1,069 research outputs found
Discovery of a new pulsating X-ray source with a 1549.1-s period, AX J183220-0840
A new pulsating X-ray source, AX J183220-0840, with a 1549.1-s period was
discovered at R.A.= 18h32m20s and Dec.=-8d40'30'' (J2000,
uncertainty=0.6degree) during an ASCA observation on the Galactic plane. The
source was observed two times, in 1997 and in 1999. A phase-averaged X-ray flux
of 1.1E-11 ergs cm-2 s-1 and pulsation period of 1549.1+/-0.4 s were
consistently obtained from these two observations. The X-ray spectrum was
represented by a flat absorbed power-law with a photon-index of =~0.8 and an
absorption column density of =~1.3E22 cm-2. Also, a signature of iron K-shell
line emission with a centroid of 6.7 keV and an equivalent width of
approximately 450 eV was detected. From the pulsation period and the iron-line
feature, AX J183220-0840 is likely to be a magnetic white dwarf binary with a
complexly absorbed thermal spectrum with a temperature of about 10 keV.Comment: 13 pages, 4 figures, accepted for publication in ApJ Letter
Correlations Between Spectral Properties and Spin-Down Rate in Soft Gamma-Ray Repeaters and Anomalous X-ray Pulsars
Anomalous x-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are x-ray
sources with unusual properties distinguishing them from both rotation-powered
and most accretion-powered pulsars. Using archival ASCA data over the energy
range 0.5-10.0 keV, we have studied the spectra of the persistent emission from
these sources and their variation with spin-down rate. Using a single power law
spectral model, we find that the overall hardness of the spectra increase with
increasing spin-down rate, and therefore the spectral and spin-down mechanism
are inextricably linked in these objects. In terms of the two-component
blackbody plus power law spectral models, this correlation is seen as an
increasing hardness of the high energy component with increasing spin-down
rate, with the temperature of the low energy blackbody component remaining
essentially constant. Also for the two component spectral model: the ratio of
the 2-10 keV power law and bolometric blackbody luminosities gradually
increases with the spin-down rate. We discuss these results in terms of the
various theoretical models for SGRs and AXPs.Comment: 12 pages with 2 figures. Accepted by ApJ Letter
X-ray observations of the high magnetic field radio pulsar PSR J1814-1744
PSR J1814-1744 is a 4 s radio pulsar with surface dipole magnetic field
strength 5.5*10^13 G, inferred assuming simple magnetic dipole braking. This
pulsar's spin parameters are very similar to those of anomalous X-ray pulsars
(AXPs), suggesting that this may be a transition object between the radio
pulsar and AXP population, if AXPs are isolated, high magnetic field neutron
stars as has recently been hypothesized. We present archival X-ray observations
of PSR J1814-1744 made with ROSAT and ASCA. X-ray emission is not detected from
the position of the radio pulsar. The derived upper flux limit implies an X-ray
luminosity significantly smaller than those of all known AXPs. This conclusion
is insensitive to the possibility that X-ray emission from PSR J1814-1744 is
beamed or that it undergoes modest variability. When interpreted in the context
of the magnetar mechanism, these results argue that X-ray emission from AXPs
must depend on more than merely the inferred surface magnetic field strength.
This suggests distinct evolutionary paths for radio pulsars and AXP, despite
their proximity in period--period derivative phase space.Comment: 11 pages, including 2 embedded figures. Accepted by Ap
Can the anomalous X-ray pulsars be powered by accretion?
The nature of the 5-12 s "anomalous" X-ray pulsars remains a mystery. Among
the models that have been proposed to explain the properties of AXPs, the most
likely ones are: (1) isolated accreting neutron stars evolved from the
Thorne-\.{Z}ytkow objects due to complete spiral-in during the common envelope
evolution of high-mass X-ray binaries, and (2) magnetars, which are neutron
stars with ultra-high ( G) surface magnetic fields. We
have critically examined the predicted change of neutron star's spin in the
accretion model, and found that it is unable to account for the steady
spin-down observed in AXPs. A simple analysis also shows that any accretion
disk around an isolated neutron star has extremely limited lifetime. A more
promising explanation for such objects is the magnetar model.Comment: 9 pages, accepted for publication in Ap
A Chandra Observation of Supernova Remnant G350.1-0.3 and Its Central Compact Object
We present a new Chandra observation of supernova remnant (SNR) G350.1-0.3.
