47 research outputs found
Superorbital modulation at GeV energies in the gamma-ray binary LS I +61 303
We report the results from our analysis of 8 years of the data for the
gamma-ray binary LS I +61 303, obtained with the Large Area Telescope onboard
the Fermi Gamma-Ray Space Telescope. We find a significant dip around the
binary's periastron in the superorbital light curves, and by fitting the light
curves with a sinusoidal function, clear phase shifts are obtained. The
superorbital modulation seen in the binary has been long known and different
scenarios have been proposed. Based on our results, we suggest that the
circumstellar disk around the Be companion of this binary has an elliptical
shape and the major axis of the disk rotates at the superorbital period of 1667
days. As a result, the density of the ambient material around the compact star
of the binary changes along the binary orbit over the superorbital period,
causing the phase shifts in the modulation, and around periastron, the compact
star probably enters the disk, causing the appearance of the dip. Numerical
simulations may be conducted in order to study the detailed physical processes
and verify our suggestion.Comment: 11 pages, 6 figure
Swift Observations of the Be/X-ray Transient System 1A 1118-615
We report results of Swift observations for the high mass Be/X-ray binary
system 1A 1118-615, during an outburst stage in January, 2009 and at a flaring
stage in March, 2009. Using the epoch-folding method, we successfully detected
a pulsed period of 407.69(2) sec in the outburst of January and of 407.26(1)
sec after the flare detection in March. We find that the spectral detection for
the source during outburst can be described by a blackbody model with a high
temperature (kT ~ 1-3 keV) and a small radius (R ~ 1 km), indicating that the
emission results from the polar cap of the neutron star. On the other hand, the
spectra obtained after the outburst can further be described by adding an
additional component with a lower temperature (kT ~ 0.1-0.2 keV) and a larger
emission radius (R ~ 10-500 km), which indicates the emission from around the
inner region of an accretion disk. We find that the thermal emission from the
hot spot of the accreting neutron star dominates the radiation in outburst; the
existence of both this X-ray contribution and the additional soft component
suggest that the polar cap and the accretion disk emission might co-exist after
the outburst. Because the two-blackbody signature at the flaring stage is a
unique feature of 1A 1118-615, our spectral results may provide a new insight
to interpret the X-ray emission for the accreting neutron star. The time
separation between the three main outbursts of this system is ~17 years and it
might be related to the orbital period. We derive and discuss the associated
physical properties by assuming the elongated orbit for this specific Be/X-ray
transient.Comment: 9 pages, 3 figures and 4 tables; accepted by MNRA
Probing the Pulsar Wind in the gamma-ray Binary System PSR B1259-63/SS 2883
The spectral energy distribution from the X-ray to the very high energy
regime ( GeV) has been investigated for the -ray binary system
PSR B1259-63/SS2883 as a function of orbital phase within the framework of a
simple model of a pulsar wind nebula. The emission model is based on the
synchrotron radiation process for the X-ray regime and the inverse Compton
scattering process boosting stellar photons from the Be star companion to the
very high energy (100GeV-TeV) regime. With this model, the observed temporal
behavior can, in principle, be used to probe the pulsar wind properties at the
shock as a function of the orbital phase. Due to theoretical uncertainties in
the detailed microphysics of the acceleration process and the conversion of
magnetic energy into particle kinetic energy, the observed X-ray data for the
entire orbit are fit using two different methods.Comment: 46 pages, 16 figures, accepted for publication in Ap
Hydrodynamic Interaction between the Be Star and the Pulsar in the TeV Binary PSR B1259-63/LS 2883
We study the interaction between the Be star and the pulsar in the TeV binary
PSR B1259-63/LS 2883, using 3-D SPH simulations of the tidal and wind
interactions in this Be-pulsar system. We first run a simulation without pulsar
wind nor Be wind, taking into account only the gravitational effect of the
pulsar on the Be disk. In this simulation, the gas particles are ejected at a
constant rate from the equatorial surface of the Be star, which is tilted in a
direction consistent with multi-waveband observations. We run the simulation
until the Be disk is fully developed and starts to repeat a regular tidal
interaction with the pulsar. Then, we turn on the pulsar wind and the Be wind.
We run two simulations with different wind mass-loss rates for the Be star, one
for a B2V type and the other for a significantly earlier spectral type.
Although the global shape of the interaction surface between the pulsar wind
and the Be wind agrees with the analytical solution, the effect of the pulsar
wind on the Be disk is profound. The pulsar wind strips off an outer part of
the Be disk, truncating the disk at a radius significantly smaller than the
pulsar orbit. Our results, therefore, rule out the idea that the pulsar passes
through the Be disk around periastron, which has been assumed in the previous
studies. It also turns out that the location of the contact discontinuity can
be significantly different between phases when the pulsar wind directly hits
the Be disk and those when the pulsar wind collides with the Be wind. It is
thus important to adequately take into account the circumstellar environment of
the Be star, in order to construct a satisfactory model for this prototypical
TeV binary.Comment: 10 pages, 5 figures, accepted for publication in PAS
GeV detection of HESS J0632+057
HESS J0632+057 is the only gamma-ray binary that has been detected at TeV
energies, but not at GeV energies yet. Based on nearly nine years of Fermi
Large Area Telescope (LAT) Pass 8 data, we report here on a deep search for the
gamma-ray emission from HESS J0632+057 in the 0.1-300 GeV energy range. We find
a previously unknown gamma-ray source, Fermi J0632.6+0548, spatially coincident
with HESS J0632+057. The measured flux of Fermi J0632.6+0548 is consistent with
the previous flux upper limit on HESS J0632+057 and shows variability that can
be related to the HESS J0632+057 orbital phase. We propose that Fermi
J0632.6+0548 is the GeV counterpart of HESS J0632+057. Considering the Very
High Energy (VHE) spectrum of HESS J0632+057, a possible spectral turnover
above 10 GeV may exist in Fermi J0632.6+0548, as appears to be common in other
established gamma-ray binaries.Comment: 17 pages, 4 figures, 1 table; Accepted for publication in Ap
The X-ray Properties of the Energetic Pulsar PSR J1838-0655
We present and interpret several new X-ray features of the X-ray pulsar PSR
J1838-0655. The X-ray data are obtained from the archival data of CHANDRA,
RXTE}, and SUZAKU. We combine all these X-ray data and fit the spectra with
different models. We find that the joint spectra are difficult to fit with a
single power law; a broken power-law model with a break at around 6.5 keV can
improve the fit significantly. The photon index changes from = 1.0
(below 6.5 keV) to = 1.5 (above 6.5 keV); this indicates a softer
spectral behaviour at hard X-rays. The X-ray flux at 2-20 keV is found to be
1.6x10^{-11} ergs cm^{-2} s^{-1}. The conversion efficiency from the spin-down
luminosity is ~ 0.9% at 0.8-10 keV, which is much higher than that (~ 10^{-3}%
- 10^{-4}%) of the pulsars that show similar timing properties. We discuss
non-thermal radiation mechanisms for the observed high X-ray conversion
efficiency and find that emission from the magnetosphere of a greatly inclined
rotator is the most favorable interpretation for the conversion rate and the
pulse profiles at X-ray bands. A line feature close to 6.65 keV is also
detected in the spectra of SUZAKU/XIS; it might be the K emission of
highly ionised Fe surrounding the pulsar.Comment: 8 pages, 6 figures and 1 tabl