9,822 research outputs found
Polarization of Thermal X-rays from Isolated Neutron Stars
Since the opacity of a magnetized plasma depends on polarization of
radiation, the radiation emergent from atmospheres of neutron stars with strong
magnetic fields is expected to be strongly polarized. The degree of linear
polarization, typically ~10-30%, depends on photon energy, effective
temperature and magnetic field. The spectrum of polarization is more sensitive
to the magnetic field than the spectrum of intensity. Both the degree of
polarization and the position angle vary with the neutron star rotation period
so that the shape of polarization pulse profiles depends on the orientation of
the rotational and magnetic axes. Moreover, as the polarization is
substantially modified by the general relativistic effects, observations of
polarization of X-ray radiation from isolated neutron stars provide a new
method for evaluating the mass-to-radius ratio of these objects, which is
particularly important for elucidating the properties of the superdense matter
in the neutron star interiors.Comment: 7 figures, to be published in Ap
Hard Extended X-ray Source in the IC 443 SNR Resolved by Chandra: A Fast Ejecta Fragment or a New Pulsar Wind Nebula?
A Chandra observation of the isolated hard X-ray source XMMU
J061804.3+222732, located in the region of apparent interaction of the
supernova remnant IC 443 with a molecular cloud, resolved the complex structure
of the source in a few bright clumps embedded in an extended emission of a ~ 30
arcsec size. The X-ray spectra of the clumps and the extended emission are
dominated by a hard power-law component with a photon index of 1.2--1.4. In
addition, we see some indications of an optically thin thermal plasma of a ~
0.3 keV temperature. The observed X-ray morphology and spectra are consistent
with those expected for an isolated supernova ejecta fragment interacting with
a dense ambient medium. A possible alternative interpretation is a pulsar wind
nebula associated with either IC 443 or another SNR, G189.6+3.3.Comment: Accepted for publication in The Astrophysical Journal Letters High
resolution Images of Fig.1 are appende
Search for the Optical Counterpart of the Vela Pulsar X-ray Nebula
Observations of the Vela pulsar region with the Chandra X-ray observatory
have revealed the fine structure of its synchrotron pulsar-wind nebula (PWN),
which showed an overall similarity with the Crab PWN. However, contrary to the
Crab, no firm detection of the Vela PWN in optical has been reported yet. To
search for the optical counterpart of the X-ray PWN, we analyzed deep optical
observations performed with different telescopes. We compared the optical
images with those obtained with the Chandra ACIS to search for extended
emission patterns which could be identified as counterparts of the X-ray nebula
elements. Although some features are seen in the optical images, we find no
correlation with the X-ray structure. Thus, we conclude that the diffuse
optical emission is more likely associated with filaments in the host Vela SNR.
The derived upper limits on the optical flux from the PWN are compatibile,
within the uncertainties, with the values expected on the basis of the
extrapolations of the X-ray data.Comment: 19 pages, 6 figures. Accepted for publication in Ap
Detection of X-ray Emission from the Very Old Pulsar J0108-1431
PSR J0108-1431 is a nearby, 170 Myr old, very faint radio pulsar near the
"pulsar death line" in the P-Pdot diagram. We observed the pulsar field with
the Chandra X-ray Observatory and detected a point source (53 counts in a 30 ks
exposure, energy flux (9+/-2)\times 10^{-15} ergs cm^{-2} s^{-1} in the 0.3-8
keV band) close to the radio pulsar position. Based on the large X-ray/optical
flux ratio at the X-ray source position, we conclude that the source is the
X-ray counterpart of PSR J0108-1431.The pulsar spectrum can be described by a
power-law model with photon index Gamma \approx 2.2 and luminosity L_{0.3-8
keV} \sim 2\times 10^{28} d_{130}^2 ergs s^{-1}, or by a blackbody model with
the temperature kT\approx 0.28 keV and bolometric luminosity L_{bol} \sim
1.3\times 10^{28} d_{130}^2 ergs s^{-1}, for a plausible hydrogen column
density NH = 7.3\times 10^{19} cm^{-2} (d_{130}=d/130 pc). The pulsar converts
\sim 0.4% of its spin-down power into the X-ray luminosity, i.e., its X-ray
efficiency is higher than for most younger pulsars. From the comparison of the
X-ray position with the previously measured radio positions, we estimated the
pulsar proper motion of 0.2 arcsec yr^{-1} (V_\perp \sim 130 d_{130} km
s^{-1}), in the south-southeast direction.Comment: 8 pages, 9 figures, accepted to ApJ; minor revisions in Sections 2.2
and 3.
Variability of the Vela Pulsar-wind Nebula Observed with Chandra
The observations of the pulsar-wind nebula (PWN) around the Vela pulsar with
the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory,
taken on 2000 April 30 and November 30, reveal its complex morphology
reminiscent of that of the Crab PWN. Comparison of the two observations shows
changes up to 30% in the surface brightness of the PWN features. Some of the
PWN elements show appreciable shifts, up to a few arcseconds (about 10^{16}
cm), and/or spectral changes. To elucidate the nature of the observed
variations, further monitoring of the Vela PWN is needed.Comment: 7 pages (incl. 3 embedded PS figures), AASTEX, uses emulateapj5.sty.
Submitted to ApJ Lett. For a high-resolution color PS image of Figure 3 (6.3
Mby), see http://www.astro.psu.edu/users/divas/velaneb_fig3.p
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