The high resolution X-ray data reveal previously unresolved filamentary
structures and allow us to perform detailed spectroscopy in the diffuse regions
of this SNR. Spectral analysis demonstrates that the region of brightest
emission is dominated by hot, metal-rich ejecta while the ambient material
along the perimeter of the ejecta region and throughout the remnant's western
half is mostly low-temperature, shocked interstellar/circumstellar medium
(ISM/CSM) with solar-type composition. The data reveal that the emission
extends far to the west of the ejecta region and imply a lower limit of 6.6 pc
on the diameter of the source (at a distance of 4.5 kpc). We show that
G350.1-0.3 is likely in the free expansion (ejecta-dominated) stage and
calculate an age of 600-1200 years. The derived relationship between the shock
velocity and the electron/proton temperature ratio is found to be entirely
consistent with that of other SNRs. We perform spectral fits on the X-ray
source XMMU J172054.5-372652, a candidate central compact object (CCO), and
find that its spectral properties fall within the typical range of other CCOs.
We also present archival 24 um data of G350.1-0.3 taken with the Spitzer Space
Telescope during the MIPSGAL galactic survey and find that the infrared and
X-ray morphologies are well-correlated. These results help to explain this
remnant's peculiar asymmetries and shed new light on its dynamics and
evolution
Effects of Strong Magnetic Fields on Neutron Star Structure
We study static neutron stars with poloidal magnetic fields and a simple
class of electric current distributions consistent with the requirement of
stationarity. For this class of electric current distributions, we find that
magnetic fields are too large for static configurations to exist when the
magnetic force pushes a sufficient amount of mass off-center that the
gravitational force points outward near the origin in the equatorial plane. (In
our coordinates an outward gravitational force corresponds to , where and are respectively time and radial
coordinates and is coefficient of in the line element.) For the
equations of state (EOSs) employed in previous work, we obtain configurations
of higher mass than had been reported; we also present results with more recent
EOSs. For all EOSs studied, we find that the maximum mass among these static
configurations with magnetic fields is noticeably larger than the maximum mass
attainable by uniform rotation, and that for fixed values of baryon number the
maximum mass configurations are all characterized by an off-center density
maximum.Comment: Submitted to the Astrophysical Journal. 37 pages, 8 figures, uses
aastex macro
The (Re-)Discovery of G350.1-0.3: A Young, Luminous Supernova Remnant and Its Neutron Star
We present an XMM-Newton observation of the long-overlooked radio source
G350.1-0.3. The X-ray spectrum of G350.1-0.3 can be fit by a shocked plasma
with two components: a high-temperature (1.5 keV) region with a low ionization
time scale and enhanced abundances, plus a cooler (0.36 keV) component in
ionization equilibrium and with solar abundances. The X-ray spectrum and the
presence of non-thermal, polarized, radio emission together demonstrate that
G350.1-0.3 is a young, luminous supernova remnant (SNR), for which archival HI
and 12-CO data indicate a distance of 4.5 kpc. The diameter of the source then
implies an age of only ~900 years. The SNR's distorted appearance, small size
and the presence of 12-CO emission along the SNR's eastern edge all indicate
that the source is interacting with a complicated distribution of dense ambient
material. An unresolved X-ray source, XMMU J172054.5-372652, is detected a few
arcminutes west of the brightest SNR emission. The thermal X-ray spectrum and
lack of any multi-wavelength counterpart suggest that this source is a neutron
star associated with G350.1-0.3, most likely a "central compact object", as
seen coincident with other young SNRs such as Cassiopeia A.Comment: 6 pages, uses emulateapj. One B/W figure, one color figure. Minor
text changes and update to Fig 2 following referee's report. ApJ Letters, in
pres
A contribution of stellar flares to the GRXE -- based on MAXI observations --
Using unbiased observations of MAXI/GSC the potential contribution of stellar
flares and CVs to GRXE luminosity is estimated in the energy range of 2 - 10
keV. Currently, a reasonable luminosity has been obtained extrapolating the
number of stellar flares and that of CVs toward the Galactic ridge from those
of the observed flares and CVs near the solar system. The ionized emission
lines of Si to Fe are also simulated making the composite thermal spectrum
which is based on MAXI observations of stellar flares and CVs. The present
estimated result strongly supports a picture that the cumulative stellar flares
contribute primarily to the GRXE in terms of the composite thermal spectrum
with emission lines and secondary contribution is due to the thermal spectrum
with high temperature from CVs
Spectral properties of anomalous X-ray pulsars
In this paper, the spectra of the persistent emission from anomalous X-ray
pulsars (AXPs) and their variation with spin-down rate is
considered. Firstly, based on an accretion-powered model, the influences of
both magnetic field and mass accretion rate on the spectra properties of AXPs
are addressed. Subsequently, the relation between the spectral property of AXPs
and mass accretion rate is investigated. The result shows that there
exists a linear correlation between the photon index and mass accretion rate,
and the spectral hardness increases with increasing . A possible
emission mechanism for the explanation of spectral properties of AXPs is also
discussed.Comment: 11pages, 3 figures, Chin. J. Astron. Astrophys. in pres
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