Suzaku Observations of PSR B1259-63: A New Manifestation of Relativistic Pulsar Wind Page: 2 of 11
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Uchiyama et al.
21-25 March 2004 (Shaw et al. 2004).
The properties of the X-ray and soft --ray emission ob-
served with ASCA and CGRO have been shown to be con-
sistent with the idea that the emission is produced by shock-
accelerated e pairs as a result of the interactions between
the pulsar wind and the Be star outflows (Tavani, Arons, &
Kaspi 1994; Kaspi et al. 1995; Tavani & Arons 1997). The
e pairs are considered to be accelerated at the inner shock
front of the pulsar wind and adiabatically expanding in the
relativistic flow of the pulsar cavity. As a scale-down version
of pulsar wind nebulae (PWNe), the PSR B1259-63 system
can be referred to as a compactified pulsar wind nebula, and
serves as a unique diagnostic of shock acceleration subject to
very fast cooling. Indeed, synchrotron radiation by the ac-
celerated e pairs offers an excellent explanation for the ob-
served X-ray emission, indicating the existence of a remark-
ably efficient acceleration mechanism at an energy of TeV on
a timescale less than ~ 100 sec (Tavani & Arons 1997). The
anisotropic inverse-Com ton scattering on the intense stellar
light of the accelerated e pairs was predicted to be detectable
by air Cherenkov telescopes (Kirk et al. 1999).
During the periastron passage in 2004, very high energy
gamma-rays have been indeed detected by the HESS tele-
scopes (Aharonian et al. 2005). The level of the TeV gamma-
ray flux and the spectral shape were in a reasonable agree-
ment with the theoretical expectation based on the compacti-
fied PWN model. However, the light curve around periastron
disagreed with the early expectation, requiring a revision of
the model6. Khangulyan et al. (2007) have introduced the
change of the maximum energy of pairs as a viable explana-
tion for the behavior of the TeV light curve. Also, they argued
that the increase of the adiabatic loss rate near periastron is
another possible way to naturally account for the reduction of
the TeV flux. Since the change of the maximum energy and/or
the adiabatic loss rate can be imprinted in the synchrotron X-
ray spectrum, the detailed X-ray observations are thus quite
important to understand the behavior of the TeV radiation as
emphasized by Khangulyan et al. (2007).
In this paper, we present the results from our monitoring
observations made with the Suzaku satellite in the X-ray and
hard X-ray bands, with an effective energy range of 0.6-50
keV, performed during the most recent periastron passage in
2007. Combined with the previous measurements in X-ray,
the Suzaku observations have clarified the spectral evolution
of the PSR B1259-63 system. Thanks to the wide band cov-
erage, we have discovered the existence of a spectral break
in a certain epoch. This paper is organized as follows. The
observations and basic data reduction are described in 2.
The results of X-ray analyses are presented in 3. In 4,
we outline our model of a compactified pulsar wind nebula,
and interpret the X-ray spectral break as a manifestation of
the Lorentz factor of the pulsar wind. Throughout this paper,
we adopt the distance to the PSR B1259-63 system from the
Earth to be D = 1.5 kpc (Johnston et al. 1994). For the cal-
culation of the pulsar orbit, we assume a mass of SS 2883 to
be M, = 10M and the pulsar mass to be Mp = 1.4M, which
implies an inclination angle of i ~ 36 (Johnston et al. 1994).
The adopted orbital elements of the pulsar are an orbital pe-
riod of 1236.72 day, eccentricity of e = 0.8699, longitude of
periastron of o = 138.7 , and the epoch of periastron of MJD
6 Even departing from the framework of the compactified PWN, a
hadronic origin of TeV gamma-rays (Kawachi et al. 2004) may replace the
synchrotron-IC model (see also Chernyakova et al. 2006).48124.34 (Manchester et al. 1995). All errors are quoted at a
90% confident level unless otherwise stated.
