46 research outputs found
Constraints on the Emission and Viewing Geometry of the Transient Anomalous X-ray Pulsar XTE J1810-197
The temporal decay of the flux components of Transient Anomalous X-ray Pulsar
XTE J1810-197 following its 2002 outburst presents a unique opportunity to
probe the emission geometry of a magnetar. Toward this goal, we model the
magnitude of the pulsar's modulation in narrow spectral bands over time.
Following previous work, we assume that the post-outburst flux is produced in
two distinct thermal components arising from a hot spot and a warm concentric
ring. We include general relativistic effects on the blackbody spectra due to
gravitational redshift and light bending near the stellar surface, which
strongly depend on radius. This affects the model fits for the temperature and
size of the emission regions. For the hot spot, the observed temporal and
energy-dependent pulse modulation is found to require an anisotropic,
pencil-beamed radiation pattern. We are able to constrain an allowed range for
the angles that the line-of-sight (psi) and the hot spot pole (xi) make with
respect to the spin-axis. Within errors, this is defined by the locus of points
in the xi-psi plane that lie along the line (xi+beta(R))(psi+beta(R)) ~
constant, where beta(R) is a function of the radius R of the star. For a
canonical value of R=12 km, the viewing parameters range from psi=xi=37 deg to
(psi,xi)=(85 deg,15 deg). We discuss our results in the context of magnetar
emission models.Comment: 8 pages, accepted to Ap
Investigating CXOU J163802.6-471358: a new pulsar wind nebula in the Norma region?
We present the first analysis of the extended source CXOU J163802.6--471358,
which was discovered serendipitously during the {\em Chandra} X-ray survey of
the Norma region of the Galactic spiral arms. The X-ray source exhibits a
cometary appearance with a point source and an extended tail region. The
complete source spectrum is fitted well with an absorbed power law model and
jointly fitting the {\em Chandra} spectrum of the full source with one obtained
from an archived {\em XMM-Newton} observation results in best fit parameters
and
(90 confidence uncertainties). The unabsorbed
luminosity of the full source is then ergs
s with kpc, where a distance of 10 kpc is a lower bound
inferred from the large column density. The radio counterpart found for the
source using data from the Molonglo Galactic Plane Survey epoch-2 (MGPS-2)
shows an elongated tail offset from the X-ray emission. No infrared counterpart
was found. The results are consistent with the source being a previously
unknown pulsar driving a bow shock through the ambient medium
Chandra Monitoring of the Candidate Anomalous X-ray Pulsar AX J1845.0-0258
The population of clearly identified anomalous X-ray pulsars has recently
grown to seven, however, one candidate anomalous X-ray pulsar (AXP) still
eludes re-confirmation. Here, we present a set of seven Chandra ACIS-S
observations of the transient pulsar AX J1845.0-0258, obtained during 2003. Our
observations reveal a faint X-ray point source within the ASCA error circle of
AX J1845.0-0258's discovery, which we designate CXOU J184454.6-025653 and
tentatively identify as the quiescent AXP. Its spectrum is well described by an
absorbed single-component blackbody (kT~2.0 keV) or power law (Gamma~1.0) that
is steady in flux on timescales of at least months, but fainter than AX
J1845.0-0258 was during its 1993 period of X-ray enhancement by at least a
factor of 13. Compared to the outburst spectrum of AX J1845.0-0258, CXOU
J184454.6-025653 is considerably harder: if truly the counterpart, then its
spectral behaviour is contrary to that seen in the established transient AXP
XTE J1810-197, which softened from kT~0.67 keV to ~0.18 keV in quiescence. This
unexpected result prompts us to examine the possibility that we have observed
an unrelated source, and we discuss the implications for AXPs, and magnetars in
general.Comment: 4 pages, 3 figures. To be published in the proceedings of the
conference "Isolated Neutron Stars: from the Interior to the Surface" (April
24-28, 2006, London, UK), eds. D. Page, R. Turolla, & S. Zan
High-Resolution X-ray Imaging Studies of Neutron Stars, Pulsar Wind Nebulae and Supernova Remnants
Supernova remnants serve as nearby laboratories relevant to many areas in
Astrophysics, from stellar and galaxy evolution to extreme astrophysics and the
formation of the heavy elements in the Universe. The Chandra X-ray mission has
enabled a giant leap forward in studying both SNRs and their compact stellar
remnants on sub-arcsecond scale. However, such high-resolution imaging studies
have been mostly limited to the nearby and/or relatively bright objects. There
is no question that we are missing a large population, especially in external
galaxies. Within our own Galaxy, we are presented with new fundamental
questions related to neutron stars' diversity, kicks, relativistic winds and
the way these objects interact with, and impact, their host environments. In
this white paper, we highlight some of the breakthroughs to be achieved with
future X-ray missions (such as the proposed AXIS probe) equipped with
sub-arcsecond imaging resolution and an order of magnitude improvement in
sensitivity.Comment: Astro2020 Science White Paper. 9 pages, 2 figure
ROSAT and ASCA observations of the Crab-Like Supernova Remnant N157B in the Large Magellanic Cloud
We report the results of ROSAT and ASCA X-ray observations of the supernova
remnant N157B (or 30 Dor B, SNR 0539-69.1) in the Large Magellanic Cloud. For
comparison, we also briefly describe the results on SNR 0540-69.3, the only
confirmed Crab-like remnant in the Cloud. The X-ray emission from N157B can be
decomposed into a bright comet-shaped feature, superimposed on a diffuse
emission region of a dimension pc. The flat and nearly featureless
spectrum of the remnant is distinctly different from those of young shell-like
remnants, suggesting a predominantly Crab-like nature of N157B. Characterized
by a power law with an energy slope , the spectrum of N157B above
keV is, however, considerably steeper than that of SNR 0540-69.3,
which has a slope of . At lower energies, the spectrum of N157B
presents marginal evidence for emission lines, which if real most likely arise
in hot gas of the diffuse emission region. The hot gas has a characteristic
thermal temperature of 0.4-0.7 keV. No significant periodic signal is detected
from N157B in the period range of s. The pulsed
fraction is (99% confidence) in the keV range. We discuss
the nature of the individual X-ray components. In particular, we suggest that
the synchrotron radiation of relativistic particles from a fast-moving () pulsar explains the size, morphology, spectrum, and energetics
of the comet-shaped X-ray feature. We infer the age of the remnant as yrs. The lack of radio polarization of the remnant may be due to
Faraday dispersion by foreground \ion{H}{2} gas.Comment: To be published in The Astrophysical Journal, 21 pages, plus 11
images in the PS, GIF, or jpeg format. Postscript files of images are
available at http://www.astro.nwu.edu/astro/wqd/paper/n157b