33 research outputs found
Suzaku Observation of the Anomalous X-ray Pulsar 1E 1841-045
We report the results of a Suzaku observation of the anomalous X-ray pulsar
(AXP) 1E 1841-045 at a center of the supernova remnant Kes 73. We confirmed
that the energy-dependent spectral models obtained by the previous separate
observations were also satisfied over a wide energy range from 0.4 to ~70 keV,
simultaneously. Here, the models below ~10 keV were a combination of blackbody
(BB) and power-law (PL) functions or of two BBs wit h different temperatures at
0.6 - 7.0 keV (Morii et al. 2003), and that above ~20 keV was a PL function
(Kuiper Hermsen Mendez 2004). The combination BB + PL + PL was found to best
represent the phase-averaged spectrum. Phase-resolved spectroscopy indicated
the existence of two emission regions, one with a thermal and the other with a
non-thermal nature. The combination BB + BB + PL was also found to represent
the phase-averaged spectrum well. However, we found that this model is
physically unacceptable due to an excessively large area of the emission region
of the blackbody. Nonetheless, we found that the temperatures and radii of the
two blackbody components showed moderate correlations in the phase-resolved
spectra. The fact that the same correlations have been observed between the
phase-averaged spectra of various magnetars (Nakagawa et al. 2009) suggests
that a self-similar function can approximate the intrinsic energy spectra of
magnetars below ~10 keV.Comment: Accepted for publication in the PAS
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
A Shock-Induced Pair of Superbubbles in the High-Redshift Powerful Radio Galaxy MRC 0406-244
We present new optical spectroscopy of the high-redshift powerful radio
galaxy MRC 0406244 at redshift of 2.429. We find that the two extensions
toward NW and SE probed in the rest-frame ultraviolet image are heated mainly
by the nonthermal continuum of the active galactic nucleus. However, each
extension shows a shell-like morphology, suggesting that they are a pair of
superbubbles induced by the superwind activity rather than by the interaction
between the radio jet and the ambient gas clouds. If this is the case, the
intense starburst responsible for the formation of superbubbles could occur
yr ago. On the other hand, the age of the radio jets may
be of the order of yr, being much shorter than the starburst age.
Therefore, the two events, i.e., the starburst and the radio-jet activities,
are independent phenomena. However, their directions of the expanding motions
could be governed by the rotational motion of the gaseous component in the host
galaxy. This idea appears to explain the alignment effect of MRC 0406244.Comment: 4 pages (emulateapj.sty), Fig. 1 (jpeg) + Fig.2 (eps). Accepted for
publications in ApJ (Letters
Spectral Lag Relations in GRB Pulses Detected with HETE-2
Using a pulse-fit method, we investigate the spectral lags between the
traditional gamma-ray band (50-400 keV) and the X-ray band (6-25 keV) for 8
GRBs with known redshifts (GRB 010921, GRB 020124, GRB 020127, GRB 021211, GRB
030528, GRB 040924, GRB 041006, GRB 050408) detected with the WXM and FREGATE
instruments aboard the HETE-2 satellite. We find several relations for the
individual GRB pulses between the spectral lag and other observables, such as
the luminosity, pulse duration, and peak energy (Epeak). The obtained results
are consistent with those for BATSE, indicating that the BATSE correlations are
still valid at lower energies (6-25 keV). Furthermore, we find that the photon
energy dependence for the spectral lags can reconcile the simple curvature
effect model. We discuss the implication of these results from various points
of view.Comment: 13 pages, 9 figures, accepted for the publication in PASJ (minor
corrections
PoGOLite - A High Sensitivity Balloon-Borne Soft Gamma-ray Polarimeter
We describe a new balloon-borne instrument (PoGOLite) capable of detecting
10% polarisation from 200mCrab point-like sources between 25 and 80keV in one 6
hour flight. Polarisation measurements in the soft gamma-ray band are expected
to provide a powerful probe into high-energy emission mechanisms as well as the
distribution of magnetic fields, radiation fields and interstellar matter. At
present, only exploratory polarisation measurements have been carried out in
the soft gamma-ray band. Reduction of the large background produced by
cosmic-ray particles has been the biggest challenge. PoGOLite uses Compton
scattering and photo-absorption in an array of 217 well-type phoswich detector
cells made of plastic and BGO scintillators surrounded by a BGO anticoincidence
shield and a thick polyethylene neutron shield. The narrow FOV (1.25msr)
obtained with well-type phoswich detector technology and the use of thick
background shields enhance the detected S/N ratio. Event selections based on
recorded phototube waveforms and Compton kinematics reduce the background to
that expected for a 40-100mCrab source between 25 and 50keV. A 6 hour
observation on the Crab will differentiate between the Polar Cap/Slot Gap,
Outer Gap, and Caustic models with greater than 5 sigma; and also cleanly
identify the Compton reflection component in the Cygnus X-1 hard state. The
first flight is planned for 2010 and long-duration flights from Sweden to
Northern Canada are foreseen thereafter.Comment: 11 pages, 11 figures, 2 table
Indications of M-Dwarf Deficits in the Halo and Thick Disk of the Galaxy
We compared the number of faint stars detected in deep survey fields with the current stellar distribution model of the Galaxy and found that the detected number in the H band is significantly smaller than the predicted number. This indicates that M-dwarfs, the major component, are fewer in the halo and the thick disk. We used archived data of several surveys in both the north and south field of GOODS (Great Observatories Origins Deep Survey), MODS in GOODS-N, and ERS and CANDELS in GOODS-S. The number density of M-dwarfs in the halo has to be 20 +/- 13% relative to that in the solar vicinity, in order for the detected number of stars fainter than 20.5 mag in the H band to match with the predicted value from the model. In the thick disk, the number density of M-dwarfs must be reduced (52 +/- 13%) or the scale height must be decreased (approximately 600 pc). Alternatively, overall fractions of the halo and thick disks can be significantly reduced to achieve the same effect, because our sample mainly consists of faint M-dwarfs. Our results imply that the M-dwarf population in regions distant from the Galactic plane is significantly smaller than previously thought. We then discussed the implications this has on the suitability of the model predictions for the prediction of non-companion faint stars in direct imaging extrasolar planet surveys by using the best-fit number densities
HETE-2 Observations of the X-Ray Flash XRF 040916
A long X-ray flash was detected and localized by the instruments aboard the
High Energy Transient Explorer II (HETE-2) at 00:03:30 UT on 2004 September 16.
The position was reported to the GRB Coordinates Network (GCN) approximately 2
hours after the burst. This burst consists of two peaks separated by 200 s,
with durations of 110 s and 60 s. We have analyzed the energy spectra of the
1st and 2nd peaks observed with the Wide Field X-Ray Monitor (WXM) and the
French Gamma Telescope (FREGATE). We discuss the origin of the 2nd peak in
terms of flux variabilities and timescales. We find that it is most likely part
of the prompt emission, and is explained by the long-acting engine model. This
feature is similar to some bright X-ray flares detected in the early afterglow
phase of bursts observed by the Swift satellite.Comment: 9 pages, 4 figures, Accepted for publication in PAS