1,004 research outputs found
Energy-Dependent Harmonic Ratios of the Cyclotron Features of X0331+53 in the 2004-2005 Outburst
We report on changes of the cyclotron resonance energies of the recurrent
transient pulsar, X0331+53 (V0332+53). The whole RXTE data acquired in the
2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between
1.7x10^36 and 3.5x10^38 ers/s, assuming a distance of 7 kpc. We confirmed that
the fundamental cyclotron resonance energy changed from ~22 to ~27 keV in a
clear anti-correlation to the source luminosity, and without any hysteresis
effects between the rising and declining phases of the outburst. In contrast,
the second harmonic energy changed from ~49 to ~54 keV, implying a weaker
fractional change as a function of the luminosity. As a result, the observed
resonance energy ratio between the second harmonic and the fundamental was ~2.2
when the source was most luminous, whereas the ratio decreased to the nominal
value of 2.0 at the least luminous state. Although the significance of this
effect is model dependent, these results suggest that the fundamental and
second harmonic resonances represent different heights in the accretion column,
depending on the mass accretion rate.Comment: 39 pages, 15 figures, 4 tables. Accepted for publication in
Astrophysical Journa
Neuro-vestibular Examination During and Following Spaceflight
Adaptation to microgravity during spaceflight causes neurological disturbances that are either directly or indirectly mediated by the vestibular system. These disturbances could include space motion sickness, spatial disorientation, cognitive impairment, as well as changes in head-eye coordination, vestibulo-ocular reflex, and strategies for controlling posture and locomotion. It seems that otolith-mediated reflex gain adapts rapidly over time during spaceflight and after landing. However, animal studies have shown that structural modifications of the vestibular sensory apparatus develop during long-duration spaceflight. To date, no studies have characterized the severity of vestibular syndromes experienced by astronauts as a function of the duration of spaceflight, or whether the effects are caused by changes at the peripheral end organs, midbrain, cerebellum, or vestibular cortex
Detection of Excess Hard X-ray Emission from the Group of Galaxies HCG62
From the group of galaxies HCG62, we detected an excess hard X-ray emission
in energies above keV with \A SCA. The excess emission is spatially
extended up to from the group center, and somewhat enhanced toward
north. Its spectrum can be represented by either a power-law of photon index
0.8-2.7, or a Bremsstrahlung of temperature keV. In the 2-10 keV range,
the observed hard X-ray flux, erg cm
s, implies a luminosity of erg s for a
Hubble constant of 50 km s Mpc. The emission is thus too luminous
to be attributed to X-ray binaries in the memb er galaxies. We discuss possible
origin of the hard X-ray emission.Comment: 6 pages, 3 Postscript figures, uses emulateapj.sty. Accepted for
publication in the Astrophysical Journal Letter
Kiso observations for 20 GRBs in HETE-2 era
We have established a GRB follow-up observation system at Kiso observatory
(Japan) in 2001. Since the east Asian area had been blank for the GRB follow-up
observational network, this observational system is very important in studying
the temporal and spectral evolution of early afterglows. Using this system, we
have performed quick observations for optical afterglows from early phase based
on HETE-2 and INTEGRAL alerts. Thanks to the quick follow-up observation
system, we have been able to use the Kiso observatory in 20 events, and conduct
their follow-up observations in optical and near infrared wavelengths.Comment: 5 pages, 4 figure. Accepted for publication into "il nuovo cimento".
Proceeding of the 4th Rome GRB conference, eds. L. Piro, L. Amati, S. Covino,
B. Gendr
Probing the stellar wind environment of Vela X-1 with MAXI
Vela X-1 is among the best studied and most luminous accreting X-ray pulsars.
The supergiant optical companion produces a strong radiatively-driven stellar
wind, which is accreted onto the neutron star producing highly variable X-ray
emission. A complex phenomenology, due to both gravitational and radiative
effects, needs to be taken into account in order to reproduce orbital spectral
variations. We have investigated the spectral and light curve properties of the
X-ray emission from Vela X-1 along the binary orbit. These studies allow to
constrain the stellar wind properties and its perturbations induced by the
compact object. We took advantage of the All Sky Monitor MAXI/GSC data to
analyze Vela X-1 spectra and light curves. By studying the orbital profiles in
the and keV energy bands, we extracted a sample of orbital light
curves (% of the total) showing a dip around the inferior
conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged
and phase-resolved spectra of both the double-peaked and the standard sample.
The dip in the double-peaked sample needs cm to
be explained by absorption solely, which is not observed in our analysis. We
show how Thomson scattering from an extended and ionized accretion wake can
contribute to the observed dip. Fitted by a cutoff power-law model, the two
analyzed samples show orbital modulation of the photon index, hardening by
around the inferior conjunction, compared to earlier and later
phases, hinting a likely inadequacy of this model. On the contrary, including a
partial covering component at certain orbital phase bins allows a constant
photon index along the orbital phases, indicating a highly inhomogeneous
environment. We discuss our results in the framework of possible scenarios.Comment: 10 pages, 9 figures, accepted for publication in A&
Is the black hole in GX 339-4 really spinning rapidly?
The wide-band Suzaku spectra of the black hole binary GX 339-4, acquired in
2007 February during the Very High state, were reanalyzed. Effects of event
pileup (significant within ~ 3' of the image center) and telemetry saturation
of the XIS data were carefully considered. The source was detected up to ~ 300$
keV, with an unabsorbed 0.5--200 keV luminosity of ~3.8 10^{38} erg/s at 8 kpc.
The spectrum can be approximated by a power-law of photon index 2.7, with a
mild soft excess and a hard X-ray hump. When using the XIS data outside 2' of
the image center, the Fe-K line appeared extremely broad, suggesting a high
black hole spin as already reported by Miller et al. (2008) based on the Suzaku
data and other CCD data. When the XIS data accumulation is further limited to
>3' to avoid event pileup, the Fe-K profile becomes narrower, and there appears
a marginally better solution that suggests the inner disk radius to be 5-14
times the gravitational radius (1-sigma), though a maximally spinning black
hole is still allowed by the data at the 90% confidence level. Consistently,
the optically-thick accretion disk is inferred to be truncated at a radius 5-32
times the gravitational radius. Thus, the Suzaku data allow an alternative
explanation without invoking a rapidly spinning black hole. This inference is
further supported by the disk radius measured previously in the High/Soft
state.Comment: 5 pages, figures, Suzaku results on GX 339-4, accepted to APJL. Nov.
11, 2009, accepted to ApJ
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