2. OBSERVATIONS AND DATA REDUCTION
We observed the PSR B1259-63 system eight times with
the Suzaku satellite each with ~ 20 ks exposure in 2007 July,
August, and September. The total exposure time amounts to
~ 170 ks after standard data screening. We note that this is
the first and possibly last chance for Suzaku to observe the
first disk transit since target visibility does not permit us to
observe it next time. Table 1 gives the log of the Suzaku obser-
vations; the eight observing epochs are referred to as Sz1-Sz8
hereafter. During the first four epochs (Sz1-Sz4) simultane-
ous observations with HESS were coordinated. The results of
the HESS observations will be presented separately.
The Suzaku observations were performed with the X-ray
Imaging Spectrometer (XIS: Koyama et al. 2007) in 0.3-12
keV and the Hard X-ray Detector (HXD: Takahashi et al.
2007) in 13-600 keV. The XIS, located at the focal plane
of the X-ray telescopes (XRT: Serlemitsos et al. 2007), con-
sists of one back-illuminated CCD camera (XIS-1) and three
front-illuminated CCDs (XIS-0, -2 and -3). One of the front-
illuminated CCDs, XIS-2, was not available at the time of our
observations, since it suffered from a fatal damage on 2006
November 9, and unusable since then. The HXD consists of
the silicon PIN photo diodes (hereafter PIN) capable of obser-
vations in the 13-70 keV band and the GSO crystal scintilla-
tors (hereafter GSO) which cover the 40-600 keV band.
The XIS instruments were operated in a normal full-frame
clocking mode, with a frame time of 8 s. In this work, we
present results from front-illuminated CCDs (XIS-0 and -3),
which have a larger effective area at high energies, and there-
fore suited for our purpose. XIS-0 and -3 have almost iden-
tical properties, so we co-added the data to improve photon
statistics. PSR B1259-63 was observed slightly off-axis (3.5'
from the optical axis), placed on the HXD nominal pointing
position, to maximize the effective area of the HXD detectors.
The HXD instruments were operated in the normal mode. The
relative normalization of the HXD-PIN spectrum to the XIS
was fixed at 1.15, as was determined by the calibration obser-
vations of the Crab Nebula (Kokubun et al. 2007).
The field-of-view (FOV) of the XIS contains another X-ray
source, 2RXP J130159.6-635806 (shortly 2RXP J1301), as
shown in Figure 1. This is an accretion-powered pulsar with
a spin period of ~ 700 s (Chernyakova et al. 2005), which is
located 10' away from PSR B1259-63. We have monitored
the 700-sec accretion-powered pulsar with the XIS by tuning
a telescope roll angle, to estimate its contribution to the hard
X-ray flux measured by the non-imaging HXD-PIN detector.
We analyzed data delivered to us via pipeline processing
version 2.0.6.13 (Szl-Sz5) or version 2.1.6.15 (Sz6-Sz8).
Basic analysis was done using the HEASOFT software pack-
age (version 6.5), with calibration files distributed on 9 July
2008. For the XIS and HXD-PIN, we made use of cleaned
event files, in which standard screening was applied. On
the other hand, we reprocessed the HXD-GSO data by our-
selves to apply appropriate gain files. The standard screen-
ing procedures include event grade selections, and removal
of time periods such as spacecraft passage of the South At-
lantic Anomaly (SAA), intervals of small geomagnetic cutoff
rigidity (COR), and those of a low elevation angle. Specif-
ically, for the XIS, the elevation angle larger than 50 above
the Earth and larger than 20 from the sunlit Earth limb are
selected. The data screening criteria for the HXD include
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Uchiyama, Yasunobu; Tanaka, Takaaki; Takahashi, Tadayuki; Mori, Koji & Nakazawa, Kazuhiro. Suzaku Observations of PSR B1259-63: A New Manifestation of Relativistic Pulsar Wind, article, April 27, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc928668/m1/2/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.