166 research outputs found
The Compact Central Object in Cas A: A Neutron Star with Hot Polar Caps or a Black Hole?
The central pointlike X-ray source of the Cas A supernova remnant was
discovered in the Chandra First Light Observation and found later in the
archival ROSAT and Einstein images. The analysis of these data does not show
statistically significant variability of the source. The power-law fit yields
the photon index 2.6-4.1, and luminosity (2-60)e34 erg/s, for d=3.4 kpc. The
power-law index is higher, and the luminosity lower, than those observed
fromvery young pulsars. One can fit the spectrum equally well with a blackbody
model with T=6-8 MK, R=0.2-0.5 km, L=(1.4-1.9)e33 erg/s. The inferred radii are
too small, and the temperatures too high, for the radiationcould be interpreted
as emitted from the whole surface of a uniformly heated neutron star. Fits with
the neutron star atmosphere models increase the radius and reduce the
temperature, but these parameters are still substantially different from those
expected for a young neutron star. One cannot exclude, however, that the
observed emission originates from hot spots on a cooler neutron star surface.
Because of strong interstellar absorption, the possible low-temperature
component gives a small contribution to the observed spectrum; an upper limit
on the (gravitationally redshifted) surface temperature is < 1.9-2.3 MK.
Amongst several possible interpretations, we favor a model of a strongly
magnetized neutron star with magnetically confined hydrogen or helium polar
caps on a cooler iron surface. Alternatively, the observed radiation may be
interpreted as emitted by a compact object (more likely, a black hole)
accreting from a fossil disk or from a late-type dwarf in a close binary.Comment: 12 pages, 2 figures, submitted to ApJ
Using Chandra to Unveil the High-Energy Properties of the High-Magnetic Field Radio Pulsar J1119-6127
(shortened) PSR J1119-6127 is a high magnetic field (B=4.1E13 Gauss), young
(<=1,700 year-old), and slow (P=408 ms) radio pulsar associated with the
supernova remnant (SNR) G292.2-0.5. In 2003, Chandra allowed the detection of
the X-ray counterpart of the radio pulsar, and provided the first evidence for
a compact pulsar wind nebula (PWN). We here present new Chandra observations
which allowed for the first time an imaging and spectroscopic study of the
pulsar and PWN independently of each other. The PWN is only evident in the hard
band and consists of jet-like structures extending to at least 7" from the
pulsar, with the southern `jet' being longer than the northern `jet'. The
spectrum of the PWN is described by a power law with a photon index~1.1 for the
compact PWN and ~1.4 for the southern long jet (at a fixed column density of
1.8E22/cm2), and a total luminosity of 4E32 ergs/s (0.5-7 keV), at a distance
of 8.4 kpc. The pulsar's spectrum is clearly softer than the PWN's spectrum. We
rule out a single blackbody model for the pulsar, and present the first
evidence of non-thermal (presumably magnetospheric) emission that dominates
above ~3keV. A two-component model consisting of a power law component (with
photon index ~1.5--2.0) plus a thermal component provides the best fit. The
thermal component can be fit by either a blackbody model with a temperature
kT~0.21 keV, or a neutron star atmospheric model with a temperature kT~0.14
keV. The efficiency of the pulsar in converting its rotational power, Edot,
into non-thermal X-ray emission from the pulsar and PWN is ~5E-4, comparable to
other rotation-powered pulsars with a similar Edot. We discuss our results in
the context of the X-ray manifestation of high-magnetic field radio pulsars in
comparison with rotation-powered pulsars and magnetars.Comment: 26 pages including 3 tables and 7 figures. Accepted for publication
in Ap
Quiescent Thermal Emission from the Neutron Star in Aql X-1
We report on the quiescent spectrum measured with Chandra/ACIS-S of the
transient, type-I X-ray bursting neutron star Aql X-1, immediately following an
accretion outburst. The neutron star radius, assuming a pure hydrogen
atmosphere and hard power-law spectrum, is =13.4{+5}{-4} (d/5 \kpc)
km. Based on the historical outburst record of RXTE/ASM, the quiescent
luminosity is consistent with that predicted by Brown, Bildsten and Rutledge
from deep crustal heating, lending support to this theory for providing a
minimum quiescent luminosity of transient neutron stars. While not required by
the data, the hard power-law component can account for 18+/-8% of the 0.5-10
keV thermal flux. Short-timescale intensity variability during this observation
is less than 15% rms (3 sigma; 0.0001-1 Hz, 0.2-8 keV). Comparison between the
Chandra spectrum and three X-ray spectral observations made between Oct 1992
and Oct 1996 find all spectra consistent with a pure H atmosphere, but with
temperatures ranging from 145--168 eV, spanning a factor of 1.87+/-0.21 in
observed flux. The source of variability in the quiescent luminosity on long
timescales (greater than years) remains a puzzle. If from accretion, then it
remains to be explained why the quiescent accretion rate provides a luminosity
so nearly equal to that from deep crustal heating.Comment: 15 pages, 1 figure, 2 tables; ApJ, accepte
Evidence for a Binary Companion to the Central Compact Object 1E 1207.4-5209
Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin
period of 424 ms that contains at least two strong absorption features in its
energy spectrum. This neutron star has been identified as a member of the
radio-quiet compact central objects in supernova remnants. It has been found
that 1E 1207.4-5209 is not spinning down monotonically suggesting that this
neutron star undergoes strong, frequent glitches, contains a fall-back disk, or
possess a binary companion. Here, we report on a sequence of seven XMM-Newton
observations of 1E 1207.4-5209 performed during a 40 day window in June/July
2005. Due to unanticipated variance in the phase measurements beyond the
statistical uncertainties, we could not identify a unique phase-coherent timing
solution. The three most probable timing solutions give frequency time
derivatives of +0.9, -2.6, and +1.6 X 10^(-12) Hz/s (listed in descending order
of significance). We conclude that the local frequency derivative during our
XMM-Newton observing campaign differs from the long-term spin-down rate by more
than an order of magnitude, effectively ruling out glitch models for 1E
1207.4-5209. If the long-term spin frequency variations are caused by timing
noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than
in other pulsars with similar period derivatives. Therefore, it is highly
unlikely that the spin variations are caused by the same physical process that
causes timing noise in other isolated pulsars. The most plausible scenario for
the observed spin irregularities is the presence of a binary companion to 1E
1207.4-5209. We identified a family of orbital solutions that are consistent
with our phase-connected timing solution, archival frequency measurements, and
constraints on the companions mass imposed by deep IR and optical observations.Comment: 8 pages, 4 figures. To be published in the proceedings of "Isolated
Neutron Stars: from the Interior to the Surface" (April 24-28, 2006) - eds.
