10 research outputs found
Soft and Hard X-Ray Emissions from the Anomalous X-ray Pulsar 4U 0142+61 Observed with Suzaku
The anomalous X-ray pulsar 4U 0142+61 was observed with Suzaku on 2007 August
15 for a net exposure of -100 ks, and was detected in a 0.4 to ~70 keV energy
band. The intrinsic pulse period was determined as 8.68878 \pm 0.00005 s, in
agreement with an extrapolation from previous measurements. The broadband
Suzaku spectra enabled a first simultaneous and accurate measurement of the
soft and hard components of this object by a single satellite. The former can
be reproduced by two blackbodies, or slightly better by a resonant cyclotron
scattering model. The hard component can be approximated by a power-law of
photon index \Gamma h ~0.9 when the soft component is represented by the
resonant cyclotron scattering model, and its high-energy cutoff is constrained
as >180 keV. Assuming an isotropic emission at a distance of 3.6 kpc, the
unabsorbed 1-10 keV and 10-70 keV luminosities of the soft and hard components
are calculated as 2.8e+35 erg s^{-1} and 6.8e+34 erg s^{-1}, respectively.
Their sum becomes ~10^3 times as large as the estimated spin-down luminosity.
On a time scale of 30 ks, the hard component exhibited evidence of variations
either in its normalization or pulse shape.Comment: 24 pages, 8 figures, accepted for publication in Publications of the
Astronomical Society of Japa
A Statistical Study of Gamma-Ray Emitting Solar Flares Observed with Yohkoh
Gamma-ray emitting solar flares observed with Yohkoh were analyzed from a
statistical viewpoint. The four-band hard X-ray (15--95 keV) photometric data,
taken with the Hard X-ray Telescope onboard Yohkoh, were utilized in
combination with the spectro-photometric gamma-ray (0.2--30 MeV) data obtained
with the Gamma-Ray Spectrometer. The GOES class was also incorporated. Out of
2788 X-ray flares observed from 1991 October to 2001 December, 178 events with
strong hard X-ray emission were selected. Among them, 40 flares were further
found to show significant gamma-ray emission. A fractal dimension analysis and
multi-band color--color plots of the 40 flares suggest that their soft X-ray to
MeV gamma-ray spectral energy distributions involve at least four independent
parameters. These are: (1) the overall flare size; (2) the relative intensities
of the thermal vs. non-thermal signals; (3) the gamma-ray to hard X-ray
intensity ratio; and (4) the hard X-ray spectral slope. These results are
examined for possible selection effects. Also, the meanings of the third
parameter are briefly considered.Comment: 23 pages, 7 figures, PASJ accepte
A Comprehensive Study of Short Bursts from SGR 1806-20 and SGR 1900+14 Detected by HETE-2
We present the results of temporal and spectral studies of the short burst
(less than a few hundred milliseconds) from the soft gamma repeaters (SGRs)
1806-20 and 1900+14 using the HETE-2 samples. In five years from 2001 to 2005,
HETE-2 detected 50 bursts which were localized to SGR 1806-20 and 5 bursts
which were localized to SGR 1900+14. Especially SGR 1806-20 was active in 2004,
and HETE-2 localized 33 bursts in that year. The cumulative number-intensity
distribution of SGR 1806-20 in 2004 is well described by a power law model with
an index of -1.1+/-0.6. It is consistent with previous studies but burst data
taken in other years clearly give a steeper distribution. This may suggest that
more energetic bursts could occur more frequently in periods of greater
activity. A power law cumulative number-intensity distribution is also known
for earthquakes and solar flares. It may imply analogous triggering mechanisms.
