (∼ 17 M ). Uhuru was the
first satellite X-ray observatory that detected this system (Giacconi et al.
1974). The neutron star has a spin period ∼ 529 s (Davison 1977; Becker et
al. 1977). The orbital period is ∼ 3.73 days (Clark 2000) and, assuming a
distance of 5.5 kpc, the X-ray luminosity is ∼ 4×1036 erg · s−1 (Becker et al.
1977; Parkes et al. 1978).
The magnetic field plays an important role in the X-ray binary systems.
The only direct evidence for neutron star magnetic field strengths is provided by X-ray cyclotron ”absorption lines”. Although these quasi-harmonic
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Figure 1: Spectrum of 4U 1538-52 and residuals with the best fit model using
the NPEX component
lines appear in absorption, they are not absorption lines in the true sense.
Cyclotron lines are spectral features due to inelastic scattering off electrons
which are quantized in the strong magnetic fields of neutron stars. In this
sense, these lines are known as Cyclotron Resonance Scattering Features
(CRSFs). To perform phase resolved spectroscopy of the CRSF, we need
first to define the continuum correctly. We found that the Negative Positive
power law EXponential (NPEX-model; Mihara 1995) describes it properly.
Then we estimated the modified Julian Date using xTime, a time conversion utility for RXTE, and used the orbital ephemeris from Makishima et
al. 1987. In this work we present the spectrum of 4U 1538-52 in the energy
band (3-100 keV). We clearly show that spectra can be well fitted when an
iron emission line at 6.4 keV and a cyclotron absorption feature at 20 keV
are added in the model. If the second harmonic is present, it is very weak
and there is no evidence yet. However, another feature may be present at a
higher energy.
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Discussion and conclusions
We have analysed spectra from the source 4U 1538-52 and used different
continuum models to fit the data. Only the functions with a CRSF near 20
keV and a Gaussian emission line near 6.4 keV give acceptable fits. This
result is in agreement with Clark et al. 1990 or Robba et al. 2001. In
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Figure 2: Spectrum of 4U 1538-52 and residuals with the best fit model
using the blackbody component plus a comptonization of soft photons in a
hot plasma
Figure 3: Spectrum of 4U 1538-52 and residuals with the best fit model
using the accretion disk consisting of multiple blackbody components plus a
comptonization of soft photons in a hot plasma
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Figure 4: Spectrum of 4U 1538-52 in the 1997 data. We show the feature
above 50 keV
our analysis, we found that absorbed power law modified by a high energy
cutoff or by a Fermi-Dirac form of the cutoff could not describe well all
phase resolved spectra. But, the absorbed NPEX continuum model describes
the continuum data properly as well as other custom models used in this
study (the absorbed blackbody plus comptonization or absorbed multiple
blackbody plus comptonization).
The fundamental CRSF, centered at ≈ 20 keV was discovered with the
Ginga satellite by Clark et al. (1990). Also has been observed with the
BeppoSAX satellite by Robba et al. (2001), centered at ≈ 21 keV. Coburn
(2001) obtained a value for the CRSF of 20.66 keV, using a modified power
law with a high energy cutoff. In this work, we found the fundamental
CRSF at 20.1±0.2, for the 2001 data, and at 20.0±0.3, for the 1997 data.
This cyclotron line change with th phase but no correlation was found.
We tried to fit the second harmonic in the spectra, but we have not found
evidence for a ≈ 40 keV harmonic in the spectra, both phase resolved and
averaged spectra (Mihara 1995, Coburn 2001). However, a non-harmonically
absorbed feature may be present at a higher energy above 50 keV. This feature at 51+4
−3 keV has been reported by Robba et al. (2001) using observations
taken with the BeppoSAX satellite observatory.
Although it is not present in al the spectra, an absorption feature might
be seen above 50 keV. As the BeppoSAX data of 4U 1538-52 also reported
this absorption feature so it is probably a real feature. However, this energy
is not compatible with the second harmonic because of is not the double
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Figure 5: Spectrum of 4U 1538-52 in the 2001 data. We show the feature
above 50 keV
of the fundamental energy. Moreover, it seems disappear in other spectra.
Unfortunately, the low level of counts and the big bar errors at high energies
have made impossible to fit a gaussian to the feature with a significance
acceptable.
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