D. Page, R. Turolla & S. Zan
Confronting Neutron Star Cooling Theories with New Observations
With the successful launch of Chandra and XMM/Newton X-ray space missions
combined with the lower-energy band observations, we are in the position where
careful comparison of neutron star cooling theories with observations will make
it possible to distinguish among various competing theories. For instance, the
latest theoretical and observational developments already exclude both nucleon
and kaon direct URCA cooling. In this way we can now have realistic hope for
determining various important properties, such as the composition, degree of
superfluidity, the equation of state and steller radius. These developments
should help us obtain better insight into the properties of dense matter.Comment: 11 pages, 1 figur
Chandra Confirmation of a Pulsar Wind Nebula in DA 495
As part of a multiwavelength study of the unusual radio supernova remnant DA
495, we present observations made with the Chandra X-ray Observatory. Imaging
and spectroscopic analysis confirms the previously detected X-ray source at the
heart of the annular radio nebula, establishing the radiative properties of two
key emission components: a soft unresolved source with a blackbody temperature
of 1 MK consistent with a neutron star, surrounded by a nonthermal nebula 40''
in diameter exhibiting a power-law spectrum with photon index Gamma =
1.6+/-0.3, typical of a pulsar wind nebula. The implied spin-down luminosity of
the neutron star, assuming a conversion efficiency to nebular flux appropriate
to Vela-like pulsars, is ~10^{35} ergs/s, again typical of objects a few tens
of kyr old. Morphologically, the nebular flux is slightly enhanced along a
direction, in projection on the sky, independently demonstrated to be of
significance in radio polarization observations; we argue that this represents
the orientation of the pulsar spin axis. At smaller scales, a narrow X-ray
feature is seen extending out 5'' from the point source, a distance consistent
with the sizes of resolved wind termination shocks around many Vela-like
pulsars. Finally, we argue based on synchrotron lifetimes in the estimated
nebular magnetic field that DA 495 represents a rare pulsar wind nebula in
which electromagnetic flux makes up a significant part, together with particle
flux, of the neutron star's wind, and that this high magnetization factor may
account for the nebula's low luminosity.Comment: 26 pages, 5 figures, AASTeX preprint style. Accepted for publication
in The Astrophysical Journa
The Complex Wind Torus and Jets of PSR B1706-44
We report on Chandra ACIS imaging of the pulsar wind nebula (PWN) of the
young Vela-like PSR B1706-44, which shows the now common pattern of an
equatorial wind and polar jets. The structure is particularly rich, showing a
relativistically boosted termination shock, jets with strong confinement, a
surrounding radio/X-ray PWN and evidence for a quasi-static `bubble nebula'.
The structures trace the pulsar spin geometry and illuminate its possible
relation to SNR G343.1-2.3. We also obtain improved estimates of the pulsar
flux and nebular spectrum, constraining the system age and energetics.Comment: To appear in the Astrophysical Journal. 15pp, 4 figures in 7 file
Powering Anomalous X-ray Pulsars by Neutron Star Cooling
Using recently calculated analytic models for the thermal structure of
ultramagnetized neutron stars, we estimate the thermal fluxes from young
( yr) ultramagnetized ( G) cooling neutron stars.
We find that the pulsed X-ray emission from objects such as 1E 1841-045 and 1E
2259+586 as well as many soft-gamma repeaters can be explained by photon
cooling if the neutron star possesses a thin insulating envelope of matter of
low atomic weight at densities g/cm. The total mass
of this insulating layer is .Comment: 8 pages, 1 figure, to appear in Ap.J. Letters (one reference entry
corrected, no other changes
XMM-Newton observations of the neutron star X-ray transient KS 1731-260 in quiescence
We report on XMM-Newton observations performed on 2001 September 13-14 of the
neutron star X-ray transient KS 1731-260 in quiescence. The source was detected
at an unabsorbed 0.5-10 keV flux of only 4 - 8 x 10^{-14} erg/s, depending on
the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10
keV X-ray luminosity of approximately 2 - 5 x 10^{32} erg/s. The September 2001
quiescent flux of KS 1731-260 is lower than that observed during the Chandra
observation in March 2001. In the cooling neutron star model for the quiescent
X-ray emission of neutron star X-ray transients, this decrease in the quiescent
flux implies that the crust of the neutron star in KS 1731-260 cooled down
rapidly between the two epochs, indicating that the crust has a high
conductivity. Furthermore, enhanced cooling in the neutron star core is also
favored by our results.Comment: Accepter for publication in ApJ Letters, 22 May 200
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