Although spectral evolution during bursts with a time scale of > 20 ms is not
common in the HETE-2 sample, spectral softening due to the very rapid (< a few
milliseconds) energy reinjection and cooling may not be excluded. The spectra
of all short bursts are well reproduced by a two blackbody function (2BB) with
temperatures ~4 and ~11 keV. From the timing analysis of the SGR 1806-20 data,
a time lag of 2.2+/-0.4 ms is found between the 30-100 keV and 2-10 keV
radiation bands. This may imply (1) a very rapid spectral softening and energy
reinjection, (2) diffused (elongated) emission plasma along the magnetic field
lines in pseudo equilibrium with multi-temperatures, or (3) a separate (located
at < 700 km) emission region of softer component (say, ~4 keV) which could be
reprocessed X-rays by higher energy (> 11 keV) photons from an emission region
near the stellar surface.Comment: 50 pages, 14 figures, accepted for publication in PAS
An Optically Dark GRB Observed by HETE-2: GRB 051022
GRB 051022 was detected at 13:07:58 on 22 October 2005 by HETE-2. The
location of GRB 051022 was determined immediately by the flight localization
system. This burst contains multiple pulses and has a rather long duration of
about 190 seconds. The detections of candidate X-ray and radio afterglows were
reported, whereas no optical afterglow was found. The optical spectroscopic
observations of the host galaxy revealed the redshift z = 0.8. Using the data
derived by HETE-2 observation of the prompt emission, we found the absorption
N_H = 8.8 -2.9/+3.1 x 10^22 cm^-2 and the visual extinction A_V = 49 -16/+17
mag in the host galaxy. If this is the case, no detection of any optical
transient would be quite reasonable. The absorption derived by the Swift XRT
observations of the afterglow is fully consistent with those obtained from the
early HETE-2 observation of the prompt emission. Our analysis implies an
interpretation that the absorbing medium could be outside external shock at R ~
10^16 cm, which may be a dusty molecular cloud.Comment: 6 pages, 2 figures, accepted for publication in PASJ lette
The incidence of double hypoglossal canal in Japanese: evaluation with multislice computed tomography.
Double hypoglossal canal, namely a hypoglossal canal bridging, is a normal variation of the hypoglossal canal. Racial differences in the prevalence of double hypoglossal canal have been reported. We evaluated the prevalence of double hypoglossal canal in a Japanese population with multidetector computed tomography (MDCT).We reviewed five hundred and ninety consecutive patients (mean age, 61 years: range, 15-94 years: 254 men, 336 women) who underwent computed tomographic angiography (CTA) of the brain for a variety of CNS abnormalities. Two radiologists achieved consensus on the canal being single or double, and measured the sizes of single canals on CT images. Kappa statistics was used to test the reliability between the 2 investigators. A logistic regression was used to evaluate the prevalence of double hypoglossal canal and the following factors: sex, age, and laterality. Student's t-test was used to evaluate the asymmetry of single hypoglossal canal diameters. Statistical significance was accepted at P < 0.05.Double hypoglossal canal was identified in 16.9% of the patients, and was bilateral in 2.2%. Double hypoglossal canal was significantly more frequent on the left side than right (P = 0.004, odds ratio = 1.79) and in males than females (P = 0.011, odds ratio = 1.67). A larger left or right-sided canal was found in 31.6% and 12.2% of the patients, respectively, following the same side preference as that of double hypoglossal canal. Almost perfect agreement was achieved between the two readers (k = 0.975).In this Japanese population, the prevalence of a double hypoglossal canal was 16.9%, of which 2.2% were bilateral. Double hypoglossal canal was more frequent in males than females, and on the left side than right
Causal relations of health indices inferred statistically using the DirectLiNGAM algorithm from big data of Osaka prefecture health checkups.
Causal relations among many statistical variables have been assessed using a Linear non-Gaussian Acyclic Model (LiNGAM). Using access to large amounts of health checkup data from Osaka prefecture obtained during the six fiscal years of years 2012-2017, we applied the DirectLiNGAM algorithm as a trial to extract causal relations among health indices for age groups and genders. Results show that LiNGAM yields interesting and reasonable results, suggesting causal relations and correlation among the statistical indices used for these analyses
Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC 1313
Two ultraluminous X-ray sources (ULXs) in the nearby Sb galaxy NGC 1313, named X-1 and X-2, were observed with Suzaku on 2005 September 15. During the observation for a net exposure of 28 ks (but over a gross time span of 90 ks), both objects varied in intensity by about 50%. The 0.4–10 keV X-ray luminosities of X-1 and X-2 were measured as 2.5×1040 ergs-1 and 5.8×1039 ergs-1, respectively, with the former exhibiting the highest ever reported for this ULX. The spectrum of X-1 can be explained by the sum of a strong and variable powerlaw component with a high-energy cutoff, and a stable multicolor blackbody with an innermost disk temperature of ∼ 0.2keV. These results suggest that X-1 was in a “very high" state, where disk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1 is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is best represented, in its fainter phase, by a multicolor blackbody model with an innermost disk temperature of 1.2–1.3 keV, and becomes flatter as the source becomes brighter. Hence, X-2 is interpreted to be in a slim-disk state. These results suggest that the two ULXs have black hole masses of some dozens to a few hundred of